Meeting Report for: The 2nd International Meeting on Esterases Reacting with Organophosphorus Compounds

Although the immediate action of organophosphorus esters is the inhibition of acetylcholinesterase, some of these compounds also produce a neurodegenerative disorder known as organophosphorus ester-induced delayed neurotoxicity (OPIDN). Tri-o-cresyl phosphate (TOCP) first produced this condition in humans and later in sensitive animal species. OPIDN is characterized by a delay period prior to onset of ataxia and paralysis. The neuropathologic lesions are Wallerian-type degeneration of the axon and myelin in the distal parts of the large tracts in both the central and peripheral nervous systems. In the past decade we have demonstrated that the pathognomonic features of OPIDN are an aberrant increase in autophosphorylation of calcium/calmodulin kinase II (CaM kinase II) and an increase in phosphorylation of cytoskeletal proteins, i.e., MAPs, tubulin, neurofilament triplet proteins, and myelin basic protein. Protein kinase-mediated phosphorylation of cytoskeletal proteins plays a critical role in regulating the growth and maintenance of the axon. We hypothesize that, in OPIDN, hyperphosphorylation of cytoskeletal proteins and axonal swelling are causally linked. Hyperphosphorylation of cytoskeletal proteins decreases their transport rate down the axon relative to their rate of entry into the axon, thus leading to their accumulation. Consistent with this hypothesis is our finding of the anomalous accumulation of phosphorylated neurofilament aggregates in the central and peripheral axons of hens treated with TOCP.

Many proteins capable of hydrolysing esters are present in biological material of all kinds (microorganisms, plants, invertebrates and vertebrates). Some serve, as indicated by their substrate specificity and distribution within organisms, a defined biological function. However for most esterases a rather general substrate specificity is found indicating that they may have a broad biological function. Their properties will be briefly reviewed with particular emphasis on inhibitors. The mechanism of hydrolysis of esters by many carboxylesterases (B-esterases) is well established largely due to the reaction of OP compounds with their catalytic centre. For others, such as enzymes hydrolysing (i) OP compounds and/or (ii) carboxyl esters which are not inhibited by a time and temperature dependent reaction by OP compounds, reaction mechanisms are still conjecture. The purpose of this presentation is to explore similarities and differences between the esterases and to discuss possible routes for progress in the A-esterase group.

A short review is presented on the key points in the development of the hypothesis for the initiation of delayed neuropathy by reaction of organophosphorus compounds with neuropathy target esterase (NTE). It is now clear from information derived from experiments showing protection and promotion and from the action of some phosphorothioamidates (which cause delayed neuropathy without aging) that the original NTE hypothesis is not generally applicable and requires modification. A suggestion is made that aging of phosphorylated and phosphonylated NTE is facilitated perhaps by two linked esterases, one being NTE.

In the conditions of chronically elevated glucocorticoid agents in plasma, a drop in AChE activity of about 45% was reported. This data suggests the possibility that among other factors glucocorticoids also control AChE activity in the skeletal muscles. The question addressed in the present investigation was if AChE activity was reduced uniformly or selectively in the rat skeletal muscles after chronic application of dexamethasone? Selective effects of glucocorticoids on the AChE activity in different muscles and/or different types or regions of muscles would suggest the potential of these agents to regulate AChE metabolism in the skeletal muscle according to the environmental demands. Specific activity of skeletal muscle AChE was reduced in sternomastoideus (SM), extensor digitorum longus (EDL) and diaphragm (D) but not in soleus (SOL) after chronic dexamethasone treatment. Axial SM (white part) was more affected than distal white muscle EDL. AChE was better preserved in red rather than in white parts of muscles. The endplate-free region lost twice as much of specific AChE activity than the endplate-rich region. Our results suggest, but do not prove that glucocorticoid agents act in a selective way on the AChE metabolism of the skeletal muscles.

Transition state stabilization is considered one means by which enzymes reduce free energy of activation. The transition state of phosphonic acid anhydrides acted on by OPA hydrolase is postulated to be pentacoordinate, which ordains either a square pyramid or a trigonal bipyramid structure. The advent of catalytic monoclonal antibodies has provided a system in which these assumptions can be tested. By immunizing mice with the protein conjugate of a trigonal bipyramid transition state analog, we have produced hybridomas secreting monoclonal antibodies which hydrolyze phosphonates. To date, activity has been shown toward pinacolyl methylphosphonofluoridic acid (soman). Preliminary results suggest that the antibody is an IgG2a with kappa light chain character. Our results support the trigonal bipyramidal transition state for this group of enzymes.

Since pharmacologic treatments of organophosphorus anticholinesterases (OPs) are nearing their practical limit other types of treatment are being sought. One approach is the prophylactic administration of scavengers that will detoxify OPs before they reach their critical target site. We have shown that mice which have been treated with a purified organophosphorus acid anhydride hydrolase (OPAH) are not measurably affected by doses of soman that are lethal to untreated animals. This result indicates that this approach is worthy of exploration and development for protecting military personnel and agriculture workers against OP intoxication.

Eptastigmine (MF 201) is a new physostigmine derivative with potent inhibitory activity on cholinesterases. Here we present a new potentiometric cholinesterase activity assay suitable for MF 201 monitoring. The analysis is performed on a differential pH system and has the following characteristics: (a) within-run precision: C.V. 2.0% (plasma cholinesterase), 1.8% (red cell cholinesterase); (b) between-run precision: C.V. 4.0% (plasma cholinesterase); (c) linearity: 1-10 kU/l (plasma cholinesterase), 6-70 U/g Hb (red cell cholinesterase); (d) comparison with a reference method (x, HITACHI 737 Boerhinger Mannheim, Italy): y = 0.785x - 0.07; n = 37; r = 0.998. The assay has been applied to the determination of plasma and red cell cholinesterase activity in volunteers over 60 years of age treated with a single oral dose of 30 mg eptastigmine. We found that red cell cholinesterase is selectively inhibited after MF 201 administration with the following kinetics (time, % of inhibition, mean +/- S.E., n = 6): 0 h, 0; 1 h, 17 +/- 4.6; 2 h, 24 +/- 4; 4 h, 23 +/- 4.4; 12 h, 14 +/- 3. Eptastigmine plasma levels were also determined by a HPLC method: maximum concentration was found one hour after drug administration.

The kinetics of time- and concentration-dependent covalent organophosphorus inhibition of carboxylesterase isoenzymes (EC 3.1.1.1) and cholinesterase isoenzymes (EC 3.1.1.7 and EC 3.1.1.8) were investigated using a wide range of organophosphate inhibitor concentrations (10(-10)-10(-3) mol/l) and different inhibition times. Computerized analysis of inhibition curves by weighted non-linear least-squares curve fitting was compared to graphic analysis by iterative elimination of exponential functions. Possible experimental errors due to inhibitor saturation kinetics and enzymatic organophosphate hydrolysis were thoroughly investigated. In mammalian heart muscle, three different cholinesterase isoenzymes were identified. High sensitivity and specificity of the classic differential inhibition test for carboxylesterase activity of hen brain neuropathy target esterase (NTE) could be confirmed independently with both methods of inhibition curve analysis.

