Abstract
Endomorphin-2 (Tyr-Pro-Phe-PheNH2) was discovered as an endogenous ligand for the μ-opioid receptor. The physiologic function of endomorphin-2 as a neurotransmitter or neuromodulator may cease after rapid enzymatic processing in the synapses of the brain, like other neuropeptides. The present study was conducted to examine the metabolism of endomorphin-2 by synaptic membranes prepared from mouse brain. Major metabolites were free tyrosine, free phenylalanine, Tyr-Pro, and PheNH2. Both the degradation of endomorphin-2 and the accumulation of major metabolites were inhibited by specific inhibitors of dipeptidyl peptidase IV, such as diprotin A and B. On the other hand, the accumulation of Phe-PheNH2 and Pro-Phe-PheNH2 was increased in the presence of bestatin, an aminopeptidase inhibitor, whereas that of free phenylalanine and PheNH2 was decreased. Furthermore, purified dipeptidyl peptidase IV hydrolyzed endomorphin-2 at the cleavage site, the Pro2-Phe3 bond. Thus degradation of endomorphin-2 by brain synaptic membranes appears to occur mainly through the cleavage of the Pro2-Phe3 bond by dipeptidyl peptidase IV, followed by the release of free phenylalanine and PheNH2 from the liberated fragment, Phe-PheNH2, by aminopeptidase. We have also examined the effects of diprotin A on the antinociception induced by intracerebroventricularly administered endomorphin-2 in the mouse paw withdrawal test. Diprotin A simultaneously injected with endomorphin-2 enhanced endomorphin-2-induced antinociception. These results indicate that dipeptidyl peptidase IV may be an important peptidase responsible for terminating endomorphin-2-induced antinociception at the supraspinal level in mice. These findings also suggest that selective dipeptidyl peptidase IV inhibitors or dipeptidyl peptidase IV-resistant endomorphin-2 analogues have the potential for the clinical use as analgesics.
