OXYGENATION REACTIONS OF PROSTAGLANDINS ENDOPEROXIDE SYNTHASE AND SOYBEAN LIPOXYGENASE - SURPRISING STOICHIOMETRY IN THE FORMATION OF HYDROPEROXY AND HYDROXY DERIVATIVES OF CIS,CIS-EICOSA-11,14-DIENOIC ACID

The stoichiometry of the oxygenation reaction of cis,cis-eicosa-11,14- dienoic acid catalyzed by prostaglandin endoperoxide synthase and soyb ean lipoxygenase has been investigated by using steady-state initial r ate measurements, The rate of product formation (conjugated diene hydr operoxy and hydroxy derivatives) was followed spectrophotometrically a t 235 nm, and the rate of oxygen consumption was measured polarographi cally. The ratio of the two rates, d[conjugated diene]/-d[O-2], is 2/1 for the prostaglandin endoperoxide synthase catalyzed reaction and 1/ 1 for the lipoxygenase reaction, The 2/1 ratio can be explained by two interrelated routes, each of which results in formation of the conjug ated diene hydroxy derivative of the acid, One route, initiated by hyd rogen atom abstraction from the acid by Compound I, results in formati on of the conjugated diene hydroperoxy derivative, The latter is conve rted to the hydroxy derivative by regenerating Compound I from the nat ive enzyme, The other route involves direct oxygen atom insertion into the acid by the tyrosyl radical form of Compound I, The decrease in a bsorbance at 235 nm obtained in the presteady-state phase suggests tha t during the initial contact of hydroperoxide and enzyme an epoxy-hydr oxy fatty acid-enzyme complex may be formed.