Oxidation mechanism of vitamin E analogue (Trolox C, 6-hydroxy-2,2,5,7,8-pentamethylchroman) and vitamin E by horseradish peroxidase and myoglobin.
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.
Nakamura
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M
Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan.
Hayashi
T
T
D000818:Animals; D001205:Ascorbic Acid / Q000737:chemistry; D002839:Chromans / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D006735:Horseradish Peroxidase / Q000737:chemistry*; D006736:Horses; D066298:In Vitro Techniques; D007700:Kinetics; D009211:Myoglobin / Q000737:chemistry*; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D014810:Vitamin E / Q000737:chemistry*
D002839:Chromans; D005609:Free Radicals; D009211:Myoglobin; D014810:Vitamin E; D006735:Horseradish Peroxidase; D001205:Ascorbic Acid; C010643:6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid