Antioxidants mechanisms

The process of oxidation inhibition has been classified into two main categories the kinetic chain breaking process and the initiation prevention mechanisms. 

The hindered phenol antioxidants Formula 4.4 and aromatic amines (e.g., p-phenylene diamines, Formula 4.1) both operate by kinetic chain-breaking processes. They donate a hydrogen to an alkyl peroxide radical and break the free radical chain with reactions such as below. 

The diarylamines were the classical first effective antioxidants. They seem to operate by a similar mechanism. 

As indicated above, a second class of antioxidation is the peroxide decomposition mechanism, which has proven to be very important for elastomers. The peroxide decomposition for tris(nonyl phenyl)phosphite antioxidant (Formula 4.2) is shown in Formula 4.11. 

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A variety of other metals and their complexes have been studied for radioprotective activity. Among these are copper glycinate, strontium chloride, ZnNa -diethylenetriaminepentaacetate (ZnDTPA), and selenium, which has been studied because of its relationship to endogenous antioxidant mechanisms, especiaHy GSH peroxidase and vitamin E. 

The basis to the chain breaking donor (CB—D) mechanism, which was the first antioxidant mechanism to be investigated, was laid down by the late 1940s [10-12]. Many reducing agents, e.g., hindered phenols and aromatic amines, which reduce the ROO to hydroperoxide in a CB—D step have already been empirically selected and used for rubbers and by this time also for the newer plastics industry (e.g., Table la, AO 1-8 and 9-12). The major mechanistic landmarks of the antioxi-... 

Sulphur compounds, e.g., thiopropionate esters and metal dithiolates (Table la, AO 16 and 17), decompose hydroperoxides catalytically, i.e., one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids [19,20]. Scheme 6 shows a simplified scheme for the antioxidant mechanism of simple alkyl sulphides. 

Scheme 6 Antioxidant mechanism of simple alkyl sulphides.

TANG s z, KERRY J P, SHEEHAN D and BUCKLY D J (2002) Antioxidative mechanisms of tea catechins in chicken meat systems , Food Chem, 76, 45-51. 

PEDRIELLI p, PEDULLi G F and SKIBSTED L H (2001a) Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-braining reaction of quercetin and epicatechin in autoxidation of methyl linoleate, JAgric Food Chem, 49, 3034-40. 

Masuda, T. et ah. Chemical studies on antioxidant mechanism of curcumin analysis of oxidative coupling products from curcumine and linoleate, J. Agric. Food Chem., 49, 2539, 2001. 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

Calcium antagonists are able to affect nitric oxide production and suppress the peroxyni-trite-induced damage. Thus, nifedipine enhanced the bioavailability of endothelial NO in porcine endothelial cell cultures supposedly through an antioxidative mechanism [288], Pretreatment with nisoldipine, a vascular-selective calcium blocker of dihydropyridine-type, of confluent bovine aortic endothelial cells suppressed the peroxynitrite-induced GSH loss and increased cell survival [289]. 

Vestibulocochlear effects Ginkgo has positive effects on vestibulocochlear function in animal models, primarly through vasoactive and antioxidant mechanisms. EGb 761 dilates the cochlear vasculature and prevents flow reduction caused by sodium salicylate (Didier et al. 1996). Oral EGb 761 reduced tinnitus induced by sodium salicylate in rats (Jastreboff et al. 1997). The recovery of balance after unilateral vestibular neurectomy is accelerated by postoperative treatment with EGb 761,... 

On the other hand, several ROS are highly cytotoxic. Consequently, eukaryotic cells have developed an elaborate arsenal of antioxidant mechanisms to neutrahze their deleterious effects (enzymes such as superoxide dismutases, catalases, glutathione peroxidases, thioredoxin inhibitors of free-radical chain reaction such as tocopherol, carotenoids, ascorbic acid chelating proteins such as lactoferrin and transferrin). It can be postulated that ROS may induce an oxidative stress leading to cell death when the level of intracellular ROS exceeds an undefined threshold. Indeed, numerous observations have shown that ROS are mediators of cell death, particularly apoptosis (Maziere et al., 2000 Girotti, 1998 Kinscherf et al., 1998 Suzuki et al., 1997 Buttke and Sanstrom, 1994 Albina et al., 1993). 

Kotamraju, S., Matalon, S., Matsunaga, T., Shang, T., Hickman-Davis, J. M., and Kalyanaraman, B. (2006). Upregulation of immunoproteasomes by nitric oxide Potential antioxidative mechanism in endothelial cells. Free Radio. Biol. Med. 40,1034—1044. 

Sang, S., Tian, S., Wang, H., Stark, R.E., Rosen, R.T., Yang, C.S., and Ho, C.T., Chemical studies of the antioxidant mechanism of tea catechins radical reaction products of epicatechin with peroxyl radicals, Bioorg. Med. Chem., 11, 3371, 2003. 

In addition to enzymatic oxidation, flavonoid oxidation can take place via autoxidation (metal-catalyzed oxidation by dioxygen) and ROS scavenging. The former process can be related to flavonoid cytotoxicity (ROS production) while the latter is one of the main antioxidant mechanisms. Both processes may be modulated by flavonoid-protein binding. Although poorly documented so far, these points could be important and, for instance, albumin-flavonoid complexes with an affinity for LDL could act as the true plasma antioxidants participating in the regeneration of a-tocopherol from the a-tocopheryl radical formed... 

Capacity, Strength, and Efficiency of Antioxidants Mechanisms of Inhibition of Initiated Chain Oxidation... 

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. 

ROS are maintained at tolerable levels through the combined efforts of antioxidant mechanisms, which include both enzymes and nonenzymatic molecules. Important antioxidant enzymes include superoxide dismutases (SODs), catalases (CATs), peroxidases, and those maintaining reduced glutathione (GSH) levels. Nonenzymatic antioxidants include GSH, a tripeptide containing a cysteinamino acid, and vitamins, such as vitamin A, E, and C. 

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-... 

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