Sodium-dependent ascorbic acid membrane transporters SLC23A1 and SLC23A2 mediate ascorbic acid (vitamin C) transport into cells. However, it is unknown how ascorbic acid undergoes cellular release, or efflux. We hypothesized that SLC23A1 and SLC23A2 could serve a dual role, mediating ascorbic acid cellular efflux as well as uptake. Renal reabsorption is required for maintaining systemic vitamin C concentrations. Because efflux from nephron cells is necessary for reabsorption, we studied whether SLC23A1 and SLC23A2 mediate efflux of ascorbic acid in the human renal nephron. We found high gene expression of SLC23A1 but no expression of SLC23A2 in the proximal convoluted and straight tubules of humans. These data rule out SLC23A2 as the ascorbic acid release protein in the renal proximal tubular epithelia cell. We utilized a novel dual transporter-based Xenopus laevis oocyte system to investigate the function of the SLC23A1 protein, and found that no ascorbate release was mediated by SLC23A1. These findings were confirmed in mammalian cells overexpressing SLC23A1. Taken together, the data for SLC23A1 show that it too does not have a role in cellular release of ascorbic acid across the basolateral membrane of the proximal tubular epithelial cell, and that SLC23A1 alone is responsible for ascorbic acid uptake across the apical membrane. These findings reiterate the physiological importance of proper functioning of SLC23A1 in maintaining vitamin C levels for health and disease prevention. The ascorbate efflux mechanism in the proximal tubule of the kidney remains to be characterized.

We tested whether the dominant intestinal sugar transporter GLUT2 was inhibited by intestinal luminal compounds that are inefficiently absorbed and naturally present in foods. Because of their abundance in fruits and vegetables, flavonoids were selected as model compounds. Robust inhibition of glucose and fructose transport by GLUT2 expressed in Xenopus laevis oocytes was produced by the flavonols myricetin, fisetin, the widely consumed flavonoid quercetin, and its glucoside precursor isoquercitrin [corrected]. IC50s for quercetin, myricetin, and isoquercitirin [corrected]were approximately 200- to 1000-fold less than glucose or fructose concentrations, and noncompetitive inhibition was observed. The two other major intestinal sugar transporters, GLUT5 and SGLT1, were unaffected by flavonoids. Sugar transport by GLUT2 overexpressed in pituitary cells and naturally present in Caco-2E intestinal cells was similarly inhibited by quercetin. GLUT2 was detected on the apical side of Caco-2E cells, indicating that GLUT2 was in the correct orientation to be inhibited by luminal compounds. Quercetin itself was not transported by the three major intestinal glucose transporters. Because the flavonoid quercetin, a food component with an excellent pharmacology safety profile, might act as a potent luminal inhibitor of sugar absorption independent of its own transport, flavonols show promise as new pharmacologic agents in the obesity epidemic.

Vitamin C in humans must be ingested for survival. Vitamin C is an electron donor, and this property accounts for all its known functions. As an electron donor, vitamin C is a potent water-soluble antioxidant in humans. Antioxidant effects of vitamin C have been demonstrated in many experiments in vitro. Human diseases such as atherosclerosis and cancer might occur in part from oxidant damage to tissues. Oxidation of lipids, proteins and DNA results in specific oxidation products that can be measured in the laboratory. While these biomarkers of oxidation have been measured in humans, such assays have not yet been validated or standardized, and the relationship of oxidant markers to human disease conditions is not clear. Epidemiological studies show that diets high in fruits and vegetables are associated with lower risk of cardiovascular disease, stroke and cancer, and with increased longevity. Whether these protective effects are directly attributable to vitamin C is not known. Intervention studies with vitamin C have shown no change in markers of oxidation or clinical benefit. Dose concentration studies of vitamin C in healthy people showed a sigmoidal relationship between oral dose and plasma and tissue vitamin C concentrations. Hence, optimal dosing is critical to intervention studies using vitamin C. Ideally, future studies of antioxidant actions of vitamin C should target selected patient groups. These groups should be known to have increased oxidative damage as assessed by a reliable biomarker or should have high morbidity and mortality due to diseases thought to be caused or exacerbated by oxidant damage.

