Acetylcholinesterase hydrolyzes substance P

Highly purified preparations of acetylcholinesterase were obtained by affinity chromatography of both fetal calf serum and a commercial preparation derived from the electroplax of the electric eel. One major molecular form was isolated, which corresponded to the secretory form of acetylcholinesterase. The specific activity ranged from 375 to 4330 U/mg protein. When several batches of acetylcholinesterase, prepared over a period of months from either source, were tested with substance P as a substrate, it was found that they had peptidase activity. Quantitative amino acid analysis showed that the most susceptible part of the substance P molecule to hydrolysis was the C-terminal pentapeptide: methionine and phenylalanine were the first amino acids liberated, with methionine being released at twice the rate of phenylalanine. The peptidase activity was inhibited by 10 −2 M acetylcholine and, in a graded fashion, by 10 −3 and 10 −4 M diisopropylfluorophosphate. The substance P hydrolyzing capacity co-migrated with acetylcholinesterase on polyacrylamide gels. Evidence is also presented of an excellent correlation between the distribution of substance P-like immunoreactivity and acetylcholinesterase end-product in the dorsal horn of chick spinal cord. Since acetylcholinesterase is concentrated in areas where substance P may be released, we suggest that one physiological function of acetylcholinesterase may be to hydrolyse this neuropeptide, thus terminating its biological activity.