Pseudocholinesterase (ChE) (acylcholineacylhydrolase, EC 3.1.1.8) has been partially purified (about 270-fold) from sheep brain. The procedure included ammonium sulfate fractionation (20-80%), DEAE-Trisacryl M chromatography and procainamide-Sepharose 4B affinity chromatography. The molecular weight of purified ChE was found to be 290,000 by gel filtration. Kinetic properties of the enzyme have been studied using the substrate analogues choline, succinylcholine and benzoylcholine. It was shown that the inhibition was partially competitive.

There are a variety of enzymes which are specifically capable of hydrolyzing organophosphorus esters with different phosphoryl bonds from the typical phosphotriester bonds of common insecticidal neurotoxins (e.g. paraoxon or coumaphos) to the phosphonate-fluoride bonds of chemical warfare agents (e.g. soman or sarin). These enzymes comprise a diverse set of enzymes whose basic architecture and substrate specificities vary dramatically, yet they appear to be ubiquitous throughout nature. The most thoroughly studied of these enzymes is the organophosphate hydrolase (opd gene product) of Pseudomonas diminuta and Flavobacterium sp. ATCC 27551, and the heterologous expression, post-translational modification, and genetic engineering studies undertaken with this enzyme are described.

Previously, a G-type nerve agent degrading enzyme activity was found in a halophilic bacterial isolate designated JD6.5. This organism was tentatively identified as an unknown species of the genus Alteromonas. In order to determine whether this type of enzyme activity was common in other species of Alteromonas, a screening program was initiated. A number of Alteromonas species and five halophilic bacterial isolates were cultured and their crude cell extracts screened for hydrolytic activity against several organophosphorus chemical agents and other related compounds. The samples were also screened for cross-reactivity with a monoclonal antibody raised against the purified enzyme from JD6.5 and for hybridization with a DNA probe based on its N-terminal amino acid sequence A wide spectrum of activities and reactivities were seen, suggesting a significant heterogeneity between the functionally similar enzymes that are present in these bacterial species. Enzymes of the type described here have considerable potential for the decontamination and demilitarization of chemical warfare agents.

The present treatment for poisoning by organophosphates consists of multiple drugs such as carbamates, antimuscarinics, and reactivators in pre- and post-exposure modalities. Recently an anticonvulsant, diazapam, has been included as a post-exposure drug to reduce convulsions and increase survival. Most regimens are effective in preventing lethality from organophosphate exposure but do not prevent toxic effects and incapacitation observed in animals and likely to occur in humans. Use of enzymes such as cholinesterases as pretreatment drugs for sequestration of highly toxic organophosphate anticholinesterases and alleviation of side effects and performance decrements was successful in animals, including non-human primates. Pretreatment of rhesus monkeys with fetal bovine serum acetylcholinesterase protected them against lethal effects of soman (up to 5 LD50) and prevented signs of OP toxicity. Monkeys pretreated with fetal bovine serum acetylcholinesterase were devoid of behavioral incapacitation after soman exposure, as measured by serial probe recognition or primate equilibrium platform performance tasks. Use of acetylcholinesterase as a single pretreatment drug provided greater protection against both lethal and behavioral effects of potent organophosphates than current multicomponent drug treatments that prevent neither signs of toxicity nor behavioral deficits. Although use of cholinesterases as single pretreatment drugs provided complete protection, its use for humans may be limited, since large quantities will be required, due to the approximately 1:1 stoichiometry between organophosphate and enzyme. Bisquaternary oximes, particularly HI-6, have been shown to reactivate organophosphate-inhibited acetylcholinesterase at a rapid rate. We explored the possibility that enzyme could be continually reactivated in animals pretreated with fetal bovine serum acetylcholinesterase, followed by an appropriate dose of reactivator, and challenged with repeated doses of sarin. In in vitro experiments, stoichiometry greater than 1:400 for enzyme:sarin was achieved; in vivo stoichiometry in mice was 1:65. Pretreatment of mice with fetal bovine serum acetylcholinesterase and HI-6 amplified the effectiveness of exogenous enzyme as a scavenger for organophosphate.

Concentrations of parent pesticide and corresponding diethylphosphorus metabolites in blood serum and urine were investigated in persons who had ingested a concentrated solution of organophosphorus pesticide chlorpyrifos. The organophosphate poisoning was indicated by a significant depression of blood cholinesterase (EC 3.1.1.7 and EC 3.1.1.8) activities. Blood and spot urine samples were collected daily after admission of the persons to hospital. Chlorpyrifos was detected only in serum samples in a period up to 15 days after poisoning. In the same samples chlorpyrifos oxygen analogue, chlorpyrifos oxon, was not detected. The presence of diethylphosphorothioate in all serum and urine samples confirmed that part of chlorpyrifos was hydrolysed before its oxidation. The maximum concentrations of chlorpyrifos in serum and of metabolites in serum and urine were measured on the day of admission. The decrease in concentrations followed the first-order kinetics with the initial rate constant faster and the later one slower. In the faster elimination phase chlorpyrifos was eliminated from serum twice as fast (t1/2 = 1.1-3.3 h) as the total diethylphosphorus metabolites (t1/2 = 2.2-5.5 h). The total urinary diethylphosphorus metabolites in six chlorpyrifos poisoned persons were excreted with an average elimination half-time of 6.10 +/- 2.25 h (mean +/- S.D.) in the faster and of 80.35 +/- 25.8 h in the slower elimination phase.

Adult White Leghorn hens were acutely exposed to 3 dosages of the following organophosphorus compounds: mipafox, tri-ortho-tolyl phosphate (TOTP), phenyl saligenin phosphate, and diisopropylphosphorofluoridate (DFP). Neuropathy target esterase (NTE) activity was measured in brain and spinal cord 4 or 48 h after exposure. Ataxia was assessed using an 8-point rating scale on days 9 through 21 after administration, and neuropathological examination was conducted on samples collected from perfusion-fixed animals on day 21. Morphological alterations were indicated by lesion scores between 0 (no lesions) and 4 (diffuse involvement of spinal cord tracts and > 25% degeneration of peripheral nerve fibers). Dosages of mipafox (30 mg/kg i.p.), TOTP (500 mg/kg p.o.), phenyl saligenin phosphate (2.5 mg/kg i.m.) and DFP (1 mg/kg s.c.) that were capable of inhibiting NTE > 80% in both brain and spinal cord preceded ataxia which reached maximal levels (scores of 7-8), and development of lesions scored as 4. Hens were notably impaired (ataxia scores of 3-4) 21 days after administration of dosages of mipafox (3 and 6 mg/kg), TOTP (90 mg/kg), phenyl saligenin phosphate (0.1 and 0.2 mg/kg), and DFP (0.4 mg/kg) when spinal cord NTE was inhibited 40-75%. Lesions were, however, only noted in spinal cord and peripheral nerves of hens given TOTP or DFP (scores 1-3). These data indicate that inhibition of spinal cord NTE > 80% was predictive of severe ataxia and extensive pathology in the hen and that less NTE inhibition was indicative of less severe ataxia and a lower score for neuropathological damage.

