Renal Tubular Transport of Proline, Hydroxyproline, and Glycine in Health and in Familial Hyperprolinemia *

In man (2) and other mammals (3), most of the amino acid load in the glomerular filtrate is reabsorbed from the proximal tubule against a chemical gradient (4, 5). There are indications that human tubular absorption of the different chemical groups of amino acids occurs by means of several transport mechanisms. In cystinuria, for example, transport of only the di-amino acids, cystine, lysine, ornithine, and arginine, is im- paired (6-9). In familial glycinuria (10) only glycine reabsorption is abnormal. In Hartnup disease (11, 12) there is impaired transport of most of the neutral amino acids; there is, however, relatively little impairment in absorption of the cystinuric group of amino acids or of the imino acids and glycine.