Dehydroascorbic acid regeneration

XU D p and wells w w (1996) Alpha-hpoic acid dependent regeneration of ascorbic acid from dehydroascorbic acid in rat liver mitochondria , J Bioenergetics Biomembranes, 28, 77-85. 

A cofartor can be any chemical required by an enzyme, that is, a metal ion, coenzyme, lipid, or accessory pmtein. A coenzyme is a small organic molecule required by an enzyme that participates in the chemistry of catalysis. Most of the coenzymes work according to the following principle. They shuttle back and forth between two or more different forms. I lere, one of the forms may be considered to be the coenzymatically active form, and the other may be seen as requiring regeneration to the active form. This is the case for folate, vitamin B, riboflavin-and niacin-based cofactors, ascorbic acid, and vitamin K. The coenzymatically active (inactive) forms of three of these coenzymes are tetrahydrofolate (dihydro-folate), ascorbic acid (dehydroascorbic acid), and vitamin KH2 (vitamin K). 

Ascorbic acid is highly reactive to all the primary water radicals, because of its carbonyl group and double bond. Reaction with ei or H reduces ascorbic acid to a ketyl radical, while reaction with OH oxidizes it to the relatively unreactive tricarbonyl radical ion [9]. Aside from a possible reaction with cytochrome-c (or ferrimyoglobin), the radical ion is most likely to undergo a complex disproportionation reaction that regenerates the ascorbic acid and produces dehydroascorbic acid, which has essentially the same vitamin activity. These reactions need to be considered, because ascorbic acid is added to foods to fortify them, to facilitate curing meats, and to enhance antioxidants. 

Kim, M., Otsuka, M., Yu, R., Kurata, T, and Arakawa, N., 1994, The distribution of ascorbic acid and dehydroascorbic acid during tissue regeneration in wounded dorsal skin of guinea pigs, Inte. J. Vitam. Nutr. Res. 64 56-59. 

FIGURE 12. The regeneration of ascorbic acid from dehydroascorbic acid by an enzymic and non-enzymatic mechanism. Dehydroascorbate reductase, using glutathione (GSH) as a cofactor, can regenerate ascorbic acid (ASA) from dehydroascorbic acid (DHAA) intracellularly. In an extracellular environment, DHAA reduction by GSH may occur nonenzymatically. Adapted from Bode et al. (1993). 

The dehydroascorbic acid of a tissue filtrate is reduced to ascorbic acid by HjS. After treatment of the filtrate for an appropriate time, the HSj is removed by passing inert gas through the solution and the regenerated ascorbic acid is measured by indophenol reduction. 

Fig. 5.18. Refolding of reduced Sepharose trypsinogen in different regeneration systems (from Sinha and Light, 1975) (A) 0.5 mM )5-mercaptoethanol (A) 0.5 mAf j -mercaptoe-thanol + 1 mM dehydroascorbic acid (O) 4 mM GSH + 0.4 mM GSSG. In all cases Tris 0.05 M CaCh buffer at pH 8.5, 35°C was used. The best regeneration mixture is the redox one containing 4 mM GSH and 0.4 mM GSSG.

In its reduced form, a-lipoic acid (Fig. 9.5) is a powerful antioxidant. It can reduce GSSG to GSH, dehydroascorbate to ascorbate, and regenerate a-tocopherol from the cy-tocopheryl radical either directly or via ascorbate. Supplementation with a-lipoic acid decreases oxidative stress and restores reduced levels of other antioxidants in vivo. However, a-lipoic acid and dihydrolipoic acid may exert pro-oxidant properties in vitro. a-Lipoic acid and dihydrolipoic acid promote the permeability... 

In addition to its function in thiol/disulfide exchange, Grx-1 also serves as an alternative electron donor to ribonucleotide reductase (Fernandes and Holmgren 2004), participates in deiodination of thyroxine to triiodothyronine (Takagi et al. 1989), and acts as a dehydroascorbate reductase for regenerating ascorbic acid (Washburn and Wells 1989). However, despite the potential role of Grx in... 

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