Demonstration of a phosphopeptide intermediate in the Mg ++ -dependent, Na + - and K + -stimulated adenosine triphosphatase reaction of the erythrocyte membrane

Backdoor phosphorylation of basolateral plasma membranes of small intestinal epithelial cells: characterization of a furosemide-induced phosphoprotein related to the second sodium pump

Enterocyte has two different Na+-stimulated ATPases, the ouabain-sensitive Na+/K+ ATPase and a furosemide-inhibitable Na+ ATPase. To identify the polypeptide associated with the Na+-ATPase, 32Pi phosphorylation into basolateral membranes of enterocyte was investigated. Both, ouabain and furosemide induced Mg2+-dependent, vanadate-sensitive 32Pi incorporation into a 100kDa polypeptide. K(m) for Pi was 17.7+/-1.82 microM and 16.8+/-0.69 microM for ouabain-induced and furosemide-induced phosphorylation, respectively. K(m) for furosemide was 1.3+/-0.21 mM. Furosemide-induced 32Pi incorporation was sensitive to alkaline pH and hydroxylamine suggesting an acyl-phosphate bond. Na+ and K+ inhibited 32Pi incorporation induced by ouabain. In contrast, Na+ stimulated furosemide-induced phosphorylation with a K(m) of 16.5+/-5.59 mM while K+ had no effect. Purified Na+/K+ ATPase only presented ouabain-induced phosphoprotein, indicating that furosemide-induced phosphorylation is not related to this enzyme and appears to correspond to a new member of P-type ATPases associated with the second Na+ pump.

Na+-ATPase from the plasma membrane of the marine alga Tetraselmis (Platymonas) viridis forms a phosphorylated intermediate

Plasma membranes isolated from the marine unicellular alga Tetraselmis (Platymonas) viridis were phosphorylated by [gamma-32P]ATP, and membrane proteins were then analyzed by PAGE in SDS, under acidic conditions. Three radioactive components with apparent molecular masses of 100 kDa, 76 kDa, and 26 kDa were detected. The phosphorylation of one of them, the 100 kDa polypeptide, was specifically stimulated by Na+. Vanadate almost completely inhibited the Na+-mediated phosphorylation of the peptide. The phosphate bound to this peptide underwent rapid turnover and was discharged by hydroxylamine. The 100 kDa phosphopeptide was sensitive to ADP. The conclusion is drawn that the 100 kDa phosphopeptide is a phosphorylated intermediate of the Na+-transporting ATPase in the T. viridis plasma membrane.

Effect of methyl isocyanate on rabbit cardiac Na+, K(+)-ATPase

The present study describes the effect of methyl isocyanate (MIC) on rabbit cardiac microsomal Na+, K(+)-ATPase. Addition of MIC in vitro resulted in dose-dependent inhibition of Na+, K(+)-ATPase, Mg(2+)-ATPase and K(+)-activated p-nitrophenyl phosphatase (K(+)-PNPPase). Activation of Na+, K(+)-ATPase by ATP in the presence of MIC showed a decrease in Vmax with no change in Km. Similarly, activation of K+ PNPPase by PNPP in the presence of MIC showed a decrease in Vmax with no change in Km. The circular dichroism spectral studies revealed that MIC interaction with Na+, K(+)-ATPase led to a conformation of the protein wherein the substrates Na+ and K+ were no longer able to bind at the Na(+)- and K(+)-activation sites. The data suggest that the inhibition of Na+, K(+)-ATPase was non-competitive and occurred by interference with the dephosphorylation of the enzyme-phosphoryl complex.

Study on the erythrocytes from myotonic dystrophy with multi-nuclear NMR

We have studied the water permeability through membranes, the function of the Na pump, and glucose metabolism of erythrocytes of patients with myotonic muscular dystrophy (MyD) using 1H--, 23Na, and 13C-NMR techniques. A significant decrease in water permeability was recognized in the MyD erythrocyte membrane, and impaired Na pumping was suspected to be correlated with the former biochemical abnormalities in band III protein of MyD erythrocyte membrane. Significant acceleration of glycolysis in the erythrocyte for the first 160 minutes was also recognized in MyD; however, the production of lactate showed no difference between MyD and controls. The increased glucose uptake in MyD may be compensatory to the diminished pumping mechanism, but further information, such as inorganic phosphate permeability and the activity of the rate-limiting enzyme of erythrocyte glycolysis, is needed.

