Mechanism of lower oxidizability of eicosapentaenoate than linoleate in aqueous micelles. II. Effect of antioxidants |
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Authors: | Kazumasa Yazu Yorihiro Yamamoto Etsuo Niki Keiji Miki Koji Ukegawa |
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Affiliation: | (1) Department of Energy Resources, National Institute for Resources and Environment, 305 Isukuba, Ibaraki, Japan;(2) Research Center for Advanced Science and Technology, University of Tokyo, Komaba, Meguro-ku, 153 Tokyo, Japan |
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Abstract: | We have reported that the peroxyl radicals derived from methyl eicosapentaenoate (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. This lowers the oxidizability for 20:5n-3 in aqueous Triton X-100 micelles by enhancing the
termination reaction rate for peroxyl radicals and by reducing the rate of propagation since there may be more polar peroxyl
radicals derived from 20:5n-3 at the surface than within the micelle 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 oxidation of lipids in aqueous micelle. Antioxidants give a clear induction 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 in aqueous phase, respectively.
However, BHT does not inhibit the oxidation 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 polar peroxyl radicals derived from 20:5n-3 are preferentially located
at the surface of the micelle. Similar results were obtained when oxidation was initiated with a lipid-soluble radical initiator
except antioxidants had lesser effect on the oxidation rate of 20:5n-3. |
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