Purification and characterization of two forms of a low-affinity Ca2+-ATPase from erythrocyte membranes

A low-affinity Ca2+-ATPase from erythrocyte membranes has been purified by agarose suspension electrophoresis and polyacrylamide gel electrophoresis in the absence of detergents. For maximal activity a calcium concentration above 10 mM is required. The activity is independent of magnesium. The Km value for ATP is about 60 microM. The enzyme appears in two forms (A and B) with similar amino acid composition. The specific activity of A is higher than that of B. Gel electrophoresis in SDS of A gives a pattern consisting of two bands. B gives the same pattern; the only difference between the patterns is the ratio of the amounts of protein in the bands. The apparent molecular weight of the proteins in the two SDS bands has been estimated at 23000 and 21000, respectively. The results obtained can be explained by assuming that the two proteins corresponding to the two bands obtained in SDS electrophoresis have a similar structure and can associate to complexes A and B. We have also shown that electrophoretic and chromatographic supporting media can induce aggregation of (membrane) proteins. Artificial complexes can thus be formed and cause misinterpretation of the data obtained. This may be the reason why some authors have speculated that Ca2+-ATPase is active only in complex with other proteins such as spectrin and actin.

Calcium transport across the plasma membrane: stimulation by calmodulin

Active transport of calcium into inside-out vesicles of red blood cell membranes was stimulated equally by (i) the purified protein activator of calcium-activated, magnesium-dependent adenosinetriphosphatase isolated from red cell hemolyzates and (ii) calmodulin, a protein activator of cylic nucleotide phosphodiesterase isolated from bovine brain. The results provide further evidence for the identity of red blood cell activator and calmodulin and show that this cytoplasmic protein may participate in the regulation of plasma membrane calcium transport.

Local anesthetics, mepacrine, and propranolol are antagonists of calmodulin

Local anesthetics such as dibucaine, QX572, tetracaine, and phenacaine, as well as other drugs with local anesthetic-like properties (e.g., mepacrine, propranolol, and SKF 525A) inhibit the specific calmodulin-dependent stimulation of erythrocyte Ca2+-ATPase (ATP phosphohydrolase, EC 3.6.1.3) and cyclic nucleotide phosphodiesterases (3',5'-cyclic-nucleotide 5'-nucleotidohydrolase, EC 3.1.4.17) from brain and heart. Basal activities of these enzymes in the absence of calmodulin are relatively unaffected by concentrations of local anesthetics that strongly inhibit the specific stimulation by calmodulin. Increasing calmodulin, but not Ca2+, overcomes the inhibitory action of the local anesthetics on brain phosphodiesterase. However, excess calmodulin does not fully restore activity of erythrocyte CA2+-stimulated ATPase. Although the mechanism(s) by which the local anesthetics act is unclear, they inhibit binding of 125I-labeled calmodulin to the erythrocyte membrane. Antagonism of calmodulin provides a molecular mechanism that may explain the inhibition of many Ca2+-dependent cellular processes by local anesthetics--e.g., Ca2+ transport, exocytosis, excitation-contraction coupling, non-muscle-cell motility, and aggregation.

A model for the regulation of the calmodulin-dependent enzymes erythrocyte Ca2+-transport ATPase and brain phosphodiesterase by activators and inhibitors

Acidic phospholipids, unsaturated fatty acids and limited proteolysis mimic the activating effect of calmodulin on erythrocyte Ca2+-transport ATPase and on brain cyclic nucleotide phosphodiesterase, as has been reported previously in several studies. Three different antagonists of calmodulin-induced activation of these enzymes were tested for their inhibitory potency on the stimulation produced by the other activators. Trifluoperazine and penfluridol were found to antagonize all the above mentioned types of activation of Ca2+-transport ATPase in the same concentration range. Both inhibitors also can reverse the activation of phosphodiesterase by oleic acid, phosphatidylserine and calmodulin at similar concentrations. However, in contrast with erythrocyte Ca2+-transport ATPase, activation of phosphodiesterase by limited tryptic digestion cannot be antagonized by penfluridol and trifluoperazine. Calmidazolium, formerly referred to as compound R 24571, was found to be a relatively specific inhibitor of calmodulin-induced activation of phosphodiesterase and Ca2+-transport ATPase, since antagonism of the other activators required much higher concentrations of the drug. The results suggest that the investigated drugs exert their inhibitory effect on calmodulin-regulated enzymes not solely via their binding to calmodulin but may also interfere directly with the calmodulin effector enzyme. In addition, a general mechanism of activation and inhibition of calmodulin-dependent enzymes is derived from our results.

