Inactivation of prostaglandin H synthase and prostacyclin synthase by phenylbutazone. Requirement for peroxidative metabolism

Phenylbutazone (PB), a nonsteroidal anti-inflammatory drug, is an efficient reducing cofactor for the peroxidase activity of prostaglandin H synthase (PHS). Most reducing cofactors for the peroxidase protect PHS and prostacyclin synthase from inactivation by hydroperoxides. PB, however, does not protect these Enzymes, but rather augments their hydroperoxide-dependent inactivation. Using ram seminal vesicle microsomes as a source of PHS and prostacyclin synthase, we have examined the interaction of PB and exogenous hydroperoxides. Chromatographic analysis of the metabolism of 14C-labeled arachidonic acid in this system revealed that PB-dependent inactivation of PHS is markedly increased in the presence of 100 microM H2O2. This inactivation is a linear function of PB concentration between 10 and 250 microM, with a half-maximal effect in this range at about 100 microM PB. Prostacyclin synthase is even more sensitive to inactivation by the combined PB and H2O2 treatment, with a corresponding half-maximal effect at PB concentrations near 25 microM. This PB- and H2O2-dependent inactivation is demonstrable whether PGH2 is generated in situ from arachidonic acid or is added exogenously, supporting a direct effect of the treatment on prostacyclin synthase. As PB undergoes peroxide-dependent co-oxygenation catalyzed by PHS, we propose that it is an oxygenated derivative of PB, rather than the parent compound, which is responsible for the inactivation of PHS and prostacyclin synthase. Nafazatrom, a competitive inhibitor of PB co-oxygenation, blocks the effects of the PB and H2O2 treatment, supporting our proposal.