Free radical lipid oxidation initiation

On the other hand, microsomes may also directly oxidize or reduce various substrates. As already mentioned, microsomal oxidation of carbon tetrachloride results in the formation of trichloromethyl free radical and the initiation of lipid peroxidation. The effect of carbon tetrachloride on microsomes has been widely studied in connection with its cytotoxic activity in humans and animals. It has been shown that CCI4 is reduced by cytochrome P-450. For example, by the use of spin-trapping technique, Albani et al. [38] demonstrated the formation of the CCI3 radical in rat liver microsomal fractions and in vivo in rats. McCay et al. [39] found that carbon tetrachloride metabolism to CC13 by rat liver accompanied by the formation of lipid dienyl and lipid peroxydienyl radicals. The incubation of carbon tetrachloride with liver cells resulted in the formation of the C02 free radical (identified as the PBN-CO2 radical spin adduct) in addition to trichoromethyl radical [40]. It was found that glutathione rather than dioxygen is needed for the formation of this additional free radical. The formation of trichloromethyl radical caused the inactivation of hepatic microsomal calcium pump [41]. 

Besides their free-radical trapping properties, flavonoids can interfere with the capacity of oxidants to reach the bilayer. A study from our laboratory demonstrated that the adsorption of water-soluble ( )-epicatechin oligomers (dimer to hexamer) to membranes prevents lipid oxidation initiated by the azocompound 2,2 -azobis (2,4-dimethylvaleronitrile), (AMVN), a hydrophobic molecule that upon its incorporation into the bilayer decomposes yielding peroxyl radicals [Verstraeten et al., 2003], In this case, given that the oxidant... 

During cooxidation, some substrates are activated to become more toxic than they were originally. In some cases substrate oxidation results in the production of free radicals, which may initiate lipid peroxidation or bind to cellular proteins or DNA. Another activation pathway involves the formation of a peroxyl radical from subsequent metabolism of prostaglandin G2. This reactive molecule can epoxidize many substates including polycyclic aromatic hydrocarbons, generally resulting in increasing toxicity of the respective substrates. 

The oxidants responsible for initiating LDL oxidation have been under intense investigation, and several possible mechanisms have been suggested. For example, Oj has been implicated as a major contributor to LDL oxidation mediated by macrophages and smooth muscle cells (H8). Here, O " is converted to H2O2 by SOD, which in turn is acted upon by a transition metal ion with the formation of HO. Another possible role for Cff is its reaction with NO to form ONOCT, which is capable of oxidizing lipids and sulfhydryl groups, even in the presence of plasma antioxidants (VI). Moreover, in vitro studies have shown that ONOCT can induce the formation of F2-isoprostanes, nonenzymatic products of the free radical-catalyzed oxidation of arachidonic acid (M13). 

Lipid peroxidation is a free radical-mediated, chain reaction resulting in the oxidative deterioration of polyunsaturated fatty acids (PUFAs) defined for this purpose as fatty acids that contain more than two double covalent carbon-carbon bonds. Singlet oxygen can produce lipid hydroperoxides in unsaturated lipids by non-radical processes (Pryor and Castle, 1984), but the reaction usually requires a radical mechanism (Porter, 1984). Polyunsaturated fatty acids are particularly susceptible to attack by free radicals. Lipid peroxidation is a complex process, and three distinct phases are recognized (a) initiation, (b) propagation and (c) termination (see Fig. 2.10). 

CLASSIC FREE RADICAL CHAIN REACTION MECHANISM OF LIPID OXIDATION Initiation (formation of ab initio lipid free radical)... 

If SA does not act through H2O2 to induce defense gene expression and resistance, then what is the significance of SA s ability to inhibit catalase and ascorbate peroxidase, and how is the SA signal perceived and transmitted One possible mechanism is that SA-mediated inhibition of catalase and ascorbate peroxidase generates SA free radicals [112], Phenolic free radicals are potent initiators of protein oxidation and lipid peroxidation... 

Iron and other transition or divalent metal ions react with polyunsaturated fats, abstracting an electron or a hydrogen atom (see Equation 1). The reaction of a polyunsaturated lipid, LH, with a prooxidant such as a transition metal ion, X, generates a lipid free radical, L. The initiation of oxidation by metals correlates to the ignition source, the match, in the combustion triangle. 

The free radical reaction may be accelerated and propagated via chain branching or homolytical fission of hydroperoxides formed to generate more free radicals (equations (11.4), (11.5)). Free radicals formed can initiate or promote fatty acid oxidation at a faster rate. Thus, once initiated, the free radical reaction is self-sustaining and capable of oxidizing large amounts of lipids. On the other hand, the free radical chain reaction may be terminated by antioxidants (AH) such as vitamin E (tocopherols) that competitively react with a peroxy radical and remove a free radical from the system (equation (11.6)). Also, the chain reaction may be terminated by self-quenching or pol)rmerization of free radicals to form non-radical dimers, trimers and polymers (equation (11.7)). 

Not all oxidants formed biolc cally have the potential to promote lipid peroxidation. The free radicals superoxide and nitric oxide [or endothelium-derived relaxing aor (EDRF)] are known to be formed in ww but are not able to initiate the peroxidation of lipids (Moncada et tU., 1991). The protonated form of the superoxide radical, the hydroperoxy radical, is capable of initiating lipid peroxidation but its low pili of 4.5 effectively precludes a major contribution under most physiological conditions, although this has been suggested (Aikens and Dix, 1991). Interestingly, the reaction product between nitric oxide and superoxide forms the powerful oxidant peroxynitrite (Equation 2.6) at a rate that is essentially difiiision controlled (Beckman eta/., 1990 Huie and Padmaja, 1993). 

Lipid peroxidation is probably the most studied oxidative process in biological systems. At present, Medline cites about 30,000 publications on lipid peroxidation, but the total number of studies must be much more because Medline does not include publications before 1970. Most of the earlier studies are in vitro studies, in which lipid peroxidation is carried out in lipid suspensions, cellular organelles (mitochondria and microsomes), or cells and initiated by simple chemical free radical-produced systems (the Fenton reaction, ferrous ions + ascorbate, carbon tetrachloride, etc). In these in vitro experiments reaction products (mainly, malon-dialdehyde (MDA), lipid hydroperoxides, and diene conjugates) were analyzed by physicochemical methods (optical spectroscopy and later on, HPLC and EPR spectroscopies). These studies gave the important information concerning the mechanism of lipid peroxidation, the structures of reaction products, etc. 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

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