The sarcoplasmic calcium pump

Structures of the Ca2+-ATPase complexes with ATP, AMPPCP and AMPPNP. An FTIR study

We studied binding of ATP and of the ATP analogs adenosine 5'-(beta,gamma-methylene)triphosphate (AMPCP) and beta,gamma-imidoadenosine 5'-triphosphate (AMPPNP) to the Ca(2+)-ATPase of the sarcoplasmic reticulum membrane (SERCA1a) with time-resolved infrared spectroscopy. In our experiments, ATP reacted with ATPase which had AMPPCP or AMPPNP bound. These experiments monitored exchange of ATP analog by ATP and phosphorylation to the first phosphoenzyme intermediate Ca(2)E1P. These reactions were triggered by the release of ATP from caged ATP. Only small differences in infrared absorption were observed between the ATP complex and the complexes with AMPPCP and AMPPNP indicating that overall the interactions between nucleotide and ATPase are similar and that all complexes adopt a closed conformation. The spectral differences between ATP and AMPPCP complex were more pronounced at high Ca(2+) concentration (10 mM). They are likely due to a different position of the gamma-phosphate which affects the beta-sheet in the P domain.

FTIR studies on the bond properties of the aspartyl phosphate moiety of the Ca2+–ATPase

As part of our work to determine the bond properties of the aspartyl phosphate moiety of the Ca(2+)-ATPase (SERCA1a) phosphoenzymes, we analyzed Morse potentials of the bridging P-O bond as well as C=O bond strengths for the model compound acetyl phosphate and the two phosphoenzyme intermediates Ca(2)E1P and E2P. Reaction-induced infrared difference spectroscopy was used and a carbonyl band of E2P at 1708 cm(-1) in the presence of mM Mg(2+) was tentatively assigned to the carbonyl group of phosphorylated Asp(351) because of its sensitivity to divalent cations. This band is found at 1716 cm(-1) with mM Ca(2+), for Ca(2)E1P at 1717 cm(-1) with Mg(2+), and at 1719 cm(-1) with Ca(2+) and at 1718 cm(-1) for acetyl phosphate in the absence of divalent cations. The similar band positions indicate similar strengths of interaction of the carbonyl oxygen in acetyl phosphate and the two phosphoenzymes. Together with information on the P-O bond strengths, this implies that the bridging oxygen exerts stronger interactions in the phosphoenzymes than in acetyl phosphate.

Interactions of phosphate groups of ATP and Aspartyl phosphate with the sarcoplasmic reticulum Ca2+-ATPase: an FTIR study

Phosphate binding to the sarcoplasmic reticulum Ca2+-ATPase was studied by time-resolved Fourier transform infrared spectroscopy with ATP and isotopically labeled ATP ([beta-18O2, betagamma-18O]ATP and [gamma-18O3]ATP). Isotopic substitution identified several bands that can be assigned to phosphate groups of bound ATP: bands at 1260, 1207, 1145, 1110, and 1085 cm(-1) are affected by labeling of the beta-phosphate, bands likely near 1154, and 1098-1089 cm(-1) are affected by gamma-phosphate labeling. The findings indicate that the strength of interactions of beta- and gamma- phosphate with the protein are similar to those in aqueous solution. Two bands, at 1175 and 1113 cm(-1), were identified for the phosphate group of the ADP-sensitive phosphoenzyme Ca2E1P. They indicate terminal and bridging P-O bond strengths that are intermediate between those of ADP-insensitive phosphoenzyme E2P and the model compound acetyl phosphate in water. The bridging bond of Ca2E1P is weaker than for acetyl phosphate, which will facilitate phosphate transfer to ADP, but is stronger than for E2P, which will make the Ca2E1P phosphate less susceptible to attack by water.

Mapping interactions between the Ca2+-ATPase and its substrate ATP with infrared spectroscopy

Infrared spectroscopy has been used to map substrate-protein interactions: the conformational changes of the sarcoplasmic reticulum Ca(2+)-ATPase upon nucleotide binding and ATPase phosphorylation were monitored using the substrate ATP and ATP analogues (2'-deoxy-ATP, 3'-deoxy-ATP, and inosine 5'-triphosphate), which were modified at specific functional groups of the substrate. Modifications to the 2'-OH, the 3'-OH, and the amino group of adenine reduce the extent of binding-induced conformational change of the ATPase, with particularly strong effects observed for the latter two. This demonstrates the structural sensitivity of the nucleotide-ATPase complex to individual interactions between nucleotide and ATPase. All groups studied are important for binding and interactions of a given ligand group with the ATPase depend on interactions of other ligand groups. Phosphorylation of the ATPase was observed for ITP and 2'-deoxy-ATP, but not for 3'-deoxy-ATP. There is no direct link between the extent of conformational change upon nucleotide binding and the rate of phosphorylation showing that the full extent of the ATP-induced conformational change is not mandatory for phosphorylation. As observed for the nucleotide-ATPase complex, the conformation of the first phosphorylated ATPase intermediate E1PCa(2) also depends on the nucleotide, indicating that ATPase states have a less uniform conformation than previously anticipated.

