Role of endogenous regucalcin in the regulation of Ca(2+)-ATPase activity in rat liver nuclei

The role of endogenous regucalcin in the regulation of Ca(2+)-ATPase, a Ca(2+) sequestrating enzyme, in rat liver nuclei was investigated. Nuclear Ca(2+)-ATPase activity was significantly reduced by the addition of regucalcin (0.1-0.5 microM) into the enzyme reaction mixture. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) caused a significant elevation of Ca(2+)-ATPase activity; this effect was completely abolished by the addition of regucalcin (0.1 microM). The effect of anti-regucalcin antibody (50 ng/ml) in increasing Ca(2+)-ATPase activity was completely prevented by the presence of thapsigargin (10(-6) M), an inhibitor of Ca(2+) sequestrating enzyme, N-ethylmaleimide (1 mM), a modifying reagent of thiol groups, or vanadate (10(-5) M), an inhibitor of phosphorylation of the enzyme by ATP, which revealed an inhibitory effect on nuclear Ca(2+)-ATPase activity. Meanwhile, the effect of anti-regucalcin antibody (50 ng/ml) was significantly enhanced by the addition of calmodulin (5 microg/ml), which could increase nuclear Ca(2+)-ATPase activity. In addition, the effect of antibody (50 ng/ml) was significantly reduced by the presence of trifluoperazine (20 microM), an antagonist of calmodulin. These results suggest that the endogenous regucalcin in liver nuclei has a suppressive effect on nuclear Ca(2+)-ATPase activity, and that regucalcin can inhibit an activating effect of calmodulin on the enzyme.     

Role of regucalcin as an activator of Ca(2+)-ATPase activity in rat liver microsomes.

The effect of Ca(2+)-binding protein regucalcin on Ca(2+)-ATPase activity in isolated rat liver microsomes was investigated. The presence of regucalcin (0.1-1.0 microM) in the enzyme reaction mixture led to a significant increase in Ca(2+)-ATPase activity. Regucalcin significantly stimulated ATP-dependent (45)Ca(2+) uptake by the microsomes. Thapsigargin (10(-6) M), a specific inhibitor of microsomal Ca(2+) pump enzyme (Ca(2+)-ATPase), clearly inhibited regucalcin (0.5 microM)-increased microsomal Ca(2+)-ATPase activity. Liver microsomal Ca(2+)-ATPase activity was markedly decreased by N-ethylmaleimide (NEM; 2.5 mM), while the activity was clearly elevated by dithiothreitol (DTT; 2.5 mM), indicating that the sulfhydryl (SH) group of the enzyme is an active site. The effect of regucalcin (0.5 microM) in increasing Ca(2+)-ATPase activity was completely inhibited by the presence of NEM (2.5 mM) or digitonin (10(-2) %), a solubilizing reagent of membranous lipids. Moreover, the effect of regucalcin on enzyme activity was seen in the presence of Ca(2+) ionophore (A23187; 10(-7) M). The present study demonstrates that regucalcin can stimulate Ca(2+) pump activity in rat liver microsomes, and that the protein may act the SH groups of microsomal Ca(2+)-ATPase.     

Stimulatory effect of regucalcin on mitochondrial ATP-dependent calcium uptake activity in rat kidney cortex

