RELATIONSHIPS BETWEEN LOWER PLASMA L-TRYPTOPHAN LEVELS AND IMMUNE-INFLAMMATORY VARIABLES IN DEPRESSION

Despite much research, the pathophysiology underlying lower L-tryptoph an (L-TRP) availability in major depression has remained elusive. The present study investigates whether lower L-TRP availability in major d epression is related to immune activation which may occur in that illn ess and is known to modulate L-TRP metabolism. Toward this end, the au thors have measured the following in depressed patients and normal con trol subjects: plasma levels of L-TRP, and the competing amino acids ( CAA) valine, leucine, isoleucine, tyrosine, and phenylalanine, togethe r with indices of immune function such as haptoglobin (Hp) and transfe rrin (Tf) plasma levels, dipeptidyl peptidase IV (DPP IV) serum activi ty, and mitogen-induced culture supernatant interleukin-6 (Il6) produc tion. Both plasma levels of L-TRP and the L-TRP/CAA ratio were signifi cantly lower in major depressed subjects as compared with healthy cont rol subjects. There were significant correlations between plasma L-TRP levels, on the one hand, and Tf plasma levels, DPP IV activity (both positive), Il-6 production, and Hp plasma levels (both negative), on t he other. Up to 63.7% of the variance in L-TRP plasma concentrations c ould be explained by DPP IV, Hp, Il-6 values; and gender. Up to 50% of the variance in the L-TRP/CAA ratio could be explained by Hp values ( negative correlation) and gender. It is hypothesized that lower plasma L-TRP availability in major depression may be related to the immune r esponse in that illness.