Transport of amino acids.

Before amino acids nourish the cell they must be transported a) across the intestinal mucosa and b) across the plasma membrane of the cell where they are to be used. In addition, full consideration of their economy in the animal organism must include also their entry into the liver and their resorption from the renal tubule. All of these events appear to have much in common and probably to represent a single type of basic process of the plasma membrane. Very likely, similar transports within the cell are necessary to the ordered metabolism occurring there. I will t ry to summarize some of the advances in the understanding of amino acid transport that have been reached in the last few years. First, we may recall that the amino acids fall into a t least three transport families, the neutral, the cationic, and the anionic. Competition between amino acids for transport is restricted within these family lines. An exception was proposed a few years ago by C. E. Dent to account for the faulty resorption of cystine, lysine, arginine, and ornithine by the kidney in cystinuria. Dent suggested that cystine might be transported by the same agency serving for the cationic amino acids because i t is a diamino acid, even though no cation. H. R. Rosenberg, S. J. Downing and S. Segal ( J . Biol. Chem. 237, 2265 (1962)) have now tested this proposal using rat kidney slices. Although competitions were observed among arginine, ornithine and lysine, cystine showed no ability to inhibit these transports, nor was its transport inhibited by them. Before we regard the result as conclusive, the demonstration needs to be made for tubular resorption per se. The defect in renal transport of diamino acids of cystinuria applies also for the intestine No. 4