Human Indoleamine 2,3-Dioxygenase Is a Catalyst of Physiological Heme Peroxidase Reactions

The heme enzyme indoleamine 2,3-dioxygenase (IDO) is a key regulator of immune responses through catalyzing l-tryptophan (l-Trp) oxidation. Here, we show that hydrogen peroxide (H 2 O 2 ) activates the peroxidase function of IDO to induce protein oxidation and inhibit dioxygenase activity. Exposure of IDO-expressing cells or recombinant human IDO (rIDO) to H 2 O 2 inhibited dioxygenase activity in a manner abrogated by l-Trp. Dioxygenase inhibition correlated with IDO-catalyzed H 2 O 2 consumption, compound I-mediated formation of protein-centered radicals, altered protein secondary structure, and opening of the distal heme pocket to promote nonproductive substrate binding; these changes were inhibited by l-Trp, the heme ligand cyanide, or free radical scavengers. Protection by l-Trp coincided with its oxidation into oxindolylalanine and kynurenine and the formation of a compound II-type ferryl-oxo heme. Physiological peroxidase substrates, ascorbate or tyrosine, enhanced rIDO-mediated H 2 O 2 consumption and attenuated H 2 O 2 -induced protein oxidation and dioxygenase inhibition. In the presence of H 2 O 2 , rIDO catalytically consumed nitric oxide (NO) and utilized nitrite to promote 3-nitrotyrosine formation on IDO. The promotion of H 2 O 2 consumption by peroxidase substrates, NO consumption, and IDO nitration was inhibited by l-Trp. This study identifies IDO as a heme peroxidase that, in the absence of substrates, self-inactivates dioxygenase activity via compound I-initiated protein oxidation. l-Trp protects against dioxygenase inactivation by reacting with compound I and retarding compound II reduction to suppress peroxidase turnover. Peroxidase-mediated dioxygenase inactivation, NO consumption, or protein nitration may modulate the biological actions of IDO expressed in inflammatory tissues where the levels of H 2 O 2 and NO are elevated and l-Trp is low. Background: Certain heme proteins exhibit a pseudo-peroxidase activity that alters their function. Results: H 2 O 2 engages the peroxidase activity of indoleamine 2,3-dioxygenase (IDO) to oxidatively inactivate its dioxygenase activity, consume nitric oxide, and promote IDO protein nitration. Conclusion: IDO is a catalyst of physiological peroxidase reactions. Significance: IDO peroxidase activity has novel implications for the control and biological actions of this important immune regulatory enzyme.