The overall objective of this project is to investigate the enzymes involved in the metabolism of the 5-met-enkephalin-6arg-7phe. 5-Met-enkephalin-6arg-7phe is an endogenous opioid peptide derived from preproenkephalin A. We have previously demonstrated that 1) this peptide is rapidly metabolized by an aminopeptidase and a dipeptidyl carboxypeptidase and 2) these two enzymes can be efficiently inhibited by Bestatin and Hoe 498, an angiotensin converting enzyme inhibitor. IC50 value of Hoe 498 diacid is 0.8 nM indicating that Hoe 498 is a highly potent inhibitor for the 5-met-enkephalin-6arg-7phe degradation. In this study, the specificity of Hoe 498 on the 5-met-enkephalin-6arg-7phe metabolism was further studied. Specifically the effect of Hoe 498 on 1) the recoveries of 5-met-enkephalin-6arg-7phe and 5-met-enkephalin-6-arg-7-gly-8-leu released by 57 mM KCl from brain slices and 2) the analgesic potencies of 5-met-enkephalin-6arg-7phe and 5-met-enkephalin-6-arg-7-gly-8-leu were studied. In the release study, Hoe 498 protected 5-met-enkephalin-6arg-7phe but not 5-met-enkephalin-6-arg-7-gly-8-leu. Hoe 498 potentiated the analgesic effect of intraventricularly injected 5-met-enkephalin-6arg-7phe but not that of 5-met-enkephalin-6-arg-7-gly-8-leu. We have previously observed that 5-met-enkephalin is not protected by the dipeptidly carboxypeptidase inhibitor. This observation and the results of the present study taken together indicate that Hoe 498 can be used to prolong the half life of 5-met-enkephalin-6arg-7phe specifically and in turn to better study the functional role of this endogenous opioid peptide.
