Correlations between ATP hydrolysis, ATP synthesis, generation and utilization of delta pH in mitochondrial ATPase-ATP synthase

Inhibition of respiratory complex I by 6-ketocholestanol: Relevance to recoupling action in mitochondria

6-Ketocholestanol (kCh) is known as a mitochondrial recoupler, i.e. it abolishes uncoupling of mitochondria by such potent agents as carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 3,5-di(tert-butyl)-4-hydroxybenzylidenemalononitril (SF6847) [Starkov et al., 1997]. Here, we report data on the kCh-induced inhibition of both NADH-oxidase and NADH-ubiquinone oxidoreductase activities of the respiratory complex I in bovine heart submitochondrial particles (SMP). Based on the absence of such inhibition with hexaammineruthenium (III) (HAR) as the complex I electron acceptor, the kCh effect could be associated with the ubiquinone-binding centre of this respiratory enzyme. In isolated rat liver mitochondria (RLM), kCh inhibited oxygen consumption with the glutamate/malate, substrates of NAD-linked dehydrogenases, while no inhibition of RLM respiration was observed with succinate, in agreement with the absence of the kCh effect on the succinate oxidase activity in SMP. Three kCh analogs (cholesterol, 6α-hydroxycholesterol, and 5α,6α-epoxycholesterol) exhibited no effect on the NADH oxidase activities in both SMP and RLM. Importantly, the kCh analogs were ineffective in the recoupling of RLM treated with CCCP or SF6847. Therefore, interaction of kCh with the complex I may be involved in the kCh-mediated mitochondrial recoupling.

Antibiotic Pyrrolomycin as an Efficient Mitochondrial Uncoupler

Pyrrolomycins C (Pyr_C) and D (Pyr_D) are antibiotics produced by Actinosporangium and Streptomyces. The mechanism of their antimicrobial activity consists in depolarization of bacterial membrane, leading to the suppression of bacterial bioenergetics through the uncoupling of oxidative phosphorylation, which is based on the protonophore action of these antibiotics [Valderrama et al., Antimicrob. Agents Chemother. (2019) 63, e01450]. Here, we studied the effect of pyrrolomycins on the isolated rat liver mitochondria. Pyr_C was found to be more active than Pyr_D and uncoupled mitochondria in the submicromolar concentration range, which was observed as the mitochondrial membrane depolarization and stimulation of mitochondrial respiration. In the case of mitoplasts (isolated mitochondria with impaired outer membrane integrity), the difference in the action of Pyr_C and Pyr_D was significantly less pronounced. By contrast, in inverted submitochondrial particles (SMPs), Pyr_D was more active as an uncoupler, which caused collapse of the membrane potential even at the nanomolar concentrations. The same ratio of the protonophoric activity of Pyr_D and Pyr_C was obtained by us on liposomes loaded with the pH indicator pyranine. The protonophore activity of Pyr_D in the planar bilayer lipid membranes (BLMs) was maximal at ~pH 9, i.e., at pH values close to pK_a of this compound. Pyr_D functions as a typical anionic protonophore; its activity in the BLM could be reduced by the addition of the dipole modifier phloretin. The difference between the protonophore activity of Pyr_C and Pyr_D in the mitochondria and BLMs can be attributed to a higher ability of Pyr_C to penetrate the outer mitochondrial membrane.

The mitochondria-targeted derivative of the classical uncoupler of oxidative phosphorylation carbonyl cyanide m-chlorophenylhydrazone is an effective mitochondrial recoupler

The synthesis of a mitochondria-targeted derivative of the classical mitochondrial uncoupler carbonyl cyanide-m-chlorophenylhydrazone (CCCP) by alkoxy substitution of CCCP with n-decyl(triphenyl)phosphonium cation yielded mitoCCCP, which was able to inhibit the uncoupling action of CCCP, tyrphostin A9 and niclosamide on rat liver mitochondria, but not that of 2,4-dinitrophenol, at a concentration of 1–2 μM. MitoCCCP did not uncouple mitochondria by itself at these concentrations, although it exhibited uncoupling action at tens of micromolar concentrations. Thus, mitoCCCP appeared to be a more effective mitochondrial recoupler than 6-ketocholestanol. Both mitoCCCP and 6-ketocholestanol did not inhibit the protonophoric activity of CCCP in artificial bilayer lipid membranes, which might compromise the simple proton-shuttling mechanism of the uncoupling activity on mitochondria.

