Redox cycles of vitamin E: hydrolysis and ascorbic acid dependent reduction of 8a-(alkyldioxy)tocopherones
- 12 December 1989
- journal article
- research article
- Published by American Chemical Society (ACS) in Biochemistry
- Vol. 28 (25), 9772-9777
- https://doi.org/10.1021/bi00451a034
Abstract
Oxidation of the biological antioxidant .alpha.-tocopherol (vitamin E; TH) by peroxyl radicals yields 8a-(alkyldioxy)tocopherones, which either may hydrolyze to .alpha.-tocopheryl quinone (TQ) or may be reduced by ascorbic acid to regenerate TH. To define the chemistry of this putative two-electron TH redox cycle, we studied the hydrolysis and reduction of 8a-[(2,4-dimethyl-l-nitrilopent-2-yl)diox]tocopherone (1) in acetonitrile/buffer mixtures and in phospholipid liposomes. TQ formation in acetonitrile/buffer mixtures, which was monitored spectrophotometrically, declined with increasing pH and could not be detected above pH 4. The rate of TQ formation from 1 first increased with time and then decreased in a first-order terminal phase. Rearrangement of 8a-hydroxy-.alpha.-tocopherone (2) to TQ displayed first-order kinetics identical with the terminal phase for TQ formation from 1. Both rate constants increased with decreasing pH. Hydrolysis of 1 in acetonitrile/H2 18O yielded [18O]TQ. These observations suggest that 1 loses the 8a-(alkyldioxy) moiety to produce the tocopherone cation (T+), which hydrolyzes to 2, the TQ-forming intermediate. Incubation of either 1 or 2 with ascorbic acid in acetonitrile/buffer yielded TH. Reduction of both 1 and 2 decreased with increasing pH. In phosphatidylcholine liposomes at pH 7, approximately 10% of the T+ generated from 1 was reduced to TH by 5 mM ascorbic acid. The results collectively demonstrate that T+ is the ascorbic acid reducible intermediate in a two-electron TH redox cycle, a process that probably would require biocatalysis to proceed in biological membranes.This publication has 7 references indexed in Scilit:
- Antioxidant protection of phospholipid bilayers by alpha-tocopherol. Control of alpha-tocopherol status and lipid peroxidation by ascorbic acid and glutathione.Journal of Biological Chemistry, 1986
- Estimation of the location of natural α-tocopherol in lipid bilayers by 13C-NMR spectroscopyBiochimica et Biophysica Acta (BBA) - Biomembranes, 1985
- Antioxidant and co-antioxidant activity of vitamin C. The effect of vitamin C, either alone or in the presence of vitamin E or a water-soluble vitamin E analogue, upon the peroxidation of aqueous multilamellar phospholipid liposomesBiochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism, 1985
- Formation of α-tocopherol radical and recycling of α-tocopherol by ascorbate during peroxidation of phosphatidylcholine liposomesAn electron paramagnetic resonance studyBiochimica et Biophysica Acta (BBA) - General Subjects, 1984
- The cooperative interaction between vitamin E and vitamin C in suppression of peroxidation of membrane phospholipidsBiochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism, 1981
- Vesicles of variable diameter prepared by a modified injection methodBiochemistry, 1977
- Reactions of Vitamin E with Peroxides. II. Reaction of Benzoyl Peroxide with d-α-Tocopherol in Alcohols*Biochemistry, 1965