Quantitative and qualitative changes of acetylcholinesterase can affect the sensitivity of insects to insecticides. First, the amount of acetylcholinesterase in the central nervous system is important in Drosophila melanogaster, flies which overexpress the enzyme are more resistant than wild-type flies. On the contrary, flies which express low levels of acetylcholinesterase are more susceptible. An overproduction of acetylcholinesterase outside the central nervous system also protects against organophosphate poisoning, that is, flies producing a soluble acetylcholinesterase, secreted in the haemolymph, are resistant to organophosphates. Second, resistance can also result from a qualitative modification of acetylcholinesterase. Four mutations have been identified in resistant strains: Phe115 to Ser, Ileu199 to Val, Gly303 to Ala and Phe368 to Tyr. Each of these mutations led to a different pattern of resistance and combinations between these mutations led to highly resistant enzymes.

Two pyridinium and two imidazolium dioximes were tested as reversible inhibitors of human erythrocyte acetylcholinesterase (AChE), as protectors of the enzyme against phosphorylation and as reactivators of the phosphorylated AChE. All four dioximes reversibly inhibited AChE, protected the enzyme against phosphorylation by soman and tabun and reactivated AChE after phosphorylation by sarin, VX and tabun. From the experimental results the enzyme/dioxime dissociation constants were evaluated for the catalytically active enzyme and for phosphorylated enzyme. The evaluation constants have shown that all four dioximes have about the same affinity for the catalytically active as for the phosphylated AChE. Obtained results also indicate that imidazolium dioximes probably bind only to the allosteric, while pyridinium dioximes bind to both, the catalytic and the allosteric site of the enzyme.

Human and rabbit paraoxonases/arylesterases were purified to homogeneity by chromatographic and gel electrophoretic/isofocusing procedures coupled with activity stains. N-terminal and peptide sequence analysis suggested retention of the secretion signal sequence and allowed design of oligonucleotide probes. The probes were used to isolate a 1294-bp rabbit paraoxonase cDNA clone, which, in turn, was used to isolate three human cDNA clones. Comparison of rabbit and human protein and cDNA sequences indicated a high degree of sequence conservation (approximately 85% identity) and verified that paraoxonase retains its signal sequence (except for the N-terminal Met). The rabbit cDNA encodes a protein of 359 amino acids and the human a protein of 355 amino acids. In situ hybridization demonstrated, as expected, that the paraoxonase gene maps to the long arm of human chromosome 7. Arginine at position 192 specifies high activity paraoxonase and glutamine low activity human paraoxonase. Variation in protein levels explains the variation of enzyme activity observed within a genetic class. Toxicity studies showed that raising rat plasma paraoxonase levels by i.v. administration of partially purified rabbit paraoxonase protected animals against cholinesterase inhibition by paraoxon and chlorpyrifos oxon. Protection correlated with the relative rates of hydrolysis of these two compounds.

Monoclonal antibodies (mAbs) were prepared against native or DFP-inhibited Torpedo californica acetylcholinesterase and native or DFP-, MEPQ-, and soman-inhibited fetal bovine serum acetylcholinesterase. The cross reactivity of these antibodies with acetylcholinesterases from various species and their ability to inhibit catalytic activity were determined. Eight antibodies were found to inhibit catalytic activity of either Torpedo or fetal bovine serum enzyme. In all cases the antibodies bound to the native form of the enzymes and in some cases even to the denatured form. None of the antibodies recognized human or horse serum butyrylcholinesterase. Sucrose density gradient centrifugation of enzyme-antibody complexes provided two types of profiles, one with multiple peaks, indicating numerous complexes between tetrameric forms of the enzyme, and the other with single peaks, demonstrating complex formation within the tetrameric form. Different antibodies appeared to interact with slightly different regions, but in all cases the binding encompassed the peripheral anionic site. Decrease in catalytic activity of the enzyme was most likely caused by conformational changes in the enzyme molecule resulting from interaction with these mAbs.

A method for the partial purification of rat liver paraoxonase is presented. The method consists of the following steps: preparation of microsomes, solubilization with Triton X-100, adsorption on hydroxylapatite and chromatography on DEAE-52 cellulose. A partially purified preparation of rat liver paraoxonase has been obtained, showing a specific activity of 422 mU/mg with a yield of about 22% and a purification factor of 77-fold.

The subcellular localization and some biochemical properties of rat liver paraoxonase have been studied in order to establish a correlation with plasma enzyme. The whole paraoxonase activity was found in the microsomal fraction. Rat plasma and liver paraoxonase showed similar optimum pH (8.5), Km (0.4 mM) and calcium requirement, but differed in the response to several inhibitors.

Neuropathy target esterase (NTE) in hen brain membranes can be labelled with tritiated di-isopropylfluorophosphate ([3H]DFP) and appears to be associated with a 155-kDa polypeptide. Using preparative SDS-PAGE, we have obtained preparations in which [3H]DFP-labelled NTE comprises 2% of the total protein. Further purification of the 155-kDa polypeptide has proved difficult. We therefore attempted to use proteases to excise smaller [3H]DFP-labelled fragments which might be more amenable to fractionation. V8 protease treatment generated a labelled fragment of about 16 kDa which could be fractionated on SDS-PAGE and contained tritium attached to both site X (putatively the active site serine) and site Z (the residue to which an isopropyl moiety is transferred during aging of [3H]DFP-inhibited NTE). Papain and thermolysin treatments generated a small labelled peptide (< 10 kDa) which could be fractionated on reverse-phase HPLC and in which tritium was attached to site X but not site Z. N-terminal sequencing of the thermolysin-generated peptide fraction indicated sample heterogeneity but also suggested that the active site of NTE may contain the serine esterase consensus sequence: Gly-Glu-Ser-Xxx-Gly.

This is a preliminary report on our attempts of synthesis by polymerase chain reaction (PCR), the cDNA probe for the determination of mRNA of the AChE catalytic subunit. As our strategy we took the advantage of the fact that sequence identity of AChE gene increases with phylogenetic proximity. Single codon usage could therefore be applied. Two non-degenerate PCR primers were synthesised corresponding to AChE regions which were highly conservative among species analyzed until now. The sequence amplified by these two primers should be 339 base pairs long as concluded from mouse AChE sequence. By determining the nucleotide sequence of the PCR product and by comparison of this sequence with the corresponding mouse AChE region, we would be able to verify the correspondence of our PCR product to the rat AChE gene fragment. Only the first four amino acids of our PCR product flanking Phe 200, which is the first amino acid from the A2 primer, are 100% homologous with the mouse AChE. However, from the next 18 amino acids towards the N-terminal, only 4 are homologous with the mouse AChE. Since we expected more than 90% homology between the phylogenetically closely related species of mouse and rat, we doubt that the DNA sequence obtained belongs to the rat AChE gene.

