Thiol chelation of Cu2+ by dihydrolipoic acid prevents human low density lipoprotein peroxidation.
Mono-thiols can act either as pro- or anti-oxidants during metal-catalyzed low density lipoprotein (LDL) peroxidation, however investigation of the role of vicinal thiols has been neglected. Therefore dihydrolipoic acid (DHLA), a vicinal dithiol, and lipoic acid, its oxidized form, were used to investigate Cu2+-mediated LDL peroxidation. We demonstrate here that DHLA inhibited Cu2+-dependent LDL peroxidation by chelating copper. DHLA (0-20 microM) increased lag-times of conjugated diene formation in LDL (100 microg/ml) oxidized with 5 microM Cu2+ in a concentration dependent manner, and this effect was saturated after 5 microM DHLA; enough to chelate all of the added Cu2+. In a similar fashion DHLA prevented LDL-mediated reduction of Cu2+ to Cu+. Lipoic acid had no effect in these systems. DHLA alone also reduced Cu2+, however this was inhibited when DHLA was in excess of the copper concentration. Hence there is complex formation between the two species. Copper:DHLA complex formation was further investigated and found to be dependent upon pH and the presence of oxygen. At low pH (<6), or in the absence of oxygen, the complex is stable, presumably due to vicinal thiol chelation. As the pH is increased, the carboxylate group also participates in copper chelation, this results in a less stable complex which is susceptible to oxidation, and copper is eventually released. Electron spin resonance studies demonstrate the formation of hydroxyl, but not superoxide, radicals during Cu2+-catalyzed DHLA oxidation. Thus in our LDL experiments at physiological pH, DHLA is able to either reductively inactivate Cu2+ when Cu2+ is in excess, or effectively chelate Cu2+ when DHLA is in excess. The Cu2+:DHLA complex eventually undergoes copper-catalyzed oxidation, copper is released and LDL peroxidation proceeds. DHLA, thus, has both pro- and antioxidant properties depending upon the ratio of Cu2+:DHLA and the pH. These results provide an additional mechanism of thiol-mediated formation of radicals and metal chelation.
Department of Molecular and Cell Biology, University of California at Berkeley, 94720-3200, USA. johnkl@socrates.berkeley.edu
J K
JK
Lodge
M G
MG
Traber
L
L
Packer
D002614:Chelating Agents; D003300:Copper / Q000737:chemistry* / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000032:analysis; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D015227:Lipid Peroxidation / Q000187:drug effects*; D008077:Lipoproteins, LDL / Q000737:chemistry*; D010084:Oxidation-Reduction; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D008063:Thioctic Acid / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology
D002614:Chelating Agents; D005609:Free Radicals; D008077:Lipoproteins, LDL; D013438:Sulfhydryl Compounds; D008063:Thioctic Acid; D003300:Copper; C015144:dihydrolipoic acid