The role of internal urease in acid resistance of Helicobacter pylori

BACKGROUND & AIMS

The relative role of internal urease for acid protection of Helicobacter pylori is unknown. The aim of this study was to determine the comparative importance of internal and external urease under acidic conditions.

METHODS

The pH optimum and measured Michaelis constant for urea of external urease and urease in intact bacteria at different medium pH (pHout) were measured using 14CO2 release from 14C-urea. The effect of urea on membrane potential and bacterial cytoplasmic pH was measured at different fixed pHout. 35S-methionine labeling and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of labeled proteins in the organism and medium measured protein synthesis at different pHout and mechanisms of urease externalization.

RESULTS

External urease had activity between pH 5.0 and 8.5 and internal urease between pHout 2.5 and 6.5, and its Michaelis constant at pHout 7.5 was 300 mmol/L but at pHout 4.5 was 0.5 mmol/L, similar to free urease. The addition of 5 mmol/L urea to bacteria at fixed pHout from 3.0 to 6.0 elevated potential to about -105 mV and periplasmic pH to about pH 6.2. Protein synthesis occurred mainly between pH 6.5 and 8.0, and urease activity resulted in increased protein synthesis at acidic pH. The labeling pattern of intrabacterial and released protein was similar.

CONCLUSIONS

Intracellular urease activity is regulated by external pH, defends against gastric acidity by increasing periplasmic pH and membrane potential, and stimulates protein synthesis at acidic pH. External urease is produced mostly by cell lysis.

Further study on the role of Mg2+ in lipid-protein interaction in reconstituted porcine heart mitochondrial H+-ATPase

Porcine heart mitochondrial H+-ATPase was reconstituted by cholate dialysis method in liposomes containing neutral (PC, PE), acidic (PG, PI, PA, PS, DPG) or neutral and acidic phospholipids. The Mg2+ effect on the ATPase activity and its sensitivity to oligomycin, ATP-induced delta psi and delta pH formation was observed for the proteoliposomes containing acidic but not neutral phospholipids. Maleimide spin labels with varying arm lengths or bromoacetamide spin probe were used to monitor the conformational difference of H+-ATPase in the Mg2+-containing and Mg2+-'free' samples. A difference in W/S ratio (weakly immobilized/strongly immobilized component in the ESR spectra) could be detected for the F0.F1-containing and F1-depleted, (F0)-containing proteoliposomes, suggesting conformational difference in the F0-F1 complex and F0 portion induced by the Mg2+ effect. A conformational change of the beta-subunits in the F1 portion was also deduced from the ATP-induced fluorescence quenching of aurovertin-complex for Mg2+-containing samples. The results obtained are in favor of our previous assumption that Mg2+ may play its role by altering the physical state of the lipid bilayer, which would induce a conformational change in F0 (buried in the lipid core), which in turn is transmitted to the catalytic F1, resulting in a higher enzyme activity.

