Tryptophan Degradation in Mice Initiated by Indoleamine 2,3-dioxygenase

Tryptophan degradation in mice initiated by indoleamine 2,3-dioxygenase was characterized, taking advantage of its induction by bacterial lipopolysaccharide. Our results demonstrated that in various tissues, N-formylkynurenine produced by the dioxygenase from tryptophan was rapidly hydrolyzed into kynurenine by a kynurenine formamidase, but it was not further metabolized. The localization in the liver and kidney of the kynurenine-metabolizing enzymes suggested that kynurenine thus formed was transported by the bloodstream to those two organs to be metabolized. In fact, the plasma kynurenine level increased in parallel with the induction of the dioxygenase by lipopolysaccharide, and kinetic analysis indicated that at the maximal induction of the enzyme there was a 3-fold increase in the kynurenine production. The major metabolic route of kynurenine was excretion in urine as xanthurenic acid. This increase in the kynurenine production was not explained by L-tryptophan 2,3-dioxygenase in the liver, because during the induction of indoleamine 2,3-dioxygenase, the hepatic enzyme level was substantially suppressed. These findings indicated that indoleamine 2,3-dioxygenase actively oxidized tryptophan in mice and that its induction resulted in an increase in tryptophan degradation.