Ascorbate free radical

Bachowski GJ, Girotti AW (1988) Light-stimulated formation of hydrogen peroxide and hydroxyl radical in the presence of uroporphyrin and ascorbate. Free Radic Biol Med 5 3-6... 

FIGURE 29.4 Ascorbic acid and ascorbate free radical. 

Asbolite, 7 2091 Ascharite, 4 1331 Ascorbate free radical, 25 769 Ascorbic acid, 12 60, 61 25 745-781, 804-805. See also L-Ascorbic acid Vitamin C... 

Wagner, B. A., Buettner, G. R., and Bums, C. P., 1993, Increased generation of hpid-derived and ascorbate free radicals by LI 210 ceUs exposed to the ether hpid edelfosine, Cancer Res. 53 711-713. 

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... 

Arrigoni, O., Dipierro, S., and Borraccino, G., Ascorbate free radical reductase, a key enzyme of the ascorbic acid system, FEBS Lett., 125, 242-244, 1981. 

The driving force, calculated from the difference in the redox potentials ( + 344 mV for the type-1 copper in ascorbate oxidase (see Table VII) +295 mV for the couple ascorbate/ascorbate-free radical (176)) is 49 mV. In the proposed modeled encounter complex (74), there is a short distance of about 7 A between the two redox centers (distance GUI—01 ASC = 6.8 A distance GUI—02 ASG = 7.5 A) and an effective parallel arrangement of the rings, with good overlap of the TT-electron density systems facilitating a rapid electron transfer (see Fig. 15). 

Semidehydro ascorbic acid (ascorbate free radical or monodehydro ascorbic acid)... 

The chemistry of ascorbic acid free radicals is reviewed. Particular emphasis is placed on identification and charac-terization of ascorbate radicals by spectrophotometric and electron paramagnetic resonance techniques, the kinetics of formation and disappearance of ascorbate free radicals in enzymatic and nonenzymatic reactions, the effect of pH upon the spectral and kinetic properties of ascorbate anion radical, and chemical reactivity of ascorbate free radicals. 

Research on ascorbate and dehydroascorbate can be performed by conventional biochemical methods, but study of the relatively short-lived ascorbate free radical requires methods such as flow techniques with rapid mixing or pulse radiolysis coupled with polarography, ESR, or spectrophotometry. Ascorbate free radicals have been generated preferentially by oxidation of ascorbic acid [enzymatic (14-17), chemical (18-20), radiation chemical (21-26), and photochemical (27)] because ascorbic acid is easily available in a high purity grade but dehydro-ascorbic acid is not. 

The primary objective of this chapter is to present an up-to-date review of the basic chemistry of the ascorbate free radical in aqueous solutions. The discussion will include such topics as spectral and kinetic... 

The spectral and kinetic properties of the ascorbate free radical have been studied extensively by optical pulse radiolysis experiments (21-24,26). When dilute aqueous solutions are exposed to ionizing radiation, essentially all of the energy is absorbed by the water yielding (28) ... 

The ascorbate free radical decays by a strictly second-order disproportionation process to ascorbic acid and dehydroascorbic acid, independent of generation method. A product analysis (22) with l-ascorbate-l- C showed dehydroascorbate to be the only new product in an irradiated solution. 

The first successful observation and characterization of the ascorbate free radical was carried out with ESR (14,15). A 1.7-G ESR doublet was reported and it was correctly concluded that the observed spectrum represented the anionic form (A ) of the radical. These measurements (14,15) showed that the enzyme-generated radical (horseradish peroxidase-hydrogen peroxide-ascorbate) was present as a free radical and decayed by second-order kinetics (see Figure 2). Recent experiments (16,17) have shown that ascorbate oxidase and dopamine-monooxygenase also generate unbound ascorbate radicals. 

Reactions that ascorbate and the ascorbate free radical undergo with molecular oxygen or its derivatives (H02/02", OH, 02) are biologically... 

On the other hand, reaction rates of ascorbate/ascorbic acid with molecular oxygen are low and pH dependent [10" -5 M h at pH 4-10 (43, 44)] detection of 02 may be diflBcult because of its low concentration. Ascorbate free radicals in such solutions could arise from a secondary reaction between dehydroascorbic acid and ascorbate ... 

An ESR study (30) showed that ascorbate free radicals are formed when ascorbate and dehydroascorbic acid are mixed under anaerobic conditions. The equilibrium constant K == [AH ] /[AH2][A] is 4.85 0.38 X 10 at pH 6.4 and 25°C. Using the numerical values from Reference 30 and Equation 28, which describe the same equilibrium in terms of concentrations of the ascorbate radical anion, ascorbic acid, and ascorbate ion at any pH, the following value is obtained ... 

The involvement of the ascorbate free radical in biological reactions is so extensive that a thorough discussion of the subject is beyond the scope of this chapter. Because in many cases A is invoked with little or no experimental evidence, only a few selected systems will be discussed to illustrate the types of reactions ascorbate free radicals undergo. 

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 believed that ascorbate free radicals have an important biological function in the reaction with destructive free radicals, particularly those associated with oxygen which often cause problems in living cells. They have been shown to quench hydroxyl radical and singlet oxygen, a function that is thought to be particularly important in the eye where comparatively large amounts of vitamin C are frequently found. 

Buettner, G.R. and Moseley, P.L. EPR spin trapping of free radicals produced by bleomycin and ascorbate. Free Radical Research Communications, 19, 589-593. 1993. 

The slope of a plot of log k against should therefore provide a value of the surface density in the vicinity of the reaction site in the membrane surface. This approach has been adopted to determine the internal and external membrane charges of chloroplasts by monitoring redox reactions, and the external membrane charge of cells by measurements of the rate of quenching of semiquinone and ascorbate-free radicals. 

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

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