On the mechanism of oxidation of cholesterol at C-7 in a lipoxygenase system.
Incubation of [7-2H2]cholesterol with soybean lipoxygenase and linoleic acid in the presence of oxygen gave a mixture of 5-cholestene-3 beta,7 alpha-diol, 5-cholestene-3 beta,7 beta-diol, 3 beta-hydroxy-5-cholesten-7-one,5 alpha,6 alpha-epoxycholestan-3 beta-ol, and 5 beta,6 beta-epoxycholestan-3 beta-ol. The conversion into the 7-oxygenated products was associated with a very high intermolecular isotope effect (KH/KD = 15-17), suggesting that the rate-limiting step in the overall conversion is likely to be the abstraction of hydrogen at C-7 in a radical reaction. Evidence that linoleic acid is to some extent directly involved was obtained with the use of [7-3H]cholesterol. Incubation of [7-3H]cholesterol resulted in a significant incorporation of 3H in the reisolated linoleic acid fraction. The isotope effect associated with conversion of [7 alpha-2H]cholesterol into 7-oxygenated products in the lipoxygenase system was 2-3, indicating that the extraction of hydrogen is nonstereospecific. Incubation of [7-2H2]cholesterol with 13-hydroperoxy-9,11-octadecadienoic acid gave the above 7-oxygenated products with relatively small isotope effects (KH/KD = 3-4). It is concluded that the most important mechanism for oxidation of cholesterol at C-7 in the lipoxygenase system involves participation of radicals and that a carbon-centered linoleic acid radical can extract hydrogen directly from cholesterol. Fatty acid hydroperoxides and their secondary products seem to be less important as initiators in connection with oxidation of cholesterol.
Lund
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Department of Clinical Chemistry, Karolinska Institutet, Huddinge Hospital, Sweden.
Diczfalusy
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U
Björkhem
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D002784:Cholesterol / Q000737:chemistry / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D008401:Gas Chromatography-Mass Spectrometry; D006859:Hydrogen / Q000737:chemistry; D007700:Kinetics; D008041:Linoleic Acids / Q000378:metabolism; D008084:Lipoxygenase / Q000378:metabolism*; D010084:Oxidation-Reduction
D008041:Linoleic Acids; D006859:Hydrogen; D002784:Cholesterol; D008084:Lipoxygenase