Characterisation of thapsigargin-releasable Ca(2+) from the Ca(2+)-ATPase of sarcoplasmic reticulum at limiting [Ca(2+)]

The Ca(2+) binding sites of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR) have been identified as two high-affinity sites orientated towards the cytoplasm, two sites of low affinity facing the lumen, and a transient occluded species that is isolated from both membrane surfaces. Binding and release studies, using (45)Ca(2+), have invoked models with sequential binding and release from high- and low-affinity sites in a channel-like structure. We have characterised turnover conditions in isolated SR vesicles with oxalate in a Ca(2+)-limited state, [Ca(2)](lim), where both high- and low-affinity sites are vacant in the absence of chelators (Biochim. Biophys. Acta 1418 (1999) 48-60). Thapsigargin (TG), a high-affinity specific inhibitor of the Ca(2+)-ATPase, released a fraction of total Ca(2+) at [Ca(2+)](lim) that accumulated during active transport. Maximal Ca(2+) release was at 2:1 TG/ATPase. Ionophore, A23187, and Triton X-100 released the rest of Ca(2+) resistant to TG. The amount of Ca(2+) released depended on the incubation time at [Ca(2+)](lim), being 3.0 nmol/mg at 20 s and 0.42 nmol/mg at 1000 s. Rate constants for release declined from 0. 13 to 0.03 s(-1). The rapidly released early fraction declined with time and k=0.13 min(-1). Release was not due to reversal of the pump cycle since ADP had no effect; neither was release impaired with substrates acetyl phosphate or GTP. A phase of reuptake of Ca(2+) followed release, being greater with shorter delay (up to 200 s) following active transport. Reuptake was minimal with GTP, with delays more than 300 s, and was abolished by vanadate and at higher [TG], >5 microM. Ruthenium red had no effect on efflux, indicating that ryanodine-sensitive efflux channels in terminal cisternal membranes are not involved in the Ca(2+) release mechanism. It is concluded that the Ca(2+) released by TG is from the occluded Ca(2+) fraction. The Ca(2+) occlusion sites appear to be independent of both high-affinity cytoplasmic and low-affinity lumenal sites, supporting a multisite 'in line' sequential binding mechanism for Ca(2+) transport.

Regulation of Ca2+ transport by sarcoplasmic reticulum Ca2+-ATPase at limiting [Ca2+]

The factors regulating Ca2+ transport by isolated sarcoplasmic reticulum (SR) vesicles have been studied using the fluorescent indicator Fluo-3 to monitor extravesicular free [Ca2+]. ATP, in the presence of 5 mM oxalate, which clamps intravesicular [Ca2+] at approximately 10 microM, induced a rapid decline in Fluo-3 fluorescence to reach a limiting steady state level. This corresponds to a residual medium [Ca2+] of 100 to 200 nM, and has been defined as [Ca2+]lim, whilst thermodynamic considerations predict a level of less than 1 nM. This value is similar to that measured in intact muscle with Ca2+ fluophores, where it is presumed that sarcoplasmic free [Ca2+] is a balance between pump and leaks. Fluorescence of Fluo-3 at [Ca2+]lim was decreased 70% to 80% by histidine, imidazole and cysteine. The K0.5 value for histidine was 3 mM, suggesting that residual [Ca2+]lim fluorescence is due to Zn2+. The level of Zn2+ in preparations of SR vesicles, measured by atomic absorption, was 0.47+/-0.04 nmol/mg, corresponding to 0.1 mol per mol Ca-ATPase. This is in agreement with findings of Papp et al. (Arch. Biochem. Biophys., 243 (1985) 254-263). Histidine, 20 mM, included in the buffer, gave a corrected value for [Ca2+]lim of 49+/-1.8 nM, which is still higher than predicted on thermodynamic grounds. A possible 'pump/leak' mechanism was tested by the effects of varying active Ca2+ transport 1 to 2 orders with temperature and pH. [Ca2+]lim remained relatively constant under these conditions. Alternate substrates acetyl phosphate and p-NPP gave similar [Ca2+]lim levels even though the latter substrate supported transport 500-fold slower than with ATP. In fact, [Ca2+]lim was lower with 10 mM p-NPP than with 5 mM ATP. The magnitude of passive efflux from Ca-oxalate loaded SR during the steady state of [Ca2+]lim was estimated by the unidirectional flux of 45Ca2+, and directly, following depletion of ATP, by measuring release of 40Ca2+, and was 0.02% of Vmax. Constant infusion of CaCl2 at [Ca2+]lim resulted in a new steady state, in which active transport into SR vesicles balances the infusion rate. Varying infusion rates allows determination of [Ca2+]-dependence of transport in the absence of chelating agents. Parameters of non-linear regression were Vmax=853 nmol/min per mg, K0.5(Ca)=279 nM, and nH(Ca)=1.89. Since conditions employed in this study are similar to those in the sarcoplasm of relaxed muscle, it is suggested that histidine, added to media in studies of intracellular Ca2+ transients, and in the relaxed state, will minimise contribution of Zn2+ to fluophore fluorescence, since it occurs at levels predicted in this study to cause significant overestimation of cytoplasmic free [Ca2+] in the relaxed state. Similar precautions may apply to non-muscle cells as well. This study also suggests that [Ca2+]lim in the resting state is a characteristic feature of Ca2+ pump function, rather than a balance between active transport and passive leakage pathways.