Molecular properties of calcium-pumping ATPase from human erythrocytes

The Ca2+-pumping ATPase from human erythrocyte membranes, purified by the method previously reported [Niggli, V., Penniston, J. T., & Carafoli, E. (1979) J. Biol. Chem. 254, 9955-9958], was freed of minor impurities by extensive washing while bound to the calmodulin-Sepharose column. The pure enzyme showed a single band of Mr 138000, which contained no stainable carbohydrate. The enzyme retained calmodulin-stimulable ATPase activity; with appropriate assay conditions, an activity of 21.2 mumol/(mg x min) was obtained. Amino acid analysis showed that the ATPase had a larger proportion of polar amino acids than do other integral membrane proteins. Despite this, the ATPase showed a tendency to form dimers and higher aggregates even in the presence of sodium dodecyl sulfate and urea. The enzyme required Mg2+ but showed little activity unless a second ion was added. With regard to this second ion, the enzyme responded to alkaline earth metal ions in the order Ca2+ greater than Sr2+ much greater than Ba2+. It was highly specific for ATP and was stimulated by Na+ or K+; in all of these properties it resembled the enzyme in unfractionated membranes. Limited proteolysis using trypsin yielded, at short times, many fragments of various molecular weights; continued proteolysis resulted in two trypsin-resistant fragments of Mr 81000 and 33500. Analysis of the time course of proteolysis indicated that the ATPase existed in two or more conformations that had differing susceptibilities to proteolysis. It is suggested that these correspond to active and inactive conformers of the enzyme.

Compound 48/80 is a selective and powerful inhibitor of calmodulin-regulated functions

Compound 48/80, a condensation product of N-methyl-p-methoxyphenethylamine with formaldehyde, is composed of a family of cationic amphiphiles differing in the degree of polymerization. Compound 48/80 was found to be a potent inhibitor of the calmodulin-activated fraction of brain phosphodiesterase and red blood cell Ca2+-transport ATPase, with IC50 values of 0.3 and 0.85 micrograms/ml, respectively. However, the basal activity of both enzymes is not at all suppressed by the drug at concentrations up to 300 micrograms/ml. Inhibition of Ca2+ transport into inside-out red blood cell vesicles by compound 48/80 follows a similar pattern in that basal, calmodulin-independent, transport is also not affected by the drug. Kinetic analysis revealed that the stimulation of Ca2+-transport ATPase induced by calmodulin is inhibited by compound 48/80 according to a competitive mechanism. It was demonstrated that the inhibitory constituents of compound 48/80 bind to calmodulin in a Ca2+-dependent fashion. Comparison of the specificity of several anti-calmodulin drugs showed that compound 48/80 is the most specific inhibitor of the calmodulin-dependent fraction of red blood cell Ca2+-transport ATPase that has been described hitherto. In addition, compound 48/80 was found to be a rather specific inhibitor of the calmodulin-induced activation of Ca2+-transport ATPase when compared with the stimulation induced by an anionic amphiphile or by limited proteolysis. Half-maximal inhibition of the activity stimulated by oleic acid or mild tryptic digestion required 8- and 32-times higher concentrations of compound 48/80, respectively, compared with the calmodulin-dependent fraction of the ATPase activity. Moreover, calmodulin-independent systems as rabbit skeletal muscle sarcoplasmic reticulum Ca2+-transport ATPase or calf cardiac sarcolemma (Na+ + K+)-transport ATPase are far less influenced by compound 48/80 as compared with trifluoperazine and calmidazolium. Because of its high specificity compound 48/80 is proposed to be a promising tool for studying calmodulin-dependent processes.

