Lipids

, 44:935

Kinetic Study of the Prooxidant Effect of α-Tocopherol. Hydrogen Abstraction from Lipids by α-Tocopheroxyl Radical

  • Aya Ouchi
  • Masaharu Ishikura
  • Kensuke Konishi
  • Shin-ichi Nagaoka
  • Kazuo Mukai
Original Article

Abstract

A kinetic study of the prooxidant effect of α-tocopherol was performed. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl stearate 1, ethyl oleate 2, ethyl linoleate 3, ethyl linolenate 4, and ethyl arachidonate 5) by α-tocopheroxyl radical in toluene were determined, using a double-mixing stopped-flow spectrophotometer. The second-order rate constants (kp) obtained are <1 × 10−2 M−1 s−1 for 1, 1.90 × 10−2 M−1 s−1 for 2, 8.33 × 10−2 M−1 s−1 for 3, 1.92 × 10−1 M−1 s−1 for 4, and 2.43 × 10−1 M−1 s−1 for 5 at 25.0 °C. Fatty acid esters 3, 4, and 5 contain two, four, and six –CH2– hydrogen atoms activated by two π-electron systems (–C=C–CH2–C=C–). On the other hand, fatty acid ester 2 has four –CH2– hydrogen atoms activated by a single π-electron system (–CH2–C=C–CH2–). Thus, the rate constants, kabstr/H, given on an available hydrogen basis are kp/4 = 4.75 × 10−3 M−1 s−1 for 2, kp/2 = 4.16 × 10−2 M−1 s−1 for 3, kp/4 = 4.79 × 10−2 M−1 s−1 for 4, and kp/6 = 4.05 × 10−2 M−1 s−1 for 5. The kabstr/H values obtained for 3, 4, and 5 are similar to each other, and are by about one order of magnitude higher than that for 2. From these results, it is suggested that the prooxidant effect of α-tocopherol in edible oils, fats, and low-density lipoproteins may be induced by the above hydrogen abstraction reaction.

Keywords

α-Tocopherol α-Tocopheroxyl radical Prooxidant effect Kinetic study Reaction rate constant Stopped-flow spectrophotometer Unsaturated lipids Fatty acids Vitamin E 

Abbreviations

ArOH

2,6-Di-tert-butyl-4(4′-methoxyphenyl)phenol

ArO·

2,6-Di-tert-butyl-4(4′-methoxyphenyl)phenoxyl (aroxyl)

AsH

Ascorbate mono anion

As·

Ascorbate mono anion radical

5,7-Di-iPr-Toc·

5,7-Di-isopropyl-tocopheroxyl

ESR

Electron spin resonance

LDL

Low-density lipoprotein

LH

Lipid (or fatty acid ethyl ester)

LOOH

Lipid hydroperoxide (or methyl linoleate hydroperoxide)

LOO·

Lipid peroxyl radical

LO·

Lipid alkoxyl radical

NRP

Non-radical products

α-TocH

α-Tocopherol

α-Toc·

α-Tocopheroxyl

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Copyright information

© AOCS 2009

Authors and Affiliations

  • Aya Ouchi
    • 1
  • Masaharu Ishikura
    • 2
  • Kensuke Konishi
    • 3
  • Shin-ichi Nagaoka
    • 1
  • Kazuo Mukai
    • 1
  1. 1.Department of Chemistry, Faculty of ScienceEhime UniversityMatsuyamaJapan
  2. 2.Life Science Business Division, Research and Development OperationsYamaha Motor Co. Ltd.ShizuokaJapan
  3. 3.Department of Physics, Faculty of ScienceEhime UniversityMatsuyamaJapan

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