Oxidation mechanism of vitamin E analogue (Trolox C, 6-hydroxy-2,2,5,7,8-pentamethylchroman) and vitamin E by horseradish peroxidase and myoglobin. |
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Authors: | M Nakamura T Hayashi |
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Affiliation: | Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan. |
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Abstract: | The oxidation of 6-hydroxy-2,2,5,7,8-pentamethylchroman, Trolox C, and alpha-tocopherol by horseradish peroxidase was examined by stopped-flow and ESR experiments. The catalytic intermediate of horseradish peroxidase during the oxidation of vitamin E analogues and vitamin E was invariably Compound II, and rate constants for the rate-determining step decreased in the order 6-hydroxy-2,2,5,7,8-pentamethylchroman > Trolox C > alpha-tocopherol. The formation of phenoxyl radicals from substrates was verified with ESR and was followed optically. Resulting 6-hydroxy-2,2,5,7,8-pentamethylchroman and Trolox C radicals decayed through a dismutation reaction, followed by formation of the quinoid form via a transient intermediate. The sequence of events after formation of 6-hydroxy-2,2,5,7,8-pentamethylchroman and Trolox C radicals was similar to that observed by pulse radiolysis (Thomas, M. J., and Bielski, B. H. J. (1989). J. Am. Chem. Soc. 111, 3315-3319). Final oxidation products of 6-hydroxy-2,2,5,7,8-pentamethylchroman and Trolox C were identified as the quinoid forms and were obtained quantitatively whether or not the analogue had a carboxyl or methyl group at the 2-position of chroman ring. In contrast, enzymatic oxidation of alpha-tocopherol gave alpha-tocopherol quinone in very low yield. Conversion of 6-hydroxy-2,2,5,7,8-pentamethylchroman, Trolox C, and alpha-tocopherol to the corresponding quinones was also catalyzed by metmyoglobin in a reaction completely inhibited by ascorbate. |
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