The effect of regucalcin, which is a regulatory protein of Ca(2+) signaling, on Ca(2+)-ATPase activity in isolated rat renal cortex mitochondria was investigated. The presence of regucalcin (50, 100, and 250 nM) in the enzyme reaction mixture led to a significant increase in Ca(2+)-ATPase activity. Regucalcin significantly stimulated ATP-dependent (45)Ca(2+) uptake by the mitochondria. Ruthenium red (10(-6) M) or lanthunum chloride (10(-6) M), an inhibitor of mitochondrial Ca(2+) uptake, markedly inhibited regucalcin (100 nM)-increased mitochondrial Ca(2+)-ATPase activity and (45)Ca(2+) uptake. The effect of regucalcin (100 nM) in elevating Ca(2+)-ATPase activity was completely prevented by the presence of digitonin (10(-2)%), a solubilizing reagent of membranous lipids, vanadate, an inhibitor of phosphorylation of ATPase, or dithiothreitol (50 mM), a protecting reagent of the sulfhydryl (SH) group of the enzyme. The activating effect of regucalcin (100 nM) on Ca(2+)-ATPase activity was not further enhanced by calmodulin (0.30 microM) or dibutyryl cyclic AMP (10(-4) M), which could increase Ca(2+)-ATPase activity. Trifluoperazine (TFP; 50 microM), an antagonist of calmodulin, significantly decreased Ca(2+)-ATPase activity. The activating effect of regucalcin on the enzyme was also seen in the presence of TFP, indicating that regucalcin's effect is not involved in mitochondrial calmodulin. The present study demonstrates that regucalcin can stimulate Ca(2+)-pump activity in rat renal cortex mitochondria, and that the protein may act on an active site (SH group) related to phosphorylation of mitochondrial Ca(2+)-ATPase.     

Stimulatory effect of regucalcin on ATP-dependent Ca(2+) uptake activity in rat liver mitochondria

The effect of Ca(2+)-binding protein regucalcin on Ca(2+)-ATPase activity in isolated rat liver mitochondria was investigated. The presence of regucalcin (0.1, 0.25, and 0.5 microM) in the enzyme reaction mixture led to a significant increase in Ca(2+)-ATPase activity. Regucalcin significantly stimulated ATP-dependent (45)Ca(2+) uptake by the mitochondria. Ruthenium red (10(-5) M) or lanthanum chloride (10(-4) M), an inhibitor of mitochondrial Ca(2+) uptake, completely inhibited regucalcin (0.25 microM)-increased mitochondrial Ca(2+)-ATPase activity and (45)Ca(2+) uptake. The effect of regucalcin (0.25 microM) in increasing Ca(2+)-ATPase activity was completely inhibited by the presence of digitonin (10(-2)%), a solubilizing reagent of membranous lipids, or vanadate (10(-5) M), an inhibitor of phosphorylation of ATPase. The activatory effect of regucalcin (0.25 microM) on Ca(2+)-ATPase activity was not further enhanced in the presence of dithiothreitol (2.5 mM), a protecting reagent of the sulfhydryl (SH) group of the enzyme, or calmodulin (0.60 microM), a modulator protein of Ca(2+) action that could increase mitochondrial Ca(2+)-ATPase activity. The present study demonstrates that regucalcin can stimulate Ca(2+) pump activity in rat liver mitochondria, and that the protein may act on an active site (SH group)-related to phosphorylation of mitochondrial Ca(2+)-ATPase.     

Effect of dietary essential fatty acid deficiency on Ca(2+)-ATPase activity in microsomal fraction of rat submandibular gland.

1. The effect of dietary essential fatty acid (EFA) deficiency on Ca(2+)-ATPase activity of rat submandibular gland microsomal fraction was studied. 2. The specific activity of Ca(2+)-ATPase per milligram of microsomal protein was depressed about 35% in rats fed the EFA-deficient diet as compared with that in those fed the control diet. 3. Lineweaver-Burk plots for Ca(2+)-ATPase activity showed no significant differences in Km values for Ca2+ and ATP, but the Vmax was decreased in the EFA-deficient rats. 4. The above results suggest that depression of the Ca(2+)-ATPase activity in rats fed the EFA-deficient diet is probably due to the decrease in the Vmax of the enzyme.     