Inactivation and phosphorylation of sarcoplasmic reticulum Ca(2+)-ATPase by Mg.ATP analogues Rh(III)-ATP and Co(III)-ATP

The interaction of sarcoplasmic reticulum Ca(2+)-ATPase with the Mg.ATP analogues Rh(H2O)4ATP and Co(NH3)4ATP have been examined. Co(NH3)4ATP slowly inactivates Ca(2+)-ATPase in a first order process, with a rate constant of 1.13 x 10(-3) s-1 and an apparent inactivation constant, KI, of 32 mM. Rh(H2O)4ATP likewise inactivates sarcoplasmic reticulum Ca(2+)-ATPase, but the plot of reciprocal apparent inactivation rate constants versus 1/[Rh(H2O)4ATP] is biphasic. The chi-intercepts of this plot yield apparent inactivation constants for the inhibition of Ca(2+)-ATPase by Rh(H2O)4ATP of KI1 = 30 microM and KI2 = 221 microM. The corresponding values of k2, the maximal first-order rate constant for inhibition in these two phases, are 1.16 and 2.19 x 10(-4)s-1. Tridentate Rh(H2O)3ATP also inhibits Ca(2+)-ATPase, but only after much longer incubation times. Ca(2+)-ATPase inactivation is accompanied by incorporation of radioactivity from gamma-32P into an acid-precipitable enzyme. Both processes were dependent on the presence of Ca2+ ions and were quenched by excess ATP. The first-order rate constant for inactivation of Ca(2+)-dependent ATPase activity in this experiment was 2.19 x 10(-4)s-1, and the first-order rate constant for Ca(2+)-dependent E-P formation was 2.07 x 10(-4)s-1, in excellent agreement with the value for inactivation. A linear relationship is observed between ATPase inactivation and E-P formation. Moreover, atomic absorption analysis demonstrates that the phosphorylation of Ca(2+)-ATPase by Rh(H2O)4ATP is accompanied by incorporation and tight binding of rhodium, with a stoichiometry of one rhodium incorporated per ATPase molecule phosphorylated. The characteristics of ATPase inactivation and phosphorylation (i.e., Ca2+ dependence, ATP competition, agreement of rate constants, and stoichiometric rhodium incorporation) suggest that Rh(H2O)4ATP is binding to the catalytic nucleotide site on Ca(2+)-ATPase and producing a highly stable, phosphorylated intermediate.

Participation of a non-covalent phosphointermediate in ATP hydrolysis by the sarcoplasmic reticulum Ca2(+)-cotransport ATPase

With increasing SDS/protein ratios, covalent phosphorylation by ATP and Pi is abolished before ATP hydrolysis (Pi production) ceases. We have shown that the SDS-dependent profiles of the decline in covalent phosphorylation by either substrate are virtually identical, reflecting a common mechanism of detergent interaction, while ATP can be hydrolysed via a non-covalent phosphointermediate. Our studies support that the transfer of both terminal Pi from ATP, as well as Pi to its final binding site, is a multistep reaction involving electrostatic interaction with one or more amino acid side chains, including a Lys residue.

Characterization of a cysteine-containing peptide after affinity labelling of Ca2+-ATPase of sarcoplasmic reticulum with the disulfide of 3'(2')-O-biotinyl-thioinosine triphosphate

3'(2')-O-Biotinyl-thioinosine triphosphate is a substrate of the Ca2+ pump of sarcoplasmic reticulum. Its disulfide inactivates the Ca2+-ATPase with two different velocities. The rapidly inactivated sulfhydryl group cannot be protected by ATP and is therefore considered to be outside the ATP binding site. The slowly reacting sulfhydryl group interacts with the disulfide of 3'(2')-O-biotinyl-thioinosine triphosphate with a dissociation constant of Kd = 137 microM and an inactivation velocity constant of 1.7 X 10(-3) s-1. It is protected by ATP with two different dissociation constants of the enzyme-ATP complex of Kd = 221 microM and 1130 microM. The slowly reacting sulfhydryl group is therefore considered to be part of the ATP binding site. Since it was impossible to isolate a tryptic peptide by affinity purification on matrix-bound avidin after affinity labelling with the disulfide of 3'(2')-O-biotinyl-thioinosine triphosphate, differential labelling with iodo[2-14C]acetic acid after affinity labelling with the disulfide of 3'(2')-O-biotinyl-thioinosine triphosphate was carried out. Tryptic digestion and FPLC purification led to the isolation of a radioactive carboxymethyl derivative of the cysteine-containing peptide ANACNSVIR. This peptide is equivalent to the cDNA-derived sequence 468-476 of Ca2+-ATPase [Brandl et al. (1986) Cell 44, 597-607] and is located between the phosphorylation site, Asp351, and Lys515, a part of the putative purine binding subsite of ATP. Although the carboxymethylation of Cys471 is hindered by (biotinyl-s6ITP)2, the strong dilution of the specific radioactivity of iodo[2-14C]acetic acid in the isolated peptide 468-476 argues against its direct interaction with the ATP analogue. It is therefore proposed that Cys471 undergoes ATP-dependent conformational changes.

Pathways of calcium release from heavy sarcoplasmic reticulum vesicles isolated from rabbit skeletal muscle

The active uptake and efflux of Ca2+ from suspensions of vesicles from heavy rabbit muscle sarcoplasmic reticulum have been examined using the antipyrylazo III dye method in the presence of various nucleotide triphosphate substrates to support active Ca2+ accumulation. On addition of ATP, Ca2+ is rapidly accumulated and maintained at high internal concentrations until the substrate for pump protein is exhausted. Ca2+-induced Ca2+ release which is inhibited by ruthenium red can be demonstrated. The kinetics of Ca2+ release via these channels is different from the Ca2+ efflux observed after substrate exhaustion. This rate was found to be dependent on the type of nucleotide triphosphate, decreasing in the order ATP greater than GTP greater than CTP greater than ITP UTP. It is suggested that different conformations of the Ca2+ pump protein induced by the different substrates may result in the creation of pathways for the facilitated diffusion of Ca2+.