Function of the sarcoplasmic reticulum and expression of its Ca2(+)-ATPase gene in pressure overload-induced cardiac hypertrophy in the rat

The reduction in Ca2+ concentration during diastole and relaxation occurs differently in normal hearts and in hypertrophied hearts secondary to pressure overload. We have studied some possible molecular mechanisms underlying these differences by examining the function of the sarcoplasmic reticulum and the expression of the gene encoding its Ca2(+)-ATPase in rat hearts with mild and severe compensatory hypertrophy induced by abdominal aortic constriction. Twelve sham-operated rats and 31 operated rats were studied 1 month after surgery. Eighteen animals exhibited mild hypertrophy (left ventricular wt/body wt less than 2.6) and 13 animals severe hypertrophy (left ventricular wt/body wt greater than 2.6). During hypertrophy we observed a decline in the function of the sarcoplasmic reticulum as assessed by the oxalate-stimulated Ca2+ uptake of homogenates of the left ventricle. Values decreased from 12.1 +/- 1.2 nmol Ca2+/mg protein/min in sham-operated rats to 9.1 +/- 1.5 and 6.7 +/- 1.1 in rats with mild and severe hypertrophy, respectively (p less than 0.001 and p less than 0.001, respectively, vs. shams). This decrease was accompanied by a parallel reduction in the number of functionally active CA2(+)-ATPase molecules, as determined by the level of Ca2(+)-dependent phosphorylated intermediate: 58.8 +/- 7.4 and 48.1 +/- 13.5 pmol P/mg protein in mild and severe hypertrophy, respectively, compared with 69.7 +/- 8.2 in shams (p less than 0.05 and p less than 0.01, respectively, vs. shams). Using S1 nuclease mapping, we observed that the Ca2(+)-ATPase messenger RNA (mRNA) from sham-operated and hypertrophied hearts was identical. Finally, the relative level of expression of the Ca2(+)-ATPase gene was studied by dot blot analysis at both the mRNA and protein levels using complementary DNA clones and a monoclonal antibody specific to the sarcoplasmic reticulum Ca2(+)-ATPase. In mild hypertrophy, the concentrations of Ca2(+)-ATPase mRNA and protein in the left ventricle were unchanged when compared with shams (mRNA, 93.8 +/- 10.6% vs. sham, NS; protein, 105.5 +/- 14% vs. sham, NS). in severe hypertrophy, the concentration of Ca2(+)-ATPase mRNA decreased to 68.7 +/- 12.9% and that of protein to 80.1 +/- 15.5% (p less than 0.001 and p less than 0.05, respectively), whereas the total amount of mRNA and enzyme per left ventricle was either unchanged or slightly increased. The slow velocity of relaxation of severely hypertrophied heart can be at least partially explained by the absence of an increase in the expression of the Ca2(+)-ATPase gene and by the relative diminution in the density of the Ca2+ pumps.(ABSTRACT TRUNCATED AT 400 WORDS)

Downregulation of surface sodium pumps by endocytosis during meiotic maturation of Xenopus laevis oocytes

During meiotic maturation, plasma membranes of Xenopus laevis oocytes completely lose the capacity to transport Na and K and to bind ouabain. To explore whether the downregulation might be due to an internalization of the sodium pump molecules, the intracellular binding of ouabain was determined. Selective permeabilization of the plasma membrane of mature oocytes (eggs) by digitonin almost failed to disclose ouabain binding sites. However, when the eggs were additionally treated with 0.02% sodium dodecyl sulfate (SDS) to permeabilize inner membranes, all sodium pumps present before maturation were recovered. Phosphorylation by [gamma-32P]ATP combined with SDS-polyacrylamide gel electrophoresis (PAGE) and autoradiography showed that sodium pumps were greatly reduced in isolated plasma membranes of eggs. According to sucrose gradient fractionation, maturation induced a shift of sodium pumps from the plasma membrane fraction to membranes of lower buoyant density with a protein composition different from that of the plasma membrane. Endocytosed sodium pumps identified on the sucrose gradient from [3H]ouabain bound to the cell surface before maturation could be phosphorylated with inorganic [32P]phosphate. The findings suggest that downregulation of sodium pumps during maturation is brought about by translocation of surface sodium pumps to an intracellular compartment, presumably endosomes. This contrasts the mechanism of downregulation of Na-dependent cotransport systems, the activities of which are reduced as a consequence of a maturation-induced depolarization of the membrane without a removal of the corresponding transporter from the plasma membrane.

