The electrophoretic properties of a Ca2+ carrier isolated from calf heart inner mitochondrial membrane

The molecular identity of the mitochondrial Ca2+ sequestration system

There is ample evidence to suggest that a dramatic decrease in mitochondrial Ca(2+) retention may contribute to the cell death associated with stroke, excitotoxicity, ischemia and reperfusion, and neurodegenerative diseases. Mitochondria from all studied tissues can accumulate and store Ca(2+) , but the maximum Ca(2+) storage capacity varies widely and exhibits striking tissue specificity. There is currently no explanation for this fact. Precipitation of Ca(2+) and phosphate in the mitochondrial matrix has been suggested to be the major form of storage of accumulated Ca(2+) in mitochondria. How this precipitate is formed is not known. The molecular identity of almost all proteins involved in Ca(2+) transport, storage and formation of the permeability transition pore is also unknown. This review summarizes studies aimed at identifying these proteins, and describes the properties of a known mitochondrial protein that may be involved in Ca(2+) transport and the structure of the permeability transition pore.

Purification of the channel component of the mitochondrial calcium uniporter and its reconstitution into planar lipid bilayers

The purification of the channel-forming component of the mitochondrial calcium uniporter and its channel properties are described. After ethanol and 50% ethanol-water extraction of mitochondria from beef heart or perfused rat liver, the extract was passed through thiopropyl-Sepharose 6B column, and absorbed components were eluted with 2-mercaptoethanol, followed by gel-filtration on Sephadex G-15. The last fraction eluted (M(r) about 2000) was then subjected to reverse-phase high-performance liquid chromatography. Of the more than 10 distinct peaks, only one showed specific Ca(2+)-channel activity in BLM with properties similar to earlier, less extensively purified preparations, i.e., conductance of 20 pS and multiples thereof, clustering of channels, participation of 2 or more subunits in channel formation, and sensitivity to 1 microM ruthenium red. Voltage sensitivity and cooperativity between channels are described. The Ca(2+)-binding glycoprotein with which the peptide was associated was found to have high homology with human acid alpha 1-glycoprotein (orosomucoid) and to show identity with beef plasma orosomucoid in the Ouchterlony immunodiffusion test.

Inhibition of the mitochondrial calcium uniporter by antibodies against a 40-kDa glycoproteinT

Polyclonal rabbit antibodies against a Ca(2+)-binding mitochondrial glycoprotein were found to inhibit the uniporter-mediated transport of Ca2+ in mitoplasts prepared from rat liver mitochondria. Spermine, a modulator of the uniporter, decreased the inhibition. This glycoprotein of M(r) 40,000, isolated from beef heart mitochondria and earlier shown to form Ca(2+)-conducting channels in black-lipid membranes, thus is a good candidate for being a component of the uniporter. Antibody-IgG was found to specifically bind to mitochondria in human fibroblasts.

Calcium uptake in human spermatozoa: characterization and mechanisms

Basal 45Ca2+ influx was analyzed in human seminal spermatozoa using a method that allows these highly reactive cells to be easily and safely handled. The uptake was a time-dependent process, with its maximum at 400 s. The kinetics of 45Ca2+ transport was saturating as a function of extracellular Ca2+ concentration with a Km of 429 microM and a Vmax of 1.6 nmol 45Ca2+/mg protein/2.5 min. Depolarizing conditions and the calcium channel blocker verapamil did not affect the uptake; based on this, the presence of operating calcium channels in seminal spermatozoa is excluded. The independence of 45Ca2+ uptake on external concentration of both Na+ and Ca2+ suggests that Na+/Ca2+ exchange does not occur in these cells. The anticalmodulin drug trifluoperazine, the mitochondrial inhibitor antimycin A, and the SH reagents N-ethylmaleimide and mersalyl all inhibited the ion transport. A calmodulin-regulated, energy-requiring, proteinaceous Ca2+ transporter seems to be the main operating mechanism of calcium uptake in human seminal gametes.

The effects of calcitonin on calcium uptake and respiration in rat-liver mitochondria

The action of calcitonin on both the transport of calcium across the mitochondrial membrane and cellular respiration has been studied in the presence and absence of added phosphate. In the presence of phosphate, both the rate of calcium entry and the amount of calcium accumulated was stimulated by calcitonin, above a threshold concentration, in a saturable manner. In the absence of phosphate, calcitonin enhanced the rate of calcium entry, but had no appreciable effect on the levels of total calcium accumulated. The minimum concentration of calcitonin necessary to produce these effects was in all cases dependent on the external calcium concentration. Mitochondrial respiration was inhibited only at calcitonin levels much higher than those affecting calcium uptake. These results are consistent with the idea that the action of calcitonin is directly related to the mechanism of calcium uptake, and not to the respiratory process.

