The two-way flux of cationic amino acids across the plasma membrane of mammalian cells is largely explained by a single transport system.

We show in this report that the inward and outward transport of cationic amino acids through the plasma membrane of cultured fibroblasts and HTC cells is mediated mostly by a single saturable transport system, here designated y+. The kinetic behavior of System y+ can be accounted for by the alternating generation of at least two conformational states, one on each side of the plasma membrane. No predictions are made as to the mechanism of these carrier oscillations. The saturable flux of cationic amino acids in both directions is strongly increased by the presence of competing substrates on the side of the membrane opposite to the measured flux. The kinetic parameters describing these interactions vary as predicted by the classical mobile carrier hypothesis. These observations support the homogeneity of mediated cationic amino acid transport. Our results do not exclude other small flux contributions by a minor, apparently passive nonsaturable migration in each direction. The mediated arginine influx is half-maximally saturated at an external substrate concentration 1/10 to 1/20 as high as the apparent intracellular concentration that half-maximally saturates efflux. The maximum flux through System y+ attained by raising the substrate concentration to a saturating level is about equal in the two directions. The test amino acids accumulate 5- to 20-fold into HTC cells and human skin fibroblasts, findings which are consistent with the kinetic asymmetry of System y+.