Ascorbate free radical reduction

Methylsilanol ascorbate free radical reduction, cosmetics anti-aging prods. 

Boron also appears to be involved in redox metabolism in cell membranes. Boron deficiency was shown to inhibit membrane H -ATPase isolated from plant roots, and H -ATPase-associated proton secretion is decreased in boron-deficient cell cultures [71]. Other studies show an effect of boron on membrane electron transport reactions and the stimulation of plasma reduced nicotinamide adenine dinucleotide (NADH) oxidase upon addition of boron to cell cultures [72, 73]. NADH oxidase in plasma membrane is believed to play a role in the reduction of ascorbate free radical to ascorbate [74]. One theory proposes that, by stimulating NADH oxidase to keep ascorbate reduced at the cell wall-membrane interface, the presence of boron is important in... 

Such ascorbate free radicals are important intermediates in a wide variety of in vitro reactions involving oxidation and reduction and there is evidence that ascorbate radicals also have an important role in living systems. Vitamin C is known to interact with the tocopheroxyl radical... 

It is possible to reduce enzymatically both AFR and DHA, regenerating ascorbate and thus decreasing nutritional requirements in animals unable to carry out the synthesis de novo. Enzymatic systems that reduce AFR usually utilize NADH as an electron donor (one-electron reduction pathway or NADH-ascorbate free radical reductase). Reduction of DHA to ascorbate involves two electrons and has been attributed to thioltransferase (glutaredoxin) and protein disulfide isomerase (Wells et aL, 1990). However, taking into consideration the reported values for DHA (millimolar range) it is unlikely that the latter enzymes are involved in the maintenance of ascorbate in its reduced state in vivo and that function is carried out by NADH-AFR reductase (Minetti et aL, 1992). 

The reduction of iridium(rv) by ascorbic acid has been studied in 80% acetone-water mixtures, the rate being first-order with respect to each reactant and inversely proportional to [H+] . The reaction involves the formation of an ascorbate free radical, AH ... 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

In 1986, the antioxidant effects of thioredoxin reductase were studied by Schallreuter et al. [81]. It has been shown that thioredoxin reductase was contained in the plasma membrane surface of human keratinocytes where it provided skin protection against free radical mediated damage. Later on, the reductive activity of Trx/thioredoxin reductase system has been shown for the reduction of ascorbyl radical to ascorbate [82], the redox regulation of NFkB factor [83], and in the regulation of nitric oxide-nitric oxide synthase activities [84,85],... 

Another type of spin traps, which have been recommended for the detection of superoxide, are the derivatives of hydroxylamine. In 1982, Rosen et al. [25] showed that superoxide is able to oxidize the hydroxylamine derivative 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazoli-dine (OXANOH) to corresponding free radical 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazolidinoxyl (OXANO). Although this radical is very stable and easily identified by its ESR spectrum, it is also easily reduced by ascorbic acid and other reductants. Furthermore, OXANOH and other hydroxylamines are oxidized by dioxygen in the presence of transition metal ions to form superoxide, and therefore, superoxide detection must be carried out in the presence of chelators. 

In the oxidation of ascorbate by Oj catalysed by ascorbate oxidase, the formation of the monodehydroascorbate free radical was demonstrated by EPR spectroscopy in a flow cell. A steady state was usually reached within 50 ms. The production of the free radical was also followed by the reduction of Fe(in)-cytochrome c. Thus the oxidation of ascorbate occurs in a one-electron step The formation of the monodehydroascorbate free radical was also measured directly by spectrophotometry at 360 nm, where the free radical shows an absorption maximum... 

Antioxidant activity involving the transfer of two electrons between the ascorbate/dehy-droascorbate redox couple or donation of one electron to inactivate highly reactive free radicals, e.g., protection of vitamin E by reduction of the tocopheryl radical Competitive inhibition in substrate binding reactions, including inhibition of the formation of carcinogenic nitrosamines... 

Neta P, Eluie RE, Mosseri S, Shastri LV, Mittal JP, Maruthamuthu P, Steenken S (1989) Rate constants for reduction of substituted methylperoxyl radicals by ascorbate ions and N,N,N, Al -tetrameth-yl-p-phenylenediamine. J Phys Chem 93 4099-4104 Netto LES, Stadtman ER (1996) The iron-catalyzed oxidation of dithiothreitol is a biphasic process hyudrogen peroxide is involved in the initiation of a free-radical chain of reactions. Arch Biochem Biophys 333 233-242... 

Several workers have shown that a high concentration of ascorbic acid added to liquid milk inhibits oxidation. Chilson (1935) suggested that added ascorbic acid acts as a reducing agent, which is oxidized more readily than milk fat. Bell et al. (1962) suggested that addition of L-ascorbic acid to cream produced a medium less conducive to oxidation by lowering the oxidation-reduction potential. Addition of an adequate level of surface-active ascorbyl palmitate to milk products may retard lipid oxidation by orientation at the lipid-aqueous interface where it intercepts free radicals (Badings and Neeter, 1980). 

According to the Chance mechanism, the interaction of H202 with the enzyme gives compound I (E,). The oxidation of the donor molecules leads to compound II (E[I) which oxidizes the second donor molecule. The radical intermediates were detected experimentally for such substrates as amines and phenols with relatively high reduction potential (Dunford and Stillman, 1976). The one-electron steps with the formation of free radicals at oxidation of amines and phenols have been proved in the ceruloplasmin, laccase and ascorbic oxidase reactions (Malsmstrom et al., 1975). 

---