Alamethicin as a permeabilizing agent for measurements of Ca(2+)-dependent ATPase activity in proteoliposomes, sealed membrane vesicles, and whole cells

Calcium occlusion in plasma membrane Ca2+-ATPase

In this work, we set out to identify and characterize the calcium occluded intermediate(s) of the plasma membrane Ca(2+)-ATPase (PMCA) to study the mechanism of calcium transport. To this end, we developed a procedure for measuring the occlusion of Ca(2+) in microsomes containing PMCA. This involves a system for overexpression of the PMCA and the use of a rapid mixing device combined with a filtration chamber, allowing the isolation of the enzyme and quantification of retained calcium. Measurements of retained calcium as a function of the Ca(2+) concentration in steady state showed a hyperbolic dependence with an apparent dissociation constant of 12 ± 2.2 μM, which agrees with the value found through measurements of PMCA activity in the absence of calmodulin. When enzyme phosphorylation and the retained calcium were studied as a function of time in the presence of La(III) (inducing accumulation of phosphoenzyme in the E(1)P state), we obtained apparent rate constants not significantly different from each other. Quantification of EP and retained calcium in steady state yield a stoichiometry of one mole of occluded calcium per mole of phosphoenzyme. These results demonstrate for the first time that one calcium ion becomes occluded in the E(1)P-phosphorylated intermediate of the PMCA.

Rb(+) occlusion stabilized by vanadate in gastric H(+)/K(+)-ATPase at 25°C

Despite its similarity with the Na(+)/K(+)-ATPase, it has not been possible so far to isolate a K(+)-occluded state in the H(+)/K(+)-ATPase at room temperature. We report here results on the time course of formation of a state containing occluded Rb(+) (as surrogate for K(+)) in H(+)/K(+)-ATPase from gastric vesicles at 25°C. Alamethicin (a pore-forming peptide) showed to be a suitable agent to open vesicles, allowing a more efficient removal of Rb(+) ions from the intravesicular medium than C(12)E(8) (a non-ionic detergent). In the presence of vanadate and Mg(2+), the time course of [(86)Rb]Rb(+) uptake displayed a fast phase due to Rb(+) occlusion. The specific inhibitor of the H(+)/K(+)-ATPase SCH28080 significantly reduces the amount of Rb(+) occluded in the vanadate-H(+)/K(+)-ATPase complex. Occluded Rb(+) varies with [Rb(+)] according to a hyperbolic function with K(0.5)=0.29±0.06mM. The complex between the Rb(+)-occluded state and vanadate proved to be very stable even after removal of free Mg(2+) with EDTA. Our results yield a stoichiometry lower than one occluded Rb(+) per phosphorylation site, which might be explained assuming that, unlike for the Na(+)/K(+)-ATPase, Mg(2+)-vanadate is unable to recruit all the Rb(+)-bound to the Rb(+)-occluded form of the Rb(+)-vanadate-H(+)/K(+)-ATPase complex.

Alamethicin permeabilizes the plasma membrane and mitochondria but not the tonoplast in tobacco (Nicotiana tabacum L. cv Bright Yellow) suspension cells

The ion channel-forming peptide AlaM (alamethicin) is known to permeabilize isolated mitochondria as well as animal cells. When intact tobacco (Nicotiana tabacum L.) Bright Yellow-2 cells were treated with AlaM, the cells became permeable for low-molecular-mass molecules as shown by induced leakage of NAD(P)+. After the addition of cofactors and substrates, activities of cytosolic as well as mitochondrial respiratory enzymes could be directly determined inside the permeabilized cells. However, at an AlaM concentration at which the cytoplasmic enzymes were maximally accessible, the vacuole remained intact, as indicated by an unaffected tonoplast proton gradient. Low-flux permeabilization of plasma membranes and mitochondria at moderate AlaM concentrations was reversible and did not affect cell vigour. Higher AlaM concentrations induced cell death. After the addition of catalase that removes the H2O2 necessary for NADH oxidation by apoplastic peroxidases, mitochondrial oxygen consumption could be measured in permeabilized cells. Inhibitor-sensitive oxidation of the respiratory substrates succinate, malate and NADH was observed after the addition of the appropriate coenzymes (ATP, NAD+). The capacities of different pathways in the respiratory electron-transport chain could thus be determined directly. We conclude that AlaM permeabilization provides a very useful tool for monitoring metabolic pathways or individual enzymes in their native proteinaceous environment with controlled cofactor concentrations. Possible uses and limitations of this method for plant cell research are discussed.

