Anomalous uncoupling of photophosphorylation by palmitic acid and by gramicidin D

Na,K-ATPase: A murzyme facilitating thermodynamic equilibriums at the membrane-interface

The redox metabolic paradigm of murburn concept advocates that diffusible reactive species (DRS, particularly oxygen-centric radicals) are mainstays of physiology, and not mere pathological manifestations. The murburn purview of cellular function also integrates the essential principles of bioenergetics, thermogenesis, homeostasis, electrophysiology, and coherence. In this context, any enzyme that generates/modulates/utilizes/sustains DRS functionality is called a murzyme. We have demonstrated that several water-soluble (peroxidases, lactate dehydrogenase, hemogoblin, etc.) and membrane-embedded (Complexes I-V in mitochondria, Photosystems I/II in chloroplasts, rhodopsin/transducin in rod cells, etc.) proteins serve as murzymes. The membrane protein of Na,K-ATPase (NKA, also known as sodium-potassium pump) is the focus of this article, owing to its centrality in neuro-cardio-musculo electrophysiology. Herein, via a series of critical queries starting from the geometric/spatio-temporal considerations of diffusion/mass transfer of solutes in cells to an update on structural/distributional features of NKA in diverse cellular systems, and from various mechanistic aspects of ion-transport (thermodynamics, osmoregulation, evolutionary dictates, etc.) to assays/explanations of inhibitory principles like cardiotonic steroids (CTS), we first highlight some unresolved problems in the field. Thereafter, we propose and apply a minimalist murburn model of trans-membrane ion-differentiation by NKA to address the physiological inhibitory effects of trans-dermal peptide, lithium ion, volatile anesthetics, confirmed interfacial DRS + proton modulators like nitrophenolics and unsaturated fatty acid, and the diverse classes of molecules like CTS, arginine, oximes, etc. These explanations find a pan-systemic connectivity with the inhibitions/uncouplings of other membrane proteins in cells.

Synergism of ammonium and palmitic acid in uncoupling of electron transfer and ATP synthesis in chloroplasts

Uncoupling by ammonium of electron transfer and ATP synthesis during linear transfer of electrons from water to photosystem 1 acceptors was studied in pea chloroplasts. It was shown that 40 microM palmitic acid decreased several-fold the ammonium concentrations necessary for 50% inhibition of ATP synthesis. The protonophore carbonyl cyanide m-chlorophenylhydrazone has no such property. The enhancement by palmitate of ammonium-induced uncoupling is accompanied by acceleration of basal electron transfer and decrease in the photoinduced uptake of hydrogen ions (H+). In the absence of ammonium, palmitate has no effect on basal transport and stimulates uptake of hydrogen ions. This means that in the case of combined action of palmitate and ammonium an additional leakage of H+ takes place, resulting in dissipation of the pH gradient. Synergic action of two metabolites, free fatty acid and ammonium, is supposed to provide for functioning of a system of mild regulation of energy coupling processes in native plant cell chloroplasts. Possible mechanisms of synergism are discussed.

Evidence that the intrinsic membrane protein LHCII in thylakoids is necessary for maintaining localized delta mu H+ energy coupling

This work tested the hypothesis that thylakoid localized proton-binding domains, suggested to be involved in localized delta mu H(+)-driven ATP formation, are maintained with the involvement of several membrane proteins, including the LHCII (Laszlo, J.A., Baker, G.M., and Dilley, R.A. (1984) Biochim. Biophys. Acta 764, 160-169), which comprises about 50% of the total thylakoid protein. The concept we have in mind is that several membrane proteins cooperate to shield a localized proton diffusion pathway from direct contact with the lumen, thus providing a physical barrier to H+ equilibration between the sequestered domains and the lumen. A barely mutant, chlorina f2, that lacks Chl b and does not accumulate some of the LHCII proteins, was tested for its capacity to carry out localized-proton gradient-dependent ATP formation. Two previously developed assays permit clear discrimination between localized and delocalized delta mu H+ gradient-driven ATP formation. Those assays include the effect of a permeable buffer, pyridine, on the number of single-turnover flashes needed to reach the energetic threshold for ATP formation and the more recently developed assay for lumen pH using 8-hydroxy-1,3,6-pyrene trisulfonic acid as a lumenally loaded pH-sensitive fluorescent probe. By those two criteria, the wild-type barley thylakoids revealed either a localized or a delocalized energy coupling mode under low- or high-salt storage conditions, respectively. Addition of Ca++ to the high-salt storage medium caused those thylakoids to maintain a localized energy-coupling response, as previously observed for pea thylakoids.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium gating of H+ fluxes in chloroplasts affects acid-base-driven ATP formation

