The high-affinity Ca(2+)-ATPase from rat parotid plasma membranes is an ectoenzyme: solubilization and characterization of the Ca(2+)-ATPase activity

A high-affinity Ca(2+)-ATPase was solubilized from rat parotid plasma membranes and purified by concanavalin A and DEAE-cellulose chromatography. The properties of the purified high-affinity Ca(2+)-ATPase are very different from those of the parotid plasma membrane ATP-dependent Ca2+ pump but appear to be similar to those of a rat liver cell adhesion protein which exhibits high-affinity ecto-Ca(2+)-ATPase activity.

Inorganic phosphate is the major component of the thermostable cytoplasmic fraction which stimulates mitochondrial anion uniport

A low molecular weight thermostable cytoplasmic fraction isolated from rat liver homogenate when pre-incubated with mitochondria increases the rate at which anions enter mitochondria via the pH-dependent anion-conducting channel in the inner membrane. The crude fraction obtained by centrifuging and heating the liver homogenate was purified by gel filtration and chromatography on DEAE-cellulose. The resulting factor is stable to heating at 100 degrees C, freeze-drying and extremes of pH. Inorganic phosphate co-purified with activity and activity was lost when the phosphate was removed by barium salt precipitation. A pure sample of KH2PO4 produced stimulation of anion conductivity. These results show that the major portion of the activity which stimulates anion uniport can be accounted for by the presence of phosphate in the crude and purified fractions. Mersalyl blocks stimulation when added before, but not when added after, incubation with phosphate which shows that the stimulation is produced by phosphate in the mitochondrial matrix. The proposed role of this factor in thyroid hormone action is discussed in the light of its identification as inorganic phosphate.

The pH-dependent anion-conducting channel of the mitochondrial inner membrane is potently inhibited by zinc ions

Zinc is a potent reversible inhibitor of the pH-dependent anion-conducting channel in the mitochondrial inner membrane, 50% inhibition was produced by 1.5 microM added Zn2+ at which point free Zn2+ was < or = 10(-8) M. Inhibition by Zn2+ is rapid but can be prevented or rapidly reversed by excess EDTA. Concentrations of Zn2+ higher than 4 microM caused reversal of inhibition to a variable extent depending on the anion. Under these conditions Zn2+ did not inhibit ribose entry, the phosphate transporter, or the pH-insensitive component of the NO3- uniport.

Effect of buffers and osmolality on anion uniport across the mitochondrial inner membrane

The effects of buffers and osmolality of the suspending medium on the pH-dependent anion uniport across the inner membrane of isolated rat liver mitochondria have been studied using the light scattering technique to measure passive osmotic swelling. In contrast to some other transport processes the rates of entry of chloride and other anions via the anion-conducting channel decreased steeply with increasing solute concentration. This effect appears to be a result of increased osmolality or decreased matrix volume rather than inhibition by the anion since it was also produced by increasing the osmolality by addition of non-penetrant solutes. The effects of some pH buffers on the mitochondrial anion-conducting channel were also investigated. Some zwitterionic buffers had little effect other than that produced by increasing osmolality but Tricine, Popso and Caps produced marked additional inhibition of anion uniport and several other zwitterionic buffers were also inhibitory. The correlation between increased anion conductivity and increased matrix volume supports the proposal that this channel functions in regulation of the volume of the mitochondrial matrix.

An evaluation of exposure time and temperature in the thermocycling of dental restorative materials

This study indicates that some current procedures used in thermocycling dental restorative materials would not produce the desired thermal gradient in the specimens. The shortfall in gradient depends on the size of the specimen, the dwell time in the baths and whether the baths are stirred or not. The desired thermal gradient of 50 degrees C can be approximated with a dwell time of 60 s in well-circulated water baths at 5 degrees C and 55 degrees C. For critical studies a dwell time of 90 s is recommended.

