In vitro methods and results of ascorbic acid absorption in epithelial tissues of fish

L-ascorbic acid (AA) is an essential nutrient for several teleost species. It plays an important role in many enzymatic reactions to maintain prosthetic metal ions in their reduced form (for example, Fe2+, Cu+) and for scavenging free radicals to protect tissues from oxidative damage. In mammals, it has recently been shown that the facilitative sugar transporters of the GLUT type (GLUT1 and GLUT3) can transport the oxidized form of the vitamin dehydro-L-ascorbic acid. However, the bulk of the vitamin, which is present in the plasma essentially in its reduced form, is carried by the Na+-dependent AA transporters SVCT1 (sodium-dependent vitamin C transporter 1) and SVCT2 (sodium-dependent vitamin C transporter 2), which have recently been functionally expressed in Xenopus oocytes, cloned and sequenced. SVCT1 is mainly confined to epithelial tissues, such as intestine, kidney and liver. In fish, many results, as obtained by different in vitro techniques, have detailed the presence of an electrogenic, Na+-coupled AA transport mechanism at the brush-border membrane of enterocytes, with kinetic characteristics similar to those found in mammals (apparent Km ranging between 0.22 and 0.75 mM), when measured using the same experimental approaches. At the basolateral level of fish intestinal absorbing epithelial cells, transport of dehydro-L-ascorbic acid (DHA) is mediated by Na+-independent transport pathway(s), presumably belonging to the GLUT type family as found in mammals, although this has not been demonstrated so far. We report data on both the kinetic characteristics of vitamin C transport through biological membranes of epithelial cells and the experimental approaches used over the time to study AA absorption in fish. The possibility of getting new theoretical information on ascorbic acid absorption and metabolism in fish by using the Xenopus laevis expression system and the perspective to develop new biotechnological applications in aquaculture by gene transfer are also pointed out.