title: Photodynamically generated 3-beta-hydroxy-5 alpha-cholest-6-ene-5- hydroperoxide: toxic reactivity in membranes and susceptibility to enzymatic detoxification.
abstract: Singlet oxygen (1O2)-mediated photooxidation of cholesterol gives three hydroperoxide products: 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide (5 alpha-OOH), 3 beta-hydroxycholest-4-ene-6 alpha-hydroperoxide (6 alpha-OOH) and 3 beta-hydroxycholest-4-ene-6 beta-hydroperoxide (6 beta-OOH). These species have been compared with respect to photogeneration rate on the one hand and susceptibility to enzymatic reduction/detoxification on the other, using the erythrocyte ghost as a cholesterol-containing test membrane and chloroaluminum phthalocyanine tetrasulfonate (AlPcS4) as a 1O2 sensitizer. Peroxide analysis was accomplished by high-performance liquid chromatography with mercury cathode electrochemical detection (HPLC-EC[Hg]). The initial rate of 5 alpha-OOH accumulation in AlPcS4/light-treated ghosts was found to be about three times greater than that of 6 alpha-OOH or 6 beta-OOH. Membranes irradiated in the presence of ascorbate and ferric-8-hydroxyquinoline (Fe[HQ]2, a lipophilic iron complex) accumulated lesser amounts of 5 alpha-OOH, 6 alpha-OOH and 6 beta-OOH but relatively large amounts of another peroxide pair, 3 beta-hydroxycholest-5-ene-7 alpha- and 7 beta-hydroperoxide (7 alpha, 7 beta-OOH), suggestive of iron-mediated free radical peroxidation. When photoperoxidized membranes containing 5 alpha-OOH, 6 alpha,6 beta-OOH and 7 alpha,7 beta-OOH (arising from 5 alpha-OOH rearrangement) were incubated with glutathione (GSH) and phospholipid hydroperoxide glutathione peroxidase (PHGPX), all hydroperoxide species underwent HPLC-EC(Hg)-detectable reduction to alcohols, the relative first order rate constants being as follows: 1.0 (5 alpha-OOH), 2.0 (7 alpha,7 beta-OOH), 2.4 (6 alpha-OOH) and 3.2 (6 beta-OOH).(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, Medical College of Wisconsin, Milwaukee 53221, USA.
authors.forename: P G
authors.initials: PG
authors.lastname: Geiger
authors.forename: W
authors.initials: W
authors.lastname: Korytowski
authors.forename: A W
authors.initials: AW
authors.lastname: Girotti
mesh_terms: D000818:Animals; D002784:Cholesterol / Q000031:analogs & derivatives* / Q000378:metabolism / Q000633:toxicity; D004910:Erythrocyte Membrane / Q000201:enzymology / Q000378:metabolism* / Q000528:radiation effects*; D006801:Humans; D008658:Inactivation, Metabolic; D007939:Leukemia L1210 / Q000378:metabolism; D008054:Lipid Peroxides / Q000378:metabolism / Q000633:toxicity; D051379:Mice; D010084:Oxidation-Reduction; D010777:Photochemistry; D014407:Tumor Cells, Cultured
chemical_list: D008054:Lipid Peroxides; C002843:cholesterol hydroperoxide; D002784:Cholesterol