Calmodulin effect on purified rat cortical plasma membrane Ca(2+)-ATPase in different phosphorylation states

The plasma membrane Ca(2+)-ATPase in neuronal tissue plays an important role in fine tuning of the intracellular Ca(2+) concentration. The enzyme exhibits a high degree of tissue specificity and is regulated by several mechanisms. Here we analysed the relationship between separate modes of Ca(2+)-ATPase regulation, i.e., reversible phosphorylation processes mediated by protein kinases A and C, protein phosphatases PP1 and PP2A, and stimulation by calmodulin. The activity of PKA- or PKC-phosphorylated Ca(2+)-ATPase was influenced by the further addition of calmodulin, and this effect was more pronounced for PKC-phosphorylated calcium pump. In both cases the fluorescence study revealed the increased calmodulin binding, and for PKA-mediated phosphorylation it was correlated with a higher affinity of calcium pump for calmodulin. The incubation of Ca(2+)-ATPase with CaM prior to protein kinases action revealed that CaM presence counteracts the stimulatory effect of PKA and PKC. Under the in vitro assay cortical Ca(2+)-ATPase was a substrate for PP1 and PP2A. Protein phosphatases decreased both the basal activity of Ca(2+)-ATPase and its affinity for calmodulin. Fluorescence analysis confirmed the lowered ability of dephosphorylated Ca(2+)-ATPase for calmodulin binding. These results may suggest that interaction of CaM with calcium pump and its stimulatory action could be a partly separate phenomenon that is dependent on the phosphorylation state of Ca(2+)-ATPase.     

Effect of eosin Y on Ca2+-dependent activity of Ca2+-Mg2+-ATPase in smooth muscle cell membrane

With the aim to elucidate mechanism of eosin Y inhibitory effect on the Ca(2+)-transporting ATPase activity of myometrial cell plasma membrane effect of this inhibitor on the maximal initial rate of ATP hydrolysis reaction, catalyzed by Ca2+, Mg(2+)-ATPase, and on the enzyme affinity for Ca2+ was studied. It was established that eosin Y decreased the rate of Ca2+, Mg(2+)-ATPase catalitic turnover determined by Ca2+ and had no effect on enzyme affinity for this cation.     

Modulation of the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by pentobarbital

The dependence of the (Ca2+ + Mg2+)-ATPase activity of sarcoplasmic reticulum vesicles upon the concentration of pentobarbital shows a biphasic pattern. Concentrations of pentobarbital ranging from 2 to 8 mM produce a slight stimulation, approximately 20-30%, of the ATPase activity of sarcoplasmic reticulum vesicles made leaky to Ca2+, whereas pentobarbital concentrations above 10 mM strongly inhibit the activity. The purified ATPase shows a higher sensitivity to pentobarbital, namely 3-4-fold shift towards lower values of the K0.5 value of inhibition by this drug. These effects of pentobarbital are observed over a wide range of ATP concentrations. In addition, this drug shifts the Ca2+ dependence of the (Ca2+ + Mg2+)-ATPase activity towards higher values of free Ca2+ concentrations and increases several-fold the passive permeability to Ca2+ of the sarcoplasmic reticulum membranes. At the concentrations of pentobarbital that inhibit this enzyme in the sarcoplasmic reticulum membrane, pentobarbital does not significantly alter the order parameter of these membranes as monitored with diphenylhexatriene, whereas the temperature of denaturation of the (Ca2+ + Mg2+)-ATPase is decreased by 4-5 C degrees, thus, indicating that the conformation of the ATPase is altered. The effects of pentobarbital on the intensity of the fluorescence of fluorescein-labeled (Ca2+ + Mg2+)-ATPase in sarcoplasmic reticulum also support the hypothesis of a conformational change in the enzyme induced by millimolar concentrations of this drug. It is concluded that the inhibition of the sarcoplasmic reticulum ATPase by pentobarbital is a consequence of its binding to hydrophobic binding sites in this enzyme.     

Exaprolol as a modulator of heart sarcolemmal (Na+ + K+)-ATPase. Evidence for interaction with an essential sulfhydryl group in the catalytic centre of the enzyme

