Ubiquinone biosynthesis by mitochondria, sonicated mitochondria, and mitoplasts of rat liver

Identification of the human mitochondrial S-adenosylmethionine transporter: bacterial expression, reconstitution, functional characterization and tissue distribution

The mitochondrial carriers are a family of transport proteins that, with a few exceptions, are found in the inner membranes of mitochondria. They shuttle metabolites and cofactors through this membrane, and connect cytoplasmic functions with others in the matrix. SAM (S-adenosylmethionine) has to be transported into the mitochondria where it is converted into S-adenosylhomocysteine in methylation reactions of DNA, RNA and proteins. The transport of SAM has been investigated in rat liver mitochondria, but no protein has ever been associated with this activity. By using information derived from the phylogenetically distant yeast mitochondrial carrier for SAM and from related human expressed sequence tags, a human cDNA sequence was completed. This sequence was overexpressed in bacteria, and its product was purified, reconstituted into phospholipid vesicles and identified from its transport properties as the human mitochondrial SAM carrier (SAMC). Unlike the yeast orthologue, SAMC catalysed virtually only countertransport, exhibited a higher transport affinity for SAM and was strongly inhibited by tannic acid and Bromocresol Purple. SAMC was found to be expressed in all human tissues examined and was localized to the mitochondria. The physiological role of SAMC is probably to exchange cytosolic SAM for mitochondrial S-adenosylhomocysteine. This is the first report describing the identification and characterization of the human SAMC and its gene.

Quinones in long-lived clk-1 mutants of Caenorhabditis elegans

Ubiquinone (UQ) (coenzyme Q) is a lipophilic redox-active molecule that functions as an electron carrier in the mitochondrial electron transport chain. Electron transfer via UQ involves the formation of semiubiquinone radicals, which causes the generation of superoxide radicals upon reaction with oxygen. In the reduced form, UQ functions as a lipid-soluble antioxidant, and protects cells from lipid peroxidation. Thus, UQ is also important as a lipophilic regulator of oxidative stress. Recently, a study on long-lived clk-1 mutants of Caenorhabditis elegans demonstrated that biosynthesis of UQ is dramatically altered in mutant mitochondria. Demethoxy ubiquinone (DMQ), that accumulates in clk-1 mutants in place of UQ, may contribute to the extension of life span. Here we elucidate the possible mechanisms of life span extension in clk-1 mutants, with particular emphasis on the electrochemical property of DMQ. Recent findings on the biochemical function of CLK-1 are also discussed.

Evidence for the presence of 5S rRNA in mammalian mitochondria

Mammalian mitochondrial ribosomes contain two prokaryotic-like rRNAs, 12S and 16S, both encoded by mitochondrial DNA. As opposed to cytosolic ribosomes, however, these ribosomes are not thought to contain 5S rRNA. For this reason, it has been unclear whether 5S rRNA, which can be detected in mitochondrial preparations, is an authentic organellar species imported from the cytosol or is merely a copurifying cytosol-derived contaminant. We now show that 5S rRNA is tightly associated with highly purified mitochondrial fractions of human and rat cells and that 5S rRNA transcripts derived from a synthetic gene transfected transiently into human cells are both expressed in vivo and present in highly purified mitochondria and mitoplasts. We conclude that 5S rRNA is imported into mammalian mitochondria, but its function there still remains to be clarified.