Characterization and use of polyclonal antibody to Na+,K+-ATPase: immunocytochemical localization in salt glands of the duck

The amount of Na+,K+-ATPase of the avian salt gland increased concomitantly with plasma membrane surface area during salt feeding of ducklings (adaptation), and both enzyme content and membrane surface area decreased upon return to fresh water (deadaptation). In a further study of the enzyme, a marker for plasma membrane biogenesis, polyvalent antibodies were raised to the denatured alpha-subunit of the purified ATPase. Antisera did not inhibit enzymatic activity but immunoprecipitated the phosphorylated intermediate of the alpha-subunit. Furthermore, the alpha-subunit, which was not glycosylated, was immunoprecipitated from homogenates of tissue slices metabolically labelled with [35S]-methionine, using antisera raised against either duck salt gland or dog kidney alpha-subunit. The former antisera also recognized the alpha-subunit in the brain, heart, kidney, liver, intestine and skeletal muscle of the duck. Immunocytochemistry with the antisera raised to the duck salt gland alpha-subunit revealed reaction at basolateral as well as apical plasma membrane in the duck salt gland principal cells, with essentially no deposits on peripheral cells, fibroblasts, erythrocytes, endothelial cells and neural elements. Within the principal cells, immunolabelling was also detected on small vesicles, multivesicular bodies and lysosomes; deposits on extracellular debris and vesicles in the lateral and lumenal spaces were also apparent. The labelling patterns were qualitatively but not quantitatively similar in salt glands of control, adapted and deadapted ducklings, and are discussed in the context of a model for plasma membrane biogenesis and turnover in which degradative events may play a major role.

Conformational states of sarcoplasmic reticulum Ca2+-ATPase as studied by proteolytic cleavage

Conformational states in sarcoplasmic reticulum Ca2+-ATPase have been examined by tryptic and chymotryptic cleavage. High affinity Ca2+ binding (E1 state) exposes a peptide bond in the A fragment of the polypeptide chain to trypsin. Absence of Ca2+ (E2 state) exposes bonds in the B fragment, which are protected by binding of Mg2+ or ATP. After phosphorylation from ATP the tryptic cleavage pattern depends on the predominant phosphoenzyme species present. ADP-sensitive E1P and ADP-insensitive E2P have cleavage patterns identical to those of unphosphorylated E1 and E2, respectively, indicating that two major conformational states are involved in Ca2+ translocation. The transition from E1P to E2P is inhibited by secondary tryptic splits in the A fragment, suggesting that parts of this fragment are of particular importance for the energy transduction process. The tryptic cleavage patterns of phosphorylated forms of detergent solubilized monomeric Ca2+-ATPase were similar to those of the membrane-bound enzyme, indicating that Ca2+ translocation depends mainly on structural changes within a single peptide chain. On the other hand, the protection of the second cleavage site as observed after vanadate binding to membranous Ca2+-ATPase could not be achieved in the soluble monomeric enzyme. Shielding of this peptide bond may therefore be due to protein-protein interactions in the semicrystalline state of the vanadate-bound Ca2+-ATPase in membranous form.

A reversal in relative mobility of the two large subunits of brine shrimp (Na+ + K+)-adenosinetriphosphatase

The two large subunits of brine shrimp Na,K-ATPase can be resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis at neutral pH and at acidic pH. These subunits appear to reverse their positions on the gel relative to each other when resolved at acidic pH relative to neutral pH. The migration of both subunits is apparently affected by charge, even in the presence of 2.5% sodium dodecyl sulfate.

A method for the preparation of low-pH dodecyl sulphate/polyacrylamide-gradient gels

1. A low-pH lithium dodecyl sulphate/polyacrylamide-gradient slab-gel system, suitable for electrophoresis, is described, and the migration properties of standard proteins are compared on this and conventional high-pH gels. 2. Cytochrome oxidase may be partially resolved into its component polypeptides. The order of migration of these is, however, dependent on the pH of the gel system.

Maltosyl isothiocyanate: an affinity label for the glucose transporter of the human erythrocyte membrane. 2. Identification of the transporter

Maltosyl isothiocyanate (MITC), a potent irreversible inhibitor of glucose transport in human erythrocytes [Mullins, R. E., & Langdon, R. G. (1980) Biochemistry (preceding paper in this issue)], has been found to react almost exclusively with band 3 of the human erythrocyte membrane. The incorporation of [14C]MITC into band 3 was found to be antagonized by transportable sugars or competitive inhibitors of transport. On the basis of [14C]MITC incorporation into band 3 and MITC inhibition of transport, it is estimated that there are 3 x 10(5) glucose transporters present in the erythrocyte membrane. It was found that [14C]MITC-labeled band 3 could be converted into 14C-labeled band 4.5 during the Triton X-100 extraction procedure described by Kasahara & Hinkle [Kasahara, M., & Hinkel, P. C. (1977) J. Biol. Chem. 252, 7384]. On the basis of the evidence presented here and in the preceding paper, it is suggested that in the native erythrocyte membrane a component of band 3 is the glucose transport protein and that during purification with nonionic detergents the transport protein may be enzymatically degraded with some retention of activity.

