Degradation and oxidation of methionine enkephalin by human neutrophils

Abstract Met 5 -enkephalin, tyr-gly-gly-phe-met, is an endogenous pentapeptide, with morphine agonist activity. In this study, we demonstrated that met 5 -enkephalin was degraded with the release of tyrosine by resting human PMN, whereas it was degraded as well as oxidized to its sulfoxide derivative, met 5 -(O)-enkephalin, by phagocytosing PMN. PMN also degraded met 5 -(O)-enkephalin but to a lesser extent. Bacitracin at 1 gm/L inhibited the degradation and oxidation of met 5 -enkephalin without affecting the production of superoxide and the viability of PMN. The oxidation of met 5 -enkephalin by phagocytosing PMN was inhibited by catalase or NaN 3 but not by SOD. This suggests that the oxidation of met 5 -enkephalin by phagocytosing PMN was, at least in part, dependent on the MPO system (MPO-H 2 O 2 -halide). Using purified canine MPO, we further demonstrated that MPO-H 2 O 2 -Cl − oxidized met 5 -enkephalin to met 5 -(O)-enkephalin. The MPO-mediated oxidation of met 5 -enkephalin was inhibited by methionine but not by methionine sulfoxide, tyrosine, glycine, or phenylalanine, confirming that it was the methionine moiety of met 5 -enkephalin which was oxidized. Since both the sulfoxide derivative and the degradation products of met 5 -enkephalin have reduced opiate agonist activity, oxidation and degradation of met 5 -enkephalin by PMN may contribute to the pain at the site of inflammation.