Soman simulator PDP is a compound that has a chemical structure identical to soman, except that the fluorine atom is replaced by a methyl group which makes PDP unable to bind covalently to the AChE active center. In rats, late mortality observed after treatment with high doses of soman could be prevented by PDP pretreatment. Such pretreatment has been much less efficient in primates. The effect of PDP in rats has been explained by blocking the deposition of soman in so-called soman depots in which soman is stored intact and subsequently released. In this paper we demonstrate that in the presence of PDP, inhibition of rat muscle AChE by soman is reduced in rat but not in human muscle homogenates. This result suggests that at least part of the beneficial effects of PDP pretreatment in rat might be due to the direct interaction of PDP with AChE resulting in reduced AChE phosphorylation by soman.

Pretreatment of rats with iso-OMPA one hour prior to each of the N-methylcarbamate insecticides, carbofuran, propoxur, or aldicarb, potentiated the toxicity of these carbamates threefold. None of these compounds alone in the dosage used produced toxic signs; however, carboxylesterase (CarbE) activity in a variety of organs including brain, muscle, liver, and plasma was significantly reduced, while acetylcholinesterase (AChE) activity was unchanged. Significant inhibition of AChE was observed after the combination of tetraisopropylpyrophosphoramide (iso-OMPA) with each one of these N-methylcarbamates. It is suggested that CarbEs are more sensitive than AChE to these N-methylcarbamates and inhibition of CarbE by iso-OMPA raises the concentration of N-methylcarbamates available to inhibit AChE resulting in increased toxicity. Other N-methylcarbamates such as physostigmine do not inhibit CarbE, nor are their toxicities potentiated by iso-OMPA.

In this study, the presence of paraoxonase activity in pericardial fluid was demonstrated. A comparison of some properties, such as optimum pH, stability versus pH, heat inactivation, effect of inhibitors, isoelectric point and kinetic parameters (Km and Vmax), between plasma and pericardial fluid paraoxonase was made. The properties studied were practically identical. The enzyme activity in pericardial fluid was tested as a marker in the postmortem diagnosis of myocardial infarction. The paraoxonase activity in the myocardial infarction group (47 cases) was lower than in the control group (40 cases), but the difference was not significant.

The influence of paraoxon and bis(4-nitrophenyl)phosphate (BNPP) on carboxylesterases of human skin is assayed. Both organophosphates have frequently been used as inhibitors of carboxylesterases of the B-esterase type. Homogenates from carefully washed skin have no paraoxon-cleaving activity and very little phosphodiesterase activity with BNPP. However, a number of skin enzymes are irreversibly inhibited by these compounds. Three zones of carboxylesterase bands can be detected in the soluble fraction of skin homogenate by isoelectric focusing. One zone containing 5 esterase bands in the pI-range of 6.7-7.0 and another zone at pI 4.9 are insensitive to organophosphate inhibition. The zone with the main esterase activities contains at least 6 bands in the range of pI 5.7-6.2. All of these are quickly and completely inhibited by paraoxon. The complex inhibition kinetics with BNPP and observations with differing substrates point to a functional heterogeneity. The esterases with pI-values in the range of 5.7 to 6.2 and the esterase with pI 4.9 can be enriched using anion exchange chromatography and FPLC. From the data presented here it is concluded that human skin contains at least four different carboxylesterases which act on simple aromatic esters.

Glucosamine oligomers--monomer through tetramer--form complexes with Cu2+ that catalyse the hydrolysis of the 'nerve gas' 1,2,2-trimethylpropyl- methylphosphonofluroidate (soman) by cleaving the P-F bond. A 1/1 glucosamine/Cu2+ ratio whether as glucosamine or glucosamine units, gives the highest hydrolytic rate over the 11.5/1 to 1/1 range. This trend also appears to hold for a glucosamine polymer, chitosan, which, when complexed with Cu2+ also hydrolyzes soman. The relatively low rate of hydrolysis by this polymer-Cu2+ complex, while not yet explainable, is consistent with an extrapolation of the monomer-through-tetramer series. The question may be raised as to whether these biopolymer metal complexes provide any clues to the involvement of Mn2+ in the functioning of one class of P-F cleaving enzymes.

Three organophosphorus acid anhydrases have been isolated from E. coli by gel filtration and ion exchange column procedures, and further identified by gel electrophoresis. All three have molecular weights in the 120,000-140,000 range. Two of them hydrolyze racemic 1,2,2-trimethylpropylmethylphosphonofluoridate (soman) to completion at a single rate and, in parallel with this, detoxify soman at a comparable rate. The third enzyme appears to show stereoselectivity with respect to the two pairs of isomers of soman in that it hydrolyzes the racemic mixture at a fast and a slow rate, the latter approaching the non-enzymatic rate, and detoxifies soman only at the slower rate. In the past, organophosphorus acid anhydrases from bacterial and mammalian sources have been assayed either as crude sonicates or homogenates, or as cold ethanol precipitated fractions. Major discrepancies among laboratories have probably been due either to the assay of mixtures of varying proportions of these three enzymes depending on the various organs or organisms used as the source, or to the purification of one of the enzymes at the expense of the others. For E. coli, a fourth organophosphorus acid anhydrase is also present but at a considerably lower activity.

Resolved isomers of O-n-hexyl-S-methylphosphorothioamidate (HXM) which had been synthesised by separate stereospecific routes were analysed by chiral glc: about 2-3% of R-(+) isomer was found in the S-(-) sample and accounted for nearly all the inhibitory power against neuropathy target esterase. Incubation of racemic HXM with rabbit serum led to slow but very specific disposal of R-(+) isomer to undetectable levels with very slight loss of S-(-): the rate of disposal was roughly estimated to be about 1% of the published rate of hydrolysis of paraoxon. Incubation with crystalline chymotrypsin caused a preferential but not totally selective disposal of S-(-) isomer.

