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 × 107 M− 1 s− 1 at pH 7.0 and 8 °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 μM 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).