Calcium ion activation of the anion-conducting channel in the rat liver mitochondrial inner membrane

Stimulation of the rat liver mitochondrial inner-membrane anion-conducting channel by aeration is dependent on the concentration of Ca2+ ions in the assay medium. Ca2+ activates anion conduction in both aerated and non-energised mitochondria but acts over a wider concentration range and produces a greater increase in anion-conductivity in aerated mitochondria. EGTA reverses Ca2+ stimulation but takes several seconds to act, indicating slow release of Ca2+ from the activation site possibly on the matrix side of the inner mitochondrial membrane. It is suggested that this channel may respond to hormone-induced changes in cytosolic Ca2+ concentration.

Respiratory inhibitors and uncouplers prevent the aeration-induced increase in mitochondrial anion conductivity

1. When mitochondria are stirred in air the rate of anion conductivity increases, this effect being enhanced by the addition of respiratory substrate. 2. This effect is reversible if the mitochondria are stored for a period of time under N2. 3. The aeration-induced increase in mitochondrial anion conductivity can also be prevented by the addition of respiratory inhibitors rotenone and antimycin A, as well as by 30 microM-cyanide. 4. A decrease in this aeration-induced anion conductivity can also be observed upon the addition of the uncouplers carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (2 microM) and 2,4-dinitrophenol (100 microM). 5. Simultaneous measurements of mitochondrial anion conductivity and membrane potential show a relationship between the level of membrane potential and anion conductivity. 6. It is suggested that the level of membrane potential is either directly or indirectly responsible for the level of mitochondrial anion conductivity.

Palmitoyl-CoA inhibits the mitochondrial inner membrane anion-conducting channel

Palmitoyl-CoA is shown here to inhibit the pH-dependent anion-conducting channel (IMAC) in the inner membrane of rat liver mitochondria, with half-maximal inhibition at 2.4 microM. It has little effect on the transport of ribose, thiocyanate and glutamate. Palmitic acid and palmitoyl-carnitine stimulate the entry of all the above metabolites. CoASH and carnitine have no effect on chloride uniport. Palmitoyl-CoA and the IMAC may have a role in controlling thermogenesis in liver mitochondria.

Respiration driven C1- uptake by submitochondrial particles

Both Mg2+ and oligomycin are required for the establishment of a membrane potential and the uptake of Cl- in submitochondrial particles prepared from rat liver. The effect of oligomycin is considered to be due to blocking of H+ conduction through exposed F0 channels of the ATPase complex whereas Mg2+ may more directly affect the anion-conducting channel.

Inhibition of electrogenic anion entry into rat liver mitochondria by N,N'-dicyclohexylcarbodiimide

The carboxyl group reagent dicyclohexylcarbodiimide inhibits the electrogenic entry of Cl- and NO3-into rat liver mitochondria at alkaline pH. The inhibition is time dependent and 50% inhibition is obtained by the addition of 3-4 nmol DCCD/mg protein. The blockage of the pH-dependent anion-conducting pore appears to be unrelated to the other known actions of DCCD on rat liver mitochondria but seems similar to its effect on the uncoupling protein of brown adipose tissue.

Studies on the nature of the high-affinity trialkyltin binding site of rat liver mitochondria

1. The proteolipid fraction isolated from rat liver mitochondria pretreated with [3H]triphenyltin chloride is enriched in triphenyltin compared with the original mitochondria. 2. Part of this [3H]triphenyltin is eluted with a protein of Mr 5000-6000 on Sephadex LH20 chromatography. 2. Mössbauer spectra of the proteolipid fraction treated with 119Sn-enriched triethyltin chloride show a doublet which corresponds closely with that assigned previously [Farrow & Dawson (1978) Eur. J. Biochem. 86. 85-95] to the absorption of triethyltin bound to the high-affinity binding site of the mitochondrial ATPase.

Permeability of the mitochondrial membrane to bicarbonate ions

Osmotic-swelling techniques show that HCO3- enters mitochondria by an electrogenic process, effectively HCO3- uniport, under non-energized conditions. This mode of translocation accounts for previous reports of non-entry of HCO3- in experiments with energy-linked Ca2+ uptake. The effects of HCO3- on mitochondrial respiration are reported and discussed.

