Tryptophan-Restriction Diets Help to Maintain L-Tryptophan Homeostasis in Tryptophan 2,3-Dioxygenase Knockout Mice

We hypothesized that the requirements of essential nutrients are dependent upon catabolic abilities. Mice lacking L-tryptophan 2,3-dioxygenase (TDO) are available. The body concentration of L-tryptophan (L-Trp) has been reported to be higher in TDO-deficient mice than in wild-type (WT) mice. We examined the requirement of an appropriate L-Trp level for TDO-deficient mice using several biomarkers. TDO-deficient mice were fed a 10% amino-acid mixture diet containing 0.06%, 0.08%, and 0.17% L-Trp. WT mice fed a 0.17% Trp diet (standard diet) were used as control mice. The concentrations of L-Trp and its metabolites via serotonin were higher in TDO-deficient mice fed the 0.17% L-Trp diet than in WT mice fed the standard diet, but the concentrations were almost identical between TDO-deficient mice fed the 0.06% L-Trp diet and WT mice fed the standard diet. Therefore, as hypothesized, requirements of essential nutrients are dependent on catabolic abilities.