Ascorbic acid free radical form

As is noted in Chapter 3 in this book, ascorbic acid reacts very rapidly with free radicals formed from water. The end products of this... 

The brown pigments which were formed through the oxidative pathway are almost twice those formed through the anaerobic pathway. This effect could be derived from the involvement of H2O2 and oxy--radicals in the aerobic oxidation pathway of ascorbic acid (20,21). The oxy-radicals may generate carbonyls and amino acid free radicals which, by polymerization, could accelerate the formation of more brown pigments. 

In order to reduce phototoxicity due to free radicals forming by photobleach-ing of the fluorophores during time-lapse microscopy, ascorbic acid (1 (rg/ml) or oxyrase (30 mM) may be added to the medium as free radical scavengers. For long-term observations (longer than two hours) the culture medium should be covered with mineral oil (from Sigma) to avoid the culture medium drying out. 

FIGURE 5. Ascorbic acid as an antioxidant. The free radical form of ascorbic acid, A , is the primary product of ascorbic acid oxidation, observed during catalytic, enzymatic, photooxidative, and free radical oxidation. It is relatively stable and, thus, can detected by electron spin resonance in ischemic reperfusion of the heart and iron overloaded blood plasma. Adapted from Bendich et al, (1986). 

Ferric chloride-photosensitized oxidations of several alcohols (e.g. ethanol, cyclohexanol, and 2-methylpropan-2-oI) have been conducted in the cavity of an e.s.r. spectrometer at temperatures between —150 and — 196°C. Except for rigid glasses of 2-methylpropan-2-ol, all the photolysed alcohols investigated yielded free-radicals detectable by the spectrometer alcohols in rigid glasses should provide better models for cellulose in the solid state than alcohols in solution. Free radicals formed on y-irradiation of single crystals of trehalose and sucrose, on oxidation of some carbohydrate imidazolines (see Scheme 60), and on treatment of L-ascorbic acid with hydrazine in oxygenated alkaline solution have been examined by e.s.r. spectroscopy. 

Study it must be produced in some way from the ascorbic acid and it need not be the hydroxyl radical. It may be remembered here that considerable evidence is available (Mathews, 1951 Kern and Racker, 1954 Nason et al., 1954) for the existence of a free radical form, probably the ascorbate anion (or semiascorbate) radical during the enzymic and nonenzymic oxidation of ascorbic acid. 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

The valency of the metal ion changes in every step so that a single atom of heavy metal (Me) may produce many free radicals. Metal chelating compounds, such as citric, tartaric or phosphoric acids, ascorbic acid, phytin or phosphatidic acids, combine with metals to form non-reactive compounds so that the oxidation reactions are inhibited and natural food antioxidants are saved. 

In the last decade numerous studies were dedicated to the study of biological role of nonenzymatic free radical oxidation of unsaturated fatty acids into isoprostanes. This task is exclusively difficult due to a huge number of these compounds (maybe many hundreds). Therefore, unfortunately, the study of several isoprostanes is not enough to make final conclusions even about their major functions. F2-isoprostanes were formed in plasma and LDL after the treatment with peroxyl radicals [98], It is interesting that their formation was observed only after endogenous ascorbate and ubiquinone-10 were exhausted, despite the presence of other antioxidants such as urate or a-tocopherol. LDL oxidation was followed by... 

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]. 

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. 

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