Transport of S-adenosylmethionine in isolated rat liver mitochondria

Mitochondria do not have the enzyme, methionine adenosyltransferase (ATP: L-methionine S-adenosyltransferase, EC 2.5.1.6), necessary for the biosynthesis of S-adenosylmethionine. Nevertheless, about 30% of total hepatic S-adenosylmethionine resides in the mitochondria and radiolabeled S-adenosylmethionine may be isolated from the mitochondria after administration of radiolabeled methionine. This leads to the hypothesis that a carrier-mediated system is responsible for S-adenosylmethionine transport from the cytosol into the mitochondria. We have characterized such a system in isolated rat liver mitochondria. Uptake of S-adenosylmethionine consisted of two components. One component was incorporation of the methyl group into phospholipids as shown by thin-layer chromatography. The second component represented uptake into the mitochondria since addition of excess unlabeled S-adenosylmethionine resulted in efflux of labeled substrate. This countertransport is characteristic of a carrier-mediated transport system. Uptake (corrected for incorporation into phospholipids) was saturable with an apparent Km = 8.9 microM and Vmax = 54.3 pmol x mg protein(-1) x min(-1). Uptake was not inhibited by methionine, adenosine, 5'-methylthioadenosine, carnitine, choline, betaine, quinine, or hemicholinium-3. Uptake was inhibited by sinefungin and by S-adenosylhomocysteine (Ki = 53.4 microM). Uptake of S-adenosylmethionine was not dependent on the electrical potential across the mitochondrial membrane. These results indicate that S-adenosylmethionine is taken up into mitochondria via a specific, carrier-mediated system.

Reduced antioxidative capacity in liver mitochondria from bile duct ligated rats

Lipid peroxidation and antioxidative mechanisms were investigated in liver mitochondria from bile duct ligated rats (BDL rats) and correlated with the activity of enzyme complexes of the electron transport chain. In comparison to pair-fed control rats, BDL rats had increased concentrations of thiobarbituric acid reacting substances (TBARS) per gram of liver and per milligram of mitochondrial protein 3, 7, 14, and 28 days after surgery. The hepatic glutathione (GSH) content was decreased in BDL rats 28 days after surgery when expressed per gram of liver but equal between BDL and control rats when expressed per liver. The mitochondrial GSH content was decreased in BDL rats by 20% to 33% from day 7 after surgery. The concentrations of ubiquinone-9 and ubiquinone-10, substances involved in electron transport and efficient antioxidants, were both decreased in BDL rats 14 and 28 days after surgery per gram of liver and per milligram of mitochondrial protein. When expressed per liver, ubiquinone-9 was decreased in BDL rats from day 7 after surgery. In comparison with controls, the decrease in total mitochondrial ubiquinone content in BDL rats averaged 52% 14 days and 38% 28 days after surgery. The activity of the succinate:ferricytochrome c oxidoreductase (complexes II and III of the electron transport chain) was decreased in BDL rats at days 7, 14, and 28 after surgery, and the activity of the ferrocytochrome c:oxygen oxidoreductase (complex IV) was reduced at 14 and 28 days after surgery. The mitochondrial concentration of TBARS showed a negative and the concentrations of GSH and ubiquinone a positive correlation with the activity of the succinate:ferricytochrome c oxidoreductase.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Hexaprenyl-4-hydroxybenzoic acid forms a predominant intermediate pool in ubiquinone biosynthesis in Saccharomyces cerevisiae

The biosynthesis of ubiquinone (coenzyme Q) was studied in Saccharomyces cerevisiae. Lipid extracts were prepared from wild-type yeast grown in the presence of p-[U-14C]- and p-[carboxy-14C]hydroxybenzoic acid. Ergosterol was removed by adsorption to digitonin-celite, and radiolabeled lipids were purified by sequential reverse-phase and normal-phase HPLC steps. Radiolabeled peaks were identified by comparison with synthetic standards using retention time and electron ionization mass spectrometric criteria. The recovery and identification of the unstable 3-hexaprenyl-4-hydroxybenzoic acid molecule were facilitated by treatment of the lipid extract with diazomethane under conditions that resulted in the formation of the stable derivatives methyl 3-hexaprenyl-4-hydroxybenzoate or methyl 3-hexaprenyl-4-methoxybenzoate. In stationary-phase yeast cultures, the major radioactive lipid products are coenzyme Q and 3-hexaprenyl-4-hydroxybenzoic acid, constituting 62 and 38% of the radioactive lipids, respectively. However, under log-phase growth conditions the biosynthetic intermediate 3-hexaprenyl-4-hydroxybenzoic acid predominates (accounting for 81% of the radioactive lipids). The data indicate that in wild-type yeast, 3-hexaprenyl-4-hydroxybenzoic acid forms a predominant intermediate pool in ubiquinone biosynthesis and that in log-phase growth this ubiquinone intermediate is present at fourfold higher abundance than the end product. The physiological rationale for this high concentration of a membrane-bound intermediate is unclear.