Reductive inactivation of soybean lipoxygenase 1 by catechols: a possible mechanism for regulation of lipoxygenase activity

Abstract

Nordihydroguaaretic acid (NDGA), one of the most efficient inhibitors of lipoxygenases, is shown, by electron paramagnetic resonance, circular dichroism, and fluorescence studies, to reduce the catalytically active ferric soybean lipoxygenase 1 (Eox) to the inactive ferrous form (Ered). In decreasing order of reactivity, the following also reduce Eox: catechol > hydroquinone > 2,6-di- tert-butyl-4-methylphenol > esculetin > caffeic acid .apprxeq. .alpha.-tocopherol > norepinephrine > dithiothreitol. The reduction of Eox by NDGA to 13(S)-hydroperoxy-9(Z), 11(E)-octadecadienoate (LOOH) and the oxidation of Ered by LOOH to give Eox. Thus, NDGA can efficiently inhibit the Eox catalyzed conversion of LH to LOOH by reducing Eox to the inactive Ered, thereby diminishing the turnover rate. Lipoxygenase catalyzes the oxidation of NDGA by LOOH at a rate that is consistent with the independently determined rate constant for the reduction of Eox by NDGA. All four reducing equivalents from the two catechol groups in NDGA can be utilized in the reduction of Eox, leading to the consumption of 4 mol of LOOH/mol of NDGA intially present. Because the catalytically inactive Ered is oxidized by fatty acid hydroperoxides (e.g. LOOH) to give the active Eox, reducing agents such as NDGA are most effective as lipoxygenase inhibitors at low hydroperoxide levels, reduction of lipoxygenase from the ferric to ferrous state may by important in the regulation of lipoxygenase activity and hence leukotriene biosynthesis.