Inhibition of enkephalin metabolism by, and antinociceptive activity of, bestatin, an aminopeptidase inhibitor.

In the presence of thiorphan an 'enkephalinase' inhibitor, bestatin an aminopeptidase inhibitor of bacterial origin potently inhibited the hydrolysis of [3H][Leu5]enkephalin by slices from rat striatum with an IC50 value of about 0.2 microM whereas puromycin was approximately 1000 times less potent on this preparation. In vivo bestatin or thiorphan (but not puromycin) significantly protected [3H][Met5]enkephalin administered intracerebroventricularly to mice from hydrolysis and co-administration of these two peptidase inhibitors resulted in a strong reduction in the appearance of hydrolysis products in brain. In a parallel fashion the antinociceptive activity of [Met5]enkephalin in the mouse hot-plate test was additively potentiated by bestatin and thiorphan but not by puromycin. Finally both bestatin and thiorphan themselves displayed antinociceptive properties on either the hot-plate jump test or the phenyl-benzo-quinone writhing test. It is concluded that a bestatin-sensitive aminopeptidase activity together with the 'enkephalinase' activity plays a critical role in the inactivation of both exogenous and endogenous enkephalins.