Interaction between peroxidase and cyclooxygenase activities in prostaglandin-endoperoxide synthase. Interpretation of reaction kinetics.

Prostaglandin-endoperoxide synthases have two distinct enzymatic activities in the biosynthesis of prostanoids: a peroxidase and a fatty acid oxygenase. Hydroperoxides, such as the cyclooxygenase reaction product, prostaglandin G2, act both as substrates for the synthase peroxidase and as obligatory initiators of the cyclooxygenase reaction itself. A mechanistic scheme was devised to describe the interactions between the two activities of the synthase. This mechanism was based on a heme-dependent peroxidase mechanism such as that observed with horseradish peroxidase and initiation of the cyclooxygenase reaction by intramolecular reaction with a peroxidase intermediate. Rate equations derived from the mechanism were numerically integrated by an interactive computer program that consolidated the diverse phenomena to provide quantitative predictions of the reaction kinetics of the synthase. The predictions agreed well with experimental observations of the purified ovine seminal vesicle enzyme under a wide variety of conditions, including inhibition by exogenous hydroperoxide scavenger and by cyanide, stimulation by exogenous hydroperoxide, and inhibition of eicosapentaenoic acid oxygenation under conditions where arachidonic acid reacts rapidly. The detailed analyses of reaction kinetics made possible with the computer simulation provide important insights into the interactions between the two catalytic activities of the synthase in the control of prostaglandin biosynthesis.