The effect of tryptophan load and vitamin B 6 supplementation on urinary excretion of tryptophan metabolites in human male subjects.

I. Urinary excretion of tryptophan metabolites of the kynurenine pathway before and after a loading dose of 2 g of L-tryptophan (both before and during supplementation with vitamin B6) was studied in normal young male subjects and in elderly male subjects with prostatic diseases. 2. The increase in urinary excretion of tryptophan metabolites in the normal male subjects after a loading dose of 2 g of L-tryptophan was unchanged after vitamin B6 supplementation, indicating that this dose does not exceed the normal capacity of the kynurenine pathway. 3. It is therefore concluded that a loading dose of 2 g of L-tryptophan is preferable when studying the capacity of the kynurenine pathway. 4. In about three-quarters of the elderly male subjects with prostatic diseases, the capacity of the kynurenine pathway was normal. 5. In about one-quarter of the subjects with prostatic diseases after a loading dose of 2 g of L-tryptophan there was abnormally high urinary excretion of one or several tryptophan metabolites, which could be normalized by vitamin B6 , indicating that the capacity of the kynurenine pathway had been exceeded in these subjects. 6. Urinary excretion of individual tryptophan metabolites after a loading dose of 2 g of L-tryptophan was constant in the same subject when studied under unchanged conditions, whether there was a normal or a decreased capacity of the kynurenine pathway. 7. It is concluded that a single determination of urinary excretion of tryptophan metabolites after a loading dose of 2 g of t-tryptophan is sufficient to determine the capacity of the kynurenine pathway.