Cleavage of an N-acetyl or N-formyl amino acid from the N-terminus of a polypeptide. Preferentially cleaves off Ac-Ala, Ac-Met and Ac-Ser Belongs to peptidase family s9C, Fujino et al Q9P0Y2: P13798 2aa dif with Q9BQ33 old human-acph. DFP (IC(50) 11-17 nM), chlorpyrifos oxon (IC(50) 21-71 nM), dichlorvos (IC(50) 230-560 nM), naled (IC(50) 370-870 nM) (Quistad et al. 2005)