Determination of the H+/site and Ca2+/site ratios of mitochondrial electron transport

Chemical Insights into Oxidative and Nitrative Modifications of DNA

This review focuses on DNA damage caused by a variety of oxidizing, alkylating, and nitrating species, and it may play an important role in the pathophysiology of inflammation, cancer, and degenerative diseases. Infection and chronic inflammation have been recognized as important factors in carcinogenesis. Under inflammatory conditions, reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from inflammatory and epithelial cells, and result in the formation of oxidative and nitrative DNA lesions, such as 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 8-nitroguanine. Cellular DNA is continuously exposed to a very high level of genotoxic stress caused by physical, chemical, and biological agents, with an estimated 10,000 modifications occurring every hour in the genetic material of each of our cells. This review highlights recent developments in the chemical biology and toxicology of 2'-deoxyribose oxidation products in DNA.

Mechanistic and phenomenological features of proton pumps in the respiratory chain of mitochondria

Various direct, indirect (kinetic and thermodynamic), and combined mechanisms have been proposed to explain the conversion of redox energy into a transmembrane protonmotive force (delta p) by enzymatic complexes of respiratory chains. The conceptual evolution of these models is examined. The characteristics of thermodynamic coupling between redox transitions of electron carriers and scalar proton transfer in cytochrome c oxidase and its possible involvement in proton pumping is discussed. Other aspects dealt with in this paper are: (i) variability of <--H+/e- stoichiometries, in cytochrome c oxidase and cytochrome c reductase and its mechanistic implications; (ii) possible models by which the reduction of dioxygen to water at the binuclear heme-copper center of protonmotive oxidases can be directly involved in proton pumping. Finally a unifying concept for proton pumping by the redox complexes of respiratory chain is presented.

Modification of the transport of protons and Ca2+ ions across mitochondrial coupling membrane by N-(phosphonomethyl)glycine

The proton permeability of mitochondrial membranes suspended in 0.15 N NH4Cl was enhanced by N-(phosphonomethyl)glycine (PMG), a broad-spectrum and a non-selective herbicide, in a concentration-dependent manner. Significant decreases in light scattering by these membranes were observed at concentrations greater than or equal to 600 microM PMG. The effect of PMG is therefore several times lower than that of FCCP, a classical uncoupler of oxidative phosphorylation. Using a sensitive pH-glass electrode, PMG significantly enhanced the movement of protons into mitochondrial matrix. Furthermore, the rate of PMG-induced release of Ca2+ ions following its accumulation by energized mitochondria was only slightly over one-half that induced by FCCP (1 microM). Whereas Ca2+ or Mg2+ only marginally reduced the effect induced by PMG, inclusion of glycine into the reaction media did not have any influence whatsoever on the effect induced by PMG. These results indicate that, although PMG increases the permeability of the mitochondrial membrane to protons and to Ca2+, the herbicide does not seem to act like a true protonophore. Its uncoupling effect may, therefore, be due to its ability to act both as a chelator and a mild protonophore.

Comparative effects of three naturally occurring furanocoumarins on mitochondrial bioenergetics

Oxygraphic measurements of the rates of mitochondrial respiration in the presence of varying amounts of chalepin, imperatorin and marmesin, three naturally occurring furanocoumarins, revealed that the oxidation of NAD(+)-linked substrates was inhibited by chalepin and imperatorin and less significantly by marmesin. The order of potency being rotenone much greater than chalepin imperatorin greater than marmesin. There was no effect whatsoever on succinate oxidation by the furanocoumarins tested (up to 60 microM). State 3 respiration was also inhibited by these furanocoumarins; by at least 80% by 10 microM chalepin and by 48 and 29% with 60 microM imperatorin and 60 microM marmesin, respectively. Consequently, ADP control of respiration was diminished by those concentrations of furanocoumarins that inhibited respiration. At 60 microM, respiratory control ratio was reduced by about 88, 49 and 28% with chalepin, imperatorin and marmesin, respectively. A measurement of the rate of proton and Ca2(+)-movements across the mitochondrial coupling membrane demonstrated that succinate-supported transport was not affected by these furanocoumarins. On the other hand, pyruvate/malate-supported proton ejection was significantly inhibited by chalepin, imperatorin and marmesin. The order of the degree of inhibition of proton flux is rotenone much greater than chalepin greater than imperatorin greater than marmesin. The pattern of the inhibition of pyruvate/malate-supported Ca2(+)-transport was identical to that seen during proton transport. A comparison of the effects of chalepin to that of rotenone suggests that chalepin might be about 10 times less potent than rotenone.

Protonmotive stoichiometry of rat liver cytochrome c oxidase: determination by a new rate/pulse method

The stoichoimetry of vectorial H+ ejection coupled to electron flow through the cytochrome c oxidase (EC 1.9.3.1) of rat liver mitochondria was determined by a new rate/pulse method. This is a modification of the oxygen-pulse method. Electron flow through the oxidase is initiated by adding oxygen to suspensions of anaerobic mitochondria at a known and constant rate. Cytochrome c oxidase was examined directly or in combination with cytochrome c reductase (ubiquinol:ferricytochrome c oxidoreductase). In both cases the----H0+/2e- ratio was found to be constant during the time-course of oxygen reduction, and thus independent of delta pH. The stoichiometries observed were consistent with mechanistic stoichiometries of 2 and 6 for cytochrome c oxidase alone and cytochrome c oxidase together with cytochrome c reductase, respectively. The stoichiometry of cytochrome c reductase alone was also examined, by using ferricyanide in place of oxygen. The results obtained were consistent with the accepted mechanistic stoichiometry of 4 for this enzyme.

Determination of the orientation of membrane vesicles derived from mitochondria

Membrane vesicles of physiological as well as inverted orientation can be isolated from mitochondria. The presence of these vesicles in a membrane can be determined and quantitated by determining the differences between the two vesicle types in terms of rates of NADH oxidation, rates of oxidation of tricarboxylate cycle intermediates, rates of ATP hydrolysis and sensitivity to inhibitors, stimulation of respiration by exogenous cytochrome c, inhibition of respiration by polycationic proteins, and visualization of the ATPase by electron microscopy. Procedures to isolate the two membrane types and characteristics of homogeneously oriented preparations are described. Differences in data obtained with homogeneous vesicle preparations and with vesicles of mixed orientation are illustrated. Nonhomogeneously oriented preparations can be enriched in the desired vesicular type by the use of immunoprecipitation, affinity chromatography, and differential centrifugation. The concept of a hybrid vesicle containing oppositely oriented regions is not supported by experimental data.

Sensitivity to NN'-dicyclohexylcarbodi-imide of proton translocation by mitochondrial NADH:ubiquinone oxidoreductase

Proton extrusion during ferricyanide reduction by NADH-generating substrates or succinate was studied in isolated rat liver mitochondria with the use of optical indicators. NN'-Dicyclohexylcarbodi-imide (DCCD) caused a decrease of 84% in the H+/e- ratio of NADH:cytochrome c reduction, but a decrease of only 49% in that of succinate:cytochrome c reduction, even though electron transfer was decreased equally in both spans. The data indicate that a DCCD-sensitive channel operates in the NADH:ubiquinone oxidoreductase region of the respiratory chain.