Red blood cell L-tryptophan uptake in depression. II. Effect of an antidepressant treatment

The evolution of the kinetic parameters, maximal velocity (Vmax) and Michaelis constant (Km), of L-tryptophan (L-TRP) uptake into red blood cells (RBC) was studied in 30 depressed patients in a drug-free state (D0) and after 1 week (D7) and 4 weeks (D28) of a treatment involving a variety of antidepressant drugs, including SSRIs and tricyclics. At D0, 76% of patients exhibited abnormal values of Vmax, which were either higher (36%) or lower (40%) than the control range (control Vmax mean +/- 1 S.D.). High and low Km values were observed in parallel with high and low Vmax values. At D7, individual values of Vmax varied drastically compared to their corresponding value at D0, whatever the pretreatment value of the parameter. The magnitude of the Vmax variation during the first week of treatment was found to be significantly larger in the treatment responders than in the non-responders. At D28, Vmax values of all the responders to treatment were within the control range, whatever their pretreatment Vmax value. On the contrary, non-responders had Vmax values that were significantly lower than those of the controls. Changes in Km followed changes in Vmax during antidepressant treatment. In conclusion, normalization of L-TRP transport kinetics was concomitant with a clear alleviation of depressive symptoms, indicating that RBC L-TRP uptake is dependent on clinical state. Moreover, early reactivity of the Vmax as soon as the first week of treatment may be useful as a predictive index of clinical outcome at D28.