Characterization of a novel Na+-independent amino acid transporter in horse erythrocytes

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Abstract
Horse erythrocytes are polymorphic with respect to L-alanine permeability. The specificity, kinetics and cation-dependence of erythrocyte amino acid transport was compared in 2 groups of thoroughbred horses, those with erythrocyte L-alanine permeabilities in the range 5-15 .mu.mol/h per -l of cells (0.2 mM extracellular L-alanine, 37.degree. C) (transport-negative type) and those with L-alanine permeabilities in the range 450-700 .mu.mol/h per -l of cells (transport-positive type). Transport-positive cells are shown to possess a novel high-affinity, stereospecific, Na+-independent transporter selective for neutral amino acids of intermediate size. This carrier system (provisional designation asc) operates preferentially in an exchange mode and is functionally absent from erythrocytes of transport-negative-type horses.