Some previous studies found decreased concentrations of L-tryptophan (TRY) and increased L-kynurenine (KYN), or its metabolites, in the body fluids of subjects with major depressive disorder (MDD), sometimes in association with suicidal behavior. Such changes might indicate a shift of TRY away from serotonin production, possibly via the effects of inflammatory peptides which activate indoleamine-2,3-dioxygenase. However, these findings have been inconsistent and require replication.
We used sensitive liquid-chromatography mass spectrometry methods to assay plasma concentrations of TRY, 5-hydroxyindoleacetic acid (5-HIAA), and KYN and its metabolites (anthranilic acid and xanthurenic acid). We compared 49 hospitalized, depressed subjects diagnosed with MDD (n = 37) or bipolar disorder (BD, n = 12), with (n = 22) or without (n = 27) previous suicide attempts, to 78 healthy, ambulatory controls of similar age and sex (total n = 127).
Contrary to expectation, TRY plasma concentrations were higher, KYN plasma concentrations were lower, and their ratio much higher in depressed subjects, with no relationship to suicidal history. Concentrations of 5-HIAA and the kynurenine metabolites did not differ between depressed and healthy subjects.
These findings are opposite to expectations and not consistent with a hypothesized increased conversion from TRY to KYN in depressed subjects. In addition, we found no evidence of altered production of serotonin as 5-HIAA concentration was unchanged. None of the observed changes was associated with a history of suicide attempt.