Triphenyltetrazolium and its derivatives are anisotropic inhibitors of energy transduction in oxidative phosphorylation in rat liver mitochondria

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.

Mitochondrial and nonmitochondrial reduction of MTT: interaction of MTT with TMRE, JC-1, and NAO mitochondrial fluorescent probes

BACKGROUND

Bioreduction of water-soluble tetrazolium salts (e.g., MTS, XTT, and MTT) to their respective formazans is generally regarded as an indicator of cell "redox activity." The reaction is attributed mainly to mitochondrial enzymes and electron carriers. However, MTT reduction may also be catalyzed by a number of other nonmitochondrial enzymes. The goal of this work was to establish the sites of MTT reduction in intact HepG2 human hepatoma cells in culture.

METHODS

In order to establish the subcellular localization of the sites of reduction of MTT, we imaged the formation of MTT-formazan deposits using backscattered light confocal microscopy. Mitochondria were visualized in viable cells using fluorescent dyes that bind in a manner dependent (JC-1 and TMRE) or independent (NAO) of mitochondrial electric potential.

RESULTS

Only 25-45% of MTT-formazan was associated with mitochondria after 25 min of incubation. No more than 25% of the mitochondrial area on images was occupied by MTT-formazan. Mitochondrial fluorescence of TMRE, NAO, and the monomeric form of JC-1 decreased rapidly in cells incubated with MTT. However, the intensity of fluorescence of JC-1 aggregates dropped by less than 30% at the onset of incubation and remained constant as reduction of MTT proceeded further.

CONCLUSIONS

(1) Most of MTT-formazan deposits are not coincident with mitochondria. (2) Monomeric JC-1, as well as TMRE and NAO, accumulating in mitochondria may be displaced by MTT. Thus, the presence of positively charged organic compounds (like MTT) may distort measurements of mitochondrial transmembrane electric potential, which are based on accumulation of fluorescent dyes.

ATP-sensitive K+ channel in the mitochondrial inner membrane

Mitochondria take up and extrude various inorganic and organic ions, as well as larger substances such as proteins. The technique of patch clamping should provide real-time information on such transport and on energy transduction in oxidative phosphorylation. It has been applied to detect microscopic currents from mitochondrial membranes and conductances of ion channels in the 5-1,000 pS range in the outer and inner membranes. These pores are not, however, selective for particular ions. Here we use fused giant mitoplasts prepared from rat liver mitochondria to identify a small conductance channel highly selective for K+ in the inner mitochondrial membrane. This channel can be reversibly inactivated by ATP applied to the matrix side under inside-out patch configuration; it is also inhibited by 4-aminopyridine and by glybenclamide. The slope conductance of the unitary currents measured at negative membrane potentials was 9.7 +/- 1.0 pS (mean +/- s.d., n = 6) when the pipette solution contained 100 mM K+ and the bathing solution 33.3 mM K+. Our results indicate that mitochondria depolarize by generating a K+ conductance when ATP in the matrix is deficient.

H(+)-ATP synthase from rat liver mitochondria. A simple, rapid purification method of the functional complex and its characterization

A novel, simple, and rapid preparative method for purification of rat liver H(+)-ATP synthase by anion-exchange HPLC was developed. The H(+)-ATP synthase purified had higher ATPase activity in the absence of added phospholipids than any preparation reported previously, and this activity was completely inhibited by oligomycin. When reconstituted into proteoliposomes, the H(+)-ATP synthase showed an ATP-dependent 8-anilinonaphthalene-1-sulfonate response and ATP-Pi exchange activity, both of which were also completely inhibited by oligomycin and an uncoupler, indicating the intactness of the H(+)-ATP synthase. An immunochemical study and a labeling experiment with N,N'-[14C]dicyclohexylcarbodiimide ([14C]DCCD) demonstrated the presence of chargerin II ( a product of mitochondrial A6L DNA) and DCCD-binding protein (subunit c) in the complex. The subunits of the complex were separated into 11 main fractions by reverse-phase HPLC, and 3 of them and the delta subunit in F1 were partially sequenced. A search for sequence homologies indicated that these components were subunit b, coupling factor 6, subunit delta, and subunit epsilon. This is the first report of the existence of subunit b, factor 6, and chargerin II in H(+)-ATP synthase purified from rat liver mitochondria.

