The ability of the mitochondrial Ca2+-binding glycoprotein to restore Ca2+ transport in glycoprotein-depleted rat liver mitochondria

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.

Calcium binding to polypeptides of rat liver and Zajdela hepatoma mitochondrial inner membranes

Composition and amount of 45Ca2+-binding proteins in the inner membrane fraction of rat liver and Zajdela hepatoma mitochondria were determined. In the inner membrane of liver mitochondria, three major 45Ca2+-binding polypeptides: a protein of approximately 130 kDa (carbamoyl-phosphate synthetase), a glycoprotein of 43-44 kDa (previously considered as the calcium uniporter), and 29-30 kDa protein were found. These components were absent (130 kDa component) or relatively reduced (43-44 kDa and 29-30 kDa components) in the inner membrane of hepatoma mitochondria. Previously unknown low molecular mass polypeptides, having very high Ca2+-binding ability, were found in the inner membrane of hepatoma mitochondria. One of them might be the natural Ca2+-binding inhibitor of H+-ATPase.

Calcium translocation in liposome systems modeled on the mitochondrial inner membrane

Ca2+ uptake by liposomes consisting of phosphatidylcholine (PC) and cardiolipin (CL) has recently been reported (Smaal, E.B. et al. (1987) Biochim. Biophys. Acta 897, 191-196). In eukaryotic cells, CL is localized exclusively in the inner mitochondrial membrane, where it occurs in the presence of equimolar amounts of PC and phosphatidyl-ethanolamine (PE). We have therefore re-examined CL-mediated Ca2+ translocation in liposomes of more nearly physiological composition, i.e., PC/PE/CL (2:2:1 and 4:4:1, mol/mol). In addition, the effect on Ca2+ uptake of plasmalogens of PE, which may account for up to 50% of mitochondrial PE, was determined. Our findings can be summarized as follows. (1) Ca2+ uptake into CL-containing liposomes was increased dramatically by PE. (2) Ca2+ entry into PC/CL liposomes was biphasic; in the presence of PE, uptake was dominated by a slow process. (3) Ca2+ uptake by PC/CL liposomes saturated at less than or equal to 2 mM external Ca2+, whereas uptake into PC/PE/CL liposomes increased with increasing Ca2+ concentration up to 10 mM or until Ca2+ release ensued. (4) Ca2+ translocation by PE-containing liposomes and the slow phase of Ca2+ uptake into PC/CL liposomes were similarly and highly dependent on temperature. It can therefore be proposed that PE amplifies the slow component of CL-mediated Ca2+ translocation. This process is characterized by a requirement for high external Ca2+ concentrations and a large apparent activation energy. Ca2+ uptake was not significantly modified by plasmalogens of PE.

Mitochondrial boundary membrane contact sites in brain: points of hexokinase and creatine kinase location, and control of Ca2+ transport

The location of hexokinase at the surface of brain mitochondria was investigated by electron microscopy using immuno-gold labelling techniques. The enzyme was located where the two mitochondrial limiting membranes were opposed and contact sites were possible. Disruption of the outer membrane by digitonin did not remove bound hexokinase and creatine kinase from brain mitochondria, although the activity of outer membrane markers and adenylate kinase decreased, suggesting a preferential location of both enzymes in the contact sites. In agreement with that, a membrane fraction was isolated from osmotically lysed rat brain mitochondria in which hexokinase and creatine kinase were concentrated. The density of this kinase-rich fraction was specifically increased by immuno-gold labelling of hexokinase, allowing a further purification by density gradient centrifugation. The fraction was composed of inner and outer limiting membrane components as shown by the specific marker enzymes, succinate dehydrogenase and NADH-cytochrome-c-oxidase (rotenone insensitive). As reported earlier for the enriched contact site fraction of liver mitochondria the fraction from brain mitochondria contained a high activity of glutathione transferase and a low cholesterol concentration. Moreover, the contacts showed a higher Ca2+ binding capacity in comparison to outer and inner membrane fractions. This finding may have regulatory implications because glucose phosphorylation via hexokinase activated the active Ca2+ uptake system and inhibited the passive efflux, resulting in an increase of intramitochondrial Ca2+.

