Identification of acylpeptide hydrolase as a sensitive site for reaction with organophosphorus compounds and a potential target for cognitive enhancing drugs

We describe here the purification and identification of a previously unreco gnized target for organophosphorus compounds. The target, acylpeptide hydro lase, was isolated as a tritiated-diisopropylfluorophosphate-reactive prote in from porcine brain and purified to homogeneity using a combination of io n-exchange and gelfiltration chromatography. Biochemical characterization a nd internal sequence analysis confirmed identity. Acylpeptide hydrolase was found to be potently inhibited by the organophosphorus compounds chlorpyri fosmethyl oxon, dichlorvos, and diisopropylfluorophosphate (20-min IC50 val ues of 18.3 +/- 2.0, 118.7 +/- 9.7, and 22.5 +/- 1.2 nM, respectively). The in vitro sensitivity of acylpeptide hydrolase toward these compounds is be tween six and ten times greater than that of acetylcholinesterase (AChE), m aking it a target of pharmacological and toxicological significance. We sho w that, in vivo, acylpeptide hydrolase is significantly more sensitive than AChE to inhibition by dichlorvos and that the inhibition is more prolonged after a single dose of inhibitor. Furthermore, using dichlorvos as a progr essive inhibitor, it was possible to show that acylpeptide hydrolase is the only enzyme in the brain capable of hydrolyzing the substrate N-acetyl-ala nyl-p-nitroanilide. A concentration of 154 +/- 27 pmol of acylpeptide hydro lase/gram of fresh rat brain was also deduced by specific labeling with tri tiated-diisopropylfluorophosphate. We also suggest that, by comparison of s tructure-activity relationships, acylpeptide hydrolase may be the target fo r the cognitive-enhancing effects of certain organophosphorus compounds. Ac ylpeptide hydrolase cleaves N-alpha-acylated amino acids from small peptide s and may be involved in regulation of neuropeptide turnover, which provide s a new and plausible mechanism for its proposed cognitive enhancement effe ct.