Proton translocation coupled to quinol oxidation in ox heart mitochondria

The suitability of ubiquinol(1) and duroquinol as pulse reductants for initiating respirationdriven proton translocation by aerobic ox heart mitochondria was investigated. At 25 degrees C the V(max.) for oxidation was close to 280nmol of quinol oxidized/min per mg of protein, and the K(m) values were 8mum for ubiquinol(1) and 28mum for duroquinol. Pulses of ubiquinol(1) and duroquinol were rapidly and completely oxidized by aerobic mitochondria with a simultaneous acidification of the suspending medium as detected with a glass electrode. The -->H(+)/2e(-) ratios (Mitchell, 1966) calculated from the observed extent of acidification and the amount of quinol added were 3.62 for ubiquinol(1) and 2.98 for duroquinol. These values are underestimates of the true value owing to proton back-flow across the membrane. An analogue computer model was used to correct the observed extent of respirationdriven acidification for proton back-flow. The corrected -->H(+)/2e(-) values were 4.01 for ubiquinol and 3.86 for duroquinol oxidation. Attempts to measure the rate of proton translocation with a pH-measuring system with a response time of 0.4s were not entirely satisfactory, owing to the relative slowness of the electrode response. Nevertheless the maximal rate of proton generation during ubiquinol(1) oxidation was about 1200ng-ions of H(+)/min per mg of mitochondrial protein. It is concluded, contrarily to Chance & Mela (1967), that mitochondria exhibit a proton-translocating ubiquinol oxidase activity with a -->H(+)/2e(-) ratio of 4.0.     

Characterization of autoantigenic sites on isolated dog heart mitochondria.

1. Anti-heart mitochondria autoantibodies were developed in serum from dogs following experimental myocardial infarction. 2. Heart mitochondria frozen and thawed repeatedly in a sucrose/Tris-chloride buffer retained both their functional integrity as measured by the respiratory control ratio and their ability to serve as an antigen in a complement fixation test. Mitochondria frozen and thawed in a potassium chloride/Tris-chloride buffer lost both their functional integrity and their autoantigenic activity after one freeze-thaw cycle. 3. Extraction of the heart mitochondria with acetone/water mixtures to remove phospholipids from the membrane led to a complete loss of the ability of the mitochondria to react in the complement fixation test but did not affect the ability of the membranes to bind autoantibody in absorption experiments. 4. Treatment of the mitochondrial membranes with increasing concentrations of trypsin caused a loss of up to approximately 50% of the membrane protein with a gradual decrease in the autoantigenic activity of the membrane without impairment of the ability of the membrane to bind autoantibody. 5. Removal of up to 90% of the sialic acid of the mitochondrial membrane with neuraminidase resulted in a considerable increase in the complement-fixing autoantigenic activity of the membrane without changing the apparent ability of the membrane to bind autoantibody in absorption experiments. 6. Exposure of mitochondrial membranes to autoantibody and complement caused an inhibition of both an inner mitochondrial membrane enzyme, i.e. cytochrome oxidase (48%) and an outer mitochondrial membrane enzyme, i.e. NADH cytochrome c reductase (rotenone insensitive) (37%).     

The molecular weight and thiol residues of acetyl-coenzyme A synthetase from ox heart mitochondria

1. A constant molecular weight of 57000 was obtained by gel filtration of highly purified acetyl-CoA synthetase over a 1000-fold range of enzyme concentrations. The amino acid analysis is reported. 2. With native enzyme at 20 degrees C the relatively rapid reaction of four thiol residues with p-hydroxymercuribenzoate caused an immediate inhibition reversible by either CoA or mercaptoethanol. Other substrates did not protect against this rapid inhibition. 3. The much slower reaction of the remaining four thiol residues was independent of the concentration of the mercurial, first-order with respect to enzyme, and had a large energy of activation (+136kJ/mol), suggesting that a conformation change in the protein was rate-limiting. This slow phase of the reaction was accompanied by an irreversible inactivation of the enzyme. 4. The effects of substrates on this irreversible inactivation at pH7.0 in 5 mm-MgCl(2) indicated strong binding of ATP and pyrophosphate by the enzyme (concentrations for half-maximal effects, K((1/2)), were <30mum and <10mum respectively) and weaker binding of acetyl-CoA (K((1/2)) about 1 mm), AMP (K((1/2)) about 2mm) and acetate. In the presence of acetate, MgCl(2) and p-hydroxymercuribenzoate, titration of the enzyme with ATP revealed at least two ATP binding sites/mol. 5. The experiments suggest that reaction of the thiol residues with mercurial causes loss of enzymic activity by altering the structure of the enzyme, rather than that the thiol residues play a direct role in the catalysis.     

