Interactions between nitric oxide and peroxynitrite during prostaglandin endoperoxide H synthase-1 catalysis: a free radical mechanism of inactivation.

Peroxynitrite (ONOO(-)) can serve either as a peroxide substrate or as an inactivator of prostaglandin endoperoxide H synthase-1 (PGHS-1). Herein, the mechanism of PGHS-1 inactivation by ONOO(-) and the modulatory role that nitric oxide (*NO) plays in this process were studied. PGHS-1 reacted with ONOO(-) with a second-order rate constant of 1.7 x 10(7) M(-1) s(-1) at pH 7.0 and 8 degrees C. In the absence of substrates, the enzyme was dose-dependently inactivated by ONOO(-) in parallel with 3-nitrotyrosine formation. However, when PGHS-1 was incubated with ONOO(-) in the presence of substrates, the direct reaction with ONOO(-) was less relevant and ONOO(-)-derived radicals became involved in enzyme inactivation. Bicarbonate at physiologically relevant concentrations enhanced PGHS-1 inactivation and nitration by ONOO(-), further supporting a free radical mechanism. Importantly, *NO (0.4-1.5 microM min(-1)) was able to spare the peroxidase activity of PGHS-1 but it enhanced ONOO(-)-mediated inactivation of cyclooxygenase. The observed differential effects of *NO on ONOO(-)-mediated PGHS-1 inactivation emphasize a novel aspect of the complex modulatory role that *NO plays during inflammatory processes. We conclude that ONOO(-)-derived radicals inactivate both peroxidase and cyclooxygenase activities of PGHS-1 during enzyme turnover. Finally, our results reconcile the proposed alternative effects of ONOO(-) on PGHS-1 (activation versus inactivation).