Dihydrolipoic acid lowers the redox activity of transition metal ions but does not remove them from the active site of enzymes.
Alpha-lipoic acid (LA) and its reduced form, dihydrolipoic acid (DHLA), have been suggested to chelate transition metal ions and, hence, mitigate iron- and copper-mediated oxidative stress in biological systems. However, it remains unclear whether LA and DHLA chelate transition metal ions in a redox-inactive form, and whether they remove metal ions from the active site of enzymes. Therefore, we investigated the effects of LA and DHLA on iron- or copper-catalyzed oxidation of ascorbate, a sensitive assay for the redox activity of these metal ions. We found that DHLA, but not LA, significantly inhibited ascorbate oxidation mediated by Fe(III)-citrate, suggesting that reduced thiols are required for iron binding. DHLA also strongly inhibited Cu(II)(histidine)(2)-mediated ascorbate oxidation in a concentration-dependent manner, with complete inhibition at a DHLA:Cu(II) molar ratio of 3:1. In contrast, no inhibition of copper-catalyzed ascorbate oxidation was observed with LA. To investigate whether LA and DHLA remove copper or iron from the active site of enzymes, Cu,Zn superoxide dismutase and the iron-containing enzyme aconitase were used. We found that neither LA nor DHLA, even at high, millimolar concentrations, altered the activity of these enzymes. Our results suggest that DHLA chelates and inactivates redox-active transition metal ions in small-molecular, biological complexes without affecting iron- or copper-dependent enzyme activities.
Suh
Jung H
JH
Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331, USA.
Zhu
Ben-Zhan
BZ
deSzoeke
Evan
E
Frei
Balz
B
Hagen
Tory M
TM
D000154:Aconitate Hydratase / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D002417:Cattle; D002614:Chelating Agents / Q000494:pharmacology; D004050:Ditiocarb / Q000494:pharmacology; D005290:Ferric Compounds / Q000037:antagonists & inhibitors / Q000494:pharmacology; D006639:Histidine / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000494:pharmacology; D009942:Organometallic Compounds / Q000037:antagonists & inhibitors / Q000494:pharmacology; D010084:Oxidation-Reduction / Q000187:drug effects; D011485:Protein Binding / Q000187:drug effects; D013482:Superoxide Dismutase / Q000737:chemistry / Q000378:metabolism*; D008063:Thioctic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology*; D028561:Transition Elements / Q000378:metabolism / Q000494:pharmacology*
D002614:Chelating Agents; D005290:Ferric Compounds; D009942:Organometallic Compounds; D028561:Transition Elements; D006639:Histidine; C025314:ferric citrate; D008063:Thioctic Acid; C015144:dihydrolipoic acid; C098468:copper histidine; D004050:Ditiocarb; D013482:Superoxide Dismutase; D000154:Aconitate Hydratase