Red blood cell L-tryptophan uptake in depression: kinetic analysis in untreated depressed patients and healthy volunteers.

Kinetic parameters (Vmax and K(m)) of L-tryptophan (TRP) uptake into red blood cells (RBC) were measured in 72 drug-free depressed inpatients and 35 healthy volunteers. Mean Vmax and K(m) values were not significantly different between patients and volunteers. The frequency distributions of Vmax values, however, largely differed in the two groups: Vmax values were homogeneous in the volunteers while they were widely scattered in the depressed patients. Only 15 out of the 72 depressed patients (21%) had Vmax values within 1 SD from the mean control value. Forty-four percent of the patients (n = 32) had Vmax values above the control mean + 1 SD in 11 patients and above the control mean + 2 SD in 21 patients. Thirty-five depressed patients (n = 25) had Vmax values below the control mean - 1 SD in 8 patients and below the control mean - 2 SD in 17 patients. High and low K(m) values were observed in combination with high and low Vmax values. The alterations in kinetic parameters were neither associated with severity of depression nor with a specific diagnostic subtype of depression. The data show abnormalities in RBC L-TRP uptake in most depressed patients that likely reflect a disturbance in peripheral availability of TRP on which central serotonin synthesis closely depends.