A Road Less Taken: Functional and Regulatory Insights into Indoleamine 2,3-Dioxygenase

Abstract

Indolamine 2,3-dioxygenase 1 (IDO1) is a heme enzyme in humans that plays a crucial role in immune system regulation by oxidizing the essential amino acid tryptophan and producing downstream metabolites such as n-formyl kynurenine. This metabolic pathway triggers a series of reactions that result in immune suppression. An example of the importance of this suppression can be found in a developing embryo, in which IDO1 activity protects the embryo from maternal immune attack.1 Unfortunately, cancer cells exploit this mechanism by upregulating IDO1 to evade immune surveillance.2 Leading inhibitors take advantage of the recently discovered regulation of IDO1.3 These compounds mimic the heme and bind to the apo form of IDO1, which has been found to be abundant in cells.The first chapter of this thesis presents a literature review focusing on the tryptophan-degrading enzymes IDO1, TDO, and IDO2. The second chapter looks at how sulfide, polysulfide and 3-mercapto indole reduces the ferric species of IDO1 to the active ferrous species. This research explores the potential benefits of suppressing the immune system in cases of overactive immune responses, such as in autoimmune diseases like multiple sclerosis. Various methods of characterizing the polysulfido IDO1 species are used to confirm the reduction and activity. Chapter three focuses on the cause of substrate inhibition in IDO1. This self-regulatory process prevents the immune system from shutting down when there is an excess of tryptophan, such as after a meal. We have shown that this substrate inhibition is caused by the off cycle stable ferryl tryptophan species. This inhibition is lifted by the addition of dopamine. Additionally, a novel method of generating ferryl IDO1 is shown by utilizing peroxynitrite. Chapter four investigates how different metal porphyrins affect the activity of IDO1 in vitro and in vivo. We have shown that IDO1 in the Fe3+ state is significantly more labile than IDO1 in the Fe2+ leading to the possibility that cellular reductants can diminish the efficacy of these leading IDO1 inhibitors. This lability allows other metal porphyrins to enter IDO1 and affect its activity. Notably, zinc(II) and manganese(III) inhibit IDO1’s activity in vitro and in vivo.