Lipid peroxidation initiation reactions

Figure 14-21. Lipid peroxidation. The reaction is initiated by an existing free radical (X ), by light, or by metal ions. Malondialdehyde is only formed by fatty acids with three or more double bonds and is used as a measure of lipid peroxidation together with ethane from the terminal two carbons ofcoB fatty acids and pentane from the terminal five carbons of cb6 fatty acids.

Nitric oxide can both promote and inhibit lipid peroxidation (Hogg and Kalyanaraman (1999). By itself, NO acts as a potent inhibitor of the lipid peroxidation chain reaction by scavenging propagatory lipid peroxyl radicals (formula [81]). It can also inhibit many potential initiators of lipid peroxidation, such as peroxidase enzymes. In the presence of 02 , NO forms peroxynitrite (see equation [46]), a powerful oxidant capable of initiating lipid peroxidation and oxidising lipid soluble antioxidants. 

However, peroxidation can also occur in extracellular lipid transport proteins, such as low-density lipoprotein (LDL), that are protected from oxidation only by antioxidants present in the lipoprotein itself or the exttacellular environment of the artery wall. It appeats that these antioxidants are not always adequate to protect LDL from oxidation in vivo, and extensive lipid peroxidation can occur in the artery wall and contribute to the pathogenesis of atherosclerosis (Palinski et al., 1989 Ester-bauer et al., 1990, 1993 Yla-Herttuala et al., 1990 Salonen et al., 1992). Once initiation occurs the formation of the peroxyl radical results in a chain reaction, which, in effect, greatly amplifies the severity of the initial oxidative insult. In this situation it is likely that the peroxidation reaction can proceed unchecked resulting in the formation of toxic lipid decomposition products such as aldehydes and the F2 isoprostanes (Esterbauer et al., 1991 Morrow et al., 1990). In support of this hypothesis, cytotoxic aldehydes such as 4-... 

Scheme 2.1 The key reactions that occur during lipid peroxidation, in this scheme, X represents the initiating species, which must be a highiy reactive oxidant, in order to abstract a H atom from a poiyunsaturated fatty-acid chain LH, the iipid substrate LO2, the peroxyi radicai L, the alkyl radical LOOH, the lipid hydroperoxide.

Since the peroxyl and alkyl radicals are regenerated, the cycle of propagation could continue indefinitely or until one or other of the substrates are consumed. However, experimentally the length of the propagation chain, which can be defined as the number of lipid molecules converted to lipid peroxide for each initiation event, is finite. This is largely because the cycle is not 100% efficient with peroxyl radicals being lost through radical-radical termination reactions (Reaction 2.4 in Scheme 2.1). 

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). 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

Oxidation of the fatty acids in an LDL particle shares many of the characteristics associated with lipid peroxidation in other biological or chemical systems. Once initiated peroxyl radicals are formed and this results in the oxidation of a-tocopherol to give the a-tocopheroyl radical (Kalyanaraman etal., 1990). This can be demonstrated by e.s.r. techniques that allow the direct observation of stable radicals such as the a-tocopheroyl radical. After the a-tocopheryl radical is consumed, lipid-derived peroxyl radicals can be detected after reaction with spin traps (Kalyanaraman etal., 1990, 1991). 

Lipid peroxidation (see Fig. 17.2) is a chain reaction that can be attacked in many ways. The chain reaction can be inhibited by use of radical scavengers (chain termination). Initiation of the chain reaction can be blocked by either inhibiting synthesis. of reactive oxygen species (ROS) or by use of antioxidant enzymes like superoxide dismutase (SOD), complexes of SOD and catalase. Finally, agents that chelate iron can remove free iron and thus reduce Flaber-Weiss-mediated iron/oxygen injury. 

Pryor, W.A. and Lightsey, J.W. 1981. Mechanisms of nitrogen dioxide reactions Initiation of lipid peroxidation and the production of nitrous acid. Science 214 435 -37. 

Hydroxyl radicals are the most reactive free-radical species known and have the ability to react with a wide number of cellular constituents including amino-acid residues, and purine and pyrimidine bases of DNA, as well as attacking membrane lipids to initiate a free-radical chain reaction known as lipid peroxidation. 

Another reason for neglecting perhydroxyl radical is a big difficulty to distinguish it from the much more abundant and more reactive peroxyl radicals. Nonetheless, in several works perhydroxyl radical was considered as a possible initiator of lipid peroxidation (see Chapter 25). It should be noted that at least two biological systems were described where the participation of perhydroxyl radicals seems to be possible. Thus, it has been shown [25,26] that perhydroxyl radical is able to abstract hydrogen atom from NADH (Reaction 6) and the glyceraldehyde-3-phosphate dehydrogenase-NADH (GAPDH-NADH) complex (Reaction 7). 

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. 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

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