A protein inhibitor of the mitochondrial adenosine triphosphatase complex of rat liver. Purification and characterization

A heat-stable protein has been purified from rat liver mitochondria which inhibits the ATP hydrolytic activity of both the soluble and membrane-bound mitochondrial F1-ATPase. The overall purification is about 2400-fold with the major purification step consisting of Sephadex "affinity" chromatography. The purified rat liver inhibitor is homogeneous as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent molecular weight of 12,300. Amino acid analysis reveals a high content of glutamic acid, lysine, and arginine and the absence of cysteine, proline and methionine. Whether tested with the rat liver or bovine heart ATPase, the liver inhibitor is equally as potent and specific as the heart inhibitor preparation of Pullman and Monroy (Pullman, M.E., and Monroy, G.C. (1963) J. Biol. Chem. 238, 3762-3769). Although the results presented show that the rat liver ATPase inhibitor resembles closely the ATPase inhibitors from other tissues with respect to specific activity and reaction specificity, it is important to note that the rat liver inhibitor is almost 2000 daltons larger than the bovine heart inhibitor, about 5000 daltons larger than ATPase inhibitors of yeast, and contains significantly more lysine residues than both the bovine heart and yeast inhibitors.

Adenosine triphosphatase from rat liver mitochondria. Crystallization and x-ray diffraction studies of the F1-component of the enzyme

The homogeneous rat liver F1-ATPase preparation of Catterall and Pedersen (Catterall, W.A., and Pedersen, P.L. (1971) J. Biol. Chem. 246, 4987-4994) has been crystallized from a solution containing phosphate and ATP by precipitation with ammonium sulfate. Most of the resultant crystals are cubes of approximately 0.3 to 0.6 mm per side. X-ray precession photographs show that the crystals are rhombohedral, space group R32 (D37 NO155) with hexagonal cell dimensions a = 148 A, c = 368 A. The molecular weight of the asymmetric unit of the crystals is 190,000 or about half the molecular weight (384,000) of the rat liver enzyme indicating that the crystallographic 2-fold axes of symmetry coincide with a molecular symmetry axis. The crystals diffract to at least 3.5 A and therefore this is the first report of an ATPase preparation in which crystals suitable for x-ray analysis have been obtained.

High aerobic glycolysis of rat hepatoma cells in culture: role of mitochondrial hexokinase

A tumorigenic anchorage-dependent cell line (H-91) was established in culture from an azo-dye-induced rat ascites hepatoma. When grown in a glucose-containing medium the cells exhibit high rates of lactic acid production characteristic of rapidly growing tumor cells. However, when glucose is replaced with galactose the cells grow equally well but exhibit only moderately elevated rates of lactic acid production. The molecular basis for this observation cannot be attributed to differences in permeability because initial rates of glucose and galactose entry into hepatoma cells are identical. Rather, the activity of hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) is found to be high in hepatoma cells, about 20-fold higher than that of control and regenerating rat liver. Moreover, tumor hexokinase activity is not inhibited by low concentrations (<0.6 mM) of the reaction product glucose 6-phosphate. Additionally, 50% of the hexokinase activity of hepatoma cells is found associated with the mitochondrial fraction. This fraction is 3-fold enriched in hexokinase activity relative to the homogenate and 4-fold enriched relative to the nuclear and postmitochondrial fractions. Tumor mitochondrial hexokinase appears to be coupled directly to oxidative phosphorylation, because addition of glucose to respiring hepatoma mitochondria (after a burst of ATP synthesis) results in stimulation of respiration. In contrast, glucose has no effect on the respiration of mitochondria from control and regenerating liver. These results suggest that the high glycolytic capacity of H-91 hepatoma cells is due, at least in part, to an elevated form of hexokinase concentrated in the mitochondrial fraction of the cell.

Adenosine triphosphatase of rat liver mitochondria: detergent solubilization of an oligomycin- and dicyclohexylcarbodiimide-sensitive form of the enzyme