Effect of short-chain primary alcohols on fluidity and activity of sarcoplasmic reticulum membranes

Intramolecular excimer formation with the fluorescent probe 1,3-di(1-pyrenyl)propane, differential scanning calorimetry, and X-ray diffraction were used to assess the effect of ethanol, 1-butanol, and 1-hexanol on the bilayer organization in model membranes, sarcoplasmic reticulum (SR) lipids and native SR membranes. These alcohols have fluidizing effects on membranes and lower the main transition temperature of dimyristoylphosphatidylcholine (DMPC), but only 1-hexanol alters the cooperativity of the phase transition and significantly increases the thickness of DMPC bilayers. The interaction of the three alcohols with the SR Ca2+ pump was also investigated. Hydrolysis of ATP and coupled Ca2+ uptake are differently sensitive to the three alcohols. Whereas ethanol and 1-butanol inhibited the Ca2+ uptake, 1-hexanol stimulated it. Nevertheless, the energetic efficiency of the pump (Ca2+/ATP) is not significantly affected by ethanol or 1-hexanol, but uncoupling was observed with 1-butanol at high concentrations. The different effects of alcohols on the activity of SR membranes rule out an unitary mechanism of action on the basis of fluidity changes induced in the lipid bilayer. Depending on the chain length, the alcohols interact with the SR membranes in different domains, perturbing differently the Ca2+-pump activity.

Retention of ellipticity between enzymatic states of the Ca2+-ATPase of sarcoplasmic reticulum

Circular dichroism spectra in the peptide region were obtained from the Ca2+-ATPase of sarcoplasmic reticulum, to establish whether transitions of intermediate states of the enzyme cycle are accompanied by large changes of secondary structure. Since membrane-bound ATPase was used to avoid denaturation, absolute estimates of secondary structural content could not be obtained, due to light scattering interference. Nevertheless, it was possible to demonstrate unambiguously that nearly constant ellipticity is retained by the enzyme following enzyme transitions produced by calcium binding or phosphorylation, even though conformational changes are revealed by other structural probes under the same conditions. We conclude that the conformational changes involved in the long-range reciprocal influence of calcium and phosphorylation sites are related to ligand-induced displacements of amino acid residues which in turn produce reorientation of whole peptide segments of the ATPase protein. This is contrasted by the behavior of calmodulin which undergoes a definite change in ellipticity upon calcium binding.

Effect of halothane on Ca2+-induced Ca2+ release from sarcoplasmic reticulum vesicles isolated from rat skeletal muscle

Halothane induces the release of Ca2+ from a subpopulation of sarcoplasmic reticulum vesicles that are derived from the terminal cisternae of rat skeletal muscle. Halothane-induced Ca2+ release appears to be an enhancement of Ca2+-induced Ca2+ release. The low-density sarcoplasmic reticulum vesicles which are believed to be derived from nonjunctional sarcoplasmic reticulum lack the capability of both Ca2+-induced and halothane-induced Ca2+ release. Ca2+ release from terminal cisternae vesicles induced by halothane is inhibited by Ruthenium red and Mg2+, and require ATP (or an ATP analogue), KCl (or similar salt) and extravesicular Ca2+. Ca2+-induced Ca2+ release has similar characteristics.

Stabilizing effect of antioxidants and inhibitors of prostaglandin synthesis on after-contractions in Ca2+-overloaded myocardium

Ca2+ overload followed by after-contractions was induced by perfusion of mammalian papillary muscles with O-K0+, high Ca2+ solution. The effect of lipotrophic agents (dexamethasone, indomethacin) or free radical scavengers (alpha-tocopherol, reduced glutathione, synthetic antioxidant) was tested at different stages of Ca2+ overloading processes. All the agents tested proved to be effective in attenuating the after-contractions. Activation of lipid peroxidation has been suggested as one of the possible molecular events underlying mechanical destabilization of Ca2+-overloaded myocardium.

Lipid requirement of the vanadate effect on the binding of calcium and ATP to the calcium transport ATPase of the sarcoplasmic reticulum

Lipid deprivation of the sarcoplasmic reticulum calcium-transport ATPase neither affects the enzyme's affinity for ATP nor that of calcium. In contrast, vanadate binding is almost completely abolished. Lipid substitution by oleic acid which at a ratio of 0.3 mg/mg protein completely reactivates the calcium-dependent ATP hydrolysis restores vanadate binding. Concomitantly the mutual interactions between vanadate and calcium or ATP and ADP, respectively are restored. The vanadate-induced disappearance of the enzyme's ATP binding sites as well as its high-affinity binding sites for calcium follow the same time course. Conversely, the displacement of vanadate by calcium proceeds in parallel with the recovery of ADP binding. In lipid-restituted preparations as well as in native membranes vanadate induces the disappearance of external high-affinity and simultaneously the appearance of internal low-affinity calcium binding sites.

The rate of Ca2+ translocation by sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase measured with intravesicular arsenazo III

Release of Ca2+ from the (Ca2+ + Mg2+)-ATPase into the interior of intact sarcoplasmic reticulum vesicles was measured using arsenazo III, a metallochromic indicator of Ca2+. Arsenazo III was placed inside the sarcoplasmic reticulum vesicles by making the vesicles transiently leaky with an osmotic gradient in the presence of arsenazo III. External arsenazo III was then removed by centrifugation. Addition of ATP to the (Ca2+ + Mg2+)-ATPase in the presence of Ca2+ causes the rapid phosphorylation of the enzyme at which time the bound Ca2+ becomes inaccessible to external EGTA. The release of Ca2+ from the (Ca2+ + Mg2+)-ATPase to the interior of the vesicle measured with intravesicular arsenazo III was much slower indicating that there is an occluded form of the Ca2+-binding site which precedes the release of Ca2+ into the vesicle. The rate of Ca2+ accumulation by sarcoplasmic reticulum vesicles is increased by K+ (5-100 mM) and ATP (50-1000 microM) but the initial rate of Ca2+ translocation measured after the simultaneous addition of ATP and EGTA to vesicles that were preincubated in Ca2+ was not influenced by these concentrations of K+ and ATP.

