Oxidative stress and mitochondrial function : role of ubiquinol as antioxidant

Abstract

Over the last three decades it has been established that biquinone (coenzyme Q), in addition to its function as an electron and proton carrier in the respiratory chain, acts in its reduced form (ubiquinol) as an antioxidant. The main theme of this thesis is a study of the antioxidant function of ubiquinol in preparations of beef heart submitochondrial particles, which consist of inverted vesicular fragments of the inner mitochondrial membrane. Incubation of these particles with ascorbate and ADP-Fe3+ caused a peroxidation of membrane phospholipids, detected by measuring the formation of thiobarbituric acid reactive substances. It was found that this process is inhibited by reduction of the endogenous ubiquinone in the particles through the addtion of succinate and antimycin. This effect was abolished when ubiquinone was removed from the particles by pentane extraction, and restored upon reincorporation of ubiquinone. Pentane extraction also removed vitamin E from the particles, showing that the antioxidant effect of ubiquinol was not dependent on vitamin E. Parallel to lipid peroxidation, treatment with ascorbate and ADP-Fe3+ also caused an oxidation of proteins in the particles, which could be detected by measuring the formation of protein carbonyls. This effect also was prevented by ubiquinol. Among the proteins undergoing oxidation, and protected by ubiquinol, the adenine nucleotide translocator was identified by SDS-PAGE analysis and immunoblotting.

Functional studies revealed an inhibition of the NADH and succinate oxidase activities in the course of lipid peroxidation, and this effect was accompanied by a degradation of ubiquinone in the particles. Maintaining ubiquinone in the reduced state prevented these effects. The degradation of ubiquinone was dependent on lipid peroxidation and was non-enzymic, which could be shown in experiments with ubiquinone-containing liposomes.

Another protein sensitive to ascorbate and ADP-Fe3+ treatment, and protected by ubiquinol, was identified as the proton-translocating nicotinamide nucleotide transhydrogenase. The enzyme was also inactivated by peroxynitrite treatment, but in this case, ubiquinol had no protective effect. Kinetic and immunologic analysis indicated that the inactivation of the enzyme accompanying lipid peroxidation occurred in the proton-translocating membrane domain, whereas peroxynitrite attacked the extramembraneous domains of the enzyme, containing its catalytic sites.

A study was also carried out of the effect of reduced plastoquinone in protecting thylakoid membranes of spinach chloroplasts against lipid peroxidation occurring during light-induced oxidative stress. It was found that the extent of lipid peroxidation was dependent on the redox state of the plastoquinone pool, the reduced form acting as an antioxidant. Similar to the ubiquinone pool in submitochondrial particles, the plastoquinone pool in the thylakoid membranes was degraded during lipid peroxidation. The discussion is focused on molecular, physiological, and biomedical aspects of the antioxidant function of ubiquinol.