EFFECT OF TROLOX-C ON THE OXYGENATION REACTION OF PROSTAGLANDIN ENDOPEROXIDE SYNTHASE WITH CIS,CIS-EICOSA-11,14-DIENOIC ACID

Trolox C, a water-soluble derivative of alpha-tocopherol, stimulates t he oxygenation of cis,cis-eicosa-11,14-dienoic acid (AH) by prostaglan din endoperoxide synthase at lower concentrations and suppresses the s timulated reaction at higher concentrations. Surprisingly, Trolox C do es not affect the stoichiometric ratio between the rate of formation o f the oxygenation product 11-hydroxy-12-trans, 14-cis-eicosadienoic ac id (AOH) and the rate of disappearance of molecular oxygen. The ratio of the two rates, d[AOH]/-d[O-2], remains constant at 2/1 for a series of Trolox C concentrations and in the absence of Trolox C. Results in dicate that AH reacts preferentially with Compound I of the enzyme and that Trolox C does not compete for Compound I. Enzyme inactivation be gins with formation of an unproductive Compound I-tyrosyl radical (Com pound I-X .) which has the same number of oxidizing equivalents as the conventional peroxidase Compound I. The stimulating effect of low con centrations of Trolox C can be explained by reduction of the oxyferryl heme so that Compound I-X . is reduced to a Compound II-X . species, the Compound II analog of Compound I-X .. Thus heme bleaching is preve nted. A further one-electron reduction by Trolox C of Compound II-X . reforms the native enzyme, which permits enzyme recycling. Large conce ntrations of Trolox C inhibit reformation of native enzyme, reducing t he extent of stimulation.