Inhibitor Report for: Molinate

Atrazine and molinate are widely used herbicides and concern has been raised about their potential deleterious impacts on aquatic ecosystems. Although there have been some studies on the toxicity of herbicides to aquatic organisms using laboratory or natural water, information on the effect of sediments, suspended particulates, and dissolved organic matter on their bioavailability is quite limited. This study aims to provide toxicity data that considers these factors and the effect that these factors have on bioavailability. In this study, the toxicity of the test chemicals was calculated following the Organisation for Economic Co-operation and Development (OECD) methods, whereas change in bioavailability was measured using EC50 values based on measured initial concentrations of the test chemicals. The acute (48-h) static toxicity of atrazine and molinate to the freshwater cladoceran Daphnia carinata was determined in cladoceran water and river water in the absence and presence of sediment. The 48-h EC50 (immobilization) values of atrazine to D. carinata ranged from 22.4 to 26.7 mg/L, while the corresponding values for molinate ranged from 18.3 to 33.6 mg/L, respectively. Both chemicals were classed as having low acute toxicity to D. carinata. The presence of dissolved organic matter and suspended particles in river water did not significantly (p > 0.05) reduce the bioavailability (measured as toxicity) of atrazine to D. carinata compared to that tested in cladoceran water. The presence of sediment, however, significantly (p < 0.05) reduced the bioavailability (48-h EC50) of atrazine in cladoceran water, from 24.6 to 30.7 mg/L, and in river water, from 22.4 to 31.0 mg/L. Similarly, the presence of sediment in cladoceran water, significantly (p < 0.05) reduced the bioavailability (48-h EC50) of molinate, from 26.6 to 46.4 mg/L, and in river water, from 22.5 to 45.6 mg/L.

Certain esterase inhibitors protect from organophosphate-induced delayed polyneuropathy (OPIDP) when given before a neuropathic organophosphate by inhibiting neuropathy target esterase (NTE). In contrast, they can exaggerate OPIDP when given afterwards and this effect (promotion) is associated with inhibition of another esterase (M200). In vitro sensitivities of hen, rat, and human NTE and M200 to the active metabolites of molinate, sulfone, and sulfoxide, were similar. NTE and M200 were irreversibly inhibited (> 78%) in brain and peripheral nerve of hens and rats given molinate (100-180 mg/kg, sc). No clinical or morphological signs of neuropathy developed in these animals. Hens and rats were protected from di-n-butyl dichlorovinyl phosphate neuropathy (DBDCVP, 1 and 5 mg/kg, sc, respectively) by molinate (180 or 100 mg/kg, sc, 24 h earlier, respectively) whereas 45 mg/kg, sc molinate causing about 34% NTE inhibition offered partial protection to hens. Hens treated with DBDCVP (0.4 mg/kg, sc) developed a mild OPIDP; molinate (180 mg/kg, 24 h later) increased the severity of clinical effects and of histopathology in spinal cord and in peripheral nerves. Lower doses of molinate (45 mg/kg, sc), causing about 47% M200 inhibition, did not promote OPIDP whereas the effect of 90 mg/kg, sc (corresponding to about 50-60% inhibition) was mild and not statistically significant. OPIDP induced by DBDCVP (5 mg/kg, sc) in rats was promoted by molinate (100 mg/kg, sc). In conclusion, protection from DBDCVP neuropathy by molinate is correlated with inhibition of NTE whereas promotion of DBDCVP neuropathy is associated with > 50% M200 inhibition