Beta-adrenoceptor blocking agents may have, in addition to their primary action, also ancillary effects on the cell membrane. In the present paper the non-specific interaction of exaprolol with the ATPase systems in isolated rat heart sarcolemmal membranes was investigated. When preincubated with sarcolemmal membranes in vitro, exaprolol in concentrations below 10(-4) mol.l-1 had no significant effect on sarcolemmal Mg2+-, Ca2+- and (Na+ + K+)-ATPase activities. At exaprolol concentration of 10(-4) mol.l-1 the Mg2+- and Ca2+-ATPase activities became inhibited whereas the (Na+ + K+)-ATPase activity was markedly stimulated. A kinetic analysis of these interactions revealed a non-competitive inhibition of Mg2+- and Ca2+-ATPase. In the case of (Na+ + K+)-ATPase a synergistic type of stimulation characterized by an exaprolol-induced conversion of an essential sulfhydryl group in the active site of the enzyme to the more reactive [S-] form has been observed thus increasing the affinity of the enzyme to ATP. Exaprolol concentrations exceeding 5 X 10(-4) mol.l-1 induced an overall depression of the investigated enzyme activities.     

Ca2+-transporting properties of myometrial plasma cell membrane Ca2+, Mg2+-ATPase reconstituted in liposomes

Purified myometrium cells plasma membrane Ca2+, Mg(2+)-ATPase was reconstitute in liposomes in functionally active state by the method of cholate dialysis: it showed ATP-hydrolase activity increased by 0.8 microM A23187 average 4 times and it showed Mg2+, ATP-dependent Ca(2+)-transporting activity. Reconstituted system transported Ca2+ at an initial rate of 114.4 +/- 16.3 nmol.min-1.mg-1 with the stoichiometry Ca2+: ATP = 1: (3.2-3.7). Calmodulin increased by 30% the initial rate of Ca(2+)-accumulation by the proteoliposomes with reconstituted Ca2+, Mg(2+)-ATPase; 0.1 mM orthovanadate decreased by 80% Ca(2+)-accumulation by this system. Ca2+, Mg(2+)-ATPase reconstituted in liposomes is just Ca(2+)-transporting ATPase of the plasma membrane. Obtained enzyme preparate can be utilised for study of the properties of this important energy-dependent Ca(2+)-transporting system of smooth muscle cell.     

The pacL gene of Synechococcus sp. strain PCC 7942 encodes a Ca(2+)-transporting ATPase.

An ATP-dependent Ca2+ uptake activity was identified in plasma membrane vesicles prepared from Synechococcus sp. strain PCC 7942. This activity was insensitive to agents which collapse pH gradients and membrane potentials but sensitive to vanadate, indicating that the activity is catalyzed by a P-type Ca(2+)-ATPase. A gene was cloned from Synechococcus sp. strain PCC 7942 by using a degenerate oligonucleotide based on a sequence conserved among P-type ATPases. This gene (pacL) encodes a product similar in structure to eukaryotic Ca(2+)-ATPases. We have shown that pacL encodes a Ca(2+)-ATPase by demonstrating that a strain in which pacL is disrupted has no Ca(2+)-ATPase activity associated with its plasma membrane. In addition, Ca(2+)-ATPase activity was restored to the delta pacL strain by introducing pacL into a second site in the Synechococcus sp. strain PCC 7942 chromosome.     

Mechanism of eosin Y action of activity of solubilized Ca2+, Mg2+- ATPase from smooth muscle sarcolemma

The eosin Y inhibitory effect on the activity of smooth muscle plasma membrane Ca(2+)-transporting ATPase was studied: effect of this inhibitor on the maximal initial rate of ATP-hydrolase reaction, catalyzed by Ca2+, Mg(2+)-ATPase, on the affinity of enzyme for the reaction reagents (Ca2+, Mg2+, ATP). Dependence of eosin Y inhibitory effect on some physicochemical factors of incubation medium was studied too. It was determined that eosin Y inhibited reversibly and with high specificity purified Ca2+, Mg(2+)-ATPase solubilized from myometrial cell plasma membrane (Ki--0.8 microM), decreased the turnover rate of this enzyme determined both by Mg2+, ATP and Ca2+. This inhibitor had no effect on the enzyme affinity for Ca2+, increased affinity for Mg2+ and decreased affinity for ATP. It was determined that inhibition of Ca2+, Mg(2+)-ATPase by eosin Y depended on pH and dielectric permeability of the incubation medium: increasing of pH from 6.5 to 8.0 reduced the apparent Ki, decreasing of dielectric permeability from 74.07 to 71.19 increased the apparent Ki.     