The tetrazolium-formazan system: design and histochemistry

Our increasing knowledge about the chemistry and the correlations between chemical structure and histochemical properties of the tetrazolium/formazan system is resulting in: a better understanding of existing histochemical tetrazolium techniques; the selection of optimal tetrazolium salts for qualified use in histochemistry, cytochemistry and biochemistry; both qualitative and quantitative improvements in histochemical techniques for purposes demonstrating the activities of various dehydrogenating enzymes; an extended insight into the "state" of the tested biological object by means of tetrazolium indicators with special properties; and the combination of histochemical enzyme determination with further morphological techniques. This article has attempted to illustrate the progress in the use of the tetrazolium/formazan-system for histochemical purposes.

Purified hydrophobic proteins, chargerins, are essential for energy transduction in oxidative phosphorylation

Studies on anisotropic inhibitors, a unique type of inhibitor of energy transduction in oxidative phosphorylation, suggested that redox reactions generate two kinds of negative charges on the outer surface of mitochondrial inner membranes, on redox complexes and on F0, and that the inhibitors inhibit energy transduction by binding to these negative charges. Recent experiments on photoaffinity labeling of mitochondria with monoazide ethidium, which is an anisotropic inhibitor, showed that the inhibitor specifically binds to a hydrophobic protein of the membranes. In the present work the mitochondrial components labeled with monoazide ethidium were further purified and two kinds of hydrophobic proteins (apparent molecular masses, 8 and 13 kDa) were found to be specifically labeled with the inhibitor. These proteins were named chargerin I and II, respectively. Redox reactions greatly increased the molar ratio of ethidium bound to chargerin I and II in mitochondria, reflecting a conformational change of the chargerins coupled with the redox reactions. It was also shown that antibody against chargerin II specifically inhibited ATP synthesis in mitoplasts (inner membranes plus matrix) prepared from rat liver mitochondria. Thus, the present findings show that chargerins have an essential role in energy transduction in oxidative phosphorylation in rat liver mitochondria, in good accord with the conformational coupling model of the H+ pumps and ATP synthesis.

Polypeptides of photosystem II and their role in oxygen evolution

The linear, four-step oxidation of water to molecular oxygen by photosystem II requires cooperation between redox reactions driven by light and a set of redox reactions involving the S-states within the oxygen-evolving complex. The oxygenevolving complex is a highly ordered structure in which a number of polypeptides interact with one another to provide the appropriate environment for productive binding of cofactors such as manganese, chloride and calcium, as well as for productive electron transfer within the photoact. A number of recent advances in the knowledge of the polypeptide structure of photosystem II has revealed a correlation between primary photochemical events and a 'core' complex of five hydrophobic polypeptides which provide binding sites for chlorophyll a, pheophytin a, the reaction center chlorophyll (P680), and its immediate donor, denoted Z. Although the 'core' complex of photosystem II is photochemically active, it does not possess the capacity to evolve oxygen. A second set of polypeptides, which are water-soluble, have been discovered to be associated with photosystem II; these polypeptides are now proposed to be the structural elements of a special domain which promotes the activities of the loosely-bound cofactors (manganese, chloride, calcium) that participate in oxygen evolution activity. Two of these proteins (whose molecular weights are 23 and 17 kDa) can be released from photosystem II without concurrent loss of functional manganese; studies on these proteins and on the membranes from which they have been removed indicate that the 23 and 17 kDa species from part of the structure which promotes retention of chloride and calcium within the oxygen-evolving complex. A third water-soluble polypeptide of molecular weight 33 kDa is held to the photosystem II 'core' complex by a series of forces which in some circumstances may include ligation to manganese. The 33 kDa protein has been studied in some detail and appears to promote the formation of the environment which is required for optimal participation by manganese in the oxygen evolving reaction. This minireview describes the polypeptides of photosystem II, places an emphasis on the current state of knowledge concerning these species, and discusses current areas of uncertainty concerning these important polypeptides.