Transport Properties of a System y+L Neutral and Basic Amino Acid Transporter

Abstract

The properties of system y+L-mediated transport were investigated on rat system y+L transporter, ry+LAT1, coexpressed with the heavy chain of cell surface antigen 4F2 in Xenopusoocytes. ry+LAT1-mediated transport of basic amino acids was Na+-independent, whereas that of neutral amino acids, although not completely, was dependent on Na+, as is typical of system y+ L-mediated transport. In the absence of Na+, lowering of pH increased leucine transport, without affecting lysine transport. Therefore, it is proposed that H+, besides Na+ and Li+, is capable of supporting neutral amino acid transport. Na+ and H+ augmented leucine transport by decreasing the apparent K m values, without affecting the V max values. We demonstrate that although ry+LAT1-mediated transport of [14C]l-leucine was accompanied by the cotransport of 22Na+, that of [14C]l-lysine was not. The Na+ to leucine coupling ratio was determined to be 1:1 in the presence of high concentrations of Na+. ry+LAT1-mediated leucine transport, but not lysine transport, induced intracellular acidification in Chinese hamster ovary cells coexpressing ry+LAT1 and 4F2 heavy chain in the absence of Na+, but not in the presence of physiological concentrations of Na+, indicating that cotransport of H+ with leucine occurred in the absence of Na+. Therefore, for the substrate recognition by ry+LAT1, the positive charge on basic amino acid side chains or that conferred by inorganic monovalent cations such as Na+ and H+, which are cotransported with neutral amino acids, is presumed to be required. We further demonstrate that ry+LAT1, due to its peculiar cation dependence, mediates a heteroexchange, wherein the influx of substrate amino acids is accompanied by the efflux of basic amino acids.