Direct measurement of intracellular free magnesium in frog skeletal muscle using magnesium-selective microelectrodes

Mg2+-selective microelectrodes have been used to measure the intracellular free Mg2+ concentration in frog skeletal muscle fibers. Glass capillaries with a tip diameter of less than 0.4 micron were backfilled with the Mg2+ sensor, ETH 1117. In the absence of interfering ions, they gave Nernstian responses between 1 and 10 mM free Mg2+. In the presence of an ionic environment resembling the myoplasm, the microelectrode response was sub Nernstian (18-24 mV) but still useful. The electrodes were calibrated before and after muscle-fiber impalements . In quiescent fibers from sartorius muscle (Rana pipiens), with resting membrane potentials not less than -82 mV, the intracellular free Mg2+ concentration was 3.8 +/- 0.41 (S.E.) mM (n = 58) at 22 degrees C. No significant change in the intracellular free Mg2+ was observed following extensive (approx. 6 h) incubation in Mg2+-free media. Increasing the external concentration of magnesium from 4 to 20 mM (approx. 15 min) produced a slow and small enhancement (1.8 mM) of [Mg2+]i, which was fully reverted when the divalent cation was removed from the bathing solution. No change in ionic magnesium resting concentration was observed when the muscle fibers were treated either with caffeine 3 mM or with Na+-free solutions. In depolarized muscle fibers (-23 +/- 2.7 mV) treated with 100 mM K+, the myoplasmic [Mg2+] was 3.7 +/- 0.45 (S.E.) mM, n = 6, immediately after the spontaneous relaxation of the contracture. Similar determinations in muscle fibers during stimulation at low frequency (5 Hz), and after fatigue development, showed no changes in the concentration of free cytosolic Mg2+. These results point out that [Mg2+]i is not modified under these three different experimental conditions.

The thermodynamic efficiency of the Ca2+-Mg2+-ATPase is one hundred percent

The thermodynamic efficiency of the Ca2+ -Mg2+ -ATPase of skeletal sarcoplasmic reticulum has been evaluated by comparing the Ca2+ gradient established with the ATP/(ADP*Pi) ratio. The evaluation was made at an external Ca2+ level (4.7 X 10(-8) M) which is below the Km value of 7 X 10(-8) M. The Mg-ATP and phosphate concentrations were held constant (0.1 mM) and the ADP concentration was varied. Maximal uptake to an internal free Ca2+ concentration of 17 mM was observed at infinite ATP/(ADP*Pi) ratio (absence of ADP). This corresponds to a [Ca2+] i/[Ca2+] 0 gradient of 3.6 X 10(5). A Ca2+ gradient one-half as large was observed at an ATP/(ADP*Pi) ratio of 3.5 X 10(3) M-1. The square of the Ca2+ gradient is shown to be proportional to the ATP/(ADP*Pi) ratio, for finite values of the latter. The proportionality constant is identical to the equilibrium constant for hydrolysis of ATP (9.02 X 10(6) M) under these conditions (0.1 mM Mg2+, 30 degrees C). The intrinsic thermodynamic efficiency of the pump is shown to be 100%, with a maximal uncertainty of 3%. The efficiency is lower under less optimal conditions, when the pump is inhibited and passive leak processes compete.

State of translocated Ca2+ by sarcoplasmic reticulum inferred from kinetic analysis of calcium oxalate precipitation

Uptake of Ca2+ by sarcoplasmic reticulum in the presence of oxalate displays biphasic kinetics. An initial phase of normal uptake is followed by a second phase coincident with precipitation of calcium oxalate inside the vesicles. The precipitation rate induced by actively transported Ca2+ is depressed by increasing the added Ca2+ concentration. This correlates linearly with the reciprocal of precipitation rate. Therefore, a maximal limit rate could be extrapolated at zero Ca2+ (V0). The rate of precipitation, also a function of added amount protein, gives a linear correlation in a double reciprocal plot. Thus, it was possible to estimate the maximal precipitation rate occurring at infinite protein concentration (V infinity). With the combined extrapolated values a maximal expected precipitation rate could be calculated (V infinity 0). Kinetics of calcium oxalate precipitation was studied in the absence of calcium uptake and empirical equations relating the rate of precipitation with the added Ca2+ were established. Entering V infinity 0 in the equations, an internal free Ca2+ concentration of approx. 2.5 mM was estimated. Additionally, it is shown that the ionophore X-537A does not suppress the Ca2+ uptake, if added during the oxalate-dependent phase, albeit the uptake proceeds at a slower rate after the release of approx. 70 nmol Ca2+/mg protein. This amount presumably equals the internal free Ca2+ not sequestered by oxalate, producing a maximal concentration approx. 14 mM. Taking into account low affinity binding of internal binding sites and the transmembrane Ca2+ gradients built up during the uptake of Ca2+, values of free Ca2+ ranging from 3 to 6 mM, approaching those estimated by the precipitation analysis, could be estimated.

