The nature of the sodium dependence of amino acid transport by system A of the S37 cell

Abstract

The present study was undertaken to answer a specific question: is the activity of amino acid transport system A entirely abolished in the absence of sodium in the medium, or does some residual activity of system A remain? A biphasic double-reciprocal plot was obtained for histidine uptake from media in which sucrose, potassium ion, or choline replaced sodium ion. The uptake of labeled N-methyl-2-amino isobutyric acid or labeled histidine from sodium-free media was inhibited by added N-methyl-2-amino isobutyric acid. N-methyl-2-amino isobutyric acid also inhibited transport of labeled alanine when system L was already blocked in a choline chloride medium. Concentrative uptake of N-methyl-2-amino isobutyric acid or alanine occurred in a choline chloride medium. Uptake of N-methyl-2-amino isobutyric acid was saturable in the absence of sodium ion, but was stimulated by the presence of sodium ion. A non -saturable uptake route was not detectable in the S37 cell. The activity of amino acid transport system A in the S37 cell is clearly enhanced by the presence of sodium ion, but sodium ion is not an obligatory requirement for transfer of amino acids through system A to occur.