Regulation of leucine transport and oxidation in peripheral human lymphocytes by glutamine

In this study we investigated the influence of physiological levels of glutamine, isoleucine, and valine on leucine oxidation and transport by peripheral lymphocytes in an in vitro system. The presence of glutamine in the incubation mixture inhibited leucine oxidation by 61%. This effect was not significantly augmented by addition of isoleucine and valine. Leucine transport revealed a Km of 124 mumol/L and a Vmax of 24 pmol/10(6) cells/30 sec. Glutamine inhibited leucine transport by 63%. The capacity of lymphocytes for leucine transport exceeded the capacity for leucine oxidation by a factor of 7.8 (13.6 +/- 0.6 v 1.74 +/- 0.1 pmol/10(6) cells/30 sec). It is concluded that glutamine is a regulator of leucine transport and oxidation in human peripheral lymphocytes, and the inhibition of leucine oxidation by glutamine is not due to an alteration of leucine transport but reflects an intracellular event.