This article introduces a Symposium devoted to Neuropathy Target Esterase (NTE). The characteristics of the disorder known as OPIDP are described and the steps by which NTE was identified as the target are summarised. Studies with many organophosphates, phosphinates and chiral phosphonates are entirely consistent with a 2-step process of initiation referred to as 'NTE (70-80%) aging': about 70-80% of available nervous system NTE is first covalently phosphylated causing inhibition of esterase activity, and then the molecules of inhibited NTE undergo a covalent bond-cleavage leaving a negative charge in the region of the still-bound phosphorus. This understanding has clarified structure/activity studies of neuropathic potential of OP esters and is now routinely applied in toxicological evaluations for regulatory purposes. However, the biological function of NTE has remained a mystery. Prospective views of the role of NTE are presented by different authors. Attempts to isolate catalytically active or radiolabelled inhibited NTE are near to success. Since the Symposium, complete isolation of NTE affinity-purified from hen brain has been reported (see M.K. Johnson & P. Glynn, Toxicologist, 13 (1993) 211, Abstr. 773). Some minor, but possibly significant, differences in properties of a soluble and a membrane-bound form of NTE in sciatic nerve has been identified. The nature of the disturbance brought about by covalent binding of organophosphoramidates at the active site of NTE and the discovery that 'non-aging' inhibitors of NTE can promote neuropathy in hens given a sub-neuropathic dose of neuropathic OPs has led to a concept of NTE inhibitors having a range of 'partial agonist' effects at the covalent binding site. Evidence is emerging that the promotion target may be 'cousin-of NTE' with very similar inhibition characteristics and a function in the processes of response to or repair of axonal damage.

O-Ethyl-O-4-nitrophenylphosphoramidate is a short-acting anticholinesterase and a possible candidate for a prophylactic agent against nerve agents since human acetylcholinesterase inhibited by this agent undergoes rapid spontaneous reactivation which can be accelerated further, if necessary, by treatment with oximes. Doses of the agent > 1 mg/kg (s.c.) given to unprotected rats were fatal in a short time but 2 rats and one hen given 0.5 mg/kg survived. Hens given 2.5 or 4 mg/kg s.c. 20 min after prophylactic physostigmine + atropine survived acute effects and were killed 4.5 or 24 h later. Brain and spinal cord neuropathy target esterase levels of these hens were depressed only 4-10% compared with levels in brains from hens given only oxime + atropine or of undosed animals. Clinical signs of neuropathy were not seen in surviving birds observed for 3 weeks. It appears there would be negligible delayed neuropathic hazard associated with administration of O-ethyl-O-4-nitrophenylphosphoramidate at subacute doses.

To initiate delayed neuropathy (DN) in adult hens organophosphates and phosphonates must inhibit most neural NTE and the inhibited NTE must undergo an 'aging' reaction. Phosphinates and those chiral isomers of phosphonates which produce non-aging NTE do not cause DN but act as prophylactic agents. Some racemic phosphoramidates cause DN although the inhibited NTE in autopsy samples can be reactivated in vitro (Johnson, Read and Vilanova, 1991, Arch. Toxicol., 65, 618-624). We now report that pure R(+)isomer of O-n-hexyl S-methyl phosphorothioamidate (5-20 mg/kg per os) caused slight acute effects but typical DN associated with high inhibition of NTE in brain, spinal cord and sciatic nerve (maximum by 6-24 h): the inhibited NTE was easily reactivated by KF (presumed not aged). For each dose the average residual NTE activity in the three tissues 24 h after dosing and the clinical ataxia severity on peak days 15-17 (score out of 4) was: 5 mg/kg: 13, 14, 27% (2,2,2,1); 10 mg/kg: 10, 14, 12%, (4,3,2); 15 mg/kg: 10,11,17%, (3,3,4); 20 mg/kg: 6, 10, 8% (3,3,3,2). The ability of this isomer and of other racemic phosphoramidates to initiate DN by covalent reaction at the active site of NTE (inhibition) without subsequent aging suggests that the chemistry (? charge distribution) in the region of the phosphorus atom determines that disturbance in the molecular environment of NTE which initiates DN.

Phenyl di-n-pentylphosphinate (PPP) is a potent inhibitor of neuropathy target esterase (NTE) with negligible effect on acetylcholinesterase: I50S at 37 degrees C for 20 min and pH 8, respectively are 0.2 microM and > 2mM. PPP is not neuropathic. This is compatible with the fact that inhibited NTE is autopsy material from hens dosed with PPP can always be reactivated in vitro, presumably because no 'aging' reaction has occurred. PPP (10 mg/kg s.c.) given to hens up to 4 days before severely neuropathic doses (1.7 mg/kg) of diisopropylphosphorofluoridate (DFP) prevented neuropathic but not cholinergic effects of DFP. Hens given PPP 3 days after a sub-neuropathic dose of DFP (0.4 mg/kg) developed severe clinical neuropathy (clinical scores of 7 and 5 compared with DFP-plus-solvent scores 0,1,3). These prophylactic and promoting effects are similar to those exerted by phenylmethanesulphonyl fluoride (PMSF) at doses which inhibit NTE. In 3 out of 4 birds a pre-dose with PMSF (15 mg/kg) prevented the promoting effect of 120 mg/kg PMSF given after DFP.

Earlier studies of OPA anhydrolase from the squid, Loligo pealei, report that the enzyme has a molecular weight near 26 kDa, despite the common observation that SDS-PAGE experiments do not support this conclusion. Recent results from protein sequencing and cloning experiments now suggest that the enzyme found in squid hepatopancreas has a molecular weight of about 42 kDa. The enzyme easily degrades into two fragments of 16 kDa and approximately 26 kDa. N-terminal sequence analyses of the intact enzyme and the 16 kDa fragment blotted from an SDS gel and sequenced from the blot have shown conclusively that the intact 42 kDa protein has a blocked N-terminus. Sequence data obtained previously are from the N-terminal portion of the 16 kDa fragment. Additional support for this interpretation has been obtained from PCR analysis of L. pealei mRNA and cDNA. The partial (30 residue) sequence presented here reveals no indication of similarity to any other OPA anhydrolase or aryldialkylphosphatase (EC 3.1.8.1.).

The complete amino acid sequence of human serum paraoxonase/arylesterase and the DNA sequence coding for that protein have recently been determined in two independent laboratories. There is now considerable evidence that the esterase exists in two genetically determined allozymic forms, and these A and B allozymes possess both paraoxonase and arylesterase activities. The B-type esterase has relatively higher paraoxonase activity and is stimulated to a greater degree by 1 M NaCl than the A allozyme. The structural basis for the distinctive isozymic properties is a single nucleotide base at position 572. Codon 191 is CAA (for glutamine) in the A-type esterase, and CGA (for arginine) in the B-type enzyme. There is a second polymorphic site which affects amino acid 54; this can be either methionine or leucine, but these alternatives have not been found to affect either the level or the quality of the allozymes. Purified A or B-type esterases are stimulated by the addition of phosphatidylcholine. The latter addition increases the maximum velocity rate, but does not alter the Km of the reaction with either paraoxon or phenylacetate. In serum, the esterase is tightly bound to the high density lipoproteins, particularly apo A-1, but the importance of this association as far as the stability and catalytic properties of the esterase is not clear, and still under study. No physiological role of the esterase has been established, but its ability to hydrolyze several potent organophosphates may be of some significance in protecting against organophosphate toxicity.