Isolation and primary characterization of two forms of inorganic pyrophosphatase from ox heart muscle mitochondria

A method allowing to obtain two highly purified forms (membrane-bound and soluble ones) of inorganic pyrophosphatase from bovine heart mitochondria is described. Both forms have the isoelectric point of 5.8, pH optimum with Mg2+ at 7--9, are maximally stable at pH 5.8 and absolutely specific to PPi and require Mg2+ or Co2+ for their activity. The soluble pyrophosphatase is also activated by Zn2+. Besides, the two forms differ in their electrophoretic mobilities, affinity for DEAE cellulose and stability in acidic (pH less than 5.8) and alkaline media.     

Associative properties of butyryl-coenzyme A synthetase from ox liver mitochondria.

Butyryl-coenzyme A synthetase (butyrate:CoA ligase (AMP-forming), EC 6.2.1.2) from an acetone-dried powder of ox liver mitochondria was found to have a molecular weight of approx. 40 000. The sedimentation equilibrium analysis suggested the presence in solution of higher molecular weight forms of the enzyme and these could also be obtained by extracting the enzyme from the mitochondrial powder in non-reducing conditions. The enzyme was inhibited by sulphydryl reagents, and was found to have at least one available thiol group/molecule. The relationship between enzymic activity and concentration was non-linear, and suggested that an inactive monomer-active dimer equilibrium was present. The 5--6-fold activation by bovine serum albumin required the presence of free thiol groups in the albumin and involved association of albumin with the enzyme.     

Characterization of the inhibitor sensitivity of the coenzyme a transport system in isolated rat heart mitochondria

The effect of protein labeling agents on coenzyme A (CoA) transport into isolated rat heart mitochondria was studied. CoA transport was substantially inhibited by sulfhydryl reagents (mersalyl, pCMB) as well as by the tyrosine-selective reagent N-acetylimidazole. The effect of pCMB was reversed by DTT. Moreover, CoA uptake was completely abolished by agents selective for lysine and amino terminal residues (pyridoxal 5-phosphate, dansyl chloride). In contrast arginine-selective reagents (2, 3-butanedione, phenylglyoxal) caused considerably less inhibiton of CoA uptake. Moreover, partial inhibition of transport was observed with the stilbene disulfonic acid derivatives DIDS and SITS. Finally, measurement of the effects of the labeling agents on the mitochondrial membrane potential indicated that the inhibition of CoA transport into mitochondria is not a secondary effect that arises from an alteration in the electric potential gradient across the inner mitochondrial membrane. These results provide the first information on the types of amino acid residues that may be essential to the CoA transport mechanism and provide additional support for the existence of a CoA transport protein within the mitochondrial inner membrane. Furthermore, the identification of effective inhibitors of the CoA transport system will greatly facilitate the functional reconstitution of this transporter in a proteoliposomal system following its solubilization and purification.     

Energy-linked nicotinamide-nucleotide transhydrogenase. Characterization of reconstituted ATP-driven transhydrogenase from beef heart mitochondria

The interaction between pure transhydrogenase and ATPase (Complex V) from beef heart mitochondria was investigated with transhydrogenase-ATPase vesicles in which the two proteins were co-reconstituted by dialysis or dilution procedures. In addition to phosphatidylcholine and phosphatidylethanolamine, reconstitution required phosphatidylserine and lysophosphatidylcholine. Transhydrogenase-ATPase vesicles catalyzed a 20-30-fold stimulation of the reduction of NADP+ or thio-NADP+ by NADH and a 70-fold shift of the apparent equilibrium expressed as the nicotinamide nucleotide ratio [NADPH][NAD+]/[NADP+][NADH]. In both of these respects, the transhydrogenase-ATPase vesicles were severalfold more efficient than beef heart submitochondrial particles. By measuring the ATP-driven transhydrogenase and the oligomycin-sensitive ATPase activities simultaneously and under the same conditions at low ATP concentrations, i.e. below 15 microM, the ATP-driven transhydrogenase/oligomycin-sensitive ATPase activity ratio was found to be about 3. This value is consistent with the stoichiometries of three protons translocated per ATP hydrolyzed and one proton translocated per NADPH formed and with a mechanism where the two enzymes interact through a delocalized proton-motive force.     

