title: Generation of superoxide and singlet oxygen from alpha-tocopherolquinone and analogues.
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.
authors.affiliation: Biomedical Sciences Research Institute, University of Salford, Salford, UK.
authors.forename: Ana G
authors.initials: AG
authors.lastname: Crisostomo
authors.forename: Raphael B
authors.initials: RB
authors.lastname: Moreno
authors.forename: Suppiah
authors.initials: S
authors.lastname: Navaratnam
authors.forename: James A
authors.initials: JA
authors.lastname: Wilkinson
authors.forename: Roger H
authors.initials: RH
authors.lastname: Bisby
mesh_terms: D000975:Antioxidants / Q000737:chemistry* / Q000378:metabolism; D002839:Chromans / Q000737:chemistry* / Q000378:metabolism; D007834:Lasers; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis; D017382:Reactive Oxygen Species / Q000378:metabolism; D026082:Singlet Oxygen / Q000378:metabolism*; D013481:Superoxides / Q000378:metabolism*; D014364:Tryptophan / Q000737:chemistry; D014810:Vitamin E / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism
chemical_list: D000975:Antioxidants; D002839:Chromans; D017382:Reactive Oxygen Species; D013481:Superoxides; D014810:Vitamin E; D026082:Singlet Oxygen; C002421:tocopherylquinone; C120726:2,2,5,7,8-pentamethyl-6-hydroxychroman; D014364:Tryptophan