Twenty questions concerning the reaction cycle of the sarcoplasmic reticulum calcium pump

The problem of "mechanism" for the calcium pump may be divided into three parts. (1) It is an enzyme catalyzing the hydrolysis of ATP. (2) At some stage of the reaction cycle it provides a pathway through the otherwise impermeable phospholipid bilayer. (3) The two properties are linked so as to provide exchange of free energy between the two substrates (ATP and Ca2+), without any exchange of matter. The third part is the most interesting, and the mechanistic problem it poses is common to all chemiosmotic free energy transducers. All three aspects of the mechanism are reviewed here, with special emphasis on the remaining experimental questions that need to be resolved. The review will show that even such fundamental questions as the exact stoichiometry of the catalyzed reaction have not yet received definitive answers.

Correlation between calmodulin-dependent increase in the rate of calcium transport and calmodulin-dependent phosphorylation of cardiac sarcoplasmic reticulum. Characterization of calmodulin-dependent phosphorylation

The aim of the present study was to prove a correlation between the calmodulin-dependent increase in the rate of calcium transport by dog cardiac sarcoplasmic reticulum and calmodulin-dependent phosphorylation. The dependence of phosphorylation on the total calmodulin concentration at 75 microM and 1 microM free calcium gave apparent calmodulin half-saturation constants Km (CaM) of 9.4 nM and 181 nM, respectively, whilst the apparent Km (CaM) for the rate of calmodulin-stimulated calcium transport carried out at 1 microM calcium, but phosphorylated prior to the calcium uptake at 75 microM or 1 microM calcium, were 12.5 nM and 127 nM, respectively. A positive correlation was obtained between calmodulin-dependent increase in the rate of calcium transport and hydroxylamine-insensitive phosphoester formed by the calcium/calmodulin-regulated, membrane-bound protein kinase. More than 90% of incorporated [32P]phosphate is confined to a 26-28-kDa or 9-11-kDa protein as determined by polyacrylamide gel electrophoresis following solubilization in sodium dodecyl sulfate at 37 degrees C and at 100 degrees C, respectively, similar to the results obtained by phosphorylation with cAMP-dependent protein kinase. The data indicate that calmodulin-dependent phosphorylation of the above protein(s) is causally related to the stimulation of the rate of calcium transport by cardiac sarcoplasmic reticulum, which is at least partially due to a shift in the calcium dependence of the rate of calcium transport to lower free calcium concentrations, K(Ca), of 1.25 microM and 0.61 microM in controls and calmodulin-dependent phosphorylation, respectively. Activation of calmodulin-dependent phosphorylation by free calcium at total calmodulin concentrations of 300 nM, 100 nM and 30 nM gave apparent K(Ca) values of 0.83 microM, 1.44 microM and 2.3 microM and Hill coefficients of 4.13, 3.76 and 3.79, respectively, indicating that all four calcium binding sites of calmodulin have to be saturated to obtain activation of the calcium/calmodulin-regulated protein kinase. The calmodulin-dependent modulation of calcium transport in vivo is, therefore, determined to great extent by the total calmodulin concentration present in the sarcoplasm.

The effect of cholecalciferol in vivo on proteins and lipids of skeletal muscle from rachitic chicks

The protein and lipid constituents of skeletal muscle subcellular fractions isolated from chicks fed a vitamin D-deficient diet for 3 weeks and chicks replated with cholecalciferol (vitamin D3) were analyzed. Administration of the sterol markedly altered the protein composition of mitochondria. The changes were localized in the inner membranes and consisted of a modification of the relative amounts of proteins of approximate mol wt of 83,000, 58,000, 42,000, and 34,000. In addition, treatment with vitamin D3 modified the distribution pattern of components of the actomyosin contractile complex. An increase in actin and troponin C was particularly noticeable. No differences between rachitic and treated animals were detected in the protein composition of sarcoplasmic reticulum membranes and postmicrosomal soluble fraction. A significant increase in the phospholipid content of sarcoplasmic reticulum (P less than 0.05), and to a lesser extent of mitochondria, was observed in repleted chicks. The relative proportions of individual phospholipids, however, were not changed. Injection of an acute dose of cholecalciferol to chicks less severely depleted in vitamin D significantly stimulated the incorporation of 32PO4 in vivo to muscle homogenates, mitochondria, and sarcoplasmic reticulum (P less than 0.05). As the increases in specific activities of sarcoplasmic inorganic P and membrane lipid P were similar whereas that of serum remained unchanged, the results are compatible with the idea that vitamin D3 stimulates phosphate fluxes across muscle membranes. The sterol produced minor modifications in the fatty acid composition of sarcoplasmic reticulum (P less than 0.05).

Inactivation of the calcium-transport ATPase in the sarcoplasmic reticulum by the combined effect of lasolocid and Triton X-100

The calcium-transport ATPase of the sarcoplasmic reticulum membranes is irreversibly inactivated by the combined action of Lasolocid and Triton X-100 at concentrations which separately do not interfere with the enzyme's activity. In the presence of Lasolocid the enzyme is most susceptible to inactivation when the Triton X-100 concentration just exceeds its critical micellar concentration, approximately, 0.2 mg X ml-1. Lasolocid becomes effective at a concentration of 10 microM and produces rapid inactivation at 100 microM. Phosphoprotein formation is less affected than phosphate liberation. The influence of the ATPase protein on the fluorescence intensity of Lasolocid passes a distinct maximum at the most effective Triton X-100 concentration of 0.2 mg X ml-1.

An extended Ca2+-hypothesis of visual transduction with a role for cyclic GMP

A model is described having the following features: Light induces Ca2+ release from vertebrate rod outer segments discs via pores composed of multimeric rhodopsin. Cytoplasmic Ca2+ reversibly blocks Na+ channels of the surface membrane, with the time course of development and amplitude of the response to light being influenced by restrictions on intradiscal Ca2+ diffusion. The falling phase of response reflects a decline in cytoplasmic [Ca2+] due to a Ca2+-binding protein controlled by cyclic GMP so that its binding capacity is increased by the reduction in cytoplasmic [cyclic GMP] which follows rhodopsin bleaching.

