The metabolism of tryptophan (TRYP) to kynurenine and its metabolites, to tryptamine, and through serotonin to 5-hydroxyindoleacetic acid was studied in nine depressed patients and six non-depressed psychiatric subjects.
In general, there were no significant differences in the basal urinary excretion of the nine metabolites measured between the two patient populations nor in the excretion of these metabolites after the administration of an oral dose of 2 gm of L-tryptophan. Also, pyridoxine (150 mg) did not differentially alter the TRYP-produced increases in urinary metabolites in the two groups of subjects.
The data in this report do not support the suggestion of increased metabolism of TRYP along the kynurenine pathway in depression.
Frazer A, Pandey GN, Mendels J. Metabolism of Tryptophan in Depressive Disease. Arch Gen Psychiatry. 1973;29(4):528–535. doi:10.1001/archpsyc.1973.04200040070012