The in vitro and in vivo resistance of synthetic enkephalin analogs to three enkephalin-hydrolyzing enzymes

Abstract

The magnitude of the in vitro and in vivo resistance of 3 synthetic enkephalin analogs, [ d-Ala 2,Met 5]-enkephalin (DAME), [ d-Ala 2,Met 5]-enkephalinamide (DAME-NH 2) and [ d-Ala 2, d-Met 5]enkephalin (DADME), to 3 enkephalin-hydrolyzing enzymes, amastatin-sensitive aminopeptidase (AsA), phosphoramidon-sensitive endopeptidase-24.11 (PsE) and captopril-sensitive dipeptidyl car☐ypeptidase I (CsD), was estimated by comparing the potency of enkephalins in the absence of the peptidase inhibitor (PI) with that in the presence of the PI. The enhancement of the potency of enkephalins in the isolated mouse vas deferens preparation by 3 PIs, amastatin, phosphoramidon, and captopril, indicated that the resistance of enkephalins to AsA, PsE, or CsD was DADME≒DAME-NH 2≒DAME > [Met 5]-enkephalin (ME), DADME > DAME-NH 2 > DAME≒ME, or DADME≒DAME-NH 2 > DAME > ME, respectively. Additionally, the data obtained by the s.c. administration of enkephalin analogs to 10-day-old rats with or without PI, showed that PsE played the most important role in the inactivation of both DAME and DAME-NH 2. In addition to PsE, both AsA and CsD, or AsA alone, played the significant role in the inactivation of DAME, or DAME-NH 2, respectively. In the inactivation of DADME, AsA alone played the significant role. These results showed that the 3 peptidases all played important roles in the inactivation of enkephalins, and therefore only an analog like DADME, which was quite resistant to the 3 enzymes, was able to produce the effect without PIs after its systemic administration. Since even DADME was not completely resistant to the 3 enzymes; however, its potency was further increased by pretreatment of infant rats with the PIs.