Interaction of F1-ATPase, from ox heart mitochondria with its naturally occurring inhibitor protein. Studies using radio-iodinated inhibitor protein

The ox heart mitochondrial inhibitor protein may be iodinated with up to 0.8 mol 125I per mol inhibitor with no loss of inhibitory activity, with no change in binding affinity to submitochondrial particles, and without alteration in the response of membrane-bound inhibitor to energisation. Tryptic peptide maps reveal a single labelled peptide, consistent with modification of the single tyrosine residue of the protein. A single type of high-affinity binding site (Kd=96 . 10 (-9)M) for the inhibitor protein has been measured in submitochondrial particles. The concentration of this site is proportional to the amount of membrane-bound F1, and there appears to be one such site per F1 molecule. The ATp hydrolytic activity of submitochondrial particles is inversely proportional to the occupancy of the high-affinity binding site for the inhibitor protein. No evidence is found for a non-inhibitory binding site on the membrane or on other mitochondrial proteins. In intact mitochondria from bovine heart, the inhibitor protein is present in an approx. 1:1 ratio with F1. Submitochondrial particles prepared by sonication of these mitochondria with MgATP contain about 0.75 mol inhibitor protein per mol F1, and show about 25% of the ATPase activity of inhibitor-free submitochondrial particles. Additional inhibitor protein can be bound to these particles to a level of 0.2 mol/mol F1, with consequent loss of ATPase activity. If MgATP is omitted from the medium, or inhibitors of ATP hydrolysis are present, the rate of combination between F1 and its inhibitor protein is very much reduced. The equilibrium level of binding is, however, unaltered. These results suggest the presence of a single, high-affinity, inhibitory binding site for inhibitor protein on membrane-bound F1. The energisation of coupled submitochondrial particles by succinate oxidation or by ATP hydrolysis results in both the dissociation of inhibitor protein into solution, and the activation of ATP hydrolysis. At least 80% of the membrane-bound F1-inhibitor complex responds to this energisation by participating in a new equilibrium between bound and free inhibitor protein. This finding suggests that a delocalised energy pool is important in promoting inhibitor protein release from F1. Dissipation of the electrochemical gradient by uncouplers, or the binding of oligomycin or efrapetin effectively blocks energised release of the inhibitor protein. Conversely, the addition of aurovertin or adenosine 5'--[beta, lambda--imido]triphosphate enhances energy-driven release. The mode of action of various inhibitors on binding and energised release of the protein inhibitor is discussed.     

Ion transport by heart mitochondria. Retention and loss of energy coupling in aged heart mitochondria

The retention and loss of energy-coupling reactions in isolated beef heart mitochondria have been examined under anaerobic conditions using suspending media chosen to mimic the intracellular milieu. In long-term incubations at 37 °C, a loose coupling develops which can be controlled by adding serum albumin. This lesion closely resembles that produced by addition of free fatty acids which has been described in previous studies. Shorter incubation times produce an increased susceptibility to hydrogen peroxide which is characterized by elevated ATPase activity, increased permeability to monovalent cations, and increased proton ejection on transition from the anaerobic to the aerobic state. This peroxide sensitivity is prevented by chelators such as EGTA and appears to involve a time-dependent release of metal ions. Of the metabolites which are known to increase in concentration in the ischemic heart cell, Na⁺, P₁, lactate, and H⁺ all promote swelling of isolated heart mitochondria and contribute to a decline in energy coupling. The relationship of these results to the pathological deterioration of mitochondria in ischemic heart tissue is discussed.     

NADH-dependent respiration in osmotically inactive swollen mitochondria: does transport replace phosphorylation in mediating respiratory control in swollen mitochondria?

Rotenone-sensitive, uncoupler-insensitive, NADH-dependent respiration was demonstrated in osmotically inactive fragments of the mitochondrial inner-membrane obtained following high amplitude (spontaneous) swelling. This NADH-dependent respiration as well as mitochondrial ATPase activity was stimulated by ligands which are known to be transported by specific transporters/mechanisms. The ligands capable of this anomalous respiratory control included several intermediates of the citric acid cycle, besides non-metabolizable ligands including lactate, cations such as K+ and Ca2+. The interaction between NADH-dependent respiration and these ligands, as manifested by stimulation of respiration, was strongly ionic strength-dependent. The thermodynamic relationship between respiratory control and stimulation of transport ATPase by the relevant transportable ligands could also be demonstrated in the conventional (rat liver) microsomes. These experimental results offer a novel experimental base for search into an intra-membranous mechanism of energy transduction.     

The isotope-exchange reactions of ox heart phosphofructokinase

1. Ox heart phosphofructokinase catalyses isotope-exchange reactions at pH6.7 between ADP and ATP, and between fructose 6-phosphate and fructose 1,6-diphosphate, the latter reaction being absolutely dependent on the presence of the magnesium complex of ADP. 2. The reaction kinetics are hyperbolic with respect to substrate concentration for both exchange reactions (within the experimental error). 3. The influence of pH, AMP and citrate suggests that the fructose 6-phosphate-fructose 1,6-diphosphate exchange is subject to effector control, and is abolished by dissociation of the enzyme. 4. These results are discussed in relation to the reaction mechanism of the enzyme.