Biochemical, physiological and medical aspects of ubiquinone function.

This presentation is a brief review of current knowledge concerning some biochemical, physiological and medical aspects of the function of ubiquinone (coenzyme Q) in mammalian organisms. In addition to its well-established function as a component of the mitochondrial respiratory chain, ubiquinone has in recent years acquired increasing attention with regard to its function in the reduced form (ubiquinol) as an antioxidant. Ubiquinone, partly in the reduced form, occurs in all cellular membranes as well as in blood serum and in serum lipoproteins. Ubiquinol efficiently protects membrane phospholipids and serum low-density lipoprotein from lipid peroxidation, and, as recent data indicate, also mitochondrial membrane proteins and DNA from free-radical induced oxidative damage. These effects of ubiquinol are independent of those of exogenous antioxidants, such as vitamin E, although ubiquinol can also potentiate the effect of vitamin E by regenerating it from its oxidized form. Tissue ubiquinone levels are regulated through the mevalonate pathway, increasing upon various forms of oxidative stress, and decreasing during aging. Drugs inhibiting cholesterol biosynthesis via the mevalonate pathway may inhibit or stimulate ubiquinone biosynthesis, depending on their site of action. Administration of ubiquinone as a dietary supplement seems to lead primarily to increased serum levels, which may account for most of the reported beneficial effects of ubiquinone intake in various instances of experimental and clinical medicine.

Uptake of dietary coenzyme Q supplement is limited in rats.

Coenzyme Q is an important mitochondrial redox component and the only endogenously produced lipid-soluble antioxidant. Its tissue concentration decreases with aging and in a number of diseases; dietary supplementation of this lipid would fulfill important functions by counteracting coenzyme Q depletion. To investigate possible uptake, rats were administered 12 mumol coenzyme Q10/100 g body wt once daily by gastric intubation. The appearance of coenzyme Q10 in various tissues and blood after 6 h, 4 d or 8 d was studied. The control group of rats received rapeseed-soybean oil (the vehicle in the experimental group). Lipids were extracted with petroleum ethermethanol, and the reduced and oxidized forms of coenzyme Q9 and Q10 were separated and quantified by reversed-phase HPLC. In the plasma, the total coenzyme Q concentration was doubled after 4 d of treatment. Coenzyme Q10 was also recovered in liver homogenates, where, as in the plasma, it was largely in the reduced form. Uptake into the spleen could be to a large extent accounted for by the blood content of this organ. No dietary coenzyme Q10 was recovered in the heart or kidney. The uptake in the whole body was 2-3% of the total dose. Coenzyme Q10 found in the liver was located mainly in the lysosomes. Dietary coenzyme Q10 did not influence the endogenous biosynthesis of coenzyme Q9. This is in contrast to dietary cholesterol, which down-regulates cholesterol biosynthesis. The dietary coenzyme Q10 level in the plasma decreased to approximately 50% after 4 d. These results suggest that dietary coenzyme Q10 may play a role primarily in the blood and that no appreciable uptake occurs into tissues.

Biosynthesis of ubiquinone and plastoquinone in the endoplasmic reticulum-Golgi membranes of spinach leaves.

The localization of ubiquinone (UQ) and plastoquinone (PQ) biosynthesis in subfractions isolated from spinach leaves has been studied. UQ-9 and UQ-10 were found mainly in mitochondria, whereas PQ was enriched in chloroplasts, but also found in Golgi membranes. alpha-Unsaturated polyprenol-11 was also present at a low concentration in chloroplasts. Autoradiography revealed the presence of nonaprenyl-4-hydroxybenzoate (NPHB) and nonaprenyl-2-methylquinol (NPMQ) transferase activities involved in quinone biosynthesis in all subfractions, but the specific activities involved in quinone biosynthesis in the total microsomal fraction were 20 times higher than those in mitochondria and chloroplasts. The isolated Golgi vesicles were particularly enriched in both activities. When the incubation medium containing total microsomes or Golgi membranes was supplemented with NADH, NADPH, S-adenosylmethionine, and an ATP-generating system, NPHB and NPMQ were transferred to UQ-9 and PQ, respectively. trans-Prenyltransferase, which synthesizes the side chain of UQ and PQ, was present in the total microsomal fraction. With farnesyl-PP as substrate, no product was formed, but with geranyl-PP, solanesyl-PP was synthesized and transferred to 4-hydroxybenzoate present in the total microsomal fraction. The results show that these membranes from spinach contain farnesyl-PP synthetase. It is concluded that the plant leaf Golgi membranes contain the enzymes for both UQ and PQ biosynthesis and that a specific transport and targeting system is required for selective transfer of UQ to the mitochondria and of PQ to the chloroplast.

