MECHANISM OF LOWER OXIDIZABILITY OF EICOSAPENTAENOATE THAN LINOLEATE IN AQUEOUS MICELLES - II - EFFECT OF ANTIOXIDANTS

We have reported that the peroxyl radicals derived from methyl eicosap entaenoate (20:5n-3) are more polar than those from methyl linoleate ( 18:2n-6) since the former peroxyl radicals have at least two molecules of oxygen in a molecule while the latter peroxyl radical has one. Thi s lowers the oxidizability for 20:5n-3 in aqueous Triton X-100 micelle s by enhancing the termination reaction rate for peroxyl radicals and by reducing the rate of propagation since there may be more polar pero xyl radicals derived from 20:5n-3 at the surface than within the micel le core. In this study, we measured the effect of three antioxidants, di-tert-butyl-4-methylphenol (BHT), 2,2,5,7,8-pentamethyl-6-chromanol (PMC) and 2-carboxy-2,5,7,8-tetramethyl-6-chromanol (Trolox), on the o xidation oi lipids in aqueous micelle. Antioxidants give a clear induc tion period during oxidation of 18:2n-6 initiated with a water-soluble radical initiator, and its induction length decreases in the order of BHT > PMC > Trolox. This is consistent with the proposed location of three antioxidants: being in the core of micelle, at the surface, or i n aqueous phase, respectively. However, BHT does not inhibit the oxida tion of 20:5n-3 efficiently, and its rate of oxidation is slower than that observed in the oxidation of 18:2n-6, supporting the idea that po lar peroxyl radicals derived from 20:5n-3 are preferentially located a t the surface of the micelle. Similar results were obtained when oxida tion was initiated with a lipid-soluble radical initiator except antio xidants had lesser effect on the oxidation rate of 20:5n-3.