Prostacyclin and sodium nitroprusside inhibit the activity of the platelet inositol 1,4,5-trisphosphate receptor and promote its phosphorylation

Prostaglandin I2 (PGI2) and sodium nitroprusside (SNP) induce a rapid decay of the thrombin-promoted increase of [Ca2+]i in aspirin-treated platelets incubated in the absence of external Ca2+. The mechanism of their effect was studied with a new method which utilizes ionomycin to increase [Ca2+]i, followed by bovine serum albumin (BSA) to remove the Ca2+ ionophore. The rapid decay of [Ca2+]i after BSA is mostly due to the reuptake into the stores, since it is strongly inhibited by the endomembrane Ca2+-ATPase inhibitor thapsigargin. PGI2 and SNP are without effect on the BSA-promoted decay both with and without thapsigargin, showing that they do not affect the activity of the Ca2+-ATPases. The fast decay of [Ca2+]i after BSA is decreased by thrombin which produces the Ca2+ releaser inositol 1,4,5-trisphosphate (InsP3), thus counteracting the activity of the endomembrane Ca2+ pump. When added after thrombin, PGI2 and SNP accelerate the BSA-activated decay of [Ca2+]i. However, under the same conditions, they do not decrease the concentration of InsP3. In saponin-permeabilized platelets, cAMP and cGMP counteract the Ca2+ release induced by exogenous InsP3. Their inhibitory effect disappears at high InsP3 concentrations. This demonstrates that PGI2 and SNP potentiate Ca2+ reuptake by inhibiting the InsP3 receptor. Two bands of approximately 260 kDa are recognized by a monoclonal antibody recognizing the C-terminal region of the InsP3 receptor. Both are phosphorylated rapidly, the heavier more intensely, in the presence of PGI2 and SNP. The phosphorylation of the InsP3 receptor is fast enough to be compatible with its involvement in the inhibition of the receptor by cyclic nucleotides.

Inhibition of red cell Ca2+-ATPase by vanadate

1. The Mg2+- plus Ca2+-dependent ATPase (Ca2+-ATPase) in human red cell membranes is susceptible to inhibition by low concentrations of vanadate. 2. Several natural activators of Ca2+-ATPase (Mg2+, K+, Na+ and calmodulin) modify inhibition by increasing the apparent affinity of the enzyme for vanadate. 3. Among the ligands tests, K+, in combination with Mg2+, had the most pronounced effect on inhibition by vanadate. 4. Under conditions optimal for inhibition of Ca2+-ATPase, the K 1/2 for vanadate was 1.5 microM and inhibition was nearly complete at saturating vanadate concentrations. 5. There are similarities between the kinetics of inhibition of red cell Ca2+-ATPase and (Na+ + K+)-ATPase prepared from a variety of sources; however, (Na+ + K+)-ATPase is approx. 3 times more sensitive to inhibition by vanadate.

Vanadate inhibition of the Ca2+-ATPase from human red cell membranes

(1) VO3(-) combines with high affinity to the Ca2+-ATPase and fully inhibits Ca2+-ATPase and Ca2+-phosphatase activities. Inhibition is associated with a parallel decrease in the steady-state of the Ca2+-dependent phosphoenzyme. (2) VO3(-) blocks hydrolysis of ATP at the catalytic site. The sites for VO3(-) also exhibit negative interactions in affinity with the regulatory sites for ATP of the Ca2+-ATPase. (3) The sites for VO39-) show positive interaactions in affinity with sites for Mg2+ and K+. This accounts for the dependence on Mg2+ and K+ of the inhibition by VO3(-). Although, with less effectiveness, Na2+ and K+ substitutes for K+ whereas Li+ does not. The apparent affinites for Mg24 and K+ for inhibiton by VO3(-) seem to be less than those for activation of the Ca2+-ATPase. (4) Inhibition by VO3(-) is independent of Ca2+ at concentrations up to 50 microM. Higher concentrations of Ca2+ lead to a progressive release of the inhibitiory effect of VO3(-).