Evidence for the involvement of dithiol groups in mitochondrial calcium transport: studies with cadmium

The effect of cadmium on some functions of mitochondria isolated from kidneys of rat was studied. Addition of cadmium chloride to mitochondria induced stimulation of both State 4 respiratory rate and ATPase activity, which are prevented by the addition of ruthenium red. We also show that cadmium inhibits competitively calcium translocation; this inhibitory effect of cadmium is reverted by the addition of dithiothreitol. From these results, it is proposed that, similarly to Ca2+, cadmium penetrates mitochondria and binds to a membrane dithiol group, which is essential for the translocation of the cation.

Characterization of calciphorin by laser-excited europium luminescence

There is some question whether the calcium binding characteristics of calciphorin are due to contaminating phospholipids. To differentiate protein ion binding by phospholipids or contaminating detergent, we describe here the use of Eu(III) as a metal-binding-site probe, and characterize the interaction of Eu(III) with calciphorin, cardiolipin, deoxycholate, and digitonin. The luminescence excitation pattern of Eu(III) bound to the calciphorin preparation clearly differentiates it from Eu(III) interactions with the possible contaminants. In addition, the effect of the luminescence decay constant of Eu(III) bound to calciphorin on the mole fraction of H2O in a mixture of H2O/2H2O indicates that all except approximately 0.8 of the 9 to 10 water molecules coordinating Eu(III) in solution are stripped off upon binding to calciphorin. This also contrasts with the data for the possible contaminants.

Characterization of calciphorin, the low molecular weight calcium ionophore, from rat liver mitochondria

Calciphorin, the putative mitochondrial calcium ionophore from rat liver mitochondria, exhibits the inherent properties of the mitochondrial calcium transport system and is similar to the calf heart preparation reported earlier. The protein has a strong selectivity for Ca2+, and has a Kd for Ca2+ of 56.5 +/- 6.6 microM and 13.9 +/- 2.1 microM in organic extraction and flow dialysis experiments, respectively. Reduction of the contaminating lipids from 23 +/- 6.5 to 1.73 +/- 0.4 moles per mole protein does not alter the affinities, Ca2+/protein stoichiometry or selectivity for Ca2+.

Ca2+-cardiolipin interaction in a model system. Selectivity and apparent high affinity

The interaction of cardiolipin with Ca2+ was assessed by measuring the cardiolipin-mediated extraction of 45Ca2+ from an aqueous to an organic (methylene chloride) phase. Cardiolipin binds Ca2+ with high affinity [Kd(apparent) = 0.70 +/- 0.17 microM (S.D.)]. Cation-cardiolipin interactions are selective. Interaction of cardiolipin with Ca2+ is insensitive to Na+, but is inhibited by divalent cations with Mn2+ greater than Zn2+ greater than Mg2+. In addition La3+ and Ruthenium red are particularly potent inhibitors of Ca2+ binding by cardiolipin. Cardiolipin-mediated extraction of Ca2+ into an aqueous phase is also inhibited by phosphatidylcholine. Inhibition of Ca2+-cardiolipin interaction by phosphatidylcholine (a phospholipid known to stabilize the bilayer conformation) may implicate inverted, non-bilayer lipid structures in the binding.

Isolation of a fraction with Ca2+ ionophore properties from rat liver mitochondria

Isolation of a small protein with properties of a Ca2+ ionophore from calf heart mitochondria has recently been reported [A. Y. Jeng and A. E. Shamoo, 1980, J. Biol. Chem. 255, 6897, 6904]. We have isolated a fraction with similar physical and chemical properties from rat liver mitochondria. In particular, the hepatic preparation is able to bind Ca2+ with high affinity in such a fashion that the resultant complex is soluble in a hydrophobic phase. It will also transport Ca2+ through a stirred organic phase (Pressman cell). Interaction of the liver preparation with Ca2+ is sensitive to inhibitors of mitochondrial Ca2+ uptake. The hepatic preparation contains both protein and lipid components. The phospholipid components were identified and the behavior of a similar mixture of commercially available phospholipids was compared to that of the ionophore fraction from rat liver mitochondria. All of the Ca2+ binding properties of the rat liver preparation could be mimicked by the lipids. In a preliminary experiment, reduction of the phospholipid content of the preparation to less than one lipid phosphate per protein molecule (assuming a molecular weight of 3000 by analogy with the calf heart case) resulted in a protein that was unable to bind Ca2+. We, therefore, suggest that the ability of the preparation to interact with Ca2+ is due to the constituent phospholipids. Measurements of phospholipid-Ca2+ interactions in the model systems and under the conditions of low (microM) Ca2+ and phospholipid concentration utilized here demonstrated an affinity for Ca2+ (Ks approximately 1 microM) and a cation selectivity that have not previously been reported.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.