title: Accumulation of vitamin C (ascorbate) and its oxidized metabolite dehydroascorbic acid occurs by separate mechanisms.
abstract: It is unknown whether ascorbate alone (vitamin C), its oxidized metabolite dehydroascorbic acid alone, or both species are transported into human cells. This problem was addressed using specific assays for each compound, freshly synthesized pure dehydroascorbic acid, the specially synthesized analog 6-chloroascorbate, and a new assay for 6-chloroascorbate. Ascorbate and dehydroascorbic acid were transported and accumulated distinctly; neither competed with the other. Ascorbate was accumulated as ascorbate by sodium-dependent carrier-mediated active transport. Dehydroascorbic acid transport and accumulation as ascorbate was at least 10-fold faster than ascorbate transport and was sodium-independent. Once transported, dehydroascorbic acid was immediately reduced intracellularly to ascorbate. The analog 6-chloroascorbate had no effect on dehydroascorbic acid transport but was a competitive inhibitor of ascorbate transport. The Ki for 6-chloroascorbate (2.9-4.4 microM) was similar to the Km for ascorbate transport (9.8-12.6 microM). 6-Chloroascorbate was itself transported and accumulated in fibroblasts by a sodium-dependent transporter. These data provide new information that ascorbate and dehydroascorbic acid are transported into human neutrophils and fibroblasts by two distinct mechanisms and that the compound available for intracellular utilization is ascorbate.
authors.affiliation: Laboratory of Cell Biology and Genetics, NIDDK, National Institutes of Health, Bethesda, Maryland 20892-0850, USA.
authors.forename: R W
authors.initials: RW
authors.lastname: Welch
authors.forename: Y
authors.initials: Y
authors.lastname: Wang
authors.forename: A
authors.initials: A
authors.lastname: Crossman
authors.forename: J B
authors.initials: JB
authors.lastname: Park
authors.forename: K L
authors.initials: KL
authors.lastname: Kirk
authors.forename: M
authors.initials: M
authors.lastname: Levine
mesh_terms: D001205:Ascorbic Acid / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D003683:Dehydroascorbic Acid / Q000138:chemical synthesis / Q000378:metabolism*; D004229:Dithiothreitol / Q000494:pharmacology; D005347:Fibroblasts / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008297:Male; D008956:Models, Chemical; D009504:Neutrophils / Q000378:metabolism; D010084:Oxidation-Reduction; D012964:Sodium / Q000494:pharmacology
chemical_list: C089412:6-chloro-6-deoxyascorbic acid; D012964:Sodium; D005947:Glucose; D001205:Ascorbic Acid; D004229:Dithiothreitol; D003683:Dehydroascorbic Acid