TRANSEPITHELIAL TRANSPORT OF AMINO ACIDS
Epithelial cells in renal tubules and intestinal mucosa use several different transport systems to move amino acids through the luminal (apical) and the antiluminal (basolateral) membranes of the cell, using sodium-dependent symporters, proton-motive forces, and concentration gradients of other amino acids. Each transporter system prefers groups of amino acids with certain physicochemical properties, but most individual amino acids can use more than 1 transporter. The transport activities have been classified into 5 main groups: (1) the “basic system” for cystine and the structurally related dibasic cationic amino acids lysine, arginine, and ornithine; (2) the “neutral system” for neutral amino acids; (3) the “acidic system” for glutamate and aspartate; (4) the “iminoglycine system” for proline, hydroxyproline, and glycine; and (5) the “β-amino acid system.”
Inherited defects in amino acid transport at the cell membrane (Fig. 139-1 and Table 139-1) are expressed as selective renal aminoaciduria (ie, the concentration of the affected amino acids is high in the urine while it is normal or low in plasma). Intestinal absorption of these amino acids is almost always impaired. The symptoms of these disorders result from an excess of certain amino acids in the urine or a lack of them in the tissues.
Simplified schematic representation of amino acid transport in proximal tubular epithelial cell including transport systems for negatively charged dicarboxylic amino acids (AA–), imino acids, (glycine, proline, and hydroxyproline); neutral amino acids (AA); and cystine and positively charged dibasic amino acids (AA+). The sites of the presently known molecular transporter defects are indicated in red.
Cystine and the dibasic amino acids are transported into the epithelial cells by a luminal transporter (in exchange for neutral amino acids), and further from the epithelial cell into tissues by an antiluminal (basolateral) transporter (in exchange for neutral amino acids and sodium). Both these transporters are heteromers of a heavy subunit (N-glycosylated type 2 membrane glycoprotein) and a light subunit (nonglycosylated polytopic membrane protein) linked by a disulfide bridge. The light subunit b0,+AT I and the heavy subunit rBAT of the luminal transporter are encoded by the genes SLC7A9 and SLC3A1, and the light subunit +LAT1 and the heavy subunit 4F2hc of the basolateral transporter by the genes SLC7A7 and SLC3A2, respectively.
The neutral transporter system (b0,+ATI encoded by the gene SLC6A19) is expressed only at the luminal border of the epithelial cells. It transports alanine, asparagine, citrulline, glutamine, histidine, isoleucine, leucine, phenylalanine, serine, threonine, tryptophan, tyrosine, and valine into the epithelial cells. Dicarboxylic amino acids are transported by SCL1A1 and proline, hydroxyproline, and glycine by SLC36A2.
Table 139-1Transport Defects of Amino Acids at the Cell Membrane