Oxidation and reduction of pyridine nucleotides in alamethicin-permeabilized plant mitochondria

The inner mitochondrial membrane is selectively permeable, which limits the transport of solutes and metabolites across the membrane. This constitutes a problem when intramitochondrial enzymes are studied. The channel-forming antibiotic AlaM (alamethicin) was used as a potentially less invasive method to permeabilize mitochondria and study the highly branched electron-transport chain in potato tuber (Solanum tuberosum) and pea leaf (Pisum sativum) mitochondria. We show that AlaM permeabilized the inner membrane of plant mitochondria to NAD(P)H, allowing the quantification of internal NAD(P)H dehydrogenases as well as matrix enzymes in situ. AlaM was found to inhibit the electron-transport chain at the external Ca2+-dependent rotenone-insensitive NADH dehydrogenase and around complexes III and IV. Nevertheless, under optimal conditions, especially complex I-mediated NADH oxidation in AlaM-treated mitochondria was much higher than what has been previously measured by other techniques. Our results also show a difference in substrate specificities for complex I in mitochondria as compared with inside-out submitochondrial particles. AlaM facilitated the passage of cofactors to and from the mitochondrial matrix and allowed the determination of NAD+ requirements of malate oxidation in situ. In summary, we conclude that AlaM provides the best method for quantifying NADH dehydrogenase activities and that AlaM will prove to be an important method to study enzymes under conditions that resemble their native environment not only in plant mitochondria but also in other membrane-enclosed compartments, such as intact cells, chloroplasts and peroxisomes.

Quantitation of cytochrome c release from rat liver mitochondria

The apoptogenic protein cytochrome c can be quantitated by reverse-phase HPLC, but this method is not utilized by those who investigate mechanisms of cell death. Here, we extend the sensitivity of the method to exceed that available from immunogenic approaches and report specific procedures for applying the method to preparations of intact mitochondria, and to supernatants and pellets that arise from mitochondrial incubations. The detection limit corresponds to 0.6% of total cytochrome c found in 100 microg of rat liver mitochondrial protein, or to all of the cytochrome c that is expected in approximately 6000 hepatocytes. A single determination can be completed in 20 min, compared to a time scale of days for Western blotting methods, or hours for ELISA-based methods. The procedures are illustrated by experiments that determine the amount of cytochrome c released following the mitochondrial permeability transition as a function of medium ionic strength, and by long-term incubations of intact mitochondria in the presence and absence of an exogenous oxidizable substrate. Swelling and the release of adenylate kinase activity have been determined simultaneously to show how the data can be applied to evaluate the role of outer membrane disruption in mechanisms that release cytochrome c.

Cytochemical localisation of calcium ATPase activity during the erythrocytic cell cycle of Plasmodium falciparum

Using a cytochemical technique, we evaluated the levels of Ca(2+)-ATPase activity in the plasmatic and in the parasitophorous vacuole membrane through the different developmental stages of the Plasmodium falciparum parasitised erythrocyte. We found that the activity is detectable and remains unaltered in the plasma membrane throughout the 48 h cell cycle. However, in the parasitophorous membrane, although the activity was very similar to that measured in the plasma membrane of the young stages (younger than 20-h-old parasites), it diminished gradually with maturation and in schizonts it was almost undetectable. These data suggest that the plasma membrane Ca(2+)-ATPase is important in the maintenance of a low erythrocyte cytoplasmic Ca(2+) concentration, and that in addition it could be a way to supply the vital cation to the parasite at the beginning of the infection, when other transport mechanisms have not yet developed.

Effect of membrane fluidity on tyrosine kinase activity of reconstituted epidermal growth factor receptor

Epidermal growth factor receptor (EGFR) was functionally reconstituted into liposome membrane. Triton X-100 was removed by Bio-beads SM-2. More than 80% of the reconstituted EGFR possessed right-side-out orientation with the EGF binding side facing the medium. The tyrosine kinase assay of the EGFR was carried out in the presence of the antibiotic alamethicin. The reconstituted EGFR tyrosine kinase was well activated by EGF. The influence of lipid composition on tyrosine kinase activity was investigated. Introduction of cholesterol into the dioleoylphophatidylcholine (DOPC) liposome membrane resulted in the decrease of tyrosine kinase activity. The tyrosine kinase activity of EGFR in distearylphosphatidylcholine liposome was much lower than that of EGFR-DOPC proteoliposome. Results indicated the importance of membrane fluidity on the apparent tyrosine kinase activity of reconstituted EGFR.