Influence of Ca2+ on the isolation from rat brain mitochondria of a fraction enriched of boundary membrane contact sites

Data have been obtained suggesting that the complex porin-hexokinase of brain mitochondria may be related to the contact sites between the outer and inner membrane. In the attempt to isolate from brain mitochondria the inner and outer membranes and the boundary membrane contacts, a procedure was developed based on swelling and shrinking of the organelles, followed by sonication and reverse discontinuous density gradient centrifugation. Three fractions were obtained by this technique, which were identified by measuring the relative specific activities of marker enzymes, namely succinate-cytochrome c reductase; NADH-cytochrome c reductase (rotenone insensitive); hexokinase and glutathione transferase, for the inner and outer membranes and contact sites, respectively. The fraction which contains the contact sites is characterized by the highest specific activity of hexokinase and glutathione transferase and by the highest calcium binding capacity; physiological concentrations of this cation produces a sharper separation of this fraction. Results indicate that both the porin-hexokinase gating system of the outer membrane and the calcium transporting complex of the inner membrane are present in the fraction which contains the contact sites.

The participation of NADP, the transmembrane potential and the energy-linked NAD(P) transhydrogenase in the process of Ca2+ efflux from rat liver mitochondria

The pyridine nucleotide specificity, the participation of delta psi, and the energy-linked transhydrogenase in the process of Ca2+ efflux stimulated by the oxidized state of NAD(P) were examined in rat liver mitochondria energized by ascorbate + TMPD. The following observations were made: The Ca2+ efflux rate is independent of the redox state of mitochondrial NAD, but is at a minimum when mitochondrial NADP is in the reduced state and accelerated several-fold when it is in the oxidized state. When the redox state of NADP is shifted to a more oxidized state, the steady-state level of Ca2+ in the medium increased and delta psi decreased in proportion to the mitochondrial NADP+ level. The activity of the energy-linked NAD(P) transhydrogenase seems to be a key element in determining the redox state of NADP and thus of Ca2+ retention and efflux from mitochondria.

The role of hexokinase as a possible modulator of Ca2+ movements in isolated rat brain mitochondria

The present study shows that in brain mitochondria the active calcium uptake and the sodium-dependent calcium efflux are modulated by the porin-bound enzyme hexokinase. The release of the enzyme, promoted by glucose-6-phosphate (G-6-P), under conditions which do not affect mitochondrial functions, is accompanied by a decrease of the rates of fluxes of the cation. This phenomenon is discussed and correlated with the formation of microcompartments between the inner and outer mitochondrial membranes, where the hexokinase-porin complex may constitute a regulating gate system for calcium.

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.

The Ca2+-binding glycoprotein as the site of metabolic regulation of mitochondrial Ca2+ movements

A change in the redox state of pyridine nucleotides such as that evoked by addition of oxaloacetate has been shown to promote Ca2+ efflux from Ca2+ pre-loaded respiring mitochondria. An affinity-chromatography-purified antibody preparation obtained against the mitochondrial Ca2+-binding glycoprotein inhibits the phenomenon. This finding suggests that the glycoprotein is involved also in the oxaloacetate-induced Ca2+ release. This conclusion is reinforced by the finding that Ca2+-binding glycoprotein shows four sites per molecule where the pyridine nucleotides may be bound. Binding of NAD+ occurs preferentially over the others and the binding shows positive cooperativity, indicating that the glycoprotein undergoes an allosteric change upon NAD+ binding. Interestingly, in addition, NAD+ lowers the affinity of the glycoprotein for Ca2+. The effect cannot be induced by NADH. Pyridine nucleotide phosphates, NADP+ and NADPH, are essentially not bound. The results are consistent with the view that the glycoprotein is the site of regulation of Ca2+ equilibration across the mitochondrial membrane and make it possible to conclude that the effector in the phenomenon is NAD+.