title: Catalytic activity of human indoleamine 2,3-dioxygenase (hIDO1) at low oxygen.
abstract: A cytokine-inducible extrahepatic human indoleamine 2,3-dioxygenase (hIDO1) catalyzes the first step of the kynurenine pathway. Immunosuppressive activity of hIDO1 in tumor cells weakens host T-cell immunity, contributing to the progression of cancer. Here we report on enzyme kinetics and catalytic mechanism of hIDO1, studied at varied levels of dioxygen (O2) and L-tryptophan (L-Trp). Using a cytochrome b5-based activating system, we measured the initial rates of O2 decay with a Clark-type oxygen electrode at physiologically-relevant levels of both substrates. Kinetics was also studied in the presence of two substrate analogs: 1-methyl-L-tryptophan and norharmane. Quantitative analysis supports a steady-state rather than a rapid equilibrium kinetic mechanism, where the rates of individual pathways, leading to a ternary complex, are significantly different, and the overall rate of catalysis depends on contributions of both routes. One path, where O2 binds to ferrous hIDO1 first, is faster than the second route, which starts with the binding of L-Trp. However, L-Trp complexation with free ferrous hIDO1 is more rapid than that of O2. As the level of L-Trp increases, the slower route becomes a significant contributor to the overall rate, resulting in observed substrate inhibition.
authors.affiliation: Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States.
authors.forename: Ayodele O
authors.initials: AO
authors.lastname: Kolawole
authors.affiliation: Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States.
authors.forename: Brian P
authors.initials: BP
authors.lastname: Hixon
authors.affiliation: Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States.
authors.forename: Laura S
authors.initials: LS
authors.lastname: Dameron
authors.affiliation: Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States.
authors.forename: Ian M
authors.initials: IM
authors.lastname: Chrisman
authors.affiliation: Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States. Electronic address: valeriy.smirnov@umontana.edu.
authors.forename: Valeriy V
authors.initials: VV
authors.lastname: Smirnov
mesh_terms: D001665:Binding Sites; D002243:Carbolines; D002384:Catalysis; D004926:Escherichia coli / Q000201:enzymology; D006247:Harmine / Q000031:analogs & derivatives / Q000737:chemistry; D006801:Humans; D007166:Immunosuppressive Agents / Q000737:chemistry; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000378:metabolism*; D007737:Kynurenine / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D011485:Protein Binding; D013379:Substrate Specificity; D014364:Tryptophan / Q000031:analogs & derivatives / Q000737:chemistry
chemical_list: D002243:Carbolines; C576710:IDO1 protein, human; D007166:Immunosuppressive Agents; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D007737:Kynurenine; D006247:Harmine; D014364:Tryptophan; C010262:norharman; D010100:Oxygen; C525396:1-methyltryptophan
keywords: Enzyme kinetics; Global fit; Indoleamine dioxygenase; Inhibition; Oxygen electrode; Substrate analog