Dehydroascorbic acid glutathione oxidation

These differences are correlated with differing functions. The intracellular reduced thioredoxins are thermodynamically the best reductants of disulfide linkages in proteins and they help keep intracellular proteins reduced. Glutaredoxin can be reduced efficiently by reduced glutathione or by NADPH and glutathione reductase and can, in turn, reduce cysteine and the oxidized form of vitamin C, dehydroascorbic acid (Box The peri-... 

The oxidative product of ascorbic acid, dehydroascorbic acid, is the preferred form of the vitamin for uptake by neutrophils, erythrocytes, and lymphocytes (27). Once within the erythrocyte, dehydroascorbic acid is reduced to ascorbic acid by a glutathione-dependent, dehydro-ascorbic-acid-reducing enzyme (20,28). However, the reduced form of ascorbic acid is found in most other tissues, that is, liver, lungs, kidneys, skin, and pituitary and adrenal glands (20,29). From these studies, ascorbic acid is taken up by several tissues by an energy-dependent and Na -sensitive process, but the transport of the oxidized vitamin form follows the principles of diflFusion. 

Whatever the true mode of action of alloxan in producing diabetes, it seems likely that it is due to its properties as an oxidant. Other oxidants might have much the same effect, as is suggested by the report (Patterson, 1949) that dehydroascorbic acid (DHA), in doses of 0.7 g./kg. body weight on three successive days, produced diabetes in rats. This could be prevented by the prior administration of cysteine, glutathione, or BAL (Patterson and Lazarow, 1950). Many other aspects of alloxan diabetes have been reviewed in an earlier volume of this series (Bailey, 1949). 

Figure 6.3. Ascorbic acid and dehydroascorbic acid. Oxidation A is catalysed by metallic ions, particularly copper, and is depressed by flavonoids In reduction B, glutathione is the usual hydrogen donor and there is some evidence that the activity of the system is enhanced...

Plants have an enzyme system linking the oxidation of,glutathione to the reduction of dehydroascorbic acid. A similar enzyme may occur in animal tissues, although in this case a facile non-en matic reaction could possibly account for the observed activity. The plant eni me provides a... 

FIGURE 1. Reversible two-electron oxidation/reduction of ascorbic acid to dehydroascorbic acid and back via reduced glutathione (GSH). 

This enzyme catalyzes the oxidation of glutathione (GSH) in the presence of dehydroascorbic acid as an H-acceptor ... 

Fig. 15.34. Reactions involved in flour improvement by ascorbic acid (according to Grosch and Wieser, 1999) Asc, ascorbic acid DHAsc, dehydroascorbic acid AO, ascorbic acid oxidase GSH-DH, glutathione dehydrogenase GSH, reduced glutathione GSSG, oxidized glutathione CSH, cysteine CSSC, cystine PSSP, gluten proteins...

Figure 3 Cyclic reactions among glutathione, ascorbic acid, and tocopherol. NADPH, nicotinamide-adenine-dinucleotide phosphate, reduced form NADP, nicotinamide-ade-nine-dinucleotide phosphate, oxidized form GSH, glutathione GSSG, glutathione disulphate DHA, dehydroascorbic acid. (From Ref. 26.)...

As shown in Figure 13.3, oxidation of ascorbic acid, for example, by the reduction of superoxide to hydrogen peroxide or Fe + to Fe +, and similar reduction of other transition metal ions, proceeds by a one-electron process, forming the monodehydroascorbate radical. The radical rapidly disproportionates into ascorbate and dehydroascorbate. Most tissues also have both nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione-dependent monodehydroascorbate reductases, which reduce the radical back to ascorbate. Ascorbate is thus an effective quencher of singlet oxygen and other radicals. 

Fig. 7.8 Top half of the figure depicts the spontaneous oxidation of ascorbate by oxygen-free radicals, peroxides and proline hydroxylase (black arrow) and the reduction of dehydroascorbate to ascorbate by dehydroascorbate via PDI, or providing reducing equivalents as a cofactor for peroxidases and other reductases (upper and lower red arrows). The oxidized form (GSSG) is reduced by NADPH (straight blue arrow). (Slightly modified from Meister A., Glutathione-ascorbic acid antioxidant system in animals. J. Biol. Chem., 269(13) 9397-9400,1994)...

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