The roles of superoxide anion and methylene blue in the reductive activation of indoleamine 2,3-dioxygenase by ascorbic acid or by xanthine oxidase-hypoxanthine

Abstract

To clarify the roles of superoxide anion (O2.-) and methylene blue in the reductive activation of the heme protein indoleamine 2,3-dioxygenase, effects of xanthine oxidase-hypoxanthine used at various oxidase concentration levels as an O2.- source and an electron donor on the catalytic activity of the dioxygenase have been examined in the presence and absence of either methylene blue or superoxide dismutase using L- and D-tryptophan as substrates. In the absence of methylene blue, initial rates of the product N-formylkynurenine formation are enhanced in parallel with the xanthine oxidase level up to approximately 100 and approximately 50% of the apparent maximal activity (approximately 2 s-1) for L- and D-Trp, respectively. Superoxide dismutase effectively inhibits the reactions by 80-98% for both isomers. Additions of methylene blue (25 microM) help to maintain the linearity of the product formation that would be rapidly lost a few minutes after the start of the reaction without the dye, especially for L-Trp. Additions of methylene blue also enhance the activity to the maximal level for D-Trp. In the presence of methylene blue, the inhibitory effects of superoxide dismutase are considerably decreased with the increase in xanthine oxidase concentration, and at near maximal dioxygenase activity levels superoxide dismutase is totally without effect. In separate anaerobic experiments leuco-methylene blue, generated either by photoreduction or by ascorbate reduction, is shown to be able to reduce the ferric dioxygenase up to 25-40%. Substrate Trp and heme ligands (CO, n-butyl isocyanide) help to shift a ferric form----ferrous form equilibrium to the right. Thus, under aerobic conditions leuco-methylene blue might similarly be able to reduce the dioxygenase in the presence of an electron donor with the aid of substrate and O2. These results strongly suggest that indoleamine 2,3-dioxygenase can be activated through different pathways either by O2.- or by an electron donor-methylene blue system. For the latter case, the dye is acting as an electron mediator from the donor to the ferric dioxygenase.