Distribution of tryptophan and tyrosine in unipolar affective disorders as defined by multicompartmental analysis

As requirements for tryptophan for synthesis of protein and 5-hydroxytryptamine were comparable in rat brain, during depletion of tryptophan there could be competition between the two pathways for the amino acid. This implied that tryptophan should be rate-limiting for protein synthesis and this was found in the short term when concentrations of the amino acid were reduced in rats. Multicompartmental studies of tryptophan and tyrosine in controls and patients subject to unipolar depression defined two main pools of the amino acid provisionally assigned to extracellular and intracellular spaces. For tyrosine, mean values for the extracellular space were comparable to those of controls. The concentration of tyrosine was low in the intracellular space in both depressed and recovered patients, but the raised fractional clearance rates for this compartment during depression had returned to normal on remission. Plasma tryptophan concentrations were significantly reduced in depression with intermediate values after recovery. This suggested that the procedure used may have been mildly stressful and that this had evoked an idiosyncratic response to the stress in the depressed patients, which was characterized by inability to maintain concentrations of this amino acid in plasma. The findings for both amino acids may have a bearing on the aetiology of unipolar affective disorder.