[Development of a new analgesic based on metabolism of endomorphin, an endogenous opioid peptide].
Endomorphin-2 (Tyr-Pro-Phe-PheNH(2)) was discovered as an endogenous ligand for the mu-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 PheNH(2). 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-PheNH(2) and Pro-Phe-PheNH(2) was increased in the presence of bestatin, an aminopeptidase inhibitor, whereas that of free phenylalanine and PheNH(2) was decreased. Furthermore, purified dipeptidyl peptidase IV hydrolyzed endomorphin-2 at the cleavage site, the Pro(2)-Phe(3) bond. Thus degradation of endomorphin-2 by brain synaptic membranes appears to occur mainly through the cleavage of the Pro(2)-Phe(3) bond by dipeptidyl peptidase IV, followed by the release of free phenylalanine and PheNH(2) from the liberated fragment, Phe-PheNH(2), 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.
Department of Biochemistry, Daiichi College of Pharmaceutical Sciences, Minami, Fukuoka, Japan. chikai@nichiyaku.ac.jp
Chikai
C
Sakurada
D000700:Analgesics / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000378:metabolism; D018819:Dipeptidyl Peptidase 4 / Q000494:pharmacology / Q000502:physiology; D015195:Drug Design; D006801:Humans; D051379:Mice; D009842:Oligopeptides / Q000378:metabolism* / Q000494:pharmacology / Q000502:physiology; D013570:Synaptic Membranes / Q000378:metabolism
D000700:Analgesics; D009842:Oligopeptides; C105294:endomorphin 2; C041445:diprotin A; D018819:Dipeptidyl Peptidase 4