A Distinct Na+-requiring Transport System for Alanine, Serine, Cysteine, and Similar Amino Acids

Abstract The Ehrlich cell has a component of transport, for which alanine, serine, and cysteine are preferred substrates, which is not reactive with α-(methylamino)isobutyric acid or N-methylalanine and is only weakly reactive with other methylamino acids and glycine. Although the new component is similar to one described before (the so-called A system) as to its rate, its dependence on the presence of Na+, its low sensitivity to irradiation, and other properties, nevertheless it shows a distinctly lower pH sensitivity, a somewhat lower sensitivity to metabolic inhibitors, and a much higher stereo-specificity than the A system. Furthermore, part of the exodus of alanine shows a stereospecificity corresponding to the new system, suggesting that the new system operates more reversibly than the A system. Systems with similar properties and substrate specificities occur in the rabbit reticulocyte and the pigeon erythrocyte, in the absence of any system reactive with N-methylalanine or α-(methylamino)isobutyric acid. From these considerations, we conclude that the new system and the A system very probably operate independently and not as a single complex transport system. Both of the amino acids that serve to suppress the A system selectively, namely α-(methylamino)isobutyric acid and N-methyl-l-alanine, show peculiarly low Vmax values for uptake by the Ehrlich cell.