Lipoxygenase treatment render low-density lipoprotein susceptible to Cu2+-catalysed oxidation.
Oxidative modification of low-density lipoprotein (LDL) has been implicated in foam-cell formation at all stages of atherosclerosis. Since transition metals and mammalian 15-lipoxygenases are capable of oxidizing LDL to its atherogenic form, a concerted action of these two catalysts in atherogenesis has been suggested. Cu2+-catalysed LDL oxidation is characterized by a kinetic lag period in which the lipophilic antioxidants are decomposed and by a complex mixture of unspecific oxidation products. We investigated the kinetics of the 15-lipoxygenase-catalysed oxygenation of LDL and found that the enzyme is capable of oxidizing LDL in the presence of the endogenous lipophilic antioxidants. In contrast with the Cu2+-catalysed reaction, no kinetic lag phase was detected. The pattern of products formed during short-term incubations was highly specific, with cholesterol-esterified (13S)-hydroperoxy-(9Z,11E)-octadecadinoic acid being the major product. However, after long-term incubations the product pattern was less specific. Preincubation with 15-lipoxygenase rendered human LDL more susceptible to Cu2+-catalysed oxidation as indicated by a dramatic shortening of the lag period. Addition of Cu2+ to lipoxygenase-treated LDL led to a steep decline in its antioxidant content and to a greatly reduced lag period. Interestingly, if normalized to a comparable hydroperoxide content, autoxidation and addition of exogenous hydroperoxy fatty acids both failed to overcome the lag period. The local peroxide concentrations in various LDL subcompartments will be discussed as a possible reason for this unexpected behaviour.
Lass
A
A
Institute of Biochemistry, University of Graz, Austria.
Belkner
J
J
Esterbauer
H
H
Kühn
H
H
D001093:Arachidonate 15-Lipoxygenase / Q000494:pharmacology*; D002338:Carotenoids / Q000494:pharmacology; D002384:Catalysis; D003300:Copper / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000378:metabolism*; D010084:Oxidation-Reduction; D014810:Vitamin E / Q000494:pharmacology
D008077:Lipoproteins, LDL; D014810:Vitamin E; D002338:Carotenoids; D003300:Copper; D001093:Arachidonate 15-Lipoxygenase