Contributions of tryptophan 2,3-dioxygenase and indoleamine 2,3-dioxygenase to the conversion of <scp>d</scp>-tryptophan to nicotinamide analyzed by using tryptophan 2,3-dioxygenase-knockout mice

We investigated the contribution percentage of tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) to the conversion of D-tryptophan to nicotinamide in TDO-knockout mice. The calculated percentage conversions indicated that TDO and IDO oxidized 70 and 30%, respectively, of the dietary L-tryptophan. These results indicate that both TDO and IDO biosynthesize nicotinamide from D-tryptophan and L-tryptophan in mice.