Reduction of ubiquinone by lipoamide dehydrogenase - An antioxidant regenerating pathway

Lipoamide dehydrogenase belongs to a family of pyridine nucleotide disulfid e oxidoreductases and is ubiquitous in aerobic organisms. This enzyme also reduces ubiquinone (the only endogenously synthesized lipid-soluble antioxi dant) to ubiquinol, the form in which it functions as an antioxidant. The r eduction of ubiquinone was linear with time and exhibited turnover numbers of 5 and 1.2 min(-1) in the presence and absence of zinc, respectively. The reaction was stimulated by zinc and cadmium but not by the other divalent ions tested. The zinc/cadmium-dependent stimulation of the reaction increas ed rapidly and linearly up to a concentration of 0.1 mm and was even furthe r increased at 0.5 mm. At pH 6, the activity was three times higher than at physiological pH. Alteration of the NADPH : NADP(+) ratio revealed that th e reaction is inhibited by higher concentrations of the oxidized cofactors. FAD reduced ubiquinone in a dose-dependent manner at a considerably lower rate, suggesting that the reduction of ubiquinone by lipoamide dehydrogenas e involves the FAD moiety of the enzyme.