The tetrapeptide, endomorphin-2 (Tyr-Pro-Phe-PheNH(2)) possesses high affin
ity for mu opioid receptors, and produces potent analgesia in mice. Its str
ucture appears to satisfy the substrate requirements of the proteinase, dip
eptidyl peptidase TV which removes dipeptides from the amino terminus of pe
ptides containing proline as the penultimate amino acid. A potent, stable a
nd specific inhibitor of this enzyme, Ala-Pyrrolidonyl-2-nitrile, has been
described which should potentiate endomorphin-2-induced analgesia. Further,
since dipeptidyl peptidase IV has an absolute requirement for L-Pro, a mor
e metabolically-stable D-Pro(2)-endomorphin-2 analog should produce longer
analgesic actions at lower doses. The present study found that endomorphin-
2 was degraded approximately twice as fast than the chromogenic substrate,
Ala-Pro-2naphthylamide, by dipeptidyl peptidase IV, whereas D-Pro(2)-endomo
rphin-2 was totally resistant to this enzyme's action. D-Pro(2)-endomorphin
-2 (ED50 = 0.05 mu g) was more potent than endomorphin-2 (ED50 = 30 mu g) i
n significantly increasing tail-flick latencies with longer durations of ac
tion. Both the peptide and analogue were equipotent (ED50 = 0.5 mu g) in si
gnificantly increasing jump thresholds. Ala-Pyrrolidonyl-2-nitrile (10-75 n
mol) elicited a dose-dependent analgesia, and potentiated the analgesic act
ions of endomorphin-2, particularly on the tail-flick test. Whereas systemi
c naltrexone (2.5, 10 mg/kg) dose-dependently eliminated each of the three
forms of analgesia on the jump test as well as the peak (15 min) effect on
the tail-flick test: analgesia elicited by either endomorphin-2, D-Pro2-end
omorphin-2 or Ala-Pyrrolidonyl-2-nitrile returned after 30-60 min in naltre
xone-treated rats on the tail-flick test. These data strongly suggest that
dipeptidyl peptidase IV plays a role in the inactivation of endomorphin-2 i
n vivo, and thereby modulates its central analgesic actions. (C) 1999 Elsev
ier Science B.V. All rights reserved.