Oxidation of ascorbic acid by lipoxygenase: effect of selected chemicals

The ability of soybean lipoxygenase to mediate ascorbic acid oxidation was examined. The oxidation of ascorbic acid was dependent on the concentration of linoleic acid, ascorbic acid and the enzyme. The optimal conditions to observe maximal enzyme velocity included the presence of 800 microM linoleic acid, 500 microM ascorbic acid and 25 nM soybean lipoxygenase in 50 mM Tris buffer, pH 8.3. The reaction displayed a K(m) value of 100 microM for ascorbic acid and an average specific activity of about 460 nmol/min/nmol enzyme under the optimal conditions. The effect of ascorbic acid on the lipoxygenase-catalysed co-oxidation of xenobiotics was also evaluated. Ascorbic acid markedly decreased the rate of oxidation of test xenobiotics by the lipoxygenase. In contrast, the rate of ascorbic acid co-oxidation was enhanced significantly by the presence of xenobiotics, which are co-oxidized simultaneously by lipoxygenase through the formation of free radicals. Superoxide generation was not observed during lipoxygenase-mediated ascorbic acid co-oxidation.