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

The kinetic characteristics of platelet L-tryptophan uptake were inves tigated 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 s ignificant change in the maximal velocity (V(max)). After 1 month of s uccessful antidepressant treatment the mean K(m) value decreased signi ficantly and reached the control value. This result raises the possibi lity that the decrease in platelet tryptophan uptake affinity is a sta te-dependent marker for depression. It is likely that the transport al teration results in a decrease in platelet tryptophan accumulation. Th e effect of this peripheral membrane defect on brain serotonin functio n is discussed.