Cholinesterase (ChE) activity was measured in plasma, whole blood [using 5,5'-dithiobis(2-nitrobenzoic acid) and 2-PDS as chromophores], brain, and whole eyes of Anguilla anguilla exposed to a sublethal concentration of 11.15 mg/L (one-third of the 96-h LC(50)) of the carbamate herbicide molinate. ChE activity was evaluated after 6, 24, 48, 72, and 96 h of pesticide exposure. Results indicated that ChE activity in eel tissues decreased as time of exposure increased, especially in eel blood. Eels exposed to molinate were transferred to a pesticide-free water for a recovery period of 4 days and ChE activity was also evaluated. Results indicated that ChE activity for those animals with preexposure to the carbamate was still different from the controls even after the recovery period. The use of hematological parameters for assessing the toxicity of molinate in A. anguilla was also studied. The parameters included the measurement of blood proteins, hematocrit, hemoglobin, erythrocytes, and leukocytes. Molinate exposure produced a decrease in all the hematological parameters tested, which was significant only during the recovery period. These results indicate that ChE activity, as well as hematological parameters, may be useful as a diagnostic test for molinate exposure in aquatic organisms.

The amidohydrolase superfamily has remarkable functional diversity, with considerable structural and functional annotation of known sequences. In microbes, the recent evolution of several members of this family to catalyze the breakdown of environmental xenobiotics is not well understood. An evolutionary transition from binuclear to mononuclear metal ion coordination at the active sites of these enzymes could produce large functional changes such as those observed in nature, but there are few clear examples available to support this hypothesis. To investigate the role of binuclear-mononuclear active-site transitions in the evolution of new function in this superfamily, we have characterized two recently evolved enzymes that catalyze the hydrolysis of the synthetic herbicides molinate (MolA) and phenylurea (PuhB). In this work, the crystal structures, mutagenesis, metal ion analysis, and enzyme kinetics of both MolA and PuhB establish that these enzymes utilize a mononuclear active site. However, bioinformatics and structural comparisons reveal that the closest putative ancestor of these enzymes had a binuclear active site, indicating that a binuclear-mononuclear transition has occurred. These proteins may represent examples of evolution modifying the characteristics of existing catalysts to satisfy new requirements, specifically, metal ion rearrangement leading to large leaps in activity that would not otherwise be possible.

Acetylcholinesterase is involved in the termination of impulse transmission by rapid hydrolysis of the neurotransmitter acetylcholine in numerous cholinergic pathways in the central and peripheral nervous systems. The enzyme inactivation, induced by various inhibitors, leads to acetylcholine accumulation, hyperstimulation of nicotinic and muscarinic receptors, and disrupted neurotransmission. Hence, acetylcholinesterase inhibitors, interacting with the enzyme as their primary target, are applied as relevant drugs and toxins. This review presents an overview of toxicology and pharmacology of reversible and irreversible acetylcholinesterase inactivating compounds. In the case of reversible inhibitors being commonly applied in neurodegenerative disorders treatment, special attention is paid to currently approved drugs (donepezil, rivastigmine and galantamine) in the pharmacotherapy of Alzheimer's disease, and toxic carbamates used as pesticides. Subsequently, mechanism of irreversible acetylcholinesterase inhibition induced by organophosphorus compounds (insecticides and nerve agents), and their specific and nonspecific toxic effects are described, as well as irreversible inhibitors having pharmacological implementation. In addition, the pharmacological treatment of intoxication caused by organophosphates is presented, with emphasis on oxime reactivators of the inhibited enzyme activity administering as causal drugs after the poisoning. Besides, organophosphorus and carbamate insecticides can be detoxified in mammals through enzymatic hydrolysis before they reach targets in the nervous system. Carboxylesterases most effectively decompose carbamates, whereas the most successful route of organophosphates detoxification is their degradation by corresponding phosphotriesterases.