Solubilization, purification and reconstitution of Ca(2+)-ATPase from bovine pulmonary artery smooth muscle microsomes by different detergents: preservation of native structure and function of the enzyme by DHPC

The properties of Ca(2+)-ATPase purified and reconstituted from bovine pulmonary artery smooth muscle microsomes {enriched with endoplasmic reticulum (ER)} were studied using the detergents 1,2-diheptanoyl-sn-phosphatidylcholine (DHPC), poly(oxy-ethylene)8-lauryl ether (C(12)E(8)) and Triton X-100 as the solubilizing agents. Solubilization with DHPC consistently gave higher yields of purified Ca(2+)-ATPase with a greater specific activity than solubilization with C(12)E(8) or Triton X-100. DHPC was determined to be superior to C(12)E(8); while that the C(12)E(8) was determined to be better than Triton X-100 in active enzyme yields and specific activity. DHPC solubilized and purified Ca(2+)-ATPase retained the E1Ca-E1*Ca conformational transition as that observed for native microsomes; whereas the C(12)E(8) and Triton X-100 solubilized preparations did not fully retain this transition. The coupling of Ca(2+) transported to ATP hydrolyzed in the DHPC purified enzyme reconstituted in liposomes was similar to that of the native micosomes, whereas that the coupling was much lower for the C(12)E(8) and Triton X-100 purified enzyme reconstituted in liposomes. The specific activity of Ca(2+)-ATPase reconstituted into dioleoyl-phosphatidylcholine (DOPC) vesicles with DHPC was 2.5-fold and 3-fold greater than that achieved with C(12)E(8) and Triton X-100, respectively. Addition of the protonophore, FCCP caused a marked increase in Ca(2+) uptake in the reconstituted proteoliposomes compared with the untreated liposomes. Circular dichroism analysis of the three detergents solubilized and purified enzyme preparations showed that the increased negative ellipticity at 223 nm is well correlated with decreased specific activity. It, therefore, appears that the DHPC purified Ca(2+)-ATPase retained more organized and native secondary conformation compared to C(12)E(8) and Triton X-100 solubilized and purified preparations. The size distribution of the reconstituted liposomes measured by quasi-elastic light scattering indicated that DHPC preparation has nearly similar size to that of the native microsomal vesicles whereas C(12)E(8) and Triton X-100 preparations have to some extent smaller size. These studies suggest that the Ca(2+)-ATPase solubilized, purified and reconstituted with DHPC is superior to that obtained with C(12)E(8) and Triton X-100 in many ways, which is suitable for detailed studies on the mechanism of ion transport and the role of protein-lipid interactions in the function of the membrane-bound enzyme.     

Effects of calcium and soluble cytoplasmic activator protein (calmodulin) on various states of (Ca2+ + Mg2+)-ATPase activity in isolated membranes of human red cells

(Ca2+ + Mg2+)-ATPase activity of red cells and their isolated membranes was investigated in the presence of various Ca2+ concentrations and cytoplasmic activator protein. Red cell ATPase activity was high at low Ca2+ concentrations, and low at moderate and high concentrations of Ca2+. In the case of isolated membranes, both low and moderate ca2+ concentrations produced higher (Ca2+ + Mg2+)-ATPase activity than high Ca2+ concentration. Membrane-free hemolysate containing soluble activator of (Ca2+ + Mg2+)-ATPase produced a significant increase in (Ca2+ + Mg2+)-ATPase activity only at low ca2+ concentration. Regardless of Ca2+ and activator concentrations, the enzyme activity in the membrane was lower than lysed red cells. The low level of (Ca2+ + Mg2+)-ATPase activity seen at high Ca2+ concentration can be augmented by lowering the Ca2+ concentration of EGTA in the assay medium. However, once the membrane was exposed to a high Ca2+ concentration, the activator could no longer exert it maximum stimulation at the low Ca2+ concentration brought about by addition of EGTA. This loss of activation was not attributable to the Ca2+-induced denaturation of activator protein but rather related to the alteration of (Ca2+ + Mg2+)-ATPase states in the membrane. On the basis of these data, it is suggested that only a small portion of (Ca2+ + Mg2+)-ATPase activity of isolated membranes can be stimulated by the soluble activator and that (ca2+ + Mg2+)ATPase most likely exists in various states depending upon ca2+ concentration and the presence of activator. The enzyme state exhibiting the high degree of stimulation by activator may undergo irreversible damage in the presence of high Ca2+ concentrations.     