The hydrolytic activity of the ATPase bound to purified inner membrane vesicles of rat liver mitochondria can be increased threefold by washing extensively with a high ionic strength phosphate buffer. The specific ATPase activities of such phosphate-washed membranes are the highest reported to date for a mitochondrial membrane preparation (21-24 mumol of ATP hydrolyzed min-1 mg-1 in bicarbonate buffer at 37 degrees C). Deoxycholate (0.1 mg/mg of protein) extracts from these membranes a soluble, cold-stable ATPase complex which exhibits a specific activity under optimal assay conditions of 12 mumol of ATP hydrolyzed min-1 mg-1. This complex is not sedimented by centrifugation at 201000 g for 90 min, and readily passes through a 250-A Millipore filter. The ATPase activity of the soluble complex is inhibited 95% by 2.4 muM oligomycin. In addition, inhibitions of 60% or better are obtained in the presence of 1-8 muM dicyclohexylcarbodiimide, p-chloromercuribenzoate, venturicidin, and aurovertin. While a similar complex may be extracted with Triton X-100 this preparation is always lower in both specific activity and in inhibitor sensitivities than the complex extracted with deoxycholate. Detergents of the Tween and Brij series and other detergents of the Triton series are also much less effective than deoxycholate in solubilizing the oligomycin-sensitive. ATPase complex of rat liver. It is concluded that deoxycholate is superior to other detergents as an extractant of the oligomycin-sensitive ATPase complex of rat liver mitochondria, and that the complex extracted with deoxycholate possesses a closer similarity to the membrane-associated ATPase than does the complex extracted with Triton X-100. These studies document the first report of a detergent-solubilized, oligomycin-sensitive ATPase preparation from rat liver mitochondria.

ATP-dependent reactions catalyzed by inner membrane vesicles of rat liver mitochondria. Kinetics, substrate specificity, and bicarbonate sensitivity

Three ATP-dependent reactions catalyzed by the inner membrane of rat liver mitochondria and the ATPase reaction catalyzed by purified mitochondrial ATPase (F1), were studied with respect to kinetic properties, substrates specificity, and sensitivity to bicarbonate. The ATP-dependent transhydrogenase reaction (reduction of NADP+ by NADH) catalyzed by inner membrane vesicles displays typical Michaelis-Menten kinetics in both Tris-Cl and Tris-bicarbonate buffers, with Km (ATP) values of 0.035 mM and 0.054 mM respectively. The Vmax of transhydrogenase activity (25 nmol min-1 mg-1) is the same in Tris-bicarbonate or Tris-Cl buffer. ITP and GTP readily substitute for ATP in the transhydrogenase reaction. The ATP-P1 exchange reaction catalyzed by inner membrane vesicles displays typical Michaelis-Menten kinetics in both Tris-Cl and Tris-bicarbonate buffers with Km (ATP) values of 1.0 mM and 1.4 mM respectively. The Vmax of exchange (200 nmol min-1 mg-1) is the same in either buffer. ITP and GTP do not effectively replace ATP in the exchange reaction.

Phosphate transport in rat liver mitochondria. Kinetics, inhibitor sensitivity, energy requirements, and labeled components

Experiments were carried out to define the kinetic parameters of the major phosphate transport processes of rat liver mitochondria, and to obtain information about the molecular properties of these systems.

Phosphate transport in rat liver mitochondria. Membrane components labeled by N-ethylmaleimide during inhibition of transport

N-ethylmaleimide (NEM) inhibits the transport of phosphate in mitochondria but is without effect on permeation of other metabolities. In spite of its specificity for inhibition of phosphate transport, NEM reacts in an unspecific manner with inner membrane proteins in general. Treatment of mitochondria with [3H]NEM just sufficient to produce inhibition of phosphate transport results in labeling of at least 10 polypeptide components of the inner membrane. A marked increase in the specificity of reaction of NEM for components of the phosphate transport system is attained by first protecting the transport system with p-mercuribenzoate (p-MB) and then by irreversibly blocking reactive sulfhydryl groups unassociated with transport by the addition of unlabeled NEM. Subsequent addition of dithiothreitol removes p-MB and restores 65 to 75 percent of the original phosphate transport activity. Reinhibition of transport with [3H]NEM results in both a 6-fold decrease in the amount of [3H]NEM bound by purified inner membrane vesicles and a substantial reduction in the number of labeled polypeptide components. Five distinct labeled species are detected by this method, one of which is a 32,000 molecular weight protein containing 40 percent of the bound radioactivity, or approximately 160 pmol/mg of inner membrane protein. Correlation of binding of [3H]NEM by inner membrane proteins with inhibition of phosphate transport suggests that the maximum concentration of the NEM-sensitive component of the phosphate transport system is 60 pmol/mg of mitochondrial protein. This value, when combined with V-max of NEM-sensitive transport of 205 nmol times min-1 times mg-1 at O degrees (Coty, W. A., and Pedersen, P. L. (1974) J. Biol. Chem. 249, 2593) yields an approximate minimum turnover for this process of 3500 min-1 at 0 degrees. This turnover number is at least 20-fold greater than similarly calculated values for adenine nucleotide transport and succinate oxidation in rat liver mitochondria at this temperature. Taken together these results suggest that the NEM-sensitive phosphate transport system in rat liver mitochondria has an unusually high catalytic activity compared to other mitochondrial processes, and that at least one of the five NEM-binding proteins is likely to be an essential component of this transport system.