In previous work, calcium ions, bound at the lumenal side of the CF0H+ channel, were suggested to keep a H+ flux gating site closed, favoring sequestered domain H+ ions flowing directly into the CF0-CF1 and driving ATP formation by a localized delta approximately mu H+ gradient. Treatments expected to displace Ca++ from binding sites had the effect of allowing H+ ions in the sequestered domains to equilibrate with the lumen, and energy coupling showed delocalized characteristics. The existence of such a gating function implies that a closed-gate configuration would block lumenal H+ ions from entering the CF0-CF1 complex. In this work that prediction was tested using as an assay the dark, acid-base jump ATP formation phenomenon driven by H+ ions derived from succinic acid loaded into the lumen. Chlorpromazine, a photoaffinity probe for many proteins having high-affinity Ca(++)-binding sites, covalently binds to the 8-kDa CF0 subunit in the largest amounts when there is sufficient Ca++ to favor the localized energy coupling mode, i.e., the "gate closed" configuration. Photoaffinity-bound chlorpromazine blocked 50% or more of the succinate-dependent acid-base jump ATP formation, provided that the ionic conditions during the UV photoaffinity treatment were those which favor a localized energy coupling pattern and a higher level of chlorpromazine labeling of the 8-kDa CF0 subunit. Thylakoids held under conditions favoring a delocalized energy coupling mode and less chlorpromazine labeling of the CF0 subunit did not show any inhibition of acid-base jump ATP formation. Chlorpromazine and calmidazolium, another Ca(++)-binding site probe, were also shown to block redox-derived H+ initially released into sequestered domains from entering the lumen, at low levels of domain H+ accumulation, but not at higher H+ uptake levels; ie., the closed gate state can be overcome by sufficiently acidic conditions. That is consistent with the observation that the inhibition of lumenal succinate-dependent ATP formation by photoaffinity-attached chlorpromazine can be reversed by lowering the pH of the acid stage from 5.5 to 4.5. The evidence is consistent with the concept that Ca++ bound at the lumenal side of the CF0 H+ channel can block H+ flux from either direction, consistent with the existence of a molecular structure in the CF0 complex having the properties of a gate for H+ flux across the inner boundary of the CF0. Such a gate could control the expression of localized or delocalized delta approximately mu H+ energy coupling gradients.

Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.

Electrical potential dissipation induced by free fatty acids in pea stem mitochondria

Linolenic, linoleic, oleic, palmitic and stearic acids (FFA) collapse the electrical potential of pea stem mitochondria in the absence or in the presence of 0.5 mM Mg2+. Higher concentrations of this cation (5 mM) lower the rate of dissipation caused by linoleic, oleic and palmitic acids, while abolishing that induced by stearic acid. Carboxyatractyloside and ADP do not reverse the FFA-induced collapse both in the presence or absence of Mg2+. EDTA, EGTA or BHT do not influence the dissipation caused by FFA that, in addition, is not linked to lipid peroxidation evaluated as malondialdehyde or conjugated diene formation. Only linolenic acid sustains a peroxidation which, however, appears to be caused by its own oxidation catalysed by lipoxygenases rather than by membrane lipoperoxidation induced by this free fatty acid. These results suggest that neither the ATP/ADP exchanger nor lipid peroxidation appear to be involved in FFA-induced uncoupling in pea stem mitochondria.