Distribution of lead-203 in human peripheral blood in vitro

In-vitro experiments using 203Pb were performed to identify the lead binding components in human peripheral blood. The distribution of lead in plasma, in the red cell membrane, and within the red cell was also investigated. Studies of the distribution of 203Pb in whole blood showed that at a lead concentration of 2.45 mumol/l (50 micrograms/100 ml) about 94% of lead had been incorporated by the erythrocytes and 6% remained in the plasma. After extraction of lipid by a methanol/chloroform mixture, about 75% of the lead was found to be associated with the protein fraction. The lipid contained about 21% of the 203Pb, the remainder being in the aqueous plasma. SDS polyacrylamide gel electrophoresis of blood plasma showed that almost 90% of the 203Pb was present in the albumin fraction; the remainder was likely to be associated with high molecular weight globulins. Several binding sites were identified on the erythrocyte membrane. The high molecular weight component, about 130 000-230 000, was the most important 203Pb binding site. Chemical modification of membrane proteins suggested that the carboxyl groups are the major ligand responsible for most of the lead binding. SH groups of the membrane may have a minor role, but amino groups did not appear to affect the lead binding. The binding of lead to erythrocytes was not confined to membranes, over 80% of lead in blood penetrates into erythrocytes and binds to intracellular components. Gel chromatography of the haemolysate showed that over 90% of the 203Pb was attached to the haemoglobin molecule.

Red cell membrane protein phosphorylation in hemolytic anemias and muscular dystrophies

We review our current understanding of membrane phosphorylation in normal and abnormal erythrocytes. The major phosphoproteins in red blood cell (RBC) membranes are spectrin (band 2), bands 3, 2.1, and 4.5, and glycophorin A. At least two protein kinases can be distinguished on the basis of their stimulation by cyclic AMP and salt; they are present both in the membrane and in the cytosol. Analysis of the conflicting data on endogenous membrane-protein phosphorylation in abnormal RBCs indicates a considerable variability of phosphorylation rates which largely depends on the manner of preparation of ghosts and enzyme assay conditions. This variability reflects differences in the partitioning of protein kinases between the membrane and cytosol, the specificity and accessibility of membrane-protein substrates, and reaction kinetics. We select examples of data from several hemolytic anemias to illustrate that precautions are needed to interpret abnormalities of membrane-protein phosphorylation properly; we discuss possible new methodological approaches.

Rate limitation of (Na+ + K+)-stimulated adenosinetriphosphatase by membrane acyl chain ordering

A somatic cell mutant (CR1) of the Chinese hamster ovary cell line (CHO-K1) that is defective in the regulation of cholesterol biosynthesis can be grown under conditions in which plasma membranes from these cells display various cholesterol contents and acyl chain order parameters. The (Na+ + K+)-stimulated adenosinetriphosphatase (ATP phosphohydrolase, EC 3.6.1.3) from these cells was shown to vary in activity by a factor of 10 as the order parameter was varied, and the activity exhibited an exponential dependence on this parameter. Under these conditions the number of Na+,K+-ATPase molecules was shown to remain constant by affinity labeling with [gamma-32P]ATP in the absence of Na+. Control experiments showed that alteration in cholesterol content without change in order parameter did not result in altered enzyme activity. It is concluded that, under our conditions, the rate of catalysis by the Na+,K+-ATPase is determined by the order parameter. These studies suggest a physical mechanism by which variation of membrane lipid composition or other factors that determine membrane lipid acyl chain order parameter can result in variation in membrane enzyme activity.

Dodecyl sulphate/polyacrylamide-gel electrophoresis at low pH values and low temperatures

A simple method is described for dodecyl sulphate/polyacrylamide-gel electrophoresis of pH- and temperature-labile biological intermediates. The method is based on a catalyst system that works at temperatures of 2--4 degrees C and pH values of 2--4 and an appropriate buffer system containing Li+ or Tris [CH2OH--C(CH2OH)2--NH3+] instead of Na+. This system does not lead to the precipitation of 1% dodecyl sulphate.

