*2.5. Vitamin C Distribution*

The absorption of both ASC and DHA takes place in all segments of the small intestine (duodenum, jejunum and ileum) [35,82,104]. The release of vitC from intestinal and renal epithelial cells after luminal absorption has not yet been conclusively determined, although both passive diffusion and facilitated diffusion through volume-sensitive anion-channels has been suggested [44]. Enterocytes express GLUT1, GLUT2 and GLUT3, although they have distinct cellular locations [105–107]. GLUT3 is found on the apical brush-border membrane, while GLUT1 is found on the basolateral membrane, and GLUT2 has been localized on both sides of the membrane. It is possible that vitC enters the blood stream by diffusion through discontinuities in the endothelial wall and circulates in the body with the blood, where it is primarily found as the ASC anion [40] (Figure 2).

The entry of ASC into the brain is hampered by the blood-brain barrier, which is impermeable to ASC and lacks the expression of SVCT2 [108–110]. Instead, ASC is thought to enter the cerebrospinal fluid of the brain through SVCT2 transporters in the choroid plexus [110,111]. DHA, on the other hand, readily crosses the barrier, due to the expression of GLUT1 [108,112]. However, due to the limited amounts of circulating DHA compared to the very high ASC concentrations maintained by the brain, this mechanism is generally believed to be of little significance [25,54]. From the extracellular space in the brain, ASC is taken up by neurons through SVCT2 [83,89,113,114]. Inside the neurons, ASC is oxidized to DHA, which is then recycled to ASC or transported out of the neurons as DHA by GLUT3 within the neuronal membrane [113]. The DHA now present in the extracellular space can then be transported to the blood stream by GLUT1 in the blood-brain barrier or taken up by astrocytes expressing GLUT1, recycled back to ASC and, concomitantly, released to the extracellular space by a yet undisclosed mechanism or possibly by diffusion [36,113].

In the renal glomeruli, ASC in the blood is filtered into the urine [115]. However, depending on the vitC status of the individual, a large proportion is reabsorbed along the proximal tubules [116,117]. This reabsorption primarily takes place at the apical side of the epithelial membrane through SVCT1 [40,118–120]. It is assumed that DHA is also reabsorbed from the glomerular filtrate, although it has not been confirmed *in vivo*, most likely due to the negligible amounts of DHA commonly found in plasma [55]. In early *in vitro* studies, DHA transport in kidney cells was found to have all the characteristics of the facilitated diffusion through the GLUTs [121,122]. In spite of this, the transport of DHA seems to have an insignificant role in the renal reabsorption, as *Slc23a1<sup>−</sup>/<sup>−</sup>* mice, unable to reabsorb ASC from the urine, showed an 18-fold increased fractional excretion of ASC that could not be replaced by the uptake of DHA [103].

A recent study in mice further revealed SVCT2 as the functional transporter in the kidney during fetal development, whereas SVCT1 expression rapidly increased in early postnatal life to become the dominant transporter in adult life [123]. The results were supported by findings of SVCT2, but not SVCT1, being expressed in human embryonic kidney cells [123]. These findings demonstrate a developmental shift in both expression and cellular localization of the SVCT transporters, with SVCT2 primarily located at the apical membrane during embryonic development, only to be translocated to the cytoplasm and the basolateral membrane within the first days of postnatal life, following which, the SVCT2 was cleared and replaced by SVCT1 [123]. A shift in SVCT-expression patterns has also been shown for other tissues. In an *in vivo* study in mice, an altered expression of

SVCT2 during fetal and postnatal development was found [29]. A marked increase in SVCT2 mRNA and protein occurred from embryonic day E15 to postnatal day P32 in both cortex and cerebellum. In liver, the increase occurred already at P1 for mRNA and P10 for protein. The changes in SVCT2 expression appeared to be directly related to the tissue content of ASC in liver, but inversely related in brain [29].

**Figure 2.** Transport mechanisms between intestines, blood and kidney*.* Ingested vitC is absorbed across the intestinal epithelium primarily by membrane transporters in the apical brush border membrane, either as ascorbate by sodium-coupled active transport via the sodium-dependent vitamin C transporter (SVCT) 1 transporter or as dehydroascorbic acid (DHA) through facilitated diffusion via glucose transporter (GLUT) 2 or GLUT3 transporters. Once inside the cell, DHA is efficiently converted to ascorbate (ASC) or transported to the blood-stream by GLUT1 and GLUT2 in the basolateral membrane, thereby maintaining a low intracellular concentration and facilitating further DHA uptake. ASC is conveyed to plasma by diffusion, possibly also by facilitated diffusion through volume-sensitive anion channels. SVCT2 located in the basolateral membrane enables re-uptake of ASC from plasma to the intestinal epithelium. In the kidney, ASC is excreted by glomerular filtration to the renal tubule lumen. Reabsorption is primarily achieved by SVCT1 transporters in the apical membrane, although diffusion from the luminal surface may also contribute to the overall uptake. While not confirmed *in vivo*, DHA is presumably re-absorbed in the renal tubule cells; however, the availability of DHA for re-absorption is thought to be negligible, due to the very low DHA concentrations in plasma. As in the intestinal epithelium, ASC can be released to the blood-stream through both passive and facilitated diffusion. GLUT2 transporters are located in the basolateral membrane, enabling transport of DHA to plasma.
