Membrane phospholipid peroxidation

Cell membranes are one of the biological structures more sensitive to damage by radicals because of the presence of polyunsaturated fatty acids in them, containing methylene groups that are simultaneously adjacent to two double bonds.

FIGURE 4.4 Radical generation in membrane phospholipids.

The C-H units in these methylenes are particularly suitable points of attack by hydroxyl and other radicals because of the stabilization of the resulting carbon radical by double resonance (Fig. 4.4).

The reaction of these polyunsaturated side chains with oxygen radicals leads to phospholipid peroxidation and subsequent membrane injury. This process is initiated by the attack of a hydroxyl radical to one of the previously mentioned bisallylic positions existing in the fatty acid side chains, leading to the generation of an alkyl radical 4.1. Superoxide radical is not sufficiently reactive to initiate lipid peroxidation, and in any case its negative charge precludes its transport across the highly lipophilic cell membrane. Carbon radical 4.1 reacts rapidly with a molecule of oxygen, which is sufficiently hydrophobic to access the interior of the membranes, generating a peroxyl radical (R-O-O*, 4.2), which can abstract a new hydrogen atom from a doubly allylic C-H bond in the adjacent fatty acid side chain. This leads to a hydroperoxide 4.3 and a new radical 4.1, allowing a self-maintained radical process that extends to an expanding area of the membrane, as long as there is sufficient oxygen (propagation phase). If traces of cations, such as Fe2+, are present, they can generate new oxygen radicals (RO* and HO*) from hydroperoxides 4.3 through Fenton chemistry, contributing to the extension of the peroxidation process (Fig. 4.5).

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