Inhibition of anion permeability by amphiphilic compounds in human red cell: evidence for an interaction of niflumic acid with the band 3 protein

In human erythrocyte, permeability to the anion is instantaneously, reversibly, and noncompetitively inhibited by the nonsteroidal anti-inflammatory drug, niflumic acid. The active form of this powerful inhibitor (I50 = 6 X 10(-7) M) is the ionic form. We demonstrated that: (i) The binding of niflumic acid to the membrane of unsealed ghosts show one saturable and one linear component over the concentration range studied. The saturable component vanishes when chloride transport is fully inhibited by covalently bound 4-acetamido-4'-isothiocyano stilbene-2,2'-disulfonic acid (SITS). Our estimate of these SITS protectable niflumate binding sites (about 9 x 10(5) per cell) agrees with the number of protein molecules per cell in band 3. These sites are half-saturated with 10(-6) M niflumic acid, a concentration very close to I50. (ii) Niflumic acid inhibits the binding reaction of SITS with anion controlling transport sites. These results indicate that niflumic acid and SITS are mutually exclusive inhibitors, suggesting that niflumic acid interacts with the protein in band 3. Niflumic acid also decreases glucose and ouabain-insensitive sodium permeabilities. However, these effects are produced at a very high concentration of niflumic acid (in millimolar range), suggesting unspecific action, possibly through lipid phase.

Erythrocyte membrane alterations in Huntington disease: effects of gamma-aminobutyric acid

The interaction of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) with erythrocyte membranes from patients with Huntington disease and normal controls has been studied by electron spin resonance. GABA affects the physical state of erythrocyte membrane proteins in control and Huntington disease differently. In addition, after exposure of spin-labeled Huntington disease erythrocyte membranes to 0.1 mM GABA, the relevant electron spin resonance parameters reflecting the physical state of membrane proteins are indistinguishable from those of untreated control membranes. These findings support the concept that this disease is associated with a generalized membrane defect.

The band 3 protein of the human red cell membrane: a review

Band 3 is the predominant polypeptide and the purported mediator of anion transport in the human erythrocyte membrane. Against a background of minor and apparently unrelated polypeptides of similar electrophoretic mobility, and despite apparent heterogeneity in its glycosylation, the bulk of band 3 exhibits uniform and characteristic behavior. This integral glycoprotein appears to exist as a noncovalent dimer of two approximately 93,000-dalton chains which span the membrane asymmetrically. The protein is hydrophobic in its composition and in its behavior in aqueous solution and is best solubilized and purified in detergent. It can be cleaved while membrane-bound into large, topographically defined segments. An integral, outer-surface, 38,000-dalton fragment bears most of the band 3 carbohydrate. A 17,000-dalton, hydrophobic glycopeptide fragment spans the membrane. A approximately 40,000-dalton hydrophilic segment represents the cytoplasmic domain. In vitro, glyceraldehyde 3-P dehydrogenase and aldolase bind reversibly, in a metabolie-sensitive fashion, to this cytoplasmic segment. The cytoplasmic domain also bears the amino terminus of this polypeptide, in contrast to other integral membrane proteins. Recent electron microscopic analysis suggests that the poles of the band 3 molecule can be seen by freeze-etching at the two original membrane surfaces, while freeze-fracture reveals the transmembrane disposition of band 3 dimer particles. There is strong evidence that band 3 mediates 1:1 anion exchange across the membrane through a conformational cycle while remaining fixed and asymmetrical. Its cytoplasmic pole can be variously perturbed and even excised without a significant alteration of transport function. However, digestion of the outer-surface region leads to inhibition of transport, so that both this segment and the membrane-spanning piece (which is selectively labeled by covalent inhibitors of transport) may be presumed to be involved in transport. Genetic polymorphism has been observed in the structure and immunogenicity of the band 3 polypeptide but this feature has not been related to variation in anion transport or other band 3 activities.

Toxic chemical agents as probes for permeation systems of the red blood cell

Chemical agents with different capacities to penetrate into the membrane and with different chemical reactivities can be used to gain information concerning the location of transport sites in the membrane structure and the particular functional ligands. If the agents are highly specific in their interactions and if their inhibitory effects are irreversible, they can also be used as probes to identify the transport components. Several examples are cited using the human red blood cells as a model. The anion transport system in particular has been studied by the use of nonpenetrating irreversible inhibitors, and more recently with a photoaffinity probe, NAP-taurine. In the dark the latter is transported in competition with the normal inorganic anions but after exposure to light, it becomes fixed in an irreversible bond that allows identification of the sites of its transport. It is proposed that anion transport involves a transmembrane protein of about 90,000 daltons that forms a channel through the lipid bilayer. The exchange of anions occurs via a gating mechanism containing a specific anion-binding site. Transport of water, cations and sugars may also involve similar transmembrane protein channels.