Atrazine and molinate are widely used herbicides and concern has been raised about their potential deleterious impacts on aquatic ecosystems. Although there have been some studies on the toxicity of herbicides to aquatic organisms using laboratory or natural water, information on the effect of sediments, suspended particulates, and dissolved organic matter on their bioavailability is quite limited. This study aims to provide toxicity data that considers these factors and the effect that these factors have on bioavailability. In this study, the toxicity of the test chemicals was calculated following the Organisation for Economic Co-operation and Development (OECD) methods, whereas change in bioavailability was measured using EC50 values based on measured initial concentrations of the test chemicals. The acute (48-h) static toxicity of atrazine and molinate to the freshwater cladoceran Daphnia carinata was determined in cladoceran water and river water in the absence and presence of sediment. The 48-h EC50 (immobilization) values of atrazine to D. carinata ranged from 22.4 to 26.7 mg/L, while the corresponding values for molinate ranged from 18.3 to 33.6 mg/L, respectively. Both chemicals were classed as having low acute toxicity to D. carinata. The presence of dissolved organic matter and suspended particles in river water did not significantly (p > 0.05) reduce the bioavailability (measured as toxicity) of atrazine to D. carinata compared to that tested in cladoceran water. The presence of sediment, however, significantly (p < 0.05) reduced the bioavailability (48-h EC50) of atrazine in cladoceran water, from 24.6 to 30.7 mg/L, and in river water, from 22.4 to 31.0 mg/L. Similarly, the presence of sediment in cladoceran water, significantly (p < 0.05) reduced the bioavailability (48-h EC50) of molinate, from 26.6 to 46.4 mg/L, and in river water, from 22.5 to 45.6 mg/L.

Certain esterase inhibitors protect from organophosphate-induced delayed polyneuropathy (OPIDP) when given before a neuropathic organophosphate by inhibiting neuropathy target esterase (NTE). In contrast, they can exaggerate OPIDP when given afterwards and this effect (promotion) is associated with inhibition of another esterase (M200). In vitro sensitivities of hen, rat, and human NTE and M200 to the active metabolites of molinate, sulfone, and sulfoxide, were similar. NTE and M200 were irreversibly inhibited (> 78%) in brain and peripheral nerve of hens and rats given molinate (100-180 mg/kg, sc). No clinical or morphological signs of neuropathy developed in these animals. Hens and rats were protected from di-n-butyl dichlorovinyl phosphate neuropathy (DBDCVP, 1 and 5 mg/kg, sc, respectively) by molinate (180 or 100 mg/kg, sc, 24 h earlier, respectively) whereas 45 mg/kg, sc molinate causing about 34% NTE inhibition offered partial protection to hens. Hens treated with DBDCVP (0.4 mg/kg, sc) developed a mild OPIDP; molinate (180 mg/kg, 24 h later) increased the severity of clinical effects and of histopathology in spinal cord and in peripheral nerves. Lower doses of molinate (45 mg/kg, sc), causing about 47% M200 inhibition, did not promote OPIDP whereas the effect of 90 mg/kg, sc (corresponding to about 50-60% inhibition) was mild and not statistically significant. OPIDP induced by DBDCVP (5 mg/kg, sc) in rats was promoted by molinate (100 mg/kg, sc). In conclusion, protection from DBDCVP neuropathy by molinate is correlated with inhibition of NTE whereas promotion of DBDCVP neuropathy is associated with > 50% M200 inhibition

Cholinesterase (ChE) activity was measured in plasma, whole blood [using 5,5'-dithiobis(2-nitrobenzoic acid) and 2-PDS as chromophores], brain, and whole eyes of Anguilla anguilla exposed to a sublethal concentration of 11.15 mg/L (one-third of the 96-h LC(50)) of the carbamate herbicide molinate. ChE activity was evaluated after 6, 24, 48, 72, and 96 h of pesticide exposure. Results indicated that ChE activity in eel tissues decreased as time of exposure increased, especially in eel blood. Eels exposed to molinate were transferred to a pesticide-free water for a recovery period of 4 days and ChE activity was also evaluated. Results indicated that ChE activity for those animals with preexposure to the carbamate was still different from the controls even after the recovery period. The use of hematological parameters for assessing the toxicity of molinate in A. anguilla was also studied. The parameters included the measurement of blood proteins, hematocrit, hemoglobin, erythrocytes, and leukocytes. Molinate exposure produced a decrease in all the hematological parameters tested, which was significant only during the recovery period. These results indicate that ChE activity, as well as hematological parameters, may be useful as a diagnostic test for molinate exposure in aquatic organisms.