Reversible thiol-dependent activation of ryanodine-sensitive Ca2+ release channel by etoposide (VP-16) phenoxyl radical

Many phenolic compounds can act as antioxidants by donating a proton to peroxyl radicals and quenching lipid peroxidation. Phenoxyl radicals produced this way or from metabolism by peroxidases, tyrosinase, or mixed-function oxidases, however, may react with sulfhydryl groups of proteins and other endogenous thiols. In this regard, phenolic compounds may have cytotoxic potential instead of antioxidant effects. We employed the anticancer drug, etoposide (VP-16), as a model phenolic compound to study the sensitivity of ryanodine-sensitive Ca2+ channel (RyR) to VP-16 phenoxyl radicals. The combination of VP-16 and tyrosinase, used to generate the etoposide phenoxyl radical, produced marked Ca2+ release from Ca2+-loaded RyR-rich vesicles prepared from terminal cisternae fraction of sarcoplasmic reticulum (SR). This effect was reversed by the SH-reagent, dithiothreitol (DTT), suggesting that cysteines within the RyR-protein complex were targets for modification by VP-16 phenoxyl radicals. VP-16/tyrosinase-induced release of Ca2+ was attenuated in vesicles prepared from longitudinal SR, which contain relatively little RyR. The effects of the VP-16 phenoxyl radical on Ca2+-ATPase in SR vesicles resembled those observed with caffeine or 4,4'-dithiodipyridine, both of which activate RyR Ca2+ release and lead to activation of Ca2+-ATPase via prolonged Ca2+ cycling. The addition of ruthenium red returned Ca2+-ATPase to its original level. Thus, under these conditions Ca2+-ATPase was not directly affected by VP-16 phenoxyl radical. The hypersensitive SH-groups on RyR are shown to be targets for oxidation of VP-16 phenoxyl radical, and suggest that other phenolic compounds could similarly disrupt Ca2+ homeostasis.

Na+/K+-ATPase activity during early development and growth of an Antarctic sea urchin

In Antarctic environments, the physiological bases for long larval life spans under natural conditions of limited food availability are not understood. The Na+ pump is likely to be involved with hypometabolic regulation in such cold environments. Changes in the activity and metabolic importance of Na+/K+-ATPase were measured in embryos of the Antarctic sea urchin Sterechinus neumayeri and in larvae reared under different feeding conditions. The rate of increase of total Na+/K+-ATPase activity was 3.9 times faster in fed than in unfed larvae. During development and growth, there was an increase in the percentage of total, potential Na+/K+-ATPase activity that was physiologically utilized. In early (10-day-old) gastrulae, 17 % was utilized in vivo, increasing to 77 % in six-arm pluteus (48-day-old) larvae. The metabolic importance of in vivo Na+/K+-ATPase activity also increased during development, accounting for 12 % of metabolic rate at day 10 and 84 % at day 48. When compared at the same enzyme assay temperature (15 degrees C), the protein-specific total Na+/K+-ATPase activities for late embryonic (prism) and early larval (pluteus) stages of S. neumayeri were 2.6 times lower than those for comparable developmental stages of two temperate sea urchin species (Strongylocentrotus purpuratus and Lytechinus pictus).

Properties of a cyclosporin-insensitive permeability transition pore in yeast mitochondria

Yeast mitochondria (Saccharomyces cerevisiae) contain a permeability transition pore which is regulated differently than the pore in mammalian mitochondria. In a mannitol medium containing 10 mM Pi and ethanol (oxidizable substrate), yeast mitochondria accumulate large amounts of Ca2+ (>400 nmol/mg of protein) upon the addition of an electrophoretic Ca2+ ionophore (ETH129). Pore opening does not occur following Ca2+ uptake, even though ruthenium red-inhibited rat liver mitochondria undergo rapid pore opening under analogous conditions. However, a pore does arise in yeast mitochondria when Ca2+ and Pi are not present, as monitored by swelling, ultrastructure, and matrix solute release. Pore opening is slow unless a respiratory substrate is provided (ethanol or NADH) but also occurs rapidly in response to ATP (2 mM) when oligomycin is present. Pi and ADP inhibit pore opening (EC50 approximately 1 and 4 mM, respectively), however, cyclosporin A (7 microg/ml), oligomycin (20 microg/ml), or carboxyatractyloside (25 microM) have no effect. The pore arising during respiration is also inhibited by nigericin or uncoupler, indicating that an acidic matrix pH antagonizes the process. Pi also inhibits pore opening by lowering the matrix pH (Pi/OH- antiport). However, inhibition of the ATP-induced pore by Pi is seen in the presence of mersalyl, suggesting a second mechanism of action. Since pore induction by ATP is not sensitive to carboxyatractyloside, ATP appears to act at an external site and Pi may antagonize the interaction. Isoosmotic polyethylene glycol-induced contraction of yeast mitochondria swollen during respiration, or in the presence of ATP, is 50% effective at a solute size of 1.0-1.1 kDa. This suggests that the same pore is induced in both cases and is comparable in size with the permeability transition pore of heart and liver mitochondria.

