Na(+)-coupled alanine transport in LLC-PK1 cells

Transport of alanine (Ala) was characterized in LLC-PK1 renal epithelial cells. Transport capability for Ala falls by 75% in postconfluent cultures, while Na(+)-coupled alpha-methylglucoside (AMG) transport rises more than fourfold during the same interval. The kinetics of Ala transport were characterized in ATP-depleted cells to allow experimental imposition of changes in Na+ gradient and control of membrane potential across the plasma membrane. At 100 microM Ala and 135 mM Na+, > 98% of the unidirectional Ala influx is dependent on the presence of Na+ in cells from postconfluent cultures. Li+ is only 1% as effective as Na+, and other monovalent cations are ineffective in supporting Ala uptake. alpha-(Methylamino)isobutyric acid (MeAIB; 5 mM) causes only a small inhibition (approximately 10%) of 100 microM Ala influx. The low selectivity for Li+; low sensitivity to competition by MeAIB or aminoisobutyric acid; pronounced inhibition by serine, homoserine, cysteine, homocysteine and threonine; moderate inhibition by valine, isoleucine, proline and histidine; and lack of inhibition by lysine, arginine, and aspartate are more consistent with those characteristics reported for entry via the ASC amino acid transport system rather than those associated with the A system. Alanine influx exhibits a hyperbolic relationship with increasing Ala or Na+ concentration. Kinetic analysis suggests a single transport pathway with a Michaelis constant (Km) for alanine of 380 microM (when Na+ is 135 mM), apparent Km for Na+ of 32 mM (with 100 microM Ala), and a maximum velocity of 7 nmol.min-1.mg cell protein-1. An interior-negative diffusion potential induces a similar enhancement of [14C]alanine or [14C]tetraphenylphosphonium influx (approximately 40%). In contrast, AMG influx is enhanced by a factor of 2.2 under the same conditions. AMG uptake also shows a sigmoidal relationship with Na+ concentration. Hill coefficients are 1.56 for AMG and 1.0 for alanine. Direct measurement of Na(+)-Ala coupling stoichiometry yields a value of 1.01 +/- 0.07. Under the same conditions, Na(+)-AMG coupling stoichiometry is 2.1 +/- 0.25. The difference in coupling stoichiometries provides an explanation for differences in intensity of interaction between Na(+)-coupled transport systems for sugars and amino acids.