EVIDENCE FOR A DEFECTIVE PLATELET L-TRYPTOPHAN TRANSPORT IN DEPRESSED-PATIENTS

The kinetic characteristics of platelet L-tryptophan uptake were investigated in 23 untreated depressed patients and 18 healthy volunteers. A significant increase of 50% in the apparent Michaelis constant (K(m)) was observed in depressed patients compared with controls, without significant change in the maximal velocity (V(max)). After 1 month of successful antidepressant treatment the mean K(m) value decreased significantly and reached the control value. This result raises the possibility that the decrease in platelet tryptophan uptake affinity is a state-dependent marker for depression. It is likely that the transport alteration results in a decrease in platelet tryptophan accumulation. The effect of this peripheral membrane defect on brain serotonin function is discussed.