The concentration of free and total tryptophan and kynurenine in plasma from 49 female depressives and 26 female controls was measured following oral loading with Ctryptophan, 100 mg/kg body weight. There was no significant difference between five depressives and six controls in the area under curve for free or total tryptophan or kynurenine in plasma. The peak concentration of kynurenine occured 4 h after loading and it correlated significantly with the area under curve for kynurenine. There was no significant correlation between the L-tryptophan dose (g) and the plasma concentration of kynurenine at 4 h in the 49 depressives or 26 controls. The mean plasma levels of tryptophan and kynurenine at 4 h in the depressives were not significantly different from control levels. There was no clear relationship between the plasma levels of tryptophan or kynurenine at 4 h and the therapeutic response in 13 depressives treated with Ltryptophan for 14 days.
It is concluded that the absorption, the plasma clearance, and the degradation to kynurenine of loading doses of ctryptophan are normal in depressed patients. Results furthermore suggest that the plasma levels of tryptophan and kynurenine at 4 h are poor predictors of the response to Ltryptophan treatment in depressives.