Degradation of kyotorphin by a purified membrane-bound-aminopeptidase from monkey brain: potentiation of kyotorphin-induced analgesia by a highly effective inhibitor, bestatin.

Kyotorphin (Tyr-Arg) was rapidly degraded in rat brain homogenates and the Vmax and Km were 29.4 nmol/mg protein/min and 16.6 microM, respectively. This degradation was effectively inhibited by bestatin (IC 50; 0.08 microM) and p-chloromercuribenzoate (IC 50; 0.70 microM). Kyotorphin was also degraded by a membrane-bound aminopeptidase from monkey brains. The Vmax and Km of kyotorphin degradation by the aminopeptidase were 20.0 nmol/mg protein/min and 29.2 microM, respectively. The degradation of kyotorphin was also inhibited effectively by bestatin (KI; 0.4 microM). Co-administration with bestatin 50 micrograms (i.cist.) potentiated the analgesic effects of kyotorphin (i.cist.) by 4.8 times, and these effects were abolished by pretreatment with naloxone 0.5 mg/kg s.c. These results suggest that potentiation of analgesia by bestatin may be due to the protection against the degradation of kyotorphin and released enkephalin by a membrane-bound aminopeptidase.