Lipid peroxidation of polyunsaturated fatty acids (PUFAs) in cell membranes is a chain reaction. The specific initiating event that converts a PUFA to a lipid peroxy radical is:

• A Hydroxyl radical (•OH) abstracts a hydrogen atom from a bis-allylic methylene group, generating a carbon-centred radical that rapidly reacts with O2 ✓
• B Superoxide radical directly oxidises the double bond of an unsaturated fatty acid
• C Hydrogen peroxide reacts with the ester bond in phosphatidylcholine initiating peroxidation
• D Singlet oxygen preferentially oxidises saturated fatty acids in ordered lipid domains

Explanation

Lipid peroxidation initiates when a highly reactive oxygen species—most commonly hydroxyl radical (•OH)—abstracts a hydrogen atom from a bis-allylic methylene carbon (–CH2– flanked by two double bonds) of a PUFA such as arachidonic or linolenic acid. This creates a carbon-centred lipid radical (L•) that is stabilised by resonance. L• reacts rapidly with molecular oxygen (O2) to form a lipid peroxyl radical (LOO•). LOO• then abstracts hydrogen from an adjacent PUFA, creating a lipid hydroperoxide (LOOH) and a new carbon-centred radical, propagating the chain reaction. Superoxide is relatively unreactive towards PUFA directly; H2O2 does not initiate peroxidation without metal ion-catalysed Fenton chemistry; singlet oxygen reacts with unsaturated rather than saturated fatty acids.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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