INFLUENCE OF THIAMINE AND TRYPTOPHAN UPON THE TRYPTOPHAN PEROXIDASE–OXIDASE ACTIVITY OF RAT LIVER

The possible role of thiamine in the oxidation of tryptophan by tryptophan peroxidase–oxidase (TPO) of liver was studied by several approaches. (1) In acute experiments parenterally administered thiamine did not cause an adaptive increase in the enzyme content of rat liver, as can be induced by similar injection of tryptophan. (2) Thiamine pyrophosphate, added to liver extracts which had been partially inactivated with respect to TPO by aging for one-half hour at 37 °C. or by prolonged dialysis, did not reactivate the enzyme oxidizing tryptophan. (3) In multifactorial dietary experiments in which thiamine, tryptophan, or both were omitted from the diet only tryptophan deficiency resulted in a decrease of the liver TPO activity. There was no effect of thiamine, either in the presence or absence of tryptophan. The D- and L-isomers of tryptophan were equally effective in promoting TPO activity. These results do not support the findings of other authors who imply a role for thiamine in the oxidation of tryptophan by TPO.