Carbamate compounds marked for their cholinesterase (ChE) inhibition are widely used as therapeutics and as insecticides. Groups of closely related carbamate molecules provide an important tool in the understanding of the domains responsible for binding these ligands to ChEs. Comparative inhibition profiles were derived for five N-methyl carbamates, mostly carbofuran derivatives, differing in length and branching of their hydrocarbonic chain towards human erythrocyte acetylcholinesterase (H.AChE), human serum butyrylcholinesterase (H.BChE) in its normal form or in a mutant form containing the point mutation Asp70-->Gly, and Drosophila nervous system ChE. Carbofuran was more toxic to all three ChEs than any of the other derivatives, with IC50 values which differed by more than 1000-fold. Drosophila ChE appeared to be most sensitive to all of the examined carbamates, and H.AChE was consistently more sensitive than H.BChE. Moreover, inhibition efficiency for H.BChE decreased more effectively than it did for H.AChE with increased length and complexity of the side chain, indicating less flexible carbamate binding site in BChE as compared with AChE. The Asp70-->Gly mutation had no apparent effect on H.BChE inhibition by N-methyl carbamates, suggesting that the Asp70 domain localized near the rim of the active site groove is not important in carbamate binding. Comparison of the carbamate IC50 values with published LD50 values demonstrated correlation between the in vivo toxicity and inhibition of BChE by carbamates, suggesting a biological in addition to scavenging importance for BChE in mammals. Pinpointing different domains characteristic of carbamate binding in each member of the ChE family can thus shed light on the variable toxicity of these inhibitors to insects and mammals, predict the toxicity of yet untested inhibitor molecules and help in designing novel and improved ChE inhibitors.

Neuropathy target esterase (NTE) was identified as the molecular target for organophosphate-induced delayed polyneuropathy several years ago but its physiological functions are still unknown. The mechanism which initiates neuropathy was thought to be a two step process: inhibition (phosphorylation) of NTE and aging of phosphorylated NTE. Depending on the occurrence of the second reaction (aging), inhibitors were ranked as neuropathic (forming an ageable NTE) and non-neuropathic (forming a non-ageable NTE). Non-neuropathic inhibitors protect from neuropathy if given before the neuropathic ones, because they occupy the catalytic centre of NTE. Thus the catalytic function of NTE seems irrelevant in maintaining the health of neurons. This paper reviews some new information concerning the interaction of NTE with its inhibitors as well as on a phenomenon called promotion of neuropathy. Some inhibitors which apparently form a non-ageable inhibited NTE were found to cause neuropathy, even though some of them must be given at very high doses. Moreover some 'non-neuropathic-protective' NTE inhibitors were found to exacerbate (promote) neuropathy when given after a neuropathic one. It is likely that the target for promotion is other than NTE. The hypothesis that NTE has some unknown receptorial functions where inhibitors act with different efficacy is discussed. NTE inhibitors have been ranked as full agonists (classic neuropathic inhibitors such as diisopropylfluorophosphate), partial agonists (protective or neuropathic, depending on the dose, such as methamidophos) and antagonists (protective, and neuropathic at the highest doses, such as phenylmethanesulfonyl fluoride). Age-related differences in the 'receptor' NTE might be responsible for the different sensitivities of juvenile and adult animals.

Human serum paraoxonase is responsible for the hydrolysis of organophosphate anticholinesterases, however, whether the enzyme has a physiological role other than the detoxication of insecticides and nerve gases has remained uncertain. Recently, evidence has begun to accumulate of a relationship between the serum activity of paraoxonase and atherosclerosis. Paraoxonase may a fundamental role in lipoprotein metabolism, preventing oxidative changes to low-density lipoprotein which render the particle atherogenic.

Liver microsomal paraoxonase, aryl esterase and fluazifop butyl esterase (carboxylesterase) were induced by pretreatment of rat with phenobarbitone but not by beta-naphthoflavone or clofibric acid. In the extrahepatic tissues lung cytosolicfluazifop butyl and phenylacetate esterase were induced.

It has been thought that the phosphorus-enzyme bond in inhibited esterases inhibited by such agents as mipafox (N,N'-di-iso-propylphosphorodiamidate) was refractory to reactivating agents either because an 'aging' reaction occurs soon after inhibition or because the bond was intrinsically very strong. We have found that both acetylcholinesterase (AChE) and neuropathy target esterase (NTE) which had been inhibited with either mipafox or with a di-n-butylphosphorodiamidate could be reactivated by prolonged treatment with aqueous potassium fluoride (KF): the reaction proceeded with first-order kinetics. Furthermore there was no time-dependent loss of reactivatability (aging). Di-isopropylphosphoro-butyrylcholinesterase could be fully reactivated by this treatment but after 18 h to allow aging the monoisopropyl phosphoro-enzyme was totally refractory to KF. We conclude that it is likely that the mipafox-enzyme bond in inhibited NTE and AChE is relatively strong but that aging has not occurred. The local disturbance around the active site of NTE caused by attachment of the phosphorodiamidate molecule appears to be sufficient to initiate delayed neuropathy without necessity for an 'aging' reaction.

Several esterase inhibitors (carbamates, phosphinates and sulfonyl halides) have been shown to promote organophosphate-induced delayed polyneuropathy (OPIDP). The mechanism of promotion is not understood, but indirect evidence suggests impairments of peripheral nerve repair. Also, other toxic neuropathies, such as those caused by 2,5-hexanedione in hens and bromophenylacetylurea in rats, have been reported to be promoted by phenylmethanesulfonyl fluoride (PMSF). Hen sciatic nerve was crushed at the bifurcation. Either mild or heavy pressure was applied by forceps obtaining a mild and rapidly recovering lesion (possibly myelinic) or a more severe, long-lasting lesion (possibly axonal), respectively. Hens were then treated with PMSF (120 mg/kg s.c. or 200 mg/kg s.c. x 2, 24 h apart) either before (5-48 h) crush or afterwards (5-48 h). Controls received vehicle only. Animals were observed for reappearance of digit movements, and standing and walking ability. PMSF treatment did not change the clinical outcome when animals received a mild crush. In hens receiving the more severe crush the reappearance of digit movements and the complete clinical recovery were observed after 43 +/- 14 and 63 +/- 9 days, respectively. In animals treated with PMSF there was a significant delay in both reappearance of digit movements (56 +/- 11 days when PMSF was given 24 and 48 h before crush, and 55 +/- 10 days, when given 24 and 48 h after crush) and in clinical recovery (75 +/- 15 and 80 +/- 18 days, respectively). It is concluded that traumatic axonopathy as well as toxic neuropathies can be promoted by PMSF. Moreover, it appears that PMSF promotion involves a target and a mechanism which are present in healthy axons and do not need to be activated by the insult to the axon.

