title: Oxidation of low density lipoprotein by thiols: superoxide-dependent and -independent mechanisms.
abstract: Oxidatively damaged low density lipoprotein (LDL) may cause macrophages to accumulate cholesterol in an unregulated manner, initiating the development of atherosclerotic lesions. Cultured smooth muscle cells oxidize LDL by a superoxide (O2.-)-dependent mechanism that requires L-cystine and redox-active transition metal ions in the incubation medium. To test the hypothesis that cellular reduction of L-cystine to a thiol might be involved, we exposed LDL to L-cysteine, glutathione, and D,L-homocysteine. In a cell-free system each thiol modified LDL by a pathway that required either Cu2+ or Fe3+. Thiol- and Cu(2+)-modified LDL underwent lipid peroxidation and exhibited a number of properties of cell-modified LDL, including increased mobility on agarose gel electrophoresis and fragmentation of apolipoprotein B-100. Superoxide dismutase inhibited modification of LDL by L-cysteine/Cu2+, whereas catalase and mannitol were without effect. In striking contrast, superoxide dismutase had little effect on oxidation of LDL by Cu2+ and either homocysteine or glutathione. Moreover, only L-cysteine/Cu(2+)-modified 125I-labeled LDL was degraded more rapidly than 125I-labeled LDL by human monocyte-derived macrophages: superoxide dismutase in the reaction mixture blocked the facilitated uptake of L-cysteine/Cu(2+)-modified 125I-labeled LDL, suggesting involvement of O2.-. These results indicate that LDL oxidation by L-cysteine and Cu2+ requires O2.- but not H2O2 or hydroxyl radical. The reaction may involve the metal ion-dependent formation of L-cystine radical anion which is oxidized by oxygen, yielding O2.- and the disulfide. LDL modified by L-cysteine and smooth muscle cells exhibit similar physical and biological properties, indicating that thiol-dependent generation of O2.- may be the oxidative mechanism in both systems. Thiols also promote lipid peroxidation by O2(.-)-independent reactions but human macrophages fail to rapidly degrade these oxidized LDLs.
authors.affiliation: Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110.
authors.forename: J W
authors.initials: JW
authors.lastname: Heinecke
authors.forename: M
authors.initials: M
authors.lastname: Kawamura
authors.forename: L
authors.initials: L
authors.lastname: Suzuki
authors.forename: A
authors.initials: A
authors.lastname: Chait
mesh_terms: D002474:Cell-Free System; D002478:Cells, Cultured; D003300:Copper / Q000494:pharmacology; D003545:Cysteine / Q000494:pharmacology; D004492:Edetic Acid / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D015227:Lipid Peroxidation / Q000187:drug effects; D008077:Lipoproteins, LDL / Q000378:metabolism*; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism; D019301:Oleic Acid; D009829:Oleic Acids / Q000378:metabolism; D010084:Oxidation-Reduction; D013438:Sulfhydryl Compounds / Q000378:metabolism*; D013482:Superoxide Dismutase / Q000494:pharmacology; D013481:Superoxides / Q000494:pharmacology*
chemical_list: D008077:Lipoproteins, LDL; D009829:Oleic Acids; D013438:Sulfhydryl Compounds; D013481:Superoxides; D019301:Oleic Acid; D003300:Copper; D004492:Edetic Acid; D013482:Superoxide Dismutase; D003545:Cysteine