Acetylcholinesterase exhibits trypsin-like and metalloexopeptidase-like activity in cleaving a model peptide

Acetylcholinesterase (EC 3.1.1.7) has been shown to possess an intrinsic peptidase activity. [Chubb et al. (1983), Neuroscience 10, 1369-1383]. To examine this activity further, the breakdown of a model hexapeptide (leu-trp-met-arg-phe-ala) LWMRFA was studied. Affinity-purified eel acetylcholinesterase rapidly cleaved the hexapeptide in a trypsin-like manner to produce two peptides (LWMR and FA). Acetylcholinesterase more slowly cleaved the C-terminal alanine residue from the peptide to yield LWMRF. Although the enzyme showed preference for cleaving the hexapeptide at its C-terminal, it was also able to cleave the N-terminal leucine residue form the tryptic product LWMR. Hydrolysis of the peptide at the tryptic site (arg4-phe5) was strongly inhibited by the trypsin inhibitor diisopropylfluorophosphate. Cleavage of the C-terminal alanine was only poorly inhibited by diisopropylfluorophosphate, but more strongly inhibited by metal-ion chelating agents, and it was increased in the presence of Zn2+ and Co2+. The pH optimum for cleavage at the tryptic site was 6, while that for the carboxypeptidase site was 8-9. These results show that acetylcholinesterase can hydrolyse peptides like a trypsin-like endopeptidase and a Zn2+- or Co2+-dependent exopeptidase, and they suggest that these two peptidase activities are associated with two separate active sites on the acetylcholinesterase molecule. As both peptidase activities eluted with acetylcholinesterase from a TSK 4000SW column when it was chromatographed by high-performance liquid chromatography, it is unlikely that the presence of either peptidase activity could be attributable to a contaminant in the acetylcholinesterase preparation. We suggest that acetylcholinesterase may be involved in the breakdown of bioactive peptides or their precursors in neuroendocrine cells.