INTERACTION BETWEEN PEROXIDASE AND CYCLOOXYGENASE ACTIVITIES IN PROSTAGLANDIN-ENDOPEROXIDE SYNTHASE - INTERPRETATION OF REACTION-KINETICS

Prostaglandin-endoperoxide syntheses have two distinct enzymatic activ ities in the biosynthesis of prostanoids: a peroxidase and a fatty aci d oxygenase. Hydroperoxides, such as the cyclooxygenase reaction produ ct, prostaglandin G(2), act both as substrates for the synthase peroxi dase and as obligatory initiators of the cyclooxygenase reaction itsel f. A mechanistic scheme was devised to describe the interactions betwe en the two activities of the synthase. This mechanism was based on a h eme-dependent peroxidase mechanism such as that observed with horserad ish peroxidase and initiation of the cyclooxygenase reaction by intram olecular reaction with a peroxidase intermediate. Rate equations deriv ed from the mechanism were numerically integrated by an interactive co mputer program that consolidated the diverse phenomena to provide quan titative predictions of the reaction kinetics of the synthase. The pre dictions agreed well with experimental observations of the purified ov ine seminal vesicle enzyme under a wide variety of conditions, includi ng inhibition by exogenous hydroperoxide scavenger and by cyanide, sti mulation by exogenous hydroperoxide, and inhibition of eicosapentaenoi c acid oxygenation under conditions where arachidonic acid reacts rapi dly. 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.