In patients with hyperlipaemia, serum paraoxonase activities were polymodally distributed with 75% individuals in the low activity mode. In the same patients the distribution of serum cholinesterase activities was unimodal, but asymmetrical. Patients with impaired glucose tolerance or non-insulin-dependent diabetes mellitus had slightly higher cholinesterase activities than patients with hyperlipaemia only.

The first step in the initiation of organophosphorus-induced delayed neuropathy (OPIDN) is proposed to be the phosphorylation of an enzyme found in the nervous system called neurotoxic esterase (neuropathy target esterase, NTE). It has been known for over twenty years that non-neuropathic inhibitors of NTE exist and can actually prevent OPIDN when given before a neuropathic organophosphate (OP). Within the last three years it has become evident that another outcome is possible following in vivo interaction between neuropathic and nonneuropathic NTE inhibitors. When administered after OP exposure, nonneuropathic inhibitors can intensify or potentiate signs of OPIDN in adult chickens. Additionally, whereas developing chickens are typically resistant to the effects of neuropathic OPs, resistant age groups will develop OPIDN when exposure to a neuropathic OP is followed by the non-neuropathic NTE inhibitor phenylmethylsulfonyl fluoride. As in the case of prevention, studies of the potentiation of OPIDN may yield insight into mechanisms involved in the pathogenesis of delayed neurotoxicity. A brief review of current knowledge regarding the role of NTE in both the prevention and potentiation of OPIDN is presented.

The hydrolysis of paraoxon (POX), phenylacetate (PA) and beta-naphthylacetate (BNA) was studied in human serum. Based upon correlations between enzyme activities, upon reversible inhibition by EDTA and upon progressive inhibition by iso-OMPA, tabun, eserine and bis-4 nitrophenylphosphate, the following conclusions were drawn about the number and specificity of enzymes involved in the hydrolysis. Two paraxonases hydrolyse paraoxon: one sensitive and the other insensitive to EDTA. The EDTA-sensitive paraoxonase also hydrolysed BNA. The EDTA-insensitive hydrolysis of BNA and PA was attributed to a serine esterase. The EDTA-sensitive hydrolysis of PA is probably due to more than one enzyme, which might be an arylesterase and a carboxylesterase.

The hydrolysis of four organophosphorus dichlorophenyl esters and of paraoxon was studied in hen brain homogenates and in rabbit serum. All compounds were hydrolysed by both preparations, but the rates were different in the two preparations. EDTA inhibited the hydrolysis almost completely in rabbit serum, but had only a small effect on the hydrolysis in hen brain homogenates.

The heat inactivation of esterases in human and rabbit serum was followed at 50 and 55 degrees C by measuring the decrease of activity with paraoxon, phenylacetate and beta-naphthylacetate as substrates. The rate of inactivation measured with the three substrates was slightly, but significantly different, indicating that the substrates are hydrolysed by different enzymes.

The organophosphorus (OP) compound O-hexyl-O-2,5-dichlorophenyl phosphoramidate (H-DCP) is hydrolysed in the plasma, liver and brain of hens and rats. We study in hen plasma the effect of tissue and substrate concentrations and of pH on the hydrolysing activity of H-DCP. The data on each tissue concentration were fitted to a double exponential mathematical model. The kinetics of this activity was not linear; in a first exponential component or fast initial phase (k1 = (1.603 +/- 0.248) x 10(-3) min-1/microliter plasma (n = 4, S.E.)) about 15% of the total compound was hydrolysed, followed by a slow second phase (k2 = (0.144 +/- 0.026) x 10(-3) min-1/microliter plasma (n = 4, S.E.)) in which the remaining 85% was hydrolysed. Both constants increased in value with pH. The hydrolytic activity and rate constants increased with the amount of plasma, but no change in kinetics was observed. The kinetic data are discussed in terms to lend support to the hypothesis of a stereospecific degradation of H-DCP.

Based on our recent X-ray crystallographic determination of the structure of acetylcholinesterase (AChE) from Torpedo californica, we can see for the first time, at atomic resolution, a protein binding pocket for the neurotransmitter, acetylcholine. It was found that the active site consists of a catalytic triad (S200-H440-E327) which lies close to the bottom of a deep and narrow gorge, which is lined with the rings of 14 aromatic amino acid residues. Despite the complexity of this array of aromatic rings, we suggested, on the basis of modelling which involved docking of the acetylcholine (ACh) molecule in an all-trans configuration, that the quaternary group of the choline moiety makes close contact with the indole ring of W84. In order to study the interaction of AChE with anticholinesterase drugs at the structural level, we have incorporated into the acetylcholinesterase crystals several different inhibitors, and have recently determined the 3-D structure of AChE:edrophonium and AChE:tacrine complexes. The crystal structures of both of these complexes are in good agreement with our model building of the ACh bound in the active site of AChE and indicate the interactions of these two drugs with the enzyme.

Acetylcholinesterase, an enzyme essential for the termination of the action of acetylcholine, is encoded by a single gene. Alternative mRNA processing gives rise to the expression of enzyme forms with three distinct carboxyl-termini. These structural differences govern the cellular disposition of the expressed enzyme but do not influence catalytic activity. Alternative polyadenylation signals give rise to distinct 3' non-coding regions which are likely to affect mRNA stability. Alternative splicing also occurs at the 5' end of the gene where two promoter regions can be identified. Hence, regulation of expression of the gene occurs at 3 levels, transcriptional through alternative promoters, translational by affecting mRNA stability and processing of distinct mRNAs and post-translationally by giving rise to distinct peptide chains which are processed differently. Recombinant DNA studies have also been extended to modifying protein structure through site-specific mutagenesis and studying the function of the mutant enzymes.

Neuropathy target esterase (NTE) is a membrane-bound protein which has been proposed as the target site in nerve tissue for initiation of organophosphate induced delayed neuropathy (OPIDN). Efforts to characterize NTE and to determine the mechanism of its involvement in OPIDN have been hampered by the lack of a suitable method for its purification. We describe here the development of a trifluoromethyl ketone liganded affinity gel which selectively binds NTE. Triton X-100/NaCl extracts of NTE from chick embryo brain microsomal membranes were adsorbed to an affinity gel prepared by attachment of 3(9'-mercaptononylthio)-1,1,1-trifluoropropan-2-one to epoxy-activated Sepharose CL4B (MNTFP-Sepharose). Typically 70-80% of NTE activity is bound under conditions in which undetectable quantities of total protein bound (< 4%). It proved difficult to elute active NTE under non-denaturing conditions, but SDS-PAGE analysis of MNTFP-Sepharose bound proteins eluted with 2% SDS identified a 155 kDa NTE-like protein that bound in a trifluoromethylketone- or mipafox-sensitive but paraoxon-insensitive manner. The levels of inhibition of binding correlated with the inhibition of activity and suggested that the 155-kDa band was composed of a single protein. MNTFP-Sepharose affinity chromatography in combination with preparative SDS-PAGE therefore holds promise as a method for obtaining microgram quantities of NTE for chemical analysis and sequencing.