Calmodulin affinity chromatography yields a functional purified erythrocyte (Ca+ + Mg2+)-dependent adenosine triphosphatase

The (Ca2+ + Mg2+)-dependent ATPase of human erythrocyte membranes was solubilized with deoxycholate and purified by calmodulin affinity chromatography to yield a functional enzyme. The method gave an enzyme purified 207-fold as compared with that of the erythrocyte membranes. The molecular weight of the ATPase was in the range 135 000-150 000, as revealed by a single major band after electrophoresis on dodecyl sulphate/polyacrylamide gels. The isolated enzyme was highly sensitive to calmodulin, since the activity was increased about 9-fold. At 37 degrees C and in the presence of calmodulin the purified ATPase had a specific activity of 10.1 mumol/min per mg of protein. Triton X-100 or deoxycholate stimulated the calmodulin-deficient enzyme in a concentration-dependent fashion whereby the calmodulin-sensitivity was lost. The purification method is suitable for studying the lipid-sensitivity of the ATPase, since the lipids can easily be exchanged without a significant loss of activity. A purification procedure described by Niggli, Penniston & Carafoli [(1979) J. Biol. Chem. 254, 9955-9958] resulted in an enzyme that indeed was pure but was lacking a predominant feature, namely the modulation by calmodulin.

Identification of a new SERCA2 splice variant regulated during monocytic differentiation

Sarco/endoplasmic reticulum-type calcium transport ATPases (SERCA enzymes) pump calcium ions from the cytosol into the endoplasmic reticulum. We report that in addition to the ubiquitously expressed SERCA2b isoform, a new splice variant of SERCA2 can be detected (SERCA2c) that arises from the inclusion of a short intronic sequence located between exons 20 and 21 of the SERCA2a isoform. Sequence analysis revealed classical splice donor and acceptor sites, as well as a branch-point site. Due to the presence in the new exon of an in-frame stop codon that is preceded by a 17 bp coding sequence, this mRNA potentially codes for a protein with a truncated C-terminus containing a short unique C-terminal peptide stretch. SERCA2c message was detected in epithelial, mesenchymal, and hematopoietic cell lines, as well as in primary human monocytes. Moreover, we found that during monocytic differentiation total SERCA2 ATPase expression is induced on the protein and mRNA level and that the novel SERCA2c messenger is also up-regulated during this process. These data indicate that the alternative splicing pattern of the 3(') region of the SERCA2 primary transcript is more complex than that previously thought and that this enzyme may be involved in the process of monocyte differentiation.

Comparison of the calmodulin antagonists compound 48/80 and calmidazolium

The two presumed calmodulin antagonists calmidazolium and compound 48/80 were compared for their effects on several calmodulin-dependent and calmodulin-independent enzyme systems. Compound 48/80 and calmidazolium were found to be about equipotent in antagonizing the calmodulin-dependent fraction of brain phosphodiesterase and erythrocyte Ca2+-transporting ATPase. Compound 48/80 combines high potency with high specificity in that: (1) the basal, calmodulin-independent, activity of calmodulin-regulated enzymes was not suppressed; (2) calmodulin-independent enzyme activities, such as Ca2+-transporting ATPases of sarcoplasmic reticulum, Mg2+-dependent ATPases of different tissues and Na+/K+-transporting ATPase of cardiac sarcolemma, were far less altered, or not altered at all, by compound 48/80 as compared with calmidazolium; and (3) antagonism of proteolysis-induced stimulation as opposed to calmodulin-induced activation of erythrocyte Ca2+-transporting ATPase required a 32 times higher concentration of compound 48/80. In all these aspects compound 48/80 was found to be a superior antagonist to calmidazolium since inhibition of calmodulin-independent events by the other agent occurred at considerably lower concentrations. Therefore compound 48/80 is proposed to be a much more specific and useful tool for studying the participation of calmodulin in biological processes than the presently used agents.

