Effect of protonation and aggregation state of (E)-resveratrol on its hydroperoxidation by lipoxygenase

The protonation and aggregation states of (E)-resveratrol were used as tools to investigate the kinetic properties of lipoxygenase (LOX). It was found that the deprotonation of the 4'-hydroxyl group at pH values higher than the pK(a1) of (E)-resveratrol produced an increase in the LOX activity, with an optimum pH of 8.5. Moreover, the results show how LOX activity is strongly affected by the aggregation state of (E)-resveratrol. When the enzyme uses monomers of (E)-resveratrol as substrate, LOX shows a Michaelian behavior and the K(m) value can be determined (44.39 microM). However, when (E)-resveratrol concentration is increased to values higher than the critical concentration determined by fluorescence methods (35 microM at pH 8.5), LOX shows strong inhibition. These results can be interpreted as a previously unreported aggregate-induced enzyme inhibition, which can be modified by the use of different modulators of the aggregation state of (E)-resveratrol, such as cyclodextrins or ethanol. Finally, when the reaction was kinetically characterized in the optimum conditions of both aggregation and protonation state, a typical induction period was observed, along with a dependence of the hydroperoxidation rate with the hydrogen peroxide concentration.