Catalytic activity of human indoleamine 2,3-dioxygenase (hIDO1) at low oxygen.

•Activity of human indoleamine 2,3-dioxygenase (hIDO1) at low O2 is measured.•Bi-substrate enzyme kinetics of hIDO1 is investigated.•Initial rates are measured as a function of concentration of both substrates.•A quantitative analysis of O2 concentration effects on initial rate is performed.•The mechanism of substrate inhibition of hIDO1 by l-tryptophan is discussed.