The control by Ca2+ of the polyphosphoinositide phosphodiesterase and the Ca2+-pump ATPase in human erythrocytes

1. Both the Ca(2+)-pump ATPase and the polyphosphoinositide phosphodiesterase of the erythrocyte membrane can, when assayed under appropriate conditions, be activated by Ca(2+) in the micromolar range. We have therefore compared the mechanisms and affinities for Ca(2+) activation of the two enzymes in human erythrocyte membranes, to see whether the polyphosphoinositide phosphodiesterase would be active in normal healthy erythrocytes. 2. At physiological ionic strength and in the presence of calmodulin, the Ca(2+)-pump ATPase was activated by Ca(2+) in a highly co-operative manner, with half-maximal activation occurring at about 0.3mum-Ca(2+). At an optimal Ca(2+) concentration, calmodulin stimulated the Ca(2+)-sensitive ATPase activity about 10-fold. 3. Ca(2+) activated the polyphosphoinositide phosphodiesterase in a non-co-operative manner. The Ca(2+) requirements for breakdown of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate were identical, which supports our previous conclusion that Ca(2+) activates a single polyphosphoinositide phosphodiesterase that degrades both lipids with equal facility. Added calmodulin did not affect the activity of the polyphosphoinositide phosphodiesterase. 4. At low ionic strength in the absence of Mg(2+), half-maximal activation of the phosphodiesterase was at about 3mum-Ca(2+). The presence of 1mm-Mg(2+) shifted the Ca(2+) activation curve to the right, as did elevation of the ionic strength. When the Ca(2+)-pump ATPase and the polyphosphoinositide phosphodiesterase were assayed in the same incubations and under conditions of intracellular ionic strength and Mg(2+) concentration, the ATPase was fully activated at 3mum-Ca(2+), whereas no polyphosphoinositide phosphodiesterase activity was detected below 100mum-Ca(2+). 5. The Ca(2+)-pump ATPase of the erythrocyte membrane normally maintains the Ca(2+) concentration of healthy erythrocytes below approx. 0.1mum. It therefore seems unlikely that the polyphosphoinositide phosphodiesterase of the erythrocyte membrane ever expresses its activity in a healthy erythrocyte.

Glucose induces lipid peroxidation and inactivation of membrane-associated ion-transport enzymes in human erythrocytes in vivo and in vitro

Erythrocytes of diabetic subjects (non-insulin dependent) were found to have eight- to ten-fold higher levels of endogenously formed thiobarbituric acid reactive malonyldialdehyde (MDA), thirteen-fold higher levels of phospholipid-MDA adduct, 15-20% reduced Na(+)-K(+)-ATPase activity with unchanged Ca+2-ATPase activity, as compared with the erythrocytes from normal healthy individuals. Incubation of normal erythrocytes with elevated concentrations (15-35 mM) of glucose, similar to that present in diabetic plasma, led to the increased lipid peroxidation, phospholipid-MDA adduct formation, reduction of Na(+)-K(+)-ATPase (25-50%) and Ca+2-ATPase (50%) activities. 2-doxy-glucose was 80% as effective as glucose in the lipid peroxidation and lipid adduct formation. However, other sugars, such as fructose, galactose, mannose, fucose, glucosamine and 3-O-methylmannoside, and sucrose, tested at a concentration of 35 mM, resulted in reduced (20-30%) lipid peroxidation without the formation of lipid-MDA adduct. Kinetic studies show that reductions in Na(+)-K(+)-ATPase and Ca+2-ATPase activities precede the lipid peroxidation as the enzyme inactivation occur within 30 min of incubation of erythrocytes with high concentration (15-35 mM) of glucose, while lipid peroxidation product, MDA appears at 4 hr and lipid-MDA adducts at 8 hr. The lipoxygenase pathway inhibitors, 5,8,11-eicosatriynoic acid and Baicalein (5,6,7-trihydroxyflavone), reduced the glucose-induced lipid peroxidation by 30% and MDA-lipid adduct formation by 26%. Indomethacin, a cyclooxygenase pathway inhibitor, had no discernible effect on the lipid peroxidation in erythrocytes. However, the inhibitors of lipid peroxidation, 3-phenylpyrazolidone, metyrapone, and the inhibitors of lipoxygenase pathways did not ablate the glucose-induced reduction of Na(+)-K(+)-ATPase and Ca+2-ATPase activities in erythrocytes. Erythrocytes produce 15-HETE (15-hydroxy-eicosatetraenoic acid), which is augmented by glucose. These results suggest that the formation of lipoxygenase metabolites potentiate the glucose-induced lipid peroxidation and that the inactivation of Na(+)-K(+)-ATPase and Ca+2-ATPase occurs as a result of non-covalent interaction of glucose with these enzymes.