The effect of high pressure on the conformation, interactions and activity of the Ca(2+)-ATPase of sarcoplasmic reticulum.

High pressure (100-150 MPa) increases the intensity and polarization of fluorescence of FITC-labeled Ca(2+)-ATPase in a medium containing 0.1 mM Ca2+, suggesting a reversible pressure-induced transition from the E1 into an E2-like state with dissociation of ATPase oligomers. Under similar conditions but using unlabeled sarcoplasmic reticulum vesicles, high pressure caused the reversible release of Ca2+ from the high-affinity Ca2+ sites of Ca(2+)-ATPase, as indicated by changes in the fluorescence of the Ca2+ indicator, Fluo-3; this was accompanied by reversible inhibition of the Ca(2+)-stimulated ATPase activity measured in a coupled enzyme system of pyruvate kinase and lactate dehydrogenase, and by redistribution of Prodan in the lipid phase of the membrane, as shown by marked changes in its fluorescence emission characteristics. In a Ca(2+)-free medium where the equilibrium favors the E2 conformation of Ca(2+)-ATPase the fluorescence intensity of FITC-ATPase was not affected or only slightly reduced by high pressure. The enhancement of TNP-AMP fluorescence by 100 mM inorganic phosphate in the presence of EGTA and 20% dimethylsulfoxide was essentially unaffected by 150 MPa pressure at pH 6.0 and was only slightly reduced at pH 8.0. As the enhancement of TNP-AMP fluorescence by Pi is associated with the Mg(2+)-dependent phosphorylation of the enzyme and the formation of Mg.E2-P intermediate, it appears that the reactions of Ca(2+)-ATPase associated with the E2 state are relatively insensitive to high pressure. These observations suggest that high pressure stabilizes the enzyme in an E2-like state characterized by low reactivity with ATP and Ca2+ and high reactivity with Pi. The transition from the E1 to the E2-like state involves a decrease in the effective volume of Ca(2+)-ATPase.     

The inhibitor of liver plasma membrane (Ca2+-Mg2+)-ATPase. Purification and identification as a mediator of glucagon action

Rat liver plasma membranes contain (Ca2+-Mg2+)-ATPase sensitive to inhibition by both glucagon and Mg2+. We have previously shown that Mg2+ inhibition is mediated by a 30,000-dalton inhibitor, originally identified as a membrane-bound protein. In fact, this inhibitor is also present in the 100,000 X g supernatant of the total liver homogenate. Its purification was achieved from this fraction by a combination of ammonium sulfate washing, gel filtration, and cationic exchange chromatography. N-Ethylmaleimide (NEM) treatment caused the inactivation of the purified inhibitor, which suggested that this protein possesses at least one NEM-sensitive sulfhydryl group essential for its activity. Treatment of the liver plasma membranes with NEM resulted in a 2- and 5-fold decrease in the affinity of the (Ca2+-Mg2+)-ATPase for glucagon and Mg2+, respectively, while the basal enzyme activity remained unchanged. This effect of NEM was concentration-, pH-, and time-dependent, optimal conditions being obtained by a 60-min treatment of plasma membranes with 50 mM NEM, at pH 7 and at 4 degrees C. The presence of 0.5 mM Mg2+ during NEM treatment of the plasma membranes prevented NEM inactivation. Reconstitution experiments showed that addition of the purified inhibitor to NEM-treated plasma membranes restored the inhibitions of the (Ca2+-Mg2+)-ATPase by both magnesium and glucagon. It is proposed that the (Ca2+-Mg2+)-ATPase inhibitor not only confers its sensitivity of the liver (Ca2+-Mg2+)-ATPase to Mg2+, but also mediates the inhibition of this system by glucagon.     

