Tightly bound calcium of adenosine triphosphatase in sarcoplasmic reticulum from rabbit skeletal muscle

Energy dissipation in slipping biological pumps

We describe active transport in slipping biological pumps, using mesoscopic nonequilibrium thermodynamics. The pump operation is characterised by its stochastic nature and energy dissipation. We show how heating as well as cooling effects can be associated with pump operation. We use as an example the well studied active transport of Ca2+ across a biological membrane by means of its ATPase, and use published data to find values for the transport coefficients of the pump under various conditions. Most of the transport coefficients of the pump, including those that relate ATP hydrolysis or synthesis to thermal effects, are estimated. This can give a quantitative description of thermogenesis. We show by calculation that all of these coupling coefficients are significant.

Coefficients for active transport and thermogenesis of Ca2+-ATPase isoforms

Coefficients for active transport of ions and heat in vesicles with Ca(2+)-ATPase from sarcoplasmic reticulum are defined in terms of a newly proposed thermodynamic theory and calculated using experiments reported in the literature. The coefficients characterize in a quantitative manner different performances of the enzyme isoforms. Four enzyme isoforms are examined, namely from white and red muscle tissue, from blood platelets, and from brown adipose mitochondria. The results indicate that the isoforms have a somewhat specialized function. White muscle tissue and brown adipose tissue have the same active transport coefficient ratio, but the activity level of the enzyme in white muscle is higher than in brown adipose tissue. The thermogenesis ratio is high in both white muscle and brown adipose tissue, in agreement with a specific role in nonshivering thermogenesis. Other isoforms do not have this ability to generate heat. A calcium-dependence of the coefficients is found, which can be understood as being in accordance with the role of this ion as a messenger in muscle contraction as well as in thermogenesis. The investigation points to new experiments related to structure as well as to function of the isoforms.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

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.

Activation of Ca2+ release in isolated sarcoplasmic reticulum

The relationship between Ca2+ release from sarcoplasmic reticulum, induced by elevated pH, tetraphenylboron (TPB-) or chemical modification, and the change in the surface charge of the membranes as measured by the fluorescence intensity of anilinonaphthalene sulfonate (ANS) is examined. The simulated Ca2+ release is inhibited by dicyclohexylcarbodiimide and external Ca2+. TPB-, but not tetraphenylarsonium (TPA+), causes a decrease in ANS- fluorescence, with 50% decrease occurring at about 5 microM TPB-. The decrease in ANS- fluorescence as well as the inhibition of Ca2+ accumulation induced by TPB- are prevented by TPA+. A linear relationship between the decrease in membrane surface potential and the extent of the Ca2+ released by TPB- is obtained. Similar levels of [3H]TPB-bound to sarcoplasmic reticulum membranes were obtained regardless of whether or not the vesicles have taken up Ca2+. The inhibition of Ca2+ accumulation and the [3H]TPB- incorporation into the membranes were correlated. Ca2+ release from sarcoplasmic reticulum, by pH elevation, chemical modification or by addition of NaSCN (0.2 to 0.5 M) or the Ca2+ ionophore ionomycin, is also accompanied by a decrease in ANS- fluorescence intensity. However, chemical modification and elevated pH affects the surface potential much less than SCN- or TPB- do. These results suggest that the enhancement of Ca2+ release by these treatments is not due to a general effect on the membrane surface potential, but rather through the modification of a specific protein. They also suggest that membrane surface charges might play an important role in the control mechanism of Ca2+ release.

Quantitative determination of the calcium involved in the regulation of the Ca2+-ATPase activity in sarcoplasmic reticulum vesicles

The dependence of the Ca2+-ATPase activity of sarcoplasmic reticulum vesicles upon the intravesicular concentration of calcium accumulated after active uptake was studied. The internal calcium concentration was modified by addition of the ionophore A23187 at the steady state of accumulation. About half of the calcium accumulated could be released at low ionophore concentration without any concomitant activation of the Ca2+-ATPase. This population of calcium might consist of calcium free in the lumen of the vesicles or bound to the bilayer at sites which do not interact with the ATPase activity. At higher concentrations of ionophore (above 1.75 nmol A23187/mg protein) the release of calcium activated this enzyme. This phenomenon was independent of the extravesicular calcium concentration and might be explained by assuming second species of calcium ions bound to the inner side of the membrane and in close functional interaction with the Ca2+-ATPase.

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.

Characterization of calcium liberation from a human platelet membrane fraction

Calcium efflux and EGTA-induced calcium release from an internal platelet membrane fraction have been studied after the oxalate-supported calcium uptake had reached steady state. Increasing external calcium concentrations stimulate the calcium efflux velocity, with an apparent half-maximal stimulation at about 5 microM outside calcium concentration and a maximal velocity of calcium efflux of 4.66 +/- 2.32 nmol X min-1 X mg-1. Moreover, the ratio of the liberated calcium on the loaded calcium seems to be independent of the increasing external calcium concentration. Increasing the calculated internal calcium concentration by varying the oxalate potassium concentration from 10 mM to 1 mM results in an increase of the liberated calcium from the membrane vesicles from 7.4% to 63%, respectively, without changing the calcium efflux velocity. Similar conclusions can be drawn from the observation of results from the calcium efflux and EGTA-induced calcium release methods. Moreover, calcium pump reversal does not seem to be responsible for the calcium efflux or calcium release. All these different points added to the previously described regulation of calcium efflux by the catalytic subunit of cAMP protein kinase suggest us that the mechanism of calcium liberation by the platelet membranes is different from the calcium uptake.

