Leucine-proton cotransport system in Chang liver cell.

The stimulatory effect of an inward H+ gradient on the Na+-independent L-leucine uptake by the plasma membrane vesicles from Chang liver cells (Mohri, T., Mitsumoto, Y., and Ohyashiki, T. (1983) Biochem. Int. 7, 159-167) has been shown to be due to the increase of the Km value without changing the Vmax value in the transport kinetics. The uptake of leucine by the vesicles is accompanied by intravesicular acidification, and a stimulated uptake of leucine by the countertransport with a high concentration of leucine in the vesicles enhances the acidification. All of these uptakes of leucine and proton and their stimulations are amplified by imposing an inward proton gradient. These results suggest appreciably different affinities of proton for the leucine transport carrier in the inner and outer sides of the plasma membrane. A rapid decrease in the cytoplasmic pH was observed only in the first minute of incubation of intact cells with leucine in Na+-containing medium. But the leucine-dependent decrease of the cytoplasmic pH persisted longer when either Na+ in the medium was replaced by choline or amiloride was present along with Na+. Addition of amiloride to Na+-containing medium was inhibitory on the leucine uptake of cells, without effect on the early phase of glycine uptake. We conclude that Chang liver cells are provided in their plasma membrane with an amino acid-H+ cotransport system, and this is coupled to the amiloride-sensitive Na+/H+ exchange system.