OXYGEN CONCENTRATION DETERMINES REGIOSPECIFICITY IN SOYBEAN LIPOXYGENASE-1 REACTION VIA A BRANCHED KINETIC SCHEME

The effect of oxygen concentration on the regiospecificity of the soyb ean lipoxygenase-l dioxygenation reaction was studied, At low oxygen c oncentrations (<5 mu M), a dramatic change in the regiospecificity of the enzyme was observed with the hydroperoxy-octadecadienoic acid (HPO D) 13:9 ratio closer to 50:50 instead of the generally reported 95:5. This alteration of regiospecificity is not an isolated phenomenon, sin ce it occurs during a reaction carried out under ''classical'' conditi ons, i.e. in a buffer saturated with air before the reaction. beta-car otene bleaching and electronic paramagnetic resonance findings provide d evidence that substrate-derived free radical species are released fr om the enzyme. The kinetic scheme proposed by Schilstra et al, (Schils tra, M. J., Veldink, G. A. & Vliegenthart, J. F. G. (1994) Biochemistr y 33, 3974-3979) was thus expanded to account for the observed variati ons in specificity, The equations describing the branched scheme show two different kinetic pathways: a fully enzymatic one leading to a reg ioisomeric composition of 13-HPOD:9-HPOD = 95:5, and a semienzymatic o ne leading to a regio-isomeric composition of 13-HPOD:9-HPOD = 50:50, The ratio between the two different pathways depends on oxygen concent ration, which thus determines the overall specificity of the reaction.