Molecular Evolution and Characterization of Fungal Indoleamine 2,3-Dioxygenases

Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are tryptophan-degrading enzymes. Mammalian IDO expression is induced by cytokines and has antimicrobial and immunomodulatory effects. A major role of mammalian TDO is to supply nicotinamide adenine dinucleotide (NAD(+)). In fungi, the IDO homologue is thought to be expressed constitutively and supply NAD(+), as TDO is absent from their genomes. Here, we reveal the distribution of IDO genes among fungal species and characterize their enzymatic activity. The yeast, Saccharomyces cerevisiae has only one IDO gene, whereas the koji-mold, Aspergillus oryzae has two genes, IDO alpha and IDO beta. The A. oryzae IDO alpha showed more similar enzymatic properties to those of S. cerevisiae IDO than IDO beta, suggesting that the A. oryzae IDO alpha is a functional homologue of the S. cerevisiae IDO. From the IDO beta gene, two isoforms, IDO beta and IDO beta(+) could be generated by alternative splicing. The latter contained a 17 amino acids insertion which were encoded by the first intron of IDO beta gene. In comparison to IDO beta(+), bacterially expressed IDO beta showed much lower K (m) value and more than five-times faster V (max) value, resulting in 85 times higher catalytic efficiency; i.e., the removal of the domain encoded by the first intron from IDO beta(+) increases its enzymatic activity drastically. This might be a unique regulation mechanism of the l-Trp metabolism in the A. oryzae. The levo-1-methyl tryptophan (l-1MT) is a good inhibitor of both IDO1 and IDO2. However, the activity of fungal IDOs tested was not inhibited at all by l-1MT.