Avian serum esterases are predominantly of the 'B' type (cholinesterases and carboxylesterases) and are inhibited by carbamates and the 'active' oxon forms of organophosphorus pesticides. Selective inhibition of mammalian serum carboxylesterase, a 'B' esterase, has shown that this enzyme may play an important role in detoxication by irreversibly binding, and thus inactivating, anticholinesterase compounds. Studies have shown differences between carnivorous and omnivorous/herbivorous avian species in the level of activity and range of forms of carboxylesterases and cholinesterases in sera. In addition, these enzymes show seasonal, diurnal and developmental variations in activity. This paper will discuss species and temporal variations in avian serum 'B' esterases in relation to their possible influence on pesticide toxicity.

The presence and elimination rate of phosalone and its diethylphosphorus metabolites in blood serum and urine were studied in persons who had ingested a concentrated phosalone solution. Phosalone was detected only in serum samples. As it was rapidly hydrolysed and eliminated from the body, its diethylphosphorus metabolites were a more sensitive indicator of exposure. The concentration decrease of phosalone in serum and of total diethylphosphorus metabolites in serum and urine followed the kinetics of a biphasic reaction. The faster elimination half-times in serum, calculated for two persons, were 2.3 and 3.4 h for phosalone and 3.4 and 38.6 h for total diethylphosphorus metabolites. In the faster phase the average elimination half-time of total urinary metabolites in five persons was 25 +/- 17 h. The kinetic data for total urinary metabolites in a person occupationally exposed to phosalone indicated an early and very fast elimination phase (elimination half-time 1.3 h), which was overlooked in poisoned persons. The proportions of single metabolites in total urinary metabolites in poisoned persons depended on whether the total amount of diethylphosphorus metabolites was above 1000 or below 1000 nmol/mg creatinine. Diethylphosphorodithioate predominated at high and diethylphosphate at low concentrations of total metabolites. The correlation between the maximum concentrations of total metabolites, measured in urine of poisoned persons on the day of admission to hospital or a day later, and the initial depression of serum cholinesterase (EC 3.1.1.8) and erythrocyte acetylcholinesterase (EC 3.1.1.7) activities was poor (r = 0.6).

NTE inhibitors cause different toxicological consequences (protection, induction or potentiation/promotion of neuropathy) depending on the order of dosing. These effects might be explained in terms of several phosphorylable sites with 'allosteric irreversible' behaviour. Brain neuropathy target esterase (NTE) has been preinhibited with phenylmethylsulphonyl fluoride (PMSF) (0, 5, 10, 15, 30 and 60 microM) or with diisopropylphoshoro fluoridate (DFP) (0, 0.2, 0.5, and 1 microM) at 37 degrees C for 30 min. After washing by centrifugation, tissues were then reinhibited with a range of PMSF (0 to 80 microM) or DFP (0 to 1 microM) concentrations. The slopes of the inhibition curves (log % activity vs. concentration) of pretreated tissues were identical to those of the non-pretreated tissues, with non-distinguishable I50 values. It is concluded that allosteric effects are not likely to be involved in membrane-bound NTE of hen brain.

NTE (neuropathy target esterase) is considered to be the target for organophosphorus-induced delayed polyneuropathy and is operationally measured by radiolabelling or by determining its esteratic activity as the paraoxon-resistant mipafox-sensitive phosphorylable site(s). From electrophoresis and density gradient centrifugation using radiolabelling techniques, several phosphorylable sites have been described in hen brain that are paraoxon-resistant mipafox-sensitive; however, only the majority electrophoresis band (155 kDa) shows properties related with the aging reaction. Kinetic criteria have also suggested two components of brain NTE (NTEA and NTEB). Most brain NTE is recovered in the particulate microsomal fraction and only about 1% in soluble fraction. In sciatic nerve about 50%/50% activity is recovered as soluble (S-NTE) or particulate (P-NTE) forms. A similar distribution were observed in hen, cat, rat and young chick. The fixed time inhibition curves show that P-NTE is more sensitive to mipafox, DFP and hexyl-DCP than S-NTE, while the reverse is true for methamidophos. P-NTE fits properly to one sensitive component while S-NTE fits better to two sensitive component models, except in the case of methamidophos. In vivo, significant differences in the inhibition of P- and S-NTE by mipafox were found only when using low non-neuropathic dosing. The possible significance of different NTE forms are discussed.

In the IUB classification of 1984, enzymes which hydrolyse paraoxon and other organophosphorous triesters were included in the category of arylesterases--enzymes which hydrolyse phenylacetate (EC 3.1.1.2). With the discovery that some forms of paraoxonase do not hydrolyse phenylacetate, a new entry was made in the revised classification of 1989, Aryldialkylphosphatase (EC 3.1.8.1) under phosphoric triester hydrolases (EC 3.1.8), to distinguish these enzymes from arylesterases. Also some enzymes that hydrolyse phenylacetate do not hydrolyse paraoxon, whereas other enzymes do. Additionally, there is growing evidence for the existence of a number of enzymes which hydrolyse P-F or P-CN bonds of organophosphorous diesters e.g., the nerve gases tabun and soman. These enzymes are in effect organophosphorous acid anhydrolases, and it has been proposed that the earlier entry of (EC 3.8.2.1) now be deleted, and a new entry diisoprophylfluorophosphatase (EC 3.1.8.2) put in its place. Within this category, there is evidence of several enzymes showing different substrate specificities, and different requirements for divalent cations as cofactors, which presents further problems of classification and nomenclature.

Mean plasma paraoxonase activity was lower in a population of all males than in a mixed sex population with no evidence of the higher activity group. It is suggested that sex differences in plasma lipid may contribute to the observed differences, and that factors other than genetics may influence observed plasma paraoxonase levels.

Paraoxonase of human and animal sera was shown to be a structural part of high density lipoproteins (HDL) by immunoprecipitation, heparin- or polyethyleneglycol fractionation, ultracentrifugation and gel chromatography. Frequency distribution of paraoxonase activity in human sera is trimodal. Human individuals, with respect to paraoxon detoxication, can be distinguished into low and high detoxicators using ratios of phenylacetate and paraoxon hydrolysis as well as activation with ethanolamine and sodium chloride. With conversion of alpha-lipoprotein subtype HDL3 to HDL2, specific activities of paraoxonase and arylesterase are increasing about 3.5-fold in low detoxicator individuals and 1.9-fold in high detoxicators, indicating that more than 90% of HDL2 particle-bound paraoxonase and arylesterase activity are incorporated during the HDL conversion process. HDL cholesterol concentrations in individual sera were shown to be positively correlated to both serum paraoxonase and arylesterase activities.