Kinetics of unidirectional leucine transport into brain: effects of isoleucine, valine, and anoxia

The rate of unidirectional uptake, v, of L [4,5 ³H]leucine was studied in 17 isolated dog brains by means of an indicator dilution technique using ²²Na as the intravascular reference. The arterial leucine concentration was varied in increments by adding unlabeled leucine to the blood, and v was determined after each change. The valine and isoleucine concentrations were varied independently to permit evaluation of their effect on leucine transport. A preliminary analysis indicated that both valine and isoleucine are competitive inhibitors. Therefore, all data were fitted to an equation that describes Michaelis Menten kinetics in the presence of 2 competitive inhibitors. These calculations yielded an apparent K(m) for leucine transport of 1.58 mM ± .28 SE, a V(max) of 0.323 μmol/g per min ± .035 SE, and an apparent K(i) for inhibition of leucine transport of 1.76 mM ± .34 SE for valine and 0.73 mM ± .14 SE for isoleucine. In 4 isolated brains perfused with blood having a constant leucine level, indicator dilution injections were made before, and at 1, 5, and 10 min after the start of perfusion with anoxic blood. These findings showed that, unlike glucose transport, the rate of leucine transport is unaffected by 10 min of anoxia.