Handbook of Vitamin C Research

220J.J.G. Marin, M.A. Serrano, M.J. Perez,et al.Regarding substrate specificity, both SVCT1 and SVCT2 are highly selective for L-ascorbic acid. These carriers are unable to transport either D-ascorbic acid, DHA or anyvitamin C derivatives. The ability of SVCTs to transport L-AA is strictly dependent on thepresence of sodium. Indeed replacing extracellular sodium by lithium, potassium or cholineresults in the abolition of vitamin C transport (Liang et al., 2001). The optimal pH for thefunction of both SVCT isoforms is 7.5. At lower pH, the affinity of these carriers for AA isdecreased and hence the efficiency of the transport is also lowered (Liang et al., 2001). Whenkinetic characteristics were measured in several experimental models, marked differencesbetween both isoforms have been found (Savini et al., 2008). Thus, the values of the apparentaffinity constant (KM) are higher for SVCT1 (65-237 µM) than for SVCT2 (8-62 µM), whichindicates that the affinity for the substrate is several-fold higher in SVCT2 than in SVCT1. Incontrast, the capacity of SVCT1, as determined by measurements of maximal velocity oftransport (Vmax), is higher than that of SVCT2. In other words, SVCT1 can be considered asa transporter of high capacity and low affinity, whereas SVCT2 is a transport of low capacityand high affinity.Non-functional variants of human isoforms SVCT1 and SVCT2 have been described.One variant of SVCT1 has been identified in the Caco-2 cell line from human colonadenocarcinoma. This variant is characterized by the addition of 12 bp during alternativesplicing, resulting in four additional amino acids – VGLH – that are inserted between E-155and V-156 of the functional protein (Wang et al., 1999). For SVCT2, a variant has been alsoreported. This was found in 293T cells derived from HEK 293 embrionary renal cells upontransfection with the T antigen from SV40 (Lutsenko et al., 2004). Regarding thecharacteristics of this variant, this was generated by the deletion of 345 bp, resulting in theloss of transmembrane domains 5 and 6 and part of 4. This truncated protein is not able tocarry out AA transport, but it may have a regulatory role regarding the function of SVCT2and, to a lesser extent, also SVCT1 (Lutsenko et al., 2004).The expression levels of vitamin C transporters are decreased in aged people even inindividuals with diets rich in vitamin C (Brubacher et al., 2000). These findings areconsistent with the experimental data obtained in rat hepatocytes isolated from aged animals.These cells have a lower ability to take up vitamin C than cells isolated from young rats, inagreement with a lower expression level of vitamin C transport proteins (Michels et al.,2003). In a recent study carried out by our group, different ontogenic patterns for Svct1 andSvct2 in the rat liver were found. The abundance of mRNA for Svct1 increased after birthand decreased in aged animals, whereas that of Svct2 was not significantly modified alongthe different life stages in this species (Vaquero et al., 2009). The expression levels of thesetransporters are also dissimilar in both sexes, at least in mice. Thus, in females of this speciesSVCT1 is expressed at higher levels and serum concentrations of AA are also consistentlyhigher, whereas urinary elimination is lower than in males (Kuo et al., 2004).There are important differences between both isoforms regarding their tissue distributionand physiological role. In general, SVCT1 is highly expressed in liver, intestine and kidney(Tsukaguchi et al., 1999), whereas SVCT2 is ubiquitously expressed, although it isparticularly abundant in the brain, retina, placenta, bone, and chondrocytes (Rajan et al.,1999; Tsukaguchi et al., 1999), as will be described in more detail below.