Evaluation of the relative potency of individual competing amino acids to tryptophan transport in endogenously depressed patients.

There is evidence that the plasma ratio of tryptophan to five competing amino acids may predict the response of depressed patients to treatment with L-tryptophan. The relative potency of the individual competitors had been evaluated retrospectively. Phenylalanine consistently reduced, and isoleucine slightly reduced the predictive value of the plasma tryptophan ratio. The ratio of tryptophan to the sum of valine, leucine, and tyrosine was identified as most predictive for the therapeutic response. The L-tryptophan responders showed a normal plasma total tryptophan concentration as did the nonresponders, whereas the concentration of the three competitors was significantly elevated. It is concluded that while the plasma ratio of tryptophan to the sum of valine, leucine, and tyrosine is a useful predictor of the course of depressives on L-tryptophan, it does not definitely separate out the L-tryptophan responders from the control subjects.