Three novel sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 isoforms. Expression, regulation, and function of the membranes of the SERCA3 family

Sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into the endoplasmic reticulum. Recently, three human SERCA3 (h3a-c) proteins and a previously unknown rat SERCA3 (r3b/c) mRNA have been described. Here, we (i) document two novel human SERCA3 splice variants h3d and h3e, (ii) provide data for the expression and mechanisms regulating the expression of all known SERCA3 variants (r3a, r3b/c, and h3a-e), and (iii) show functional characteristics of the SERCA3 isoforms. h3d and h3e are issued from the insertion of an additional penultimate exon 22 resulting in different carboxyl termini for these variants. Distinct distribution patterns of the SERCA3 gene products were observed in a series of cell lines of hematopoietic, epithelial, embryonic origin, and several cancerous types, as well as in panels of rat and human tissues. Hypertension and protein kinase C, calcineurin, or retinoic acid receptor signaling pathways were found to differently control rat and human splice variant expression, respectively. Stable overexpression of each variant was performed in human embryonic kidney 293 cells, and the SERCA3 isoforms were fully characterized. All SERCA3 isoforms were found to pump Ca2+ with similar affinities. However, they modulated the cytosolic Ca2+ concentration ([Ca2+]c) and the endoplasmic reticulum Ca2+ content ([Ca2+]er) in different manners. A newly generated polyclonal antibody and a pan-SERCA3 antibody proved the endogenous expression of the three novel SERCA3 proteins, h3d, h3e, and r3b/c. All these data suggest that the SERCA3 gene products have a more widespread role in cellular Ca2+ signaling than previously appreciated.

Regulation of calcium accumulation and efflux from platelet vesicles. Possible role for cyclic-AMP-dependent phosphorylation and calmodulin

Calcium-accumulating vesicles were isolated by differential centrifugation of sonicated platelets. Such vesicles exhibit a (Ca2+ + Mg2+)-ATPase activity of about 10 nmol (min . mg)-1 and an ATP-dependent Ca2+ uptake of about 10 nmol (min . mg)-1. When incubated in the presence of Mg[gamma-32P]ATP, the pump is phosphorylated and the acyl phosphate bond is sensitive to hydroxylamine. The [32P]phosphate-labeled Ca2+ pump exhibits a subunit molecular weight of 120 000 when analyzed by lithium dodecyl sulfate-polyacrylamide gel electrophoresis. Platelet calcium-accumulating vesicles contain a 23 kDa membrane protein that is phosphorylatable by the catalytic subunit of cAMP-dependent protein kinase but not by protein kinase C. This phosphate acceptor is not phosphorylated when the vesicles are incubated in the presence of either Ca2+ or Ca2+ plus calmodulin. The latter protein is bound to the vesicles and represents 0.5% of the proteins present in the membrane fraction. Binding of 125I-labeled calmodulin to this membrane fraction was of high affinity (16 nM), and the use of an overlay technique revealed four major calmodulin-binding proteins in the platelet cytosol (Mr = 94 000, 87 000, 60 000 and 43 000). Some minor calmodulin-binding proteins were enriched in the membrane fractions (Mr = 69 000, 57 000, 39 000 and 37 000). When the vesicles are phosphorylated in the presence of MgATP and of the catalytic subunit of cAMP-dependent protein kinase, the rate of Ca2+ uptake is essentially unaltered, while the Ca2+ capacity is diminished as a consequence of a doubling in the rate of Ca2+ efflux. Therefore, the inhibitory effect of cAMP on platelet function cannot be explained in such simple terms as an increased rate of Ca2+ removal from the cytosol. Calmodulin, on the other hand, was observed to have no effect on the initial rate of calcium efflux when added either in the absence or in the presence of the catalytic subunit of the cyclic AMP-dependent protein kinase, nor did the addition of 0.5 microM calmodulin result in increased levels of vesicle phosphorylation.