Calcium-dependent interaction of chlorpromazine with the chloroplast 8-kilodalton CF0 protein and calcium gating of H+ fluxes between thylakoid membrane domains and the lumen

Earlier work suggested that Ca2+ ions in the chloroplast thylakoid lumen interact with thylakoid membrane proteins to produce a proton flux gating structure which functions to regulate the expression of the energy-coupling H+ gradient between localized and delocalized modes [Chiang, G., & Dilley, R. A. (1987) Biochemistry 26, 4911-4916]. In this work, one of the phenothiazine Ca2+ antagonists, chlorpromazine, was used as a photoaffinity probe to test for Ca(2+)-dependent binding of the probe to thylakoid proteins. [3H]Chlorpromazine photoaffinity-labels thylakoid polypeptides of Mr 8K and 6K, with generally much less label occurring in other proteins (some experiments showed labeled proteins at Mr 13K-15K). More label was incorporated in circumstances where it is expected that Ca2+ occupies the putative H+ flux gating site, compared to when the gating site is not occupied by calcium. The photoaffinity labeling of the 8-kDa protein was also influenced by the energization level of the thylakoids (less labeling under H+ uptake energization). The 8-kDa protein was identified by partial amino acid sequence data as subunit III of the thylakoid CF0 H+ channel complex. The partial amino acid sequence of the 6-kDa protein (19 residues were determined with some uncertainties) was compared to data in the GCG sequence analysis data base, and no clear identity to a known sequence was revealed. Neither the exact site of putative Ca2+ binding to the CF0 proteolipid nor the site of covalent attachment of the chlorpromazine to the CF0 component has been identified. Evidence for gating of energy-linked H+ fluxes by the hypothesized Ca(2+)-CF0 gating site came from the correlation between Ca(2+)-dependent binding of chlorpromazine to the CF0 8-kDa protein with inhibition of light-driven H+ uptake into the lumen but no inhibition of H+ uptake into sequestered membrane domains. When conditions favored a delocalized delta mu H+ coupling mode, less chlorpromazine was bound to the CF0 structure, and much larger amounts of H+ ions were accumulated in the lumen. The data support the hypothesis that Ca2+ ions act in concert with the 8-kDa CF0 protein (and perhaps another protein, the 6-kDa polypeptide?) in a gating mechanism for regulating the expression of the energy-coupling H+ gradient between localized or delocalized coupling modes.

Fatty acid circuit as a physiological mechanism of uncoupling of oxidative phosphorylation

Free fatty acids, natural uncouplers of oxidative phosphorylation, are shown to differ from artificial ones in that they fail to increase conductance of phospholipid bilayers which are permeable for the protonated form of fatty acids but impermeable for their anionic form. Recent studies have revealed that uncoupling by fatty acids in mitochondria is mediated by the ATP/ADP antiporter and, in brown fat, by thermogenin which is structurally very similar to the antiporter. It is suggested that both the ATP/ADP antiporter and thermogenin facilitate translocation of the fatty anions through the mitochondrial membrane.

Evidence that localized energy coupling in thylakoids can continue beyond the energetic threshold onset into steady illumination