Molecular dynamics simulation of conformational flexibility of alamethicin fragments in aqueous and membranous environment

We present here results on molecular dynamics (MD) simulation on two fragments of channel forming antibiotic peptide Alamethicin, containing isoamino butyric acid (Aib). Simulations are carried out in aqueous and membranous environment in a bilayer of 39 molecules of Dimyristoyl phosphatidyl choline (DMPC). The peptides Boc-Pro-Aib-Ala-Aib-OBzl (Alam 1) and Boc-Leu-Aib-Pro-OBzl (Alam 2) were simulated from their crystallographic coordinates. The bilayers were built from two different conformations (A and B) of DMPC reported in crystal data. The P-N dipoles were arranged hexagonally with surface area per lipid molecule 66.5 A degrees 2 and P-P separation across the bilayer 34 A degrees. They were hydrated by 28.6 and 25.5 water molecules per DMPC molecule. Simulations are done using AMBER 4.0 package in constant number volume temperature (NVT) condition for 100 pico seconds (ps) in aqueous environment and 250 ps of equilibrated bilayer. Geometric parameters of lipids as: bilayer thickness, order parameter of the chains, transfraction of chain torsional angles were monitored. We also monitored geometric parameters of the peptides as backbone torsional angles, distances amongst C alpha atoms, angles between C alpha atoms, movement of center of gravity (CG) along and perpendicular to bilayer normal. We find that membrane bilayer is slightly disturbed due to the presence of peptides. In case of alam 2 in water angles phi 1 and phi 3 showed larger variation in water compared to same in the bilayer. The peptide conformation is more stable in DMPC bilayer. However the peptides showed movement along and perpendicular to bilayer normal. This we believe is due to hydrophobic nature of these peptides.

Pulmonary microsomes contain a Ca(2+)-transport system sensitive to oxidative stress

A variety of events, including inhalation of atmospheric chemicals, trauma, and ischemia-reperfusion, may cause generation of reactive oxygen species in the lung and result in airways constriction. The specific metabolic mechanisms that translate oxygen radical production into airways constriction are yet to be identified. In the lung, calcium homeostasis is central to release of bronchoactive and vasoactive chemical mediators and to regulation of smooth muscle cell contractility, i.e., airway constriction. In the present work, we characterized Ca(2+)-transport in the microsomal fraction of mouse lungs, and determined how reactive oxygen species, generated by Fe2+/ascorbate and H2O2/hemoglobin, affected Ca2+ transport. The microsomal fraction of pulmonary tissue accumulated 90 +/- 5 nmol Ca2+/mg protein by an ATP-dependent process in the presence of 15 mM oxalate, and 16 +/- 2 nmol Ca2+ in its absence. In the presence of oxalate, the rate of Ca2+ uptake was 50 +/- 5 nmol Ca2+/min per mg protein at pCa 5.9 (37 degrees C). The Ca(2+)-ATPase activity was 50-60 nmol Pi/min per mg protein (pCa 5.9, 37 degrees C) in the presence of alamethicin. Inhibitors of mitochondrial H(+)-ATPase had no effect on the Ca2+ transport. Half-maximal activation of Ca2+ transport was produced by 0.4-0.5 microM Ca2+. Endoplasmic reticulum Ca(2+)-pump (SERC-ATPase) was found to be predominantly responsible for the Ca(2+)-accumulating capacity of the pulmonary microsomes. Incubation of the microsomes in the presence of either Fe2+/ascorbate or H2O2/hemoglobin resulted in a time-dependent accumulation of peroxidation products (TBARS) and in inhibition of the Ca2+ transport. The inhibitory effect of Fe2+/ascorbate on Ca2+ transport strictly correlated with the inhibition of the Ca(2+)-ATPase activity. These results are the first to indicate a highly active microsomal Ca2+ transport system in murine lungs which is sensitive to endogenous oxidation products. The importance of this system to pulmonary disorders exacerbated by oxidative chemicals remains to be studied.