Scavenging of free radicals

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

Polyakov, N. E., T. V. Leshina et al. (2001c). Carotenoids as scavengers of free radicals in a Fenton reaction, antioxidants or pro-oxidants. Free Rad. Biol. Med. 31 398-404. 

Ressmeyer, A. R Mayo, J. C., Zelosko, V. et al. (2003). Antioxidant properties of the melatonin metabolite N -acetyl-S-melhoxykynuramine (AMK) scavenging of free radicals and prevention of protein destruction. Redox Rep. 8, 205 13. 

During the chain oxidation of hydrocarbons, sulfides and disulfides terminate chains by reacting with peroxyl radicals [40,42,44], which, as opposed to phenols, are weak inhibitors (see Table 17.6). The mechanism and stoichiometry of the termination reaction by sulfides remain yet unclear. Since sulfenic acid is an efficient scavenger of free radicals, the oxidation of tetralin in the presence of dialkylsulfoxide occurs only if the initiation rate v > vimin is proportional to the concentration of sulfoxide [5], so that the rate of oxidation is... 

Since glutathione is synthesized in cells in relatively huge amounts, it is seldom applied as pharmacological antioxidant. Furthermore, the mechanism of its antioxidant activity is not so simple as that of vitamins E and C. The major reason is that the GS radical formed during scavenging of free radicals by GSH does not disappear by dimerization but participates in the chain reaction, producing superoxide (Reactions (20)-(23)). Furthermore, it has recently been shown that contrary to previous findings the rate constant for the reaction of GSH with superoxide is relatively small (200-1000 lmol-1 s-1) [211,223],... 

Very interesting mechanism of the enhanced glutathione antioxidant activity in the presence of SOD was offered by Winterbourn [227]. She pointed out that the interaction of all free radicals with glutathione resulted in the formation of superoxide as the only final active species a free radical sink. Therefore, the mixture of glutathione together with SOD may be considered as a universal scavenger of free radicals of any structure. 

As already mentioned, another mechanism of antioxidant activity of ubiquinones is scavenging of free radicals by ubihydroquinones (Reaction (26)) ... 

Despite the conclusions in the cited literature about direct MT interaction with free radicals, the mechanism of MT antioxidant activity remains obscure. Markant and Pallauf [339] concluded that cysteine groups and not zinc are responsible for the inhibition of lipid peroxidation in hepatocytes. Maret and Vallee [340,341] also questioned the possibility of direct scavenging of free radicals by MT and suggested that zinc release is a major mechanism of antioxidant effects of metallothioneins. 

As the AO with a direct nonspecific mechanism of action we have chosen Hypoxene - sodium poly(2,5-dihydroxiphenyl)-4-thiosulfonate. Besides a direct AO effect as a scavenger of free radicals it exerts an anti-hypoxic effect shunting I and II complexes of mitochondrial respiratory chain, which are inhibited as a consequence of hypoxia (Eropkin et al., 2007). Hypoxene was introduced into cell incubation media before illumination and left during cells further incubation. Hypoxene in the concentration of 40pg/ml, comparable to doses applied in vivo, completely blocked C60-induced phototoxicity (Table 7.3). Cellular viability has completely recovered to control level, which is a convincing evidence of free radical nature of cellular damage in photodynamic effect of fullerene. 

It is obvious that lazaroids cannot be effective scavengers of free radicals excepting highly reactive hydroxyl radicals. Therefore, the second generation lazaroid U-78517F (Figure... 

Nitric oxide is the simplest stable molecule with an odd number of electrons. Because of its unpaired electron, it is paramagnetic at room temperature (the lowest lying energy level, X2U/z, is diamagnetic), ionizes relatively easily, and is highly reactive with other paramagnetic species, such as NOz and Oa. It is an efficient scavenger of free radicals and triplet states and has been used frequently for this purpose in photochemical and thermal studies. 

In most cases two different additives are used, one acting as an internal filter or excited state quencher and the other one being a scavenger of free radicals. These two additives have a synergic effect, that is they are much more effective together than either of them separately. 

The second additive acts most probably as a scavenger of free radicals. A class of compounds known as hindered piperidines is one of the most widely used for this purpose. Their chemical structure is shown in Figure 6.17. 

Scavenging of free radicals associated with a significant enhancement in glutathione peroxidase activity... 

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