Soybean lipoxygenase-promoted oxidation of free and esterified linoleic acid in the presence of deoxycholate

Lipoxygenase (EC 1.13.11.12) catalyzes the reaction between oxygen and polyunsaturated fatty acids to give fatty acid hydroperoxides. Recent work showed that soybean lipoxygenase 1 can oxidize diacylglycerols when deoxycholate is present in the reaction medium. Conditions were sought to maximize 1,3-dilinolein oxidation with a commercial soybean lipoxygenase preparation. It was found that dilinolein was oxidized most rapidly in a multicomponent buffer medium that contained 10 mM deoxycholate between pH 8 and 9. When dilinolein oxidation was conducted in the individual components of the multicomponent buffer, the oxidation rate decreased two- to threefold. Addition of 0.2 M NaCl to one of the components, Tricine buffer, caused a twofold increase in the oxidation rate, demonstrating that high ionic strength is a major factor promoting rapid oxidation in the multicomponent buffer. In the deoxycholate multicomponent buffer, the order of reactivity toward oxidation was monolinolein>methyl linoleate≈ linoleic acid>dilinolein. Competition experiments in which mixtures of the substrates were presented simultaneously to lipoxygenase in the presence of deoxycholate showed that linoleic acid was the most reactive substrate. When no surfactant was present or when the surfactant was Tween 20, linoleic acid was the most rapidly oxidized substrate. Overall, the results demonstrate that monolinolein and methyl linoleate are just as reactive, or more so, as linoleic acid to oxidation by lipoxygenase under specified reaction conditions. In competition experiments, linoleic acid oxidation predominates, probably because its free carboxyl functionality allows it to be preferentially bound to the active site of lipoxygenase.