Anisodamine causes the changes of structure and function in the transmembrane domain of the Ca(2+)-ATPase from sarcoplasmic reticulum

The effects of anisodamine on the Ca(2+)-ATPsae of sarcoplasmic reticulum (SR) were investigated by using differential scanning calorimetry to measure the ability of anisodamine to denature the transmembrane domain and the cytoplasmic domain. Anisodamine significantly altered the thermotropic phase behaviors of the transmembrane domain of purified Ca(2+)-ATPase. Specifically, the melting temperature of the transmembrane domain moved toward lower temperatures with the concentrations of anisodamine increasing and the thermotropic phase peak was abolished at 10 mM, indicating that the stabilized structure of the transmembrane domain in the presence of Ca2+ could be destabilized by anisodamine. Decreases of the intrinsic fluorescence and increases of the extrinsic fluorescence of ANS, a fluorescent probe, showed the exposure of tryptophan and hydrophobic region, respectively, suggesting again that anisodamine caused a less compact conformation in the transmembrane domain. A marked inhibition of the Ca2+ uptake activity of SR Ca(2+)-ATPase was observed when the addition of anisodamine. The drug did not affect the cytoplasmic domain of the enzyme and only slightly decreased the ATPase activity of the enzyme at concentrations up to 10 mM. This was likely due to the destabilized protein transmembrane domain. To sum up, our results revealed that anisodamine interacted specifically with the transmembrane domain of SR Ca(2+)-ATPase and inhibited the Ca2+ uptake activity of the enzyme.     

Low N-ethylmaleimide concentrations activate ryanodine receptors by a reversible interaction, not an alkylation of critical thiols

Previous studies proposed that N-ethylmaleimide (NEM) alkylates 3 classes of thiols on skeletal muscle ryanodine receptors (RyRs) producing 3 phases of channel modification, as function of time and concentration. NEM (5 mm) decreased, increased, and then decreased the open probability (P(o)) of the channel by thiol alkylation, a reaction not reversed by reducing agents. We now show that low NEM concentrations (20-200 microm) elicit Ca(2+) release from sarcoplasmic reticulum (SR) vesicles, but contrary to expectations, the effect was fully reversed by reducing agents or by washing SR vesicles. In bilayers, NEM (0.2 mm) increased P(o) of RyRs within seconds when added to the cis (not trans) side, and dithiothreitol (DTT; 1 mm) decreased P(o) in seconds. High (5 mm) NEM concentrations elicited SR Ca(2+) release that was not reversed by DTT, as expected for an alkylation reaction. A non-sulfhydryl reagent structurally related to NEM, N-ethylsuccinimide (0.1-0.5 mm), also elicited SR Ca(2+) release that was not reversed by DTT (1 mm). Other alkylating agents elicited SR Ca(2+) release, which was fully (N-methylmaleimide) or partially (iodoacetic acid) reversed by DTT and inhibited by ruthenium red. Nitric oxide (NO) donors at concentrations that did not activate RyRs inhibited NEM-induced Ca(2+) release, most likely by an interaction of NO with NEM rather than an inactivation of RyRs by NO. Thus, at low concentrations, NEM does not act as a selective thiol reagent and activates RyRs without alkylating critical thiols indicating that the multiple phases of ryanodine binding are unrelated to RyR activity or to NEM alkylation of RyRs.     

The effect of ionizing radiation on the Ca2+, Mg2(+)-ATPase activity of plasma membranes]

A study was made of a change in Ca2+, Mg(2+)-ATPase activity induced by the effect of ionizing radiation (5-10(4) Gy) on a thymocyte plasma membrane suspension. The Michaelis' constant and maximum rate of enzymic reactions were determined. With a dose of 10(3) Gy the structural changes in Ca2+, Mg(2+)-ATPase were shown to reduce the affinity of the substrate to an active enzyme center and to decrease the rate of the enzyme/substrate complex degradation.