*2.4. Active Transport of ASC*

The existence of specific vitC transporters were proposed long before the actual transporters could be identified [42,74–82]. The transport was found to be concentration-, energy-, temperatureand sodium-dependent, satiable and mediated by two different components [42,74–82]. Two specific transporters were defined by Tsukaguchi and co-workers as sodium-dependent vitamin C transporter (SVCT) 1 and SVCT2 [83]. These enable the active transport of ASC against a concentration gradient, allowing an accumulation in cells of concentrations more than 50-fold that of extracellular fluids [76,77]. The sodium-dependency of the transport has been shown to have a stoichiometry of two Na+ -ions to one ASC anion [84–86], demonstrating the transport as a secondary active transport, with the sodium gradient driving the transport of ASC, which, in turn, is maintained by the sodium/potassium-ATPase.

In humans, SVCT1 is a 598 amino acid protein, while SVCT2 measures 650 amino acids [87]. The proteins are encoded by the genes, *SLC23A1* (located at chromosome 5q31.2–31.3) and *SLC23A2* (at 20p12.2–12.3), respectively [88–91]. The transporters share 65% identity [83] and are differentially distributed within the body. The SVCT1 is primarily expressed in epithelial cells [90,92,93]. It has a relatively high *K*m of 65–252 μM [52,87,94–97] and a *V*max of approximately 15 pmol/min/cell [87,90], establishing SVCT1 as a low affinity/high capacity transporter and corresponding well to its function believed to be maintaining whole-body homeostasis [52,87,96–98]. The SVCT2 is expressed in various organ systems [83]. SVCT2 has high affinity (*K*m-values of 8–69 μM [52,87,94–97]), but low capacity (approximately 1 pmol/min/cell [87,90]) for vitC transport [52,87,96,98], enabling cells of peripheral organs to take up ASC from the extracellular fluid [40,97]. The SVCT2 transporter is believed to be the primary transporter of ASC to the brain, enabling this organ to obtain and preserve a strikingly high vitC concentration, even during states of (severe) deficiency [37–39]. The distribution of the two transporters taken together with their kinetic properties suggests a distinct polarity of expression with SVCT1 expressed on the apical side of the epithelial cell membrane, while SVCT2 is suggested to be located in the basolateral membrane [52]. This has been confirmed by *in vitro* studies displaying apical SVCT1 expression and SVCT2 expression in the basolateral membrane of enterocytes and renal tubule cells [99,100], supporting the distinctive roles of the two transporters in regulating overall vitC concentration; SVCT1 adhering to luminal surfaces in the intestinal tract and kidney and SVCT2 being associated with the further distribution and/or re-uptake of ASC [97,101]. Findings in Caco-2 cells, an *in vitro* model of human enterocytes, suggest that the basolateral SVCT2 primarily transports ASC into the enterocytes from the blood stream, while not contributing to the absorption of dietary vitC as such [99].

Importantly, recent knowledge of human genetic variation in the SVCTs and its impact on vitC homeostasis has complicated the interpretation of the existing clinical literature (for a comprehensive review, see [102]. Using pooled human genotype data, Corpe and coworkers calculated average population prevalences of *SLC23A1* polymorphisms, and following an extensive experimental *in vivo* study in mice, they modeled dose *vs.* plasma concentrations in an attempt to predict human steady-state levels as a function of known allelic genotypes [103]. Their results surprisingly suggest that, e.g., the A772G allele results in a vitC deficient phenotype regardless of the vitC intake (up to 2500 mg/day). Genetic variation is also well known in *SLC23A2*, with about 2200 identified single-nucleotide polymorphisms (SNPs) [102]. However, changes in *SLC23A2* can be expected to primarily influence tissue rather than plasma levels of ASC, and indeed, little is known about their impact on vitC homeostasis in general. Future human studies of vitC homeostasis should preferably include genotyping, as well as assessment of tissue/cellular concentrations through biopsy or leukocyte analysis, in addition to that of plasma.