The vanadate complex of the calcium-transport ATPase of the sarcoplasmic reticulum, its formation and dissociation

Vanadate binding to different sarcoplasmic reticulum membrane preparations was determined by measuring bound vanadate colorimetrically and by phosphorylating the vanadate-free enzyme fraction with [gamma-32P] ATP. Colorimetry allowed the study of the dependence of equilibrium vanadate binding on ionized magnesium and the displacing effect of ionized calcium at vanadate concentrations greater than 0.1 mM only. At saturating magnesium concentration the enzyme binds 6-8 nmol vanadate/mg protein and half-maximum saturation is reached at 40 microM. Vanadate is displaced from the enzyme when its high-affinity calcium-binding sites are saturated and conversely calcium is solely displaced from its high-affinity binding sites by vanadate. The phosphorylation procedure allowed the measurement of equilibrium binding as well as the kinetics of vanadate binding and release at vanadate concentrations below 0.1 mM. Half-times of 30s and 3s were observed for vanadate release induced by 0.1 mM and 1 mM calcium respectively. Millimolar concentrations of ATP are required for vanadate displacement. Under equilibrium conditions the enzyme displays an affinity for vanadate of 1.6 X 10(6) M-1. The dependence on the concentration of vanadate of the rate of vanadate binding yielded an affinity of only 1 X 10(4) M-1. Closed vesicles bind vanadate much more slowly than calcium-permeable preparations. The initial rate of calcium-induced vanadate dissociation is accelerated considerably when the vesicles are made calcium permeable. The rate of vanadate dissociation from calcium-permeable vesicles reaches half-maximum values at 1-2 mM calcium indicating that the internal low-affinity calcium-binding sites must first be occupied in order to release bound vanadate. The results suggest that vanadate binding leads to a transition of the external high to internal low-affinity calcium-binding sites.

The ADP- and Mg2+-reactive calcium complex of the phosphoenzyme in skeletal sarcoplasmic reticulum Ca2+-ATPase

The effects of ATP on Ca2+ binding in the absence of added Mg2+ to the purified sarcoplasmic reticulum Ca2+-ATPase were studied at pH 7.0 and 0 degrees C. ATP increased the number of Ca2+-binding sites of the enzyme from 2 to 3 mol per mol of phosphorylatable enzyme. The association constant for the ATP-induced Ca2+ binding was 4 X 10(5) M-1, which was not significantly different from that obtained in the absence of ATP. AdoP[CH2]PP had little effect on the Ca2+-binding process. The amount of phosphoenzyme formed was equivalent to the level of ATP-induced Ca2+ binding. ADP decreased the level of ATP-induced Ca2+ binding and phosphoenzyme by the same amount. These results suggest that ATP-induced Ca2+ binding exists in the form of an ADP-reactive phosphoenzyme . Ca complex. In addition, the Ca2+ bound to the enzyme in the presence of ATP was released on the addition of 1 mM MgCl2; after the release of Ca2+, the phosphoenzyme decayed. These observations suggest that Mg2+, added after the ATP-induced Ca2+-binding process, may replace the Ca2+ on the phosphoenzyme and initiate phosphoenzyme decomposition.

Reversible loss of cooperative calcium ion binding by sarcoplasmic reticulum calcium adenosinetriphosphatase

Incubation of rabbit skeletal muscle sarcoplasmic reticulum vesicles in solutions of very low [Ca2+] caused Ca2+ to bind noncooperatively, as determined by the dependence of the intrinsic tryptophan fluorescence intensity on added increments of Ca2+. Cooperative Ca2+ binding was obtained if the ATPase was incubated in [Ca2+] high enough (25 microM) to saturate the two high-affinity Ca2+ binding sites and then titrated with [ethylenebis(oxyethylenenitrilo)]tetraacetic acid. The cooperative binding had an apparent association constant of 6.3 X 10(6) M-1 and a Hill coefficient of 2.6; these constants for the noncooperative binding case were 5.0 X 10(5) M-1 and 1.2, respectively. The transitions from the noncooperative to the cooperative Ca2+ binding forms of the enzyme were slow compared to the time required for Ca2+ binding to reach equilibrium. Thus, it appears that sarcoplasmic reticulum CaATPase is a hysteretic enzyme. Intrinsic association constants for Ca2+ binding and equilibrium constants for the transitions between the two forms in low and high [Ca2+] were estimated from analyses of a general scheme for cooperative and noncooperative binding.