The structure of the major protein of the human erythrocyte membrane. Characterization of the intact protein and major fragments

Polypeptide 3, the major membrane-penetrating protein of the human erythrocyte membrane, was characterized, together with two major fragments derived by specific proteolysis of the native protein in the membrane. One fragment (fragment 3f) was obtained from thermolysin cleavage in the extracellular region of the protein, and the other (fragment T1) was derived from tryptic cleavage in the intracellular region of the protein. The results of N- and C-terminal group analysis suggest that fragment 3f contains the N-terminal region of polypeptide 3 and fragment T1 contains the C-terminal part of the molecule. The carbohydrate contents of the polypeptides suggest that carbohydrates are present in three regions of the molecule, much of this carbohydrate being present in the C-terminal part of the molecule. This region of the protein also contains the receptors for concanavalin and the lectins from Phaseolus vulgaris and Ricinis communis, and our results suggest that there is heterogeneity in the carbohydrate chains present in the C-terminal region of polypeptide 3. These data are related to the folding of polypeptide 3 in the erythrocyte membrane.

Regulation of plasma membrane protein phosphorylation in two mammalian cell types

The appreciation of protein phosphorylation as a ubiquitous mechanism for the post-translational control of protein function has drawn our attention to the phosphorylation of plasma membrane proteins. We have studied this phenomenon in the human erythrocyte and rat adipocyte, and have observed several features, common to the two systems, which may be of general significance. In examining protein phosphorylation in intact cells incubated with 32Pi, it is evident that the 32P-polypeptides of the plasma membrane are among the most highly labelled species in the cell, despite their minor contribution to overall protein content. The addition of epinephrine (to adipocytes) or cAMP (to erythrocytes) increases the phosphorylation of certain peptides, whereas others are unaffected. The protein kinases mediating these phosphorylations are present in the plasma membrane as isolated, and can be divided into two groups--cAMP dependent and cAMP independent. These two classes of kinase differ markedly in their substrate specificity toward endogenous and exogenous polypeptide substrates. Two classes of protein kinases with similar properties can be detected in the cytoplasm. The relationship between the membrane-bound and cytoplasmic enzymes is uncertain. The potential roles of the plasma membrane cAMP dependent protein kinases are evident from the diverse effects of cAMP on surface properties; however, the prevalence of plasma membrane proteins phosphorylated via cAMP independent pathways is striking. Thus, elucidation of the regulatory properties of the plasma membrane cAMP independent protein kinases may give new insight into the control of a variety of surface phenomena not mediated by cAMP.

The binding of nucleotides and bivalent cations to the calcium-and-magnesium ion-dependent adenosine triphosphatase from rabbit muscle sarcoplasmic reticulum

The binding of MgATP to purified Ca2+Mg2+-dependent adenosine triphosphatase from rabbit muscle sarcoplasmic reticulum was studied by using a flow-dialysis method. Phosphoryl-enzyme formation and catalytic activity were also measured, and all three processes demonstrated negative co-operativity, with half-saturation of all three parameters at a MgATP concentration of 40-50muM, and a Hill coefficient (h) of 0.8. The variation of the binding constant with with pH was measured and showed tighter binding of MgATP with increasing pH over the range 6.8-8.5. Binding parameters for ATP analogues were also measured. The binding of Ca2+ in the presence and absence of ATP analogues gave half saturation at a Ca2+ concentration of 1.2-1.3muM. Hill plots of Ca2+-binding data gave a slope of 0.8. These results show that the binding of MgATP and Ca2+ can occur in a random manner, with neither substrate influencing the affinity of the enzyme for the other.

Erythrocyte ghosts (Na+ + K+) ATPase activity in Duchenne's dystrophy and myotonia

In Duchenne muscular dystrophy the activity of (Na+ + K+)ATPase in erythrocyte ghosts is reduced and its reaction to ouabain is paradoxical both in low sodium and high sodium systems. No such changes were seen in a case of Becker dystrophy, in limb-girdle dystrophy, and in neurogenic atrophy of muscles. In myotonic dystrophy and congenital myotonia the activity of ATPase and its inhibition by ouabain were depressed.