Phosphoenzyme decomposition in sarcoplasmic reticulum isolated from cat caudofemoralis, tibialis and soleus

Decomposition of phosphoenzyme (E approximately P) in sarcoplasmic reticulum isolated from caudofemoralis, tibialis and soleus of cat hind leg skeletal muscles was studied under various conditions of monovalent cations. In the presence of Li+, Na+, and K+ chosen for E approximately P formation and decomposition after quenching of E approximately P with EGTA, E approximately P in the caudofemoralis and tibialis sarcoplasmic reticulum decomposed faster than that in the soleus sarcoplasmic reticulum. Quenching the E approximately P formation with EGTA and ADP revealed that 30-40% of the total E approximately P formed in all types of sarcoplasmic reticulum was 'ADP sensitive'. Decomposition of the remaining E approximately P in caudofemoralis and tibialis sarcoplasmic reticulum was enhanced by ADP, which resulted in a multiphasic decomposition pattern. A larger portion of the remaining E approximately P in the soleus sarcoplasmic reticulum, on the other hand decomposed in a monophasic manner and was not significantly influenced by ADP. The data on E approximately P decomposition clearly differentiate between the fast and slow muscle types.

Influence of monovalent cations on the Ca2+-ATPase of sarcoplasmic reticulum isolated from rabbit skeletal and dog cardiac muscles. An interpretation of transient-state kinetic data

Transient-state kinetics of phosphorylation and dephosphorylation of the Ca2+-ATPase of sarcoplasmic reticulum vesicles from rabbit skeletal and dog cardiac muscles were studied in the presence of varying concentrations of monovalent and divalent cations. Monovalent cations affect the two types of sarcoplasmic reticulum differently. When the rabbit skeletal sarcoplasmic reticulum was Ca2+ deficient, preincubation with K+ (as compared with preincubation with choline chloride) did not affect initial phosphorylation at various concentrations of Ca2+, added with ATP to phosphorylate the enzyme. This is in contrast to preincubation with K+ of the Ca2+-deficient dog cardiac sarcoplasmic reticulum, which resulted in an increase in the phosphoenzyme level. When Ca2+ was bound to the rabbit skeletal sarcoplasmic reticulum, K+ inhibited E - P formation; but under the same conditions, E - P formation of dog cardiac sarcoplasmic reticulum was activated by K+ at 12 microM Ca2+ and inhibited at 0.33 and 1.3 microM Ca2+. Li+, Na+ and K+ also have different effects on E - P decomposition of skeletal and cardiac sarcoplasmic reticulum. The latter responded less to these cations than the former. Studies with ADP revealed differences between the two types of sarcoplasmic reticulum. For rabbit skeletal sarcoplasmic reticulum, 40% of the phosphoenzyme formed was 'ADP sensitive', and the decay of the remaining E - P was enhanced by K+ and ADP. Dog cardiac sarcoplasmic reticulum yielded about 40--48% ADP-sensitive E - P, but the decomposition rate of the remaining E - P was close to the rate measured in the absence of ADP. Thus, these studies showed certain qualitative differences in the transformation and decomposition of phosphoenzymes between skeletal and cardiac muscle which may have bearing on physiological differences between the two muscle types.

Formation of magnesium-phosphoenzyme and magnesium-calcium-phosphoenzyme in the phosphorylation of adenosine triphosphatase by orthophosphate in sarcoplasmic reticulum. Models of a reaction sequence

The aim of the present study was to test simple reaction sequences which describe calcium-independent plus calcium-dependent phosphorylation of sarcoplasmic reticulum transport. ATPase by orthophosphate including the function of magnesium in phosphoenzyme formation. The reaction schemes considered were based on the reaction sequence for calcium-independent phosphorylation proposed previously; namely that the transport enzyme (E) forms a ternary complex (Mg . E . Pi), by random binding of free magnesium and free orthophosphate, which is in equilibrium with the magnesium-phosphoenzyme (Mg . E-P). Phosphorylation, performed at pH 7.0 20 degrees C and a constant free orthophosphate concentration using sarcoplasmic reticulum vesicles either unloaded or loaded passively with calcium in the presence of 5 mM or 40 mM CaCl2, resulted in a gradual decrease in the apparent magnesium half-saturation constant and an increase in maximum phosphoprotein formation with increasing calcium loads. When phosphorylation of sarcoplasmic reticulum vesicles preloaded in the presence of 5 mM CaCl2 was performed at a constant free magnesium concentration, a decrease in the apparent orthophosphate half-saturation constant and an increase in maximum phosphoprotein formation was observed as compared with vesicles from which calcium inside has been removed by ionophore X-537A plus EGTA treatment; however, both parameters remained unchanged by increasing free magnesium from 20 mM to 30 mM. When phosphorylation of sarcoplasmic reticulum vesicles passively loaded with calcium in the presence of 40 mM CaCl2, at which the saturation of the low-affinity calcium binding sites of the ATPase is presumably near maximum, was performed at increasing concentrations of free orthophosphate, there was a parallel shift of phosphoprotein formation as a function of free magnesium and vice versa, with no change in the maximum phosphoenzyme formation. Comparison of the experimental data with the pattern of phosphoprotein formation predicted from model equations for various theoretical possible reaction sequences suggests that phosphoenzyme formation from orthophosphate possesses the following features. Firstly, calcium present at the inside of the sarcoplasmic reticulum membrane binds to the free enzyme and in sequential order to E . Mg . Pi or Mg . E-P or to both, but neither to E. Mg nor to E . Pi. Secondly, calcium-independent and calcium-dependent phosphoproteins are magnesium-phosphoenzymes. Calcium-dependent phosphoenzyme is a magnesium-calcium-enzyme phosphate complex with 1 magnesium, 2 calciums and 1 orthophosphate (the last covalently) bound to the enzyme [Mg . E-P . (Cai)2], and not a 'calcium-phosphoprotein' without bound magnesium.