Efficient reconstitution of mitochondrial energy-transfer reactions from depleted membranes and F1-ATPase as a function of the amount of bound oligomycin sensitivity-conferring protein (OSCP)

Pig heart mitochondrial membranes depleted of F1 and OSCP by various treatments were analyzed for their content in alpha and beta subunits of F1 and in OSCP using monoclonal antibodies. Membrane treatments and conditions of rebinding of F1 and OSCP were optimized to reconstitute efficient NADH- and ATP-dependent proton fluxes, ATP synthesis and oligomycin-sensitive ATPase activity. F1 and OSCP can be rebound independently to depleted membranes but to avoid unspecific binding of F1 to depleted membranes (ASUA) which is not efficient for ATP synthesis, F1 must be rebound before the addition of OSCP. The rebinding of OSCP to depleted membranes reconstituted with F1 inhibits the ATPase activity of rebound F1, while it restores the ATP-driven proton flux measured by the quenching of ACMA fluorescence. The rebinding of OSCP also renders the ATPase activity of bound F1 sensitive to uncouplers. The rebinding of OSCP alone or F1 alone, does not modify the NADH-dependent proton flux, while the rebinding of both F1 and OSCP controls this flux, inducing an inhibition of the rate of NADH oxidation. Similarly, oligomycin, which seals the F0 channel even in the absence of F1 and OSCP, inhibits the rate of NADH oxidation. OSCP is required to adjust the fitting of F1 to F0 for a correct channelling of protons efficient for ATP synthesis. All reconstituted energy-transfer reactions reach their optimal value for the same amount of OSCP. This amount is consistent with a stoichiometry of two OSCP per F1 in the F0-F1 complex.

Structural aspects and orientation mechanism of mitochondrial F1 adenosinetriphosphatase. Evidence for a negative electric birefringence due to a permanent moment perpendicular to the long axes of the particle

The electric birefringence technique was used to investigate the steady-state birefringence, the orientational relaxation time, and the orientation mechanism of pig heart mitochondrial F1 adenosine-5'-triphosphatase (F1-ATPase). The electrooptical properties of this enzyme in solution were studied as functions of pH, protein concentration, and applied electric field. The F1-ATPase exhibits a surprising negative electric birefringence with a specific Kerr constant of -1.5 X 10(-3) esu cgs. The field-independent relaxation time was found to be 0.65 +/- 0.05 microseconds, corresponding to a rotational diffusion constant of 2.55 X 10(5) s-1. The overall size and shape of F1-ATPase have been calculated from both translational and rotational diffusion constants. The enzyme may be assumed to be an oblate ellipsoid of revolution with dimensions of about 170 X 170 X 70 A. The orientation mechanism of F1-ATPase was analyzed by fitting experimental birefringence rising curves with theoretical rising functions. The ratio of the permanent to induced dipole moment is found to be very high; therefore, the birefringence of F1-ATPase is due to a strong permanent dipole moment in a direction perpendicular to the long axes of the particle. These particular electric properties can be explained by the oligomeric structure of the protein and seem likely to play a role in its mechanism of functioning.

Structure and function of the ATPase-ATP synthase complex of mitochondria as compared to chloroplasts and bacteria

An overview of the structure and function of the mitochondrial ATPase-ATP synthase complex is presented. Attempts are made to identify the analogies and differences between mitochondrial, chloroplastic and bacterial complexes. The relatively more precise information available on the structure of the E. coli enzyme is used to try and understand the apparently more complex structure of the mitochondrial enzyme. Recent ideas on the mechanism of ATP hydrolysis and ATP synthesis will be summarized.

Monoclonal antibodies to mitochondrial F1-ATPase and oligomycin sensitivity conferring protein (OSCP). Tools for recognition of well conserved and essential antigenic sites

The preparation of anti-OSCP monoclonal antibodies is described for the first time. One of these antibodies prevents the activating effect of OSCP in reconstitution experiments. These antibodies and antibodies previously obtained against the alpha- and beta-subunits of pig heart mitochondrial F1-ATPase have been used to look for well conserved epitopes in various species. One anti-beta antibody can recognize all species tested while the anti-OSCP antibodies only recognize the pig or beef enzyme. The above anti-beta antibody inhibits ATP synthesis without modifying the rate of ATP hydrolysis. This antibody also prevents the ADP-induced hysteretic inhibition of F1-ATPase.