|
| Section2 = }} Dehydroascorbic acid (DHA) is an oxidized form of ascorbic acid (vitamin C). It is actively imported into the endoplasmic reticulum of cells via glucose transporters. It is trapped therein by reduction back to ascorbate by glutathione and other thiols. The (free) chemical radical semidehydroascorbic acid (SDA) also belongs to the group of oxidized ascorbic acids. ==Structure and Physiology== Top: ascorbic acid (reduced form of vitamin C) Bottom: dehydroascorbic acid (nominal oxidized form of vitamin C) Although a sodium-dependent transporter for vitamin C exists, it is present mainly in specialized cells, whereas the glucose transporters, the most notable being GLUT1, transport Vitamin C (in its oxidized form, DHA)〔 in most cells, where recycling back to ascorbate generates the necessary enzyme cofactor and intracellular antioxidant, (see Transport to mitochondria). The structure shown here for DHA is the commonly shown textbook structure. This 1,2,3-tricarbonyl is too electrophilic to survive more than a few milliseconds in aqueous solution, however. The actual structure shown by spectroscopic studies is the result of rapid hemiacetal formation between the 6-OH and the 3-carbonyl groups. Hydration of the 2-carbonyl is also observed.〔Kerber, R.C.; "'As Simple as Possible, but not Simpler' -- The Case of Dehydroascorbic Acid," ''J. Chem. Ed.'' 85 ''(2008)'' 1237-1242.〕 The lifetime of the stabilized species is commonly said to be about 6 minutes under biological conditions.〔May, J.M.; "Ascorbate Function and Metabolism in the Human Erythrocyte," ''Frontiers in Bioscience,'' 3 ''(1981)'' d1-10.〕 Destruction results from irreversible hydrolysis of the ester bond, with additional degradation reactions following.〔Kimoto, E.; Tanaka, H.; Ohmoto, T.; Choami, M.; "Analysis of the Transformation Products of Dehydro-L-Ascorbic Acid by Ion-Pairing High-Performance Liquid Chromatography," ''Anal. Biochem.'' 214 ''(1993)'' 38-44.〕 Crystallization of solutions of DHA gives a pentacyclic dimer structure of indefinite stability. Recycling of ascorbate via active transport of DHA into cells, followed by reduction and reuse, mitigates the inability of humans to synthesize it from glucose.〔Montel-Hagen, A.; Kinet, S.; Manel, N.; Mongellaz, C.; Prohaska,R.; Battini, J.L.; Delaunay,J.; Sitbon, M.; Taylor, N.; "Erythrocyte GLUT1 triggers Dehydroascorbic Acid Uptake in Mammals unable to Synthesize Vitamin C," ''Cell'' 132 ''(2008)'' 1039-48.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dehydroascorbic acid」の詳細全文を読む スポンサード リンク
|