Interaction of homogeneous mitochondrial ATPase from rat liver with adenine nucleotides and inorganic phosphate

Mitochondrial ATPase from rat liver mitochondria contains multiple nucleotide binding sites. At low concentrations ADP binds with high affinity (1 mole/mole ATPase, KD = 1-2 muM). At high concentrations, ADP inhibits ATP hydrolysis presumably by competing with ATP for the active site (KI = 240-300 muM). As isolated, mitochondrial ATPase contains between 0.6 and 2.5 moles ATP/mole ATPase. This "tightly bound" ATP can be removed by repeated precipitations with ammonium sulfate without altering hydrolytic activity of the enzyme. However, the ATP-depleted enzyme must be redissolved in high concentrations of phosphate to retain activity. AMP-PNP (adenylyl imidodiphosphate) replaces tightly bound ATP removed from the enzyme and inhibits ATP hydrolysis. AMP-PNP has little effect on high affinity binding of ADP. Kinetics studies of ATP hydrolysis reveal hyperbolic velocity vs. ATP plots, provided assays are done in bicarbonate buffer or buffers containing high concentrations of phosphate. Taken together, these studies indicate that sites on the enzyme not directly associated with ATP hydrolysis bind ATP or ADP, and that in the absence of bound nucleotide, Pi can maintain the active form of the enzyme.

Deficiency of uncoupler-stimulated adenosine triphosphatase activity in tightly coupled hepatoma mitochondria

Tightly coupled mitochondria from the well-differentiated hepatoma 7800 failed to exhibit a significant 2,4-dinitrophenol-activated ATPase activity at concentrations of uncoupler sufficient to completely inhibit oxidative phosphorylation. ATPase activity could not be maximally activated by uncoupling agents more potent than 2,4-dinitrophenol, such as carbonylcyanide p-trifluoromethoxyphenylhydrazone and 5-chloro, 3-tert-butyl, 2'-chloro, 4'-nitrosalicylanilide, nor by Mg(++) after the following treatments: sonication, freezing, detergent lysis, and digestion with trypsin. Gel electrophoresis patterns of the membrane proteins of the hepatome mitochondria revealed neither an absence of any one of the three different types of ATPase subunits characteristic of the homogeneous enzyme purified from normal liver mitochondria, nor a deficiency of the oligomeric molecule. Taken together, these data strongly suggest that the supramolecular structure of the membrane ATPase complex of mitochondria from hepatoma 7800 is altered in such a way that its capacity for ATP hydrolysis is severely diminished.

Biochemical and ultrastructural properties of a mitochondrial inner membrane fraction deficient in outer membrane and matrix activities

Treatment of the inner membrane matrix fraction of rat liver mitochondria with the nonionic detergent Lubrol WX solubilized about 70% of the total protein and 90% or more of the following matrix activities: malate dehydrogenase, glutamate dehydrogenase, and isocitrate dehydrogenase (NADP). The Lubrol-insoluble fraction was enriched in cytochromes, phospholipids, and a Mg(++)-stimulated ATPase activity. Less than 2% of the total mitochondrial activity of monoamine oxidase, an outer membrane marker, or adenylate kinase, an intracristal space marker could be detected in this inner membrane fraction. Electron micrographs of negatively stained preparations showed vesicles (</=0.4 micro diameter) literally saturated on the periphery with the 90 A ATPase particles. These inner membrane vesicles, which appeared for the most part to be inverted with respect to the normal inner membrane configuration in intact mitochondria, retained the succinicoxidase portion of the electron-transport chain, an intact phosphorylation site II with a high affinity for ADP, and the capacity to accumulate Ca(++). A number of biochemical properties characteristic of intact mitochondria and the inner membrane matrix fraction, however, were either absent or markedly deficient in the inner membrane vesicles. These included stimulation of respiration by either ADP or 2,4-dinitrophenol, oligomycin-sensitive ADP-ATP exchange activity, atractyloside sensitivity of adenine nucleotide requiring reactions, and a stimulation of the Mg(++)-ATPase by 2,4-dinitrophenol.