Energy transduction from proton gradients into ATP formation in chloroplast thylakoids has been hypothesized to be driven equally efficiently by localized domain delta mu H+ or by a delocalized delta mu H+ (Beard, W. A. and Dilley, R. A. (1988) J. Bioenerg. Biomembr. 20, 129-154). An important question is whether the apparent localized protonmotive force energy coupling mode can be observed only in the dark-to-light transient in the flash excitation protocol commonly used, or whether the localized energy coupling gradient can be maintained under conditions of continuous illumination ATP formation. The assay in the previous work was to use permeable amines, added to thylakoids in the dark, and observe the effect of the amine on the length of the energization lag (number of single-turnover flashes) required to initiate ATP formation in the dark-to-light transition. Amine buffers delayed the ATP onset in high-salt-stored membranes but did not delay the onset with low salt-stored membranes. This work tested whether permeable amines show the different effects in low- or high-salt-stored thylakoids which had attained a steady-state ATP formation rate (in continuous light) for 20-40 s prior to adding the amine. Hydroxyethylmorpholine was the preferred amine for such experiments, a suitable choice inasmuch as it behaves similarly to pyridine in the flash-induced ATP formation onset experiments, but it permeates more rapidly than pyridine and it has a higher pKa, which enhances its buffering effects. With high-salt-stored thylakoids, 0.5 or 1.0 mM hydroxyethylmorpholine added after 40 s of continuous illumination caused a marked, but transient, slowing of the ATP formation rate, but little or no slowing of the rate was observed with low-salt-stored thylakoids (at similar phosphorylation rates for the two thylakoid samples). Those data indicate that in continuous illumination conditions the proton gradient driving ATP formation in thylakoids from the low-salt-stored treatment did not equilibrate with the lumen, but in thylakoids stored in high-salt the delta mu H+ freely equilibrated with the lumen. That suggestion was supported by measurement of the luminal pH under coupling conditions by the [14C]methylamine distribution method using low- or high-salt-stored thylakoids. Further supportive evidence was obtained from measuring the effect of permeable amine buffers on H+ uptake under coupled and basal conditions with both types of thylakoid.(ABSTRACT TRUNCATED AT 400 WORDS)

Duramycin effects on the structure and function of heart mitochondria. II. Energy conversion reactions

The polypeptide antibiotic duramycin has been reported to interact selectively with phosphatidylethanolamine (PE) and monogalactosyldiacylglycerol (Navarro et al., 1985, Biochemistry 24, 4645-4650). PE is a major component of mitochondrial membranes. Duramycin was used to probe the role of PE in mitochondrial energy conversion reactions with the following results: (i) Duramycin uncoupled mitochondrial respiration, decreasing the respiratory control ratio to 1 at 5 microM. At concentrations of duramycin in excess of 10 microM, ADP addition inhibited electron transport. (ii) Duramycin inhibited oxidative phosphorylation (C50 less than 2 microM). (iii) Duramycin stimulated mitochondrial ATP hydrolysis modestly. The antibiotic was 7- to 16-fold less effective in this regard than concentrations of carbonylcyanide p-trifluoromethoxyphenylhydrazone (F-CCP) which produced comparable uncoupling. (iv) Duramycin inhibited uncoupled ATPase activity (C50 = 8 microM). Inhibition of the ATPase activity of intact mitochondria was blocked by 1 mM MgCl2 and 5 mM CaCl2; inhibition persisted in sub-mitochondrial particles assayed in the presence of 3 mM MgCl2. The effects on mitochondrial function of free fatty acids (FFA) and duramycin are similar in many respects. It is suggested that duramycin, like FFA, uncouples via a nonclassical mechanism, possibly by disrupting intramembrane H+ transfer between redox and ATPase complexes. In addition, interaction of duramycin, either direct or indirect, with the F0 moiety of the mitochondrial ATPase and with one or more components of the respiratory electron transport chain is proposed.

Transfer of tightly-bound tritium from the chloroplast membranes to CF1 is activated by the photophosphorylation process

Isolated thylakoids were incubated for 14-16 h in the buffer containing 3-6 mCi T2O/ml and then resedimented and suspended in non-radioactive medium. It was found that illumination of thylakoids induced an increase in radioactivity level in CF1 isolated from these thylakoids. Such effect was observed only if photophosphorylation substrates (ADP and phosphate or arsenate) were added to the medium during illumination. The light-induced ADP and arsenate-dependent incorporation of tritium into CF1 was suppressed by DCCD and inhibited by low gramicidin concentrations to the same extent as photophosphorylation.

Effects of cyclohexane, an industrial solvent, on the yeast Saccharomyces cerevisiae and on isolated yeast mitochondria

Little information on the effects of cyclohexane at the cellular or subcellular level is available. In Saccharomyces cerevisiae, cyclohexane inhibited respiration and diverse energy-dependent processes. In mitochondria isolated from S. cerevisiae, oxygen uptake and ATP synthesis were inhibited, although ATPase activity was not affected. Cyclohexane effects were similar to those reported for beta-pinene and limonene, suggesting that the cyclohexane ring in these monoterpenes may be a determinant for their biological activities.

