OXIDATION-KINETICS OF CAFFEIC ACID BY PROSTAGLANDIN-H SYNTHASE - POTENTIAL ROLE IN REGULATION OF PROSTAGLANDIN BIOSYNTHESIS

The naturally occurring catechol derivative caffeic acid is a moderate stimulator of prostaglandin H synthase cyclooxygenase activity and a good reducing substrate for prostaglandin H synthase-compounds I and I I. The discrepancy between the two properties is explained by a specif ic peroxidative mechanism that includes the formation of an inhibitory complex of caffeic acid with native enzyme followed by a three-step i rreversible ping-pong peroxidation. The concentration of caffeic acid necessary to produce 50% stimulation of 0.2 mM arachidonic acid oxidat ion is 0.8 +/- 0.1 mM. The rate constant for the reaction of prostagla ndin H synthase with hydrogen peroxide, determined from steady-state r esults, is (5.68 +/- 0.1) x 10(5) M(-1) s(-1). The rate constant for t he reaction of prostaglandin H synthase-compound II with caffeic acid is (1.25 +/- 0.1) x 10(6) M(-1) s(-1). The dissociation constant of ca ffeic acid from the inhibitory complex is 35 +/- 10 mu M In diluted en zyme solutions, caffeic acid binding is diminished and the enzyme exhi bits higher peroxidase activity. Our results suggest that caffeic acid is not a O-demethylation product of ferulic acid degradation catalyze d by prostaglandin H synthase, nor a chelating agent for the heme iron . The oxidation of caffeic acid could be important in the regulation o f both prostaglandin H synthase and lipoxygenase activities and hence prostaglandin and leukotriene biosynthesis.