Differentiation between Ca2+ transport and ATP-induced Ca2+ binding by sarcoplasmic reticulum

The Ca2+ actively accumulated by sarcoplasmic reticulum isolated from skeletal muscle is composed of two fractions; one represented by intravesicular free Ca2+ and another represented by Ca2+ selectively bound to the membranes. Both of these Ca2+ fractions depend on ATP, although it is not clear whether ATP hydrolysis is essential for accumulation of the second Ca2+ fraction. The existence of the membrane-bound Ca2+ induced by ATP is clearly shown in experiments in which the Ca2+ retention by sarcoplasmic reticulum is measured in the presence and in the absence of X-537A, a Ca2+ ionophore, which makes the membrane permeable to Ca2+. Thus, in the presence of X-537A all Ca2+ accumulated due to ATP is bound to the membranes. This membrane-bound Ca2+ represents about 30 nmol/mg protein in the range of external pCa values of 7 to 3.5. The magnitude of this Ca2+ fraction is slightly higher whether or not the experiments are performed in the presence of oxalate, which greatly increased the intravesicular Ca2+ accumulation. Furthermore, taking advantage of the impermeability of sarcoplasmic reticulum to EGTA, it is possible to show the existence of the membrane-bound Ca2+ as a distinct fraction from that which exists intravesicularly.

Variable Ca2+ transport:phosphoprotein ratios in the early part of the GTP-driven calcium-transport reaction of the sarcoplasmic reticulum

Initial Ca2+ transport and phosphoprotein formation of the sarcoplasmic reticulum membrane with GTP were investigated in a comparative study. While saturation of the high-affinity sites for Ca2+ binding and transporting as well as for GTP binding on the external surface of the membrane resulted in Ca2+ transport and phosphoprotein formation in a molar ratio of 2, the variation of the concentrations of the two reactants yielded ratios between 1.7 and 5.7. The ratios varied with a similar dependence on the concentrations of Ca2+ and GTP, except at 500 microM Ca2+, if the reaction was started by Ca2+ instead of GTP but the overall rates decreased. 1 mM DL-propranolol in the preincubation medium selectively inhibited Ca2+ transport but had no effect on initial phosphoprotein formation. These observations indicate that:L (a) phosphorylation of one enzyme molecule induces Ca2+ transport by a variable but limited number of neighbouring molecules, (b) not all Ca2+ bound is essential for phosphorylation but can be transported in parallel, (c) Ca2+ bound to low-affinity sites occupied at 500 microM Ca2+ in the reaction medium is also transported initially, (d) the accessibility of the high-affinity Ca2+ binding sites for DL-propranolol differs, (e) DL-propranolol interacts with Ca2+ binding and transporting sites only in that conformation of the enzyme that can be phosphorylated by the nucleotide.

Uptake and energy-dependent extrusion of calcium in neural cells in culture

The metabolism of Ca2+ was studied in a neuronal model system, the clonal mouse neuroblastoma x rat glioma hybrid cell line 108CC5. 1. Homogenates of the hybrid cells exhibit a specific activity of Ca2+-ATPase considerably higher than that of homogenates of the parental cells. 2. Uptake and release of 45Ca2+ by the hybrid cells display two and three distinct phases, respectively, and indicate that 40--50% of the cell-associated Ca2+ is located at the cell surface. 3. The influx of 45Ca2+ is insignificantly affected by Mg2+ or Na+, slightly diminished by Ba2+ or Sr2+, strongly inhibited by La3+, Co2+ or prenylamine, and considerably enhanced by high (i.e., depolarizing) concentrations of K+. The efflux of 45Ca2+ is reduced by La3+. 4. The hybrid cells tend to maintain Ca2+ homeostasis with an overall cellular Ca2+ concentration of 0.5--0.7 mM. At 1.8 mM Ca2+ in the medium this implies the necessity of an extrusion pump in the plasma membrane. 5. A reduction in the hybrid cells of the level of ATP is paralleled by a decline in the content of Ca2+. This can only be explained by the existence of energy-dependent intracellular Ca2+ stores that effectively compete for Ca2+ with a Ca2+ pump located in the plasma membrane. The internal stores are not identical with the mitochondria because mitochondrial inhibitors hardly change Ca2+ metabolism. 6. Micromolar concentrations of the ionophore A23187 can switch off the internal Ca2+ stores without affecting considerably the influx of Ca2+ through the plasma membrane. 7. With switched-off Ca2+ stores it is possible to increase the cellular Ca2+ content distinctly and to bring it back again to the control values in an ATP-dependent manner, i.e. to demonstrate the action of a Ca2+-extrusion pump in the plasma membrane. 8. Under some conditions active extrusion of Ca2+ depends not only on ATP but also on the presence of extracellular Na+. 9. Similar results as with hybrid cells are also obtained with rat glioma cells. The methodology of combining energy deprivation with the application of the ionophore A23187 is possibly generally applicable to obtain insight into the Ca2+ metabolism of various cell types.