Peroxide dependent and independent lipid peroxidation: Site‐specific mechanisms of initiation by chelated iron and inhibition by α‐tocopherol

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Abstract
Peroxidation of linoleic acid (LA) was catalyzed by Fenton reagent (H2O2, and Fe2+) in positively charged tetradecyltrimethylammonium bromide (TTAB) micelles, but not in negatively charged sodium dodecylsulfate (SDS) micelles. However, more hydroxyl radicals formedvia the Fenton reaction were trapped byN-t-butyl-α-phenylnitrone (PBN) in SDS micelles than in TTAB micelles. Generation of linoleic acid alkoxy (LO) radicals by Fe2+ via reductive cleavage of linoleic acid hydroperoxide (LOOH) resulted in peroxidation of LA and formation of PBN-LO· adducts in SDS micelles, but not in TTAB micelles. This LOOH dependent lipid peroxidation could be catalyzed in TTAB micelles in the presence of a negatively charged iron chelator, nitrilotriacetic acid (NTA). LO radicals formed by the LOOH dependent Fenton reaction were also trapped by PBN at the surface of TTAB micelles in the presence of NTA, but not in its absence. The consumption of a spin probe, 16-(N-oxyl-4,4′-dimethyloxazolidin-2-yl)stearic acid (16-NS) during the LOOH dependent Fenton reaction in the presence of NTA was higher in TTAB micelles of LA than in those of lauric acid (LauA), although the rates and amounts of LO radicals formed in the two types of fatty acid micelles were similar. The rates of 5-NS consumption in LA and LauA micelles were almost the same, and were lower than the rate of 16-NS in LA micelles. NTA-Fe2+ initiated peroxidation of LA in TTAB micelles without a lag time in the presence of LOOH, but after a lag period, peroxidation occurred without LOOH. α-Tocopherol inhibited peroxidation of LA catalyzed by Fenton reagent by scavenging OH radicals in TTAB micelles. In contrast, α-tocopherol enhanced free Fe2+ induced LOOH dependent lipid peroxidation through the regeneration of Fe2+ in SDS micelles. However, it inhibited NTA-Fe2+ induced LOOH dependent lipid peroxidation in TTAB micelles. The rate and amount of α-tocopherol oxidized by the Fe2+ induced, H2O2 dependent Fenton reaction were almost the same in TTAB micelles of LA and LauA. The oxidation of α-tocopherol by the NTA-Fe2+ induced, LOOH dependent Fenton reaction was greater and faster in LA micelles than in LauA micelles, although the rates of LO radical production in the two types of micelles were the same. During NTA-Fe2+ induced, LOOH dependent lipid peroxidation, α-tocopherol inhibited more effectively the consumption of 16-NS than 5-NS. The results are discussed in relation to the location of iron, the unsaturated bonding region of LA, the OOH group of LOOH, the radical trapping site of PBN, the spin sites of 5-NS and 16-NS, and the phenolic hydroxyl group of α-tocopherol in micelles with different charges.

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