The indoleamine 2,3-dioxygenase (EC 1.13.11.17) activity in human T24 cells has been investigated in cell extracts by using a non-radioactive assay. It is enhanced in a dose-dependent manner up to 25-fold by interferon-gamma. The maximum reaction velocity is increased rather than the Km, which remains at 4 mumol/l. Induction of activity starts 3 h after stimulation and reaches a plateau at 21-48 h. Decreased stimulation was observed in the presence of high L-tryptophan concentrations.