Activation of the Ca2+-ATPase of human erythrocyte membrane by an endogenous Ca2+-dependent neutral protease

Limited proteolysis of the plasma membrane calcium transport ATPase (Ca2+-ATPase) from human erythrocytes by trypsin produces a calmodulin-like activation of its ATP hydrolytic activity and abolishes its calmodulin sensitivity. We now demonstrate a similar kind of activation of the human erythrocyte membrane Ca2+-ATPase by calpain (calcium-dependent neutral protease) isolated from the human red cell cytosol. Upon incubation of red blood cell membranes with purified calpain in the presence of Ca2+ the membrane-bound Ca2+-ATPase activity was increased and its sensitivity to calmodulin was lost. In contrast to the action of other proteases tested, proteolysis by calpain favors activation over inactivation of the Ca2+-ATPase activity, except at calpain concentrations more than 2 orders of magnitude higher. Exogenous calmodulin protects the Ca2+-ATPase against calpain-mediated activation at concentrations which also activate the Ca2+-ATPase activity. Calcium-dependent proteolytic modification of the Ca2+-ATPase could provide a mechanism for the irreversible activation of the membrane-bound enzyme.

Role of calmodulin in thyroid hormone stimulation in vitro of human erythrocyte Ca2+-ATPase activity

Because human erythrocyte membrane Ca2+-ATPase is a calmodulin-dependent enzyme, and because physiological levels of thyroid hormone stimulate this enzyme system in vitro, we have studied the role of calmodulin in this model of extranuclear thyroid hormone action. Ca2+-ATPase activity in the absence of thyroid hormone ("basal activity") was increased by inclusion in the preassay incubation mixture of purified calmodulin or hypothyroid erythrocyte hemolysate that contained calmodulin (39 micrograms calmodulin/ml packed cells, determined by radioimmunoassay); addition of L-thyroxine or 3,5,3'-triiodo-L-thyronine (10(-10)M) significantly enhanced (P less than 0.001) enzyme activity in the presence of calmodulin or hemolysate. The stimulatory effects of thyroid hormone, calmodulin, and hemolysate were additive. At 5-10 microM, trifluoperazine, an antagonist of calmodulin, inhibited thyroid hormone stimulation of Ca2+-ATPase activity. Higher concentrations of trifluoperazine (50-100 microM) inhibited basal and hormone-stimulated enzyme activity, with or without added calmodulin. Anti-calmodulin antibody (10-50 micrograms antibody/mg membrane protein) inhibited basal, calmodulin-stimulated and thyroid hormone-stimulated Ca2+-ATPase activity. Membrane preparations were shown by radioimmunoassay to contain residual endogenous calmodulin (0.27 +/- 0.02 micrograms/mg membrane protein). The latter accounts for the effect of trifluoperazine and calmodulin antibody on membrane Ca2+-ATPase activity in the absence of added purified calmodulin. These results support the conclusion that the in vitro action of physiological levels of iodothyronines on human erythrocyte Ca2+-ATPase activity requires the presence of calmodulin.

Progressive inhibition of the Ca pump and Ca:Ca exchange in sickle red cells

Sickle cell anaemia red cells (SS) were reported to have a high Ca content and an increased Ca uptake on deoxygenation, but their Ca-pump activity was described as normal. This seemed puzzling because the saturated Ca-extrusion rate of the normal, high Ca-affinity Ca pump is about 10 mmol per 1 cells per h (refs 3, 4) and the highest sickling-induced Ca influx reported in SS cells and observed in ATP-depleted sickle-trait (SA) red cells never exceeded 0.2 mmol per 1 cells per h. Normal pump performance is, therefore, incompatible with Ca accumulation unless SS cells have abnormally high Ca-binding capacity. We provide here evidence which suggests that SS cells have normal Ca-buffering capacity and probably genetically normal Ca pumps, but that the sickling process causes progressive Ca-pump failure and a marked reduction in Ca:Ca exchange.