Antioxidant mechanisms involvement

The final possible mode of action for an antioxidant is as a peroxide decomposer. In the sequences that lead to photodegradation of a polymer the ready fragmentation of the hydroperoxide groups to free radicals is the important step. If this step is interfered with because the peroxide has undergone an alternative decomposition this major source of initiation is removed. The additives which act by decomposing hydroperoxide groups include compounds containing either divalent sulfur or trivalent phosphorus. The mechanism involves... 

The additional protection given to nylon by antioxidants has already been mentioned. Since the need is to protect against oxidation by free radicals, antioxidants are essentially of two types peroxide decomposers and radical scavengers. Reviews of these products are available [409,410,413] these should be consulted for details of the mechanisms involved. Peroxide decomposer types include compounds of manganese (II) or copper(I) and copper(II) complexes, such as azomethine bridge derivatives of the type represented by 10.160, of which numerous water-soluble or water-insoluble variants are possible [409]. These products have a catalytic action and are therefore used in very small amounts. 

A new interesting branch of the modern antioxidant chemistry deals with the cyclic mechanisms involving acid catalysis. The first inhibiting system of this type was discovered in 1988 [44]. It consisted of an alcohol (primary or secondary), a stable nitroxyl radical TEMPO, and... 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

An inhibition mechanism involving electron transfer between a chain-propagating radical and the antioxidant has frequently been suggested but has rarely been identified with any certainty. This process remains one of the least understood of all inhibition mechanisms. Probably the most clear-cut example of inhibition by one electron transfer (either partial or complete) has come from studies of metal-catalyzed oxidations. Many workers have reported that under certain conditions transition metals may inhibit rather than catalyze oxidations. Cobalt, manganese, and copper are particularly prominent in this respect. 

No readily acceptable mechanism has been advanced in reasonable detail to account for the decomposition of hydroperoxides by metal dialkyl dithiophosphates. Our limited results on the antioxidant efficiency of these compounds indicate that the metal plays an important role in the mechanism. So far it seems, at least for the catalytic decpmposition of cumene hydroperoxide on which practically all the work has been done, that the mechanism involves electrophilic attack and rearrangement as shown in Scheme 4. This requires, as commonly proposed, that the dithiophosphate is first converted to an active form. It does seem possible, on the other hand, that the original dithiophosphate could catalyze peroxide decomposition since nucleophilic attack could, in principle, lead to the same chain-carrying intermediate as in Scheme 4 thus,... 

The enzyme copper, zinc superoxide dismutase (Cu,Zn-SOD, EC 1.15.1.1) catalyzes the disproportionation of superoxide anion to dioxygen and hydrogen peroxide (equations 1 and 2). Crystallographic data can be found in References 41-46. This antioxidant enzyme is present in the cytosol and mitochondrial intermembrane space of eukaryotic cells and in the periplasmic space of bacterial cells as a homodimer of 32 kDa. Each monomer binds one copper and one zinc ion. The reaction mechanism involves the... 

Oxidative stress involves an imbalance between cellular reactive oxygen species (ROS) and antioxidant mechanisms that keep these in check. ROS include hydrogen peroxide (H202), superoxide anion radical (02- ), and the hydroxyl radical ( OH). 02 and OH are free radicals, that is, they possess an unpaired electron. As such, free radicals are extremely reactive and will seek an electron from nearby electron-rich macromolecules, for example, proteins, lipids, and DNA, which can lead to disruption of cellular functions. 

The mode of action of these compounds, which are under investigation, is via an antioxidative mechanism. Since COX enzyme catalysis involves radical intermediates, a radical scavenging moiety such as a di-fenf-butylphenol interferes with the cyclooxygenase reaction. Linkage of phenolic substructure with a thiazolone, oxazolone, thiadiazole or oxadiazole derivative produces non-... 

Although the acute vasodilator effects, as shown in in vitro studies (see above), may participate in the antihypertensive effects, the reduced blood pressure persisted even 42-48 h after the last administration of quercetin, when the plasma quercetin concentration and its metabolites fell bellow 25% of the peak post-administration levels [43]. Furthermore, the antihypertensive effects of quercetin did not appear to be related to its antioxidant properties since quercetin did not lower the urinary isoprostane F20 excretion, a prostaglandin-like compound produced in a non enzymatic reaction of arachidonic acid in membrane lipids and superoxide, which is currently used as a reliable marker of oxidative stress. The mechanisms involved in the antihypertensive effects and protection from organ damage... 

For some time, the effects of and responses to vitamin E have been interpreted in terms of an antioxidant mechanism of action. However, several observations have raised the question as to whether other mechanisms could be involved. For example, the effects of selenium and vitamin E on growth and polyunsaturated fatty acid synthesis in cultured mouse fibroblasts could not be reproduced by artificial antioxidants [198, 199]. The specific requirement of (+ )-a-toco-pherol for the phenotypic differentiation of the rotifer [200] may not be through an antioxidant mechanism. The effects of vitamin E on differentiation of neuroblastoma cells [201] and metamorphosis of various species [202] are likely to be due to a growth-factor-like action. A study on the interaction... 

The mechanism of melatonin s interaction with reactive species probably involves donation of an electron to form the melatoninyl cation radical or through a radical addition at the site C3. Other possibilities include hydrogen donation from the nitrogen atom or substitution at position C2, C4, and C7 and nitrosation [169]. The mechanisms by which melatonin protects against LP most likely involve direct or indirect antioxidant and free-radical scavenging activities of this indoleamine [169,171]. 2-Phenyl indole derivatives have redox properties because of the presence of an electron-rich aromatic ring system that allows the indoleamine to easily function as an electron donor. For these derivatives, the possible antioxidant mechanism might be most probably toward carbon-centered radicals described by Antosiewicz et al. [172]. 

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