The mode of action of lipid-soluble antioxidants in biological membranes: relationship between the effects of ubiquinol and vitamin E as inhibitors of lipid peroxidation in submitochondrial particles.

The effects of ubiquinol and vitamin E on ascorbate- and ADP-Fe3+-induced lipid peroxidation were investigated by measuring oxygen consumption and malondialdehyde formation in beef heart submitochondrial particles. In the native particles, lipid peroxidation showed an initial lag phase, which was prolonged by increasing concentrations of ascorbate. Lipid peroxidation in these particles was almost completely inhibited by conditions leading to a reduction of endogenous ubiquinone, such as the addition of succinate or NADH in the presence of antimycin. Lyophilization of the particles followed by three or four consecutive extractions with pentane resulted in a complete removal of vitamin E and a virtually complete removal of ubiquinone, as revealed by reversed-phase high pressure liquid chromatography. In these particles, lipid peroxidation showed no significant lag phase and was not inhibited by either increasing concentrations of ascorbate or conditions leading to ubiquinone reduction. Treatment of the particles with a pentane solution of vitamin E (alpha-tocopherol) restored the lag phase and its prolongation by increasing ascorbate concentrations. Treatment of the extracted particles with pentane containing ubiquinone-10 resulted in a restoration of the inhibition of lipid peroxidation by succinate or NADH in the presence of antimycin, but not the initial lag phase or its prolongation by increasing concentrations of ascorbate. Malonate and rotenone, which prevent the reduction of ubiquinone by succinate and NADH, respectively, abolished, as expected, the inhibition of the initiation of lipid peroxidation in both native and ubiquinone-10-supplemented particles. Reincorporation of both vitamin E and ubiquinone-10 restored both effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the mode of action of erucic acid on heart metabolism.

The effects of erucic acid on the oxidative metabolism of rat-heart mitochondria have been investigated using intact animals, perfused beating heart, isolated mitochondria and mitochondrial extracts. Feeding rats with a diet containing erucic acid was found to lead to a diminished ability of the isolated heart mitochondria to oxidize various substrates, in accordance with previous reports (Houtsmuller et al., Biochim. Biophys. Acta 218 (1970) 564). This effect was almost pronounced with palmitylcarnitine as substrate, in which case the rate of oxidation was decreased by more than 50% at such a low erucic acid content in the diet as 1.4% given over 2-4 weeks. Oxidation of palmitylcarnitine was also found to be inhibited when erucylcarnitine was added to isolated heart mitochondria from control animals, in agreement with earlier observations (Christophersen and Bremer, FEBS Lett. 23 (1972) 230; Biochim. Biophys. Acta 280 (1972) 506). The inhibition was accompanied by a decrease in the rate and extent of reduction of mitochondrial flavoprotein. Experiments with perfused beating rat-heart likewise revealed an inhibition of flavoprotein reduction, as well as nicotinamide nucleotide reduction, when erucate was added to the perfusing medium of the beating heart respiring with oleate--but not with octanoate--as substrate. These data together with those earlier published in the literature indicate that erucic acid may interfere with the enzyme system involved in the mitochondrial oxidation of long-chain fatty acids, probably at the level of acyl-CoA dehydrogenase. Kinetic data supporting this conclusion, obtained with extracts of rat-heart mitochondria containing the acyl-CoA dehydrogenase and electron-transferring flavoprotein system, are presented. The possible implications of these results for the known effect of dietary erucic acid in causing an accumulation of fat in the heart are discussed.