Effects of triphenylsulphonium ions on mitochondria. Inhibition of adenosine triphosphatase activity

Triphenylsulphonium ions inhibit mitochondrial oxidative phosphorylation and adenosine triphosphatase activity. The site of action is on the soluble F1 adenosine triphosphatase component. Triphenylsylphonium ions also inhibit electron transfer in the NAD-cytochrome b region of the respiratory chain. In both types of inhibition, triphenylsulphonium ions are effective at low concentrations, half-maximal inhibition being produced by a concentration of about 20-30 muM. These effects resemble the effects of alkylguanidines on mitochondria and are discussed in relation to the effects of alkylguanidines and other lipophilic cations such as ethidium and dibenzyldimethylammonium ions. A modification of the purification procedure for the soluble mitochondrial adenosine triphosphatase [Beechey, Hubbard, Linnett, Mitchell & Munn (1975) Biochem. J. 148, 533-537] IS DESCRIBED, WHICH YIELDS A PREPARATION WITH A HIGHER SPECIFIC ACTIVITY AND SHOWING FEWER BANDS IN GEL ELECTROPHORESIS.

The action of tributyltin on energy coupling in coupling-factor-deficient submitochondrial particles

1. Tributyltin at concentrations of approx. 1nmol/mg of protein induces respiratory control and lessens the protein permeability of coupling-factor-deficient submitochondrial particles. 2. At these concentrations or lower, it increases the P/O ratio of the particles to a small extent and inhibits the adenosine triphosphatase activity without greatly increasing its sensitivity to uncoupling agents. 3. It fails to stimulate ATP-driven reversed electron transport or transhydrogenase, but stimulates the transhydrogenase driven by aerobic succinate oxidation. 4. The results indicate that, unlike oligomycin, tributyltin does not discriminate between damaged and intact ATP-synthesizing complexes. 5. The relationship between the oligomycin- and tributyltin-binding sites is discussed.

Inhibition of the soluble adenosine triphosphatase from mitochondria by adenylyl imidodiphosphate

1. Adenylyl imidodiphosphate is an inhibitor with high affinity for the soluble ATPase (adenosine triphosphatase) from mitochondria. 2. The reaction of the inhibitor with the ATPase is slow and estimates for the association and dissociation reaction rate constants are given. 3. The number of binding sites for the inhibitor appears to be doubled in the presence of 2,4-dinitrophenol. 4. Adenylyl imidodiphosphate is less effective as an inhibitor of the ATPase activity of this enzyme than of the inosine triphosphatase activity. It is also less effective on the ATPase of frozen-thawed or intact mitochondria and did not inhibit ADP-stimulated respiration by intact mitochondria.

Inhibition and uncoupling of photophosphorylation in isolated chloroplasts by organotin, organomercury and diphenyleneiodonium compounds

1. Trialkyltin, triphenyltin and diphenyleneiodonium compounds inhibited ADP-stimulated O(2) evolution by isolated pea chloroplasts in the presence of phosphate or arsenate. Tributyltin and triphenyltin were the most effective inhibitors, which suggests a highly hydrophobic site of action. Phenylmercuric acetate was a poor inhibitor of photophosphorylation, which suggests that thiol groups are not involved. 2. Triethyltin was a potent uncoupler of photophosphorylation by isolated chloroplasts in media containing Cl(-), but had little uncoupling activity when Cl(-) was replaced by NO(3) (-) or SO(4) (2-), which are inactive in the anion-hydroxide exchange. It is suggested that uncoupling by triethyltin is a result of the Cl(-)-OH(-) exchange together with a natural uniport of Cl(-). Tributyltin, triphenyltin and phenylmercuric acetate had low uncoupling activity, probably because in these compounds the uncoupling activity is partially masked by inhibitory effects. 3. At high concentrations the organotin compounds caused inhibition of electron transport uncoupled by carbonyl cyanide m-chlorophenylhydrazone or NH(4)Cl. At these high concentrations the organotin compounds may be producing a detergent-like disorganization of the membrane structure. In contrast, diphenyleneiodonium sulphate inhibited uncoupled electron transport at low concentrations; however, this inhibition is less than the inhibition of photophosphorylation, which suggests that the compound also inhibits the phosphorylation reactions as well as electron transport. 4. The effects of these compounds on basal electron transport were complex and depended on the pH of the reaction media. However, they can be explained on the basis of three actions: inhibition of the phosphorylation reactions, uncoupling and direct inhibition of electron transport. 5. The inhibition of cyclic photophosphorylation in the presence of phenazine methosulphate by diphenyleneiodonium sulphate shows that it inhibits in the region of photosystem 1.

