Energy-linked quinacrine fluorescence changes in submitochondrial particles from skeletal muscle mitochondria. Evidence for intramembrane H+ transfer as a primary reaction of energy coupling

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Carbonic anhydrase in guinea pig skeletal muscle mitochondria

The presence of carbonic anhydrase activity was demonstrated in guinea pig skeletal muscle mitochondria purified by Percoll gradient centrifugation such that contamination by sarcoplasmic reticulum vesicles was less than 5%. Assay of purified heavy sarcoplasmic reticulum vesicles for carbonic anhydrase activity showed these to have somewhat less activity than the mitochondria, so that any contribution by sarcoplasmic reticulum vesicles to mitochondrial activity would be negligible. In agreement with this observation, rabbit skeletal muscle mitochondria prepared by the Percoll method had no detectable activity. Assay of the guinea pig muscle mitochondrial enzyme activity in the presence of Triton X-100 showed a sixfold greater activity than in its absence, indicating a matrix location for the carbonic anhydrase. The enzyme is highly sensitive to the sulfonamide inhibitor ethoxzolamide, with Ki = 8.7 nM. The activation energy obtained from the rate constant for CO2 hydration, kenz with units (mg/ml)-1 s-1, over the range 4 to 37 degrees C was 12.8 kcal/mol. These properties are those expected for a carbonic anhydrase of the CA II class of isozymes, rather than for CA I, CA III, and the liver mitochondrial enzyme CA V.

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.

Mechanistic differences in the energy-linked fluorescence decreases of 9-aminoacridine dyes associated with bovine heart submitochondrial membranes

(1) The pH dependence of the fluorescence intensities of 9-aminoacridines associated with energized submitochondrial membranes suggests that a mechanism(s) other than protonation of the dye molecules, as is the case with quinacrine, is responsible for the energy-linked fluorescence decreases of 9-aminoacridine and 9-amino-3-chloro-7-methoxyacridine (9-ACMA). (2) That the fluorescence polarization of quinacrine associated with submitochondrial membranes more than doubles upon energization of the membranes is attributed to: (i) the bulky side chain at the 9-position of the acridine moiety which hinders the molecular rotation of quinacrine and (ii) electrostatic forces resulting from the protonation of quinacrine . H+ which induce tight binding between the dye molecules and the membranes. (3) The protonation of quinacrine associated with energized membranes, from the monoprotonated to the diprotonated species, takes place in the membrane phase, as evidence by the observation of a 'break' in both the Arrhenius plot of the respiratory rate and the plot of fluorescence polarization as a function of temperature. (4) That the measured fluorescence polarization of both 9-aminoacridine and 9-ACMA associated with both energized and nonenergized membranes is nearly zero suggests that the emitting species of these dye molecules are those in the 'free' form and that the membrane-bound molecules have formed nonfluorescent complexes; consequently no polarization can be measured.