Handbook of Vitamin C Research

66Yasuo Kagawa, Shizu Higasa, Masaru Tsujimura et al.membrane is negative inside, unless co-transported with sodium ion. Known VC transportmechanisms include the facilitated diffusion of DAsA through glucose transporters (GLUTs,Km = 0.8 mM) [26, 40, 69], and secondary active transport of AsA through high-affinity (Km= 0.2 mM) [69] sodium-dependent vitamin C transporters SVCT1 and SVCT2 proteins,which are encoded by Slc23a1 and Slc23a2, respectively [44]. Three members of the glucosetransporter family, GLUT1, GLUT3 and GLUT4 are DAsA transporters [40]. GLUT2 islocated in the plasma membrane of hepatocytes and pancreatic cells to respond changes inblood sugar by its very high Km. GLUT4 is insulin sensitive low Km transporter in muscleand adipocytes. GLUT 6 is a low affinity DAsA transporter, and GLUT8, GLUT10 andGLUT12 belong to the same class as GLUT6. On the other hand, GLUT9 and GLUT 11belong to the same class as GLUT 5, a fructose transporter in enterocytes unable to transportDAsA. The maximal rates of uptake for AsA and DAsA are similar [40, 69]. AsA isreabsorbed from the renal lumen by SVCTs. SVCT1 and SVCT2 correspond to intestinal andrenal sodium-dependent glucose transporter (SGLUT), but the substrate specificity isconfined to AsA. Slc23a1 mRNA has been detected in intestine and liver and the S3 segmentof the renal proximal tubule to synthesize SVCT1. Its distribution is broader, and all threeproximal tubule segments of mouse and human were found to express the transporter, but theS3 segment showed the highest expression [44]. AsA transport in these cells was regulated bya single kinetic component that depends on sodium concentration, pH and temperature.Decreases in AsA concentration increase the apical expression of Slc23a1, perhaps as a resultof a feedback system for regulating SVCT1 abundance at the luminal membrane [44].The VC transport system allows intracellular VC levels to be in the range of 2–4 mM(35-70 mg/dl), despite human plasma VC levels are significantly lower (40–120 µM=0.7-2.1mg/dl) [43]. Usually, cultured cells accumulate extracellular VC as DAsA, not as AsA [43].There is dose-dependent increase in intracellular AsA (8 mM) in response to extracellularlyadministered DAsA (1mM) [43]. However, incubation with extracellular AsA (1mM) did notelevate intracellular AA levels after 30 min of incubation [43].VC accumulation in activatedhuman neutrophils is increased as much as 10-fold above the mM concentrations present innormal neutrophils [45]. Internal concentrations as high as 14 mM are achieved whenexternal VC is at physiologic concentration. The mechanism is by oxidation of external AsAto DAsA by hydrogen peroxide formation by NADP oxidase stimulated by the addition ofseveral stimulants, preferential transmembrane translocation of DAsA via GLUTs, andintracellular reduction to AsA with glutathione-dependent reactions within minutes. Thesedata indicate that VC accumulation is enhanced in activated human neutrophils and thathuman neutrophils utilize and recycle external DAsA under physiologic conditions [45].There is species specificity in DAsA transport. Of all cells, human erythrocytes express thehighest level of the GLUT1 glucose transporter [26]. Erythrocyte GLUT1 and associatedDAsA uptake are unique traits of humans and the few other mammals that have lost theability to synthesize AA from glucose. Stomatin, an integral erythrocyte membrane protein, isregulating the switch from glucose to DAsA transport. Mice, a species capable ofsynthesizing AA, express GLUT4 but not GLUT1 in mature erythrocytes. Thus, erythrocytespecificcoexpression of GLUT1 with stomatin constitutes a compensatory mechanism inmammals that are unable to synthesize VC [26].