The action of trialkyltin compounds on mitochondrial respiration. The effect of pH

1. Inhibition of 2,4-dinitrophenol-stimulated respiration by trialkyltins is dependent on the presence of Cl(-) in the assay medium and is only apparent at acid pH values. It appears to be a result of the Cl(-)-OH(-) exchange mediated by trialkyltins. 2. In a KCl medium at alkaline pH values, the maximum rate of respiration produced by uncouplers is further increased by the presence of trialkyltins. 3. The inhibition of uncoupled succinate oxidation at acid pH values is not reversed by increasing the external substrate concentration, suggesting that depletion of intramitochondrial succinate is not an important factor in the inhibition. 4. It is suggested that the probable explanation for these observations is that in the presence of Cl(-) trialkyltins alter the internal pH to a more acid value and this directly affects the activity of one or more steps in succinate oxidation. 5. The oligomycin-like action of trialkyltins in a Cl(-)-free medium shows considerable pH-dependence over the pH range 6.6-7.6 in the presence of 10mm-phosphate, but very much less pH-dependence in the presence of 1mm-phosphate. 6. The binding of triethyltin to mitochondria shows a pK at pH6.3 and does not change greatly over the pH range 6.6-7.6. 7. It is suggested that the pH-dependence of the oligomycin-like action described by Coleman & Palmer (1971) is the result of the pH-dependence of the formation of a hydrophilic complex between trialkyltins and P(i).

Use of progress curves to investigate product inhibition in enzyme-catalysed reactions. Application to the soluble mitochondrial adenosine triphosphatase

1. Several methods of analysing progress curves of enzyme-catalysed reactions are discussed briefly in relation to their usefulness in a situation where a reaction product has a K(i) much lower than the K(m) for the substrate 2. A comparison is made of different methods of estimating initial rates in this situation. 3. The use of a computer curve-fitting routine capable of handling functions of more than one variable for the extraction of kinetic parameters from progress curves is described. 4. This method and that of fitting time as a polynomial in product concentration are applied to progress curves for the soluble mitochondrial adenosine triphosphatase and the results are compared with values obtained by more conventional methods.

Preparation and general properties of a soluble adenosine triphosphatase from mitochondria

1. The purification of an adenosine triphosphatase present in aqueous extracts of acetone-dried ox-heart mitochondria is described. 2. No evidence was found for the presence of more than one protein having adenosine-triphosphatase activity in these extracts. 3. The enzyme is less stable at 0 degrees than at 25 degrees but is stabilized by glycerol. 4. The activity is dependent on the presence of Mg(2+) or certain other bivalent metal cations. 5. The adenosine-triphosphatase activity of the Mg(2+)-activated enzyme is enhanced by 2,4-dinitrophenol. 6. The kinetics of Mg(2+) activation indicate that the ATP-Mg(2+) complex is the important substrate: free ATP and Mg(2+) are inhibitory. 7. This preparation of mitochondrial adenosine triphosphatase has many properties in common with the adenosine triphosphatase coupling factor from mitochondria (Racker, 1961).