0-Scission lipid radicals

The reactions described so far do not require the involvement of the apo-B protein, neither would they necessarily result in a significant amount of protein modification. However, the peroxyl radical can attack the fatty acid to which it is attached to cause scission of the chain with the concomitant formation of aldehydes such as malondialdehyde and 4-hydroxynonenal (Esterbauer et al., 1991). Indeed, complex mixtures of aldehydes have been detected during the oxidation of LDL and it is clear that they are capable of reacting with lysine residues on the surface of the apo-B molecule to convert the molecule to a ligand for the scavenger receptor (Haberland etal., 1984 Steinbrecher et al., 1989). In addition, the lipid-derived radical may react directly with the protein to cause fragmentation and modification of amino acids. 

The distillation method (see Alternate Protocol 1) involves recovering malonaldehyde from an acidified food product. It is similar to that of the direct heating approach (see below), except that TBA is reacted only with an aliquot of the distillate. Consequently, physical and chemical interference by extraneous food constituents in the reaction with TBA is minimized because the food is never directly in contact with TBA. Unfortunately, direct heating of a food under acidic conditions enhances the degradation of existing lipid hydroperoxides as malonaldehyde precursors, and generates additional reactive radicals and scission products other than malonaldehyde that can react with TBA (Raharjo and Sofos, 1993). More malonaldehyde or reactive substances will be gener-... 

Lipid alkoxy radicals (LO ) decompose in chain scission reaction to a great variety of reactive aldehydes such as malonaldehyde, hydroxyalkenals, 2-alkenals, 2,4-alkadienals and alkanals. Lipid alkoxy radicals also cause the degradation of the apolipoprotein B (apoB) to smaller peptide fragments. 

This presents an interesting analytical quandary. Epoxides are major products of lipid oxidation and derive from LO cyclization as well as LOO additions (see Section 3.2.2). Consequently, it may be difficult to determine the mechanism that is operative in a given reaction system, and indeed, both may contribute. For example, Hendry (283) reacted a series of ROO with their parent compounds at 60°C and found 40% of the products were epoxides. Rate constants of k = 20 to 1130 M sec were calculated assuming the reactions were aU additions, but at the elevated temperature of the study, hydrogen abstraction to form the hydroperoxides, followed by homolytic scission to alkoxyl radicals, could also have contributed to the yields. 

Hydrogen abstraction by LO to propagate free radical chains is facile also in nonpolar aprotic solvents when lipids are at high concentrations. However, at moderate lipid concentrations, H abstraction must compete with internal rearrangements and scission (304), and at low concentrations it may become insignificant (305). 

This is a very fast reaction that, under some conditions, can even exceed rates of H abstraction (309). Cyclization of LO to epoxides is the dominant reaction in aprotic solvents (including neat lipids), when lipids are at low concentration (275) or highly dispersed on a surface (315, 316), at room temperature (147, 308, 317), and at low oxygen pressures (275, 278) and the reaction accelerates with increasing polarity of the aprotic solvent (308-310). However, the stability of LO is reduced considerably in polar solvents (309, 310). Although epoxyallylic radicals from cyclization have been observed in pulse radiolysis studies of LO in aqueous solutions (308), H abstraction and scission reactions are much faster. This pattern can be seen in the change of cyclic products yields when oxidation was conducted in different solvents (Table 8). The change in competition over time is also apparent. 

It is important to recognize that scission does not necessarily stop after reaction of initial alkoxyl radicals. Scissions of secondary products generated during lipid oxidation also contribute to propagation and to the ultimate product mix (346). Malonaldehyde is perhaps the best known example of this, as will be discussed further in Section 4.2. 

Ultimately, production of lipid hydroperoxides, even by circuitous routes, becomes the major process driving the oxidation reaction forward. LOOK are the first stable products of lipid oxidation, accumulating in the absence of pro-oxidant heat, metals, hemes, ultraviolet light, peroxyl radicals, or antioxidant acids or nucleophiles. However, from a practical standpoint, one or more of these or other decomposing factors are nearly always present, so the low energy 0—0 and O—H bonds undergo a variety of scission reactions. Indeed, a large proportion of the... 

Alkoxyl and Alkyl Radical Recombinations A wide variety of alkoxyl and alkyl radical recombinations have been proposed to explain lipid oxidation products observed in model reaction systems and in food or biological materials. Many are hypothetical, based on detailed studies with simple compound, but not necessarily verified in lipid oxidation. Nevertheless, the radical recombinations outlined below do provide a pathway to products not generated in the reactions already discussed. Obviously, recombinations lead to polymers. Perhaps just as importantly, however, recombinations of the fragment radicals formed in a and (3 scissions of alkoxyl radicals generate low levels of volatile compounds and flavor components that augment those produced in scission reactions and provide the undertones and secondary notes that round out flavors (340). 

Current information raises questions about the literal application of the classic free radical chain sequence to lipid oxidation. Observed products do not match those predicted Many studies have now shown that hydroperoxides are not exclusive products in early stages and lipid alcohols are not even major products after hydroperoxide decomposition. Product distributions are consistent with multiple pathways that compete with each other and change dominance with reaction conditions and system composition. Rate constants show no strong preference for H abstraction, cyclization, addition, or scission, which partially explains the mixmre of products usually observed with oxidizing lipids. It could be argued that the reactions in Figure 1 accurately describe early processes of lipid oxidation, but LOO rate constants considerably higher for cyclization than for abstraction contradict this. 

Organic mercurials are capable of inducing nephrotoxicity in S2 and S3 segments of the proximal tubule. Part of the S3 damage results from the biotransformation of the organic mercurial to release mercuric ions. Methylmercury (CH3Hg + ) readily concentrates in renal proximal tubular cells and alters mitochondrial function and lysosomes. At least part of methylmercury-induced nephrotoxicity may be due to homolytic scission of methylmercury to release methyl radicals and to lipid peroxidative toxicity. 

In biological systems, free radicals can react with cellular macromolecules in a variety of ways, the most important of which is hydrogen abstraction from DNA leading to chain scission or cross-linking. In proteins, tryptophan is the amino acid residue most susceptible to free radical attack. Lipid peroxidation by free radicals in turn is liable to cause alteration in cell membranes. 

Studies on prostaglandin biosynthesis in the early 1970s have shown that molecular oxygen is incorporated into polyunsaturated lipids. It was shown that autoxidation of polyunsaturated species leads to peroxyl radical intermediates that can undergo yff-scission, H-atom abstraction, and allylic rearrangement or/and cyclization. Beckwith looked into the oxygenation of dienes initiated by phenylthiyl radicals... 

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