Neutral amino acid transport has been characterized in the mouse peritoneal macrophages elicited by thioglycolate broth. Serine, alanine, cysteine, and leucine were transported mainly in a Na+-independent manner and, within the concentration ranges tested, the transport of each amino acid was mediated by a single, saturable system. The transport of these amino acids was strongly inhibited by a wide variety of neutral amino acids, 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid, a putative System L substrate, but not by glycine, proline, or 2-methylaminoisobutyrate. In this study the transport of serine has been characterized in detail. Inhibition of the transport of serine by 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid or by leucine was competitive. The transport of serine which exhibited trans-stimulation was slightly stimulated by lowering the pH and was little affected by N-ethylmaleimide. It was also shown that most of the neutral amino acids were concentrated only slightly (less than 2-fold) in the macrophages, unlike in the lymphocytes and granulocytes. From these results we have concluded that in the macrophages serine, cysteine, alanine, leucine, and presumably many other neutral amino acids are transported almost exclusively through a common transport system analogous to System L.
Alanine Amino Acids