Disorders of Amino-Acid Transport

Abstract

Transport mechanisms of essential metabolites are now recognized as being of great importance in physiology and disease. There are three sites in the body which are of especial importance in this respect: the jejunum, where substances are transported from the intestinal lumen to the portal blood; the renal tubules, where transport mechanisms are essential in the reabsorption of substances filtered at the glomeruli; and the cellular membranes, which control the passage of metabolites between the extracellular and intracellular fluids. Historically, glucose was the substance in which transport mechanisms were first studied in detail because of the use of phlorhizin as a physiological tooL Phlorhizin was first isolated from apple-tree bark by de Koninck (1835), and its action in-causing glycosuria was recognized by von Mering (1888). The investigations of Mayrs (1923) and Poulsson (1930) suggested that this action was due to failure of reabsorption of glucose from the glomerular filtrate, a view finally confirmed by Jolliffe et al. (1932), who showed that the glucose, xylose, sucrose, and rafflinose clear- ances were identical in the phlorhizinized dog, although widely different in the normal animal.