Vitamin C transport systems of mammalian cells

Vitamin C is essential for many enzymatic reactions and also acts as a free radical scavenger. Specific non-overlapping transport proteins mediate the transport of the oxidized form of vitamin C, dehydroascorbic acid, and the reduced form, L-ascorbic acid, across biological membranes. Dehydroascorbi c acid uptake is via the facilitated-diffusion glucose transporters, GLUT 1 , 3 and 4, but under physiological conditions these transporters are unlike ly to play a major role in the uptake of vitamin C due to the high concentr ations of glucose that will effectively block influx. L-ascorbic acid enter s cells via Na+-dependent systems, and two isoforms of these transporters ( SVCT1 and SVCT2) have recently been cloned from humans and rats. Transport by both isoforms is stereospecific, with a pH optimum of similar to7.5 and a Na+ : ascorbic acid stoichiometry of 2: 1. SVCT2 may exhibit a higher aff inity for ascorbic acid than SVCT1 but with a lower maximum velocity. SVCT1 and SVCT2 are predicted to have 12 transmembrane domains, but they share n o structural homology with other Na+ co-transporters. Potential sites for p hosphorylation by protein kinase C exist on the cytoplasmic surface of both proteins, with an additional protein kinase A site in SVCT1. The two isofo rms also differ in their tissue distribution: SVCT1 is present in epithelia l tissues, whereas SVCT2 is present in most tissues with the exception of l ung and skeletal muscle.