Uncoupler-resistant mutants of bacteria

The chemiosmotic model of energy transduction offers a satisfying and widely confirmed understanding of the action of uncouplers on such processes as oxidative phosphorylation; the uncoupler, by facilitating the transmembrane movement of protons or other compensatory ions, reduces the electrochemical proton gradient that is posited as the energy intermediate for many kinds of bioenergetic work. In connection with this formulation, uncoupler-resistant mutants of bacteria that neither exclude nor inactivate these agents represent a bioenergetic puzzle. Uncoupler-resistant mutants of aerobic Bacillus species are, in fact, membrane lipid mutants with bioenergetic properties that are indeed challenging in connection with the chemiosmotic model. By contrast, uncoupler-resistant mutants of Escherichia coli probably exclude uncouplers, sometimes only under rather specific conditions. Related phenomena in eucaryotic and procaryotic systems, as well as various observations on uncouplers, decouplers, and certain other membrane-active agents, are also briefly considered.

Chloroplast thylakoid proteins associated with sequestered proton-buffering domains. Plastocyanin contributes buffering groups to localized proton domains

Thylakoid membrane proteins are organized so as to shield 30-50 nmol H+ (mg Chl)-1 from freely equilibrating with either the external or the lumen aqueous phases. Amine groups provide binding sites for this metastable buffering array and can be quantitatively measured by acetylation using [3H]acetic anhydride. The principle of the assay is that a metastable acidic domain will have relatively less of the reactive neutral form of the amine compared to the amount present after addition of an uncoupler. The extent of the acetylation reaction is strongly influenced by whether the lumen pH comes to complete equilibrium with the external pH prior to adding the acetic anhydride. Determination of the lumen pH by [14C]methylamine distribution after the standard 3 or 5 min equilibration in pH 8.6 buffer indicated that the lumen may have been 0.2 to 0.3 pH more acidic than the external phase. This effect was taken into account by determining the pH dependence, in the pH 8.2-8.6 range, of acetylation of the membrane proteins studied, and the labeling data were conservatively corrected for this possible contribution. Experiments were carried out to identify the thylakoid proteins that contribute such metastable domain amine groups, using the above conservative correction. Surprisingly, plastocyanin contributes buried amine groups, but cytochrome f did not give evidence for such a contribution, if the conservative correction in the labeling was applied. If the correction was too conservative, cytochrome f may contribute amines to the sequestered domains. The new methodology verified earlier results suggesting that three Tris-releasable photosystem II-associated proteins also contribute significantly to the sequestered amine-buffering array.

Effect of high KCl concentrations on membrane-localized metastable proton buffering domains in thylakoids

Recent work showed that chloroplast thylakoid membranes stored in 100 mM KCl-containing media have delocalized energy coupling consistent with a rapid equilibration of the proton gradient between the proton-producing redox steps and the lumen bulk phase (Beard and Dilley 1986). Thylakoids stored in low salt media showed localized energy coupling. A related thylakoid membrane property is the occurrence of sequestered, metastable, acidic domains, associated with pK a ≈7.5 amine groups. For low salt-stored membranes the domain protons appear to be in the direct (localized) diffusion pathway of protons involved in energizing ATP formation, whereas in thylakoids stored in high KCl, domain protons equilibrated with the lumen during the development of the ATP energization threshold (Theg et al. 1988). This work tested whether the 100 mM KCl storage treatment did or did not cause the dissipation of the metastable acidic domain protons in the dark, storage period. By three criteria, it was found that the 100 mM KCl storage treatment had only a slight tendency to dissipate the acidic domain protons into alkaline media under dark conditions. Storage in KCl does not cause the dissipation of the acidic domains in the dark, but allows domain protons to equilibrate with the lumen after the redox system begins turning over, but before the ATP energization threshold ΔpH is reached. These results must be considered in models of how the thylakoid structure can accommodate metastable acidic domains and how such domain protons diffuse to the CF0-CF1 complexes in energy coupling.