Vitamin C and flavonoids, polyphenols with uncertain function, are abundant in fruits and vegetables. We postulated that flavonoids have a novel regulatory action of delaying or inhibiting absorption of vitamin C and glucose, which are structurally similar. From six structural classes of flavonoids, at least 12 compounds were chosen for studies. We investigated the effects of selected flavonoids on the intestinal vitamin C transporter SVCT1(h) by transfecting and overexpressing SVCT1(h) in Chinese hamster ovary cells. Flavonoids reversibly inhibited vitamin C transport in transfected cells with IC50 values of 10–50 μm, concentrations expected to have physiologic consequences. The most potent inhibitor class was flavonols, of which quercetin is most abundant in foods. Because Chinese hamster ovary cells have endogenous vitamin C transport, we expressed SVCT1(h) inXenopus laevis oocytes to study the mechanism of transport inhibition. Quercetin was a reversible and non-competitive inhibitor of ascorbate transport; Ki 17.8 μm. Quercetin was a potent non-competitive inhibitor of GLUT2 expressed inXenopus oocytes; Ki 22.8 μm. When diabetic rats were administered glucose with quercetin, hyperglycemia was significantly decreased compared with administration of glucose alone. Quercetin also significantly decreased ascorbate absorption in normal rats given ascorbate plus quercetin compared with rats given ascorbate alone. Quercetin was a specific transport inhibitor, because it did not inhibit intestinal sugar transporters GLUT5 and SGLT1 that were injected and expressed inXenopus oocytes. Quercetin inhibited but was not transported by SVCT1(h). Considered together, these data show that flavonoids modulate vitamin C and glucose transport by their respective intestinal transporters and suggest a new function for flavonoids.

BioFactorsVolume 15, Issue 2-4 p. 71-74 Article Ideal vitamin C intake Mark Levine, Corresponding Author Mark Levine Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USABuilding 10 Room 4D52 MSC 1372, National Institutes of Health, Bethesda MD 20892-1372, USA. Tel.: +1 301 402 5588; Fax: +1 301 402 6436; E-mail: [email protected]Search for more papers by this authorYaohui Wang, Yaohui Wang Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorArie Katz, Arie Katz Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorPeter Eck, Peter Eck Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorOran Kwon, Oran Kwon Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorShenglin Chen, Shenglin Chen Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorJe-Hyuk Lee, Je-Hyuk Lee Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorSebastian J. Padayatty, Sebastian J. Padayatty Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this author Mark Levine, Corresponding Author Mark Levine Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USABuilding 10 Room 4D52 MSC 1372, National Institutes of Health, Bethesda MD 20892-1372, USA. Tel.: +1 301 402 5588; Fax: +1 301 402 6436; E-mail: [email protected]Search for more papers by this authorYaohui Wang, Yaohui Wang Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorArie Katz, Arie Katz Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorPeter Eck, Peter Eck Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorOran Kwon, Oran Kwon Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorShenglin Chen, Shenglin Chen Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorJe-Hyuk Lee, Je-Hyuk Lee Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this authorSebastian J. Padayatty, Sebastian J. Padayatty Molecular and Clinical Nutrition Section, Digestive Diseases Branch, Building 10 Room 4D52, MSC 1372, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-1372, USASearch for more papers by this author First published: 16 December 2008 https://doi.org/10.1002/biof.5520150203Citations: 8AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat References 1 M. Levine, S. C. Rumsey, R. C. Daruwala, J. B. Park and Y. Wang, Criteria and recommendations for vitamin C intake, J.A.M.A. 281 (1999), 1415–1423. 2 M. Levine, C. Conry-Cantilena and Y. Wang et al., Vitamin C pharmacokinetics in healthy volunteers: evidence for a Recommended Dietary Allowance, Proc. Natl. Acad. Sci. USA 93 (1996), 3704–3709. 3 J. King, Y. Wang, R. W. Welch, K. R. Dhariwal, C. Conry-Cantilena and M. Levine, Use of a new vitamin C-deficient diet in a depletion/repletion clinical trial, Am. J. Clin. Nutr. 65 (1997), 1434–1440. 4 J. F. Graumlich, T. M. Ludden, C. Conry-Cantilena, L. R. Jr. Cantilena L. R., Y. Wang and M. Levine, Pharmacokinetic model of ascorbic acid in healthy male volunteers during depletion and repletion, Pharmaceutical Research 14 (1997), 1133–1139. 5 M. Levine, New concepts in the biology and biochemistry of ascorbic acid, N. Engl. J. Med. 314 (1986), 892–902. Citing Literature Volume15, Issue2-42001Pages 71-74 ReferencesRelatedInformation