EFFECT OF NONIONIC DETERGENTS ON LIPOXYGENASE CATALYSIS

In many studies on lipoxygenase catalysis, nonionic detergents are use d to obtain an optically transparent solution of the fatty acid substr ate. In order to resolve some controversies that exist with regard to the interpretation of kinetic data obtained with solutions containing nonionic detergents, a systematic investigation was undertaken into th e effects of Lubrol, Tween-20 and Triton X-100 (0-0.8 g/L) on the kine tics of linoleate (2.5-110 mu M) dioxygenation, catalyzed by lipoxygen ase-1 or lipoxygenase-2 from soybean, at pH 9 or 10, at 25 degrees C. Under most conditions, it was found that the detergents slowed down th e reaction. However, at high linoleate concentrations, where substrate inhibition of lipoxygenase is significant, small amounts of detergent increased the dioxygenation rate. In a quantitative analysis of the r esults, a kinetic model in which the incorporation of linoleate in the detergent micelles is formulated as a simple reversible equilibrium, and in which both lipoxygenase-1 and -2 interact with free linoleate, but not with linoleate incorporated in the micelles, appeared to be su fficient to predict experimental results over a wide range of experime ntal conditions. According to this model, the changes in the dioxygena tion kinetics caused by the presence of nonionic detergents are simila r (but not equal) to those caused by competitive inhibitors. The concl usions that monomeric, nonmicellar Linoleate is the preferred substrat e for lipoxygenase and that the observed inhibition and stimulation ar e solely due to changes in the effective linoleate concentration stron gly corroborate the earlier observations by Galpin and Allen [Biochim. Biophys. Acta 488 (1977), 392-401].