CRISOSTOMO ANAG, MORENO RAPHAELB, NAVARATNAM SUPPIAH, WILKINSON JAMESA, BISBY ROGERH. Generation of superoxide and singlet oxygen from alpha-tocopherolquinone and analogues.
Free Radic Res 2007;
41:730-7. [PMID:
17516246 PMCID:
PMC2080821 DOI:
10.1080/10715760701324075]
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Abstract
Three potential routes to generation of reactive oxygen species (ROS) from alpha-tocopherolquinone (alpha-TQ) have been identified. The quinone of the water-soluble vitamin E analogue Trolox C (Trol-Q) is reduced by hydrated electron and isopropanol alpha-hydroxyalkyl radical, and the resulting semiquinone reacts with molecular oxygen to form superoxide with a second order rate constant of 1.3 x 10(8) dm(3)/mol/s, illustrating the potential for redox cycling. Illumination (UV-A, 355 nm) of the quinone of 2,2,5,7,8-pentamethyl-6-hydroxychromanol (PMHC-Q) leads to a reactive short-lived (ca. 10(- 6) s) triplet state, able to oxidise tryptophan with a second order rate constant greater than 10(9) dm(3)/mol/s. The triplet states of these quinones sensitize singlet oxygen formation with quantum yields of about 0.8. Such potentially damaging reactions of alpha-TQ may in part account for the recent findings that high levels of dietary vitamin E supplementation lack any beneficial effect and may lead to slightly enhanced levels of overall mortality.
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