Reduced ascorbate

Ascorbate is known to act as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, reduced ascorbate can react non-enzymatically with molecular oxygen to produce dehydroascorbate and hydrogen peroxide. Also, ascorbate in the presence of light, hydrogen peroxide and riboflavin, or transition metals (e.g. Fe, Cu " ), can give rise to hydroxyl radicals (Delaye and Tardieu, 1983 Ueno et al., 1987). These phenomena may also be important in oxidative damage to the lens and subsequent cataract formation. 

Reduce ascorbic acid dosage gradually because abrupt discontinuation may produce rebound deficiency... 

Cytochromes b of mitochondrial membranes are involved in passing electrons from succinate to ubiquinone in complex II138 and also from reduced ubiquinone to cytochrome c, in the 248-kDa complex III (Fig. 18-8). A similar complex is present in photosynthetic purple bacteria.123 139 Cytochrome b560 functions in the transport of electrons from succinate dehydrogenase to ubiquinone,138 and cytochrome b561 of secretory vesicle membranes has a specific role in reducing ascorbic acid radicals.140... 

The ease of oxidation of reduced ascorbic acid is the basis for a simple method of analysis by dye titration (58j. Ascorbic acid as it occurs in citrus juice is in the reduced form. When subjected to oxidation, ascorbic acid changes to the dehydro form. Dehydroascorbic acid has nearly the same physiological activity as the reduced form and is easily converted to the latter. Further oxidation of the dehydroascorbic acid converts it to 2,3-diketo-gulonic acid. This reaction is irreversible, and the oxidized product is devoid of biological activity. These reactions are shown in Figure 2. Nearly 90 percent or more of the vitamin C found in citrus juice and citrus products is in the reduced form (Table X) (59). 

Table X. Reduced Ascorbic Acid and Dehydroascorbic Acid in Citrus Products...

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

In studies on the anaerobic reduction of tree laccase by hydroquinone and ascorbate (49), the existence of a plateau phase at low substrate concentration was reported for the reaction of the type 1 copper. This observation was explained in terms of an intramolecular reoxidation by the type 3 copper pair. A similar plateau phase is a dominant feature of the reduction of both chromophores of ascorbate oxidase by reductate (Figure 7). However, the plateau phase is only observed in the presence of "contaminating dioxygen rigorous removal of these dioxygen traces removes the plateau phase at all wavelengths. The reaction of reduced ascorbate oxidase with dioxygen is very rapid, k = 5 X 10 at pH... 

The effects of stress on the depression of serum/leucocyte levels of vitamin C were recently summarized (60). Included were myocardial infarction, and other conditions involving physical, infective, or traumatic insult, including cold virus infection, and intravenous tetracosactrin. Associated with the reduced ascorbate level in myocardial infarction was an increased cortisol level over a period of 56 d. The authors recommended 1 g of ascorbic acid/d for 1 month following the traumatic event, with a comment concerning the need for additional study (60). Plasma ascorbate was also depressed following iv administration of adrenalin this depression was prevented by prior administration of propranolol, a )3-blocker (61). Smoking not only reduces blood levels of vitamin C (62,63), but increases the proportion of the population in the scorbutic range of blood values fourfold (64). These effects are probably caused by increased... 

Reduced ascorbic acid (average of that originally present) inside leaves average of 12 determinations. 

The vitamin C activity of L-ascorbic acid or reduced ascorbic acid (RAA) and its oxidized form, dehydroascorbic acid (DHA) is essentially the same, while D-ascorbic acid (isoascorbic acid or erythroascorbic acid) has little of the vitamin s biological potency (1). The readiness with which RAA is reversibly oxidized to DHA is the basis of its physiological activity, and of its use as an antioxidant in food systems. 

The data indicate that zinc-induced metallothionein binds mercury in the renal cortex and shifts the distribution of mercury from its site of toxicity at the epithelial cells of the proximal tubules. Thus, the renal content of mercury is increased, yet less is available to cause toxicity. In contrast, the renal toxicity of mercuric chloride is exacerbated in zinc-deficient animals (Fukino et al. 1992). In the zinc-deficient state, less mercury accumulates in the kidneys, but the toxicity is greater. The mechanism of the protection appears to involve more than simply a redistribution of renal mercury, because in the absence of mercury exposure, zinc deficiency increases renal oxidative stress (increased lipid peroxidation, decreased reduced ascorbate). When mercury exposure occurs, the oxidative stress is compounded (increased lipid peroxidation and decreased glutathione and glutathione peroxidase). Thus, zinc appears to affect the biochemical protective mechanisms in the kidneys as well. 

The American workers studied liver tyrosine oxidase with the aid of 2,6-dichlorophenolindophenol, which destroys reduced ascorbic acid. The inhibition was only partly reversed by ascorbic acid, and glutathione was found also to be involved (928). Under certain conditions the dichloro-phenolindophenol could be stimulatory and was thought to replace yet another factor (927). (The work of the Japanese authors discussed below... 

There are a number of low-molecular-weight species present in root nodules that might act as reductants for Fe(III) Lb. In mature root nodules there are significant levels of NADH and NADPH (150-200 /mM), free cysteine (200 /uM), reduced glutathione (40-150 /uM) and reduced ascorbate (1-2 mM). The exact concentrations depend on the age of the nodules and the method by which the extraction and quan-... 

Fig. / P. 1II situ estimates of the rate of photochemistry, the rate of thermal energy dissipation, and the level of energy dissipation activity from the noiipliotocliemical quenching of F, and the levels or activities of Z+A, reduced ascorbate, ascorbate peroxidase (APX), superoxide dismutase (SOD), glutathione reductase (GR), catalase, and a-tocopherol in leaves of Finca major growing under four different PFDs or pumpkin growing under two PFDs in the field in Boulder, Colorado during the summer of 1995. Data from Logan et al.(1998).

Although ascorbate (vitamin C) is an oxidation-reduction coenzyme that functions in collagen synthesis and other reactions, it also plays a role in free radical defense. Reduced ascorbate can regenerate the reduced form of vitamin E through donating electrons in a redox cycle (Fig. 24.19). It is water-soluble and circulates unbound in blood and extracellular fluid, where it has access to the lipid-soluble vitamin E present in membranes and lipoprotein particles. 

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. 

H6. Harris, A. B., Hartley, J., and Moor, A., Reduced ascorbic acid excretion and oral contraceptives. Lancet 1,201-202 1973). 

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