Handbook of Vitamin C Research

Molecular Bases of the Cellular Handling of Vitamin C 219Figure 3. Schematic representation of the transporters responsible for the uptake of ascorbic acid anddehydroascorbic acid.In spite of the existence of differences in size and chromosomal localization, there areimportant similarities in the genomic organization of the genes encoding SVCT1 and SVCT2.Thus, SLC23A1 has a size of 16,096 bp and is localized in chromosome at 5q31.2-32.3,whereas the size of SLC23A2 is approximately ten-fold bigger (158,398 bp). Thechromosomal localization of SLC23A2 is 20p12.2-12.3. However, the size of the ORFs ofboth genes is similar (1,791 and 1,953 bp for SLC23A1 and SLC23A2, respectively).Moreover, both nucleotide sequences have 58% similarity (Stratakis et al., 2000; Wang et al.,2000). There is a marked analogy (86-95%) with the mRNA sequence of some SCVT1 andSVCT2 orthologues, such as those of mice, rats and guinea pigs (Daruwala et al., 1999; Clarket al., 2002).In humans, the structure of both proteins is also similar. The amino acid sequencecomprises 598 and 650 aa in the case of SVCT1 and SVCT2, respectively, with 65%similarity to each other (Daruwala et al., 1999). This level of similarity is also found in theisoforms of mice and rats (Faaland et al., 1998; Tsukaguchi et al., 1999). Regardingmolecular weight, there is some degree of controversy. Thus, in transfected cells, analysis byelectrophoresis revealed the existence of bands between 65 and 75 KDa, which probablydepends on the degree of protein glycosylation (Lutsenko et al., 2004; Kang et al., 2007),whereas in vivo, the band detected is of approximately 50 KDa (Savini et al., 2007; Savini etal., 2008).Hydropathy plot analyses predict a similar structure for both isoforms of SVCTs. Theyhave 12 transmembrane domains, with both the amine and carboxyl ends locatedintracellularly. SVCT1 and SVCT2 differ in the fact that the latter contains two additionalregions of 12 and 44 aa located at positions 2 and 38, respectively (Liang et al., 2001).Regarding glycosylation sites, there are two possibilities in the extracellular loop betweentransmembrane domains 3 and 4 - Asn 138 and Asn 144 for SVCT1 and Asn 188 and Asn 196 forSVCT2 - and a third site (Asn 230 ) only for SVCT1 in the extracellular loop betweentransmembrane domains 5 and 6 (Liang et al., 2001). The glycosylation of SVCTs plays animportant role in their maturation, which determines the functionality of these transporters(Subramanian et al., 2008). Both isoforms contains several sites for potentialphosphorylation. Thus, in SVCT1 there are five sites for protein kinase C (PKC)-dependentphosphorylation and another site for PKA-dependent phosphorylation, whereas in SVCT2there are six sites for PKC-dependent phosphorylation (Daruwala et al., 1999; Wang et al.,1999).