Ca-2+-stimulated membrane phosphorylation and ATPase activity of the human erythrocyte.

1. Human erythrocyte membranes were preincubated with ethyleneglycolbis-(beta-aminoethyl)-N,N' tetraacetate (EGTA) and subsequently labelled for short periods with micromolar concentrations of [8-3-H, gamma-32-P]ATP. Under these conditions, and at temperatures smaller than or equal to 22 degrees C, both ATP hydrolysis and membrane phosphorylation were stimulated by Ca-2+. 2. The properties of the Ca-2+-stimulated ATP hydrolysis and associated phosphorylation of a 150 000 molecular weight protein component, previously described (Knauf, P. A., Proverbio, F. and Hoffman, J. F. (1974) J. Gen. Physiol. 63, 324-336), have been studied. The behavior of the phosphorylated component, ECaP, has properties consistent with its role as a phosphorylated intermediate of Ca-2+-ATPase activity, including: (1) similar dependence of the steady-state level of ECaP and Ca-2+-ATPase on ATP concentration; (2) rapid turnover apparent upon the addition of excess non-radioactive ATP; and (3) good correlation between the steady-state levels of Ca-2+-dependent phosphorylation and Ca-2+-ATPase activity in separate preparations possessing variable specific activity. Addition of excess EGTA to ECaP caused only partial dephosphorylation. Sensitivity of Ca-2+-stimulated ATP hydrolysis and associated phosphorylation to micromolar concentrations of Ca-2+ implicates this activity in the "high-affinity" Ca-2+-pump system of the human erythrocyte (Schatzmann, H. J. (1973) J. Physiol. London 235, 551-569).     

Characterization of human erythrocyte cytoskeletal ATPase

Human erythrocyte cytoskeletal ATPase was extracted with 0.2 mM ATP (pH 8.0) from Triton X-100 treated ghosts. The ATPase fraction contained mainly spectrin, actin, and band 4.1. When the ATPase fraction was applied to a Sepharose 4B column, 90% of the ATPase activity was recovered in a spectrin, actin, and band 4.1 complex fraction and none was detected in the spectrin fraction. A specific activity of the complex ATPase was 60-120 nmol/(mg protein X h). No ATPase activity was detected in the presence of EDTA. The presence of magnesium in the incubation medium was essential for the ATPase activity. The activity was activated by KCl and was almost completely inhibited by 10(-5) M free calcium in the presence of 0.2 mM MgCl2. The Ki for Ca2+ was 7 X 10(-7) M. Phalloidin and DNase 1, which affect actin, inhibited this K,Mg-ATPase activity by 95%, but cytochalasin B did not inhibit it. N-Ethylmaleimide activated the ATPase 1.6-fold. The order of affinity for nucleotides was ATP greater than ITP greater than CTP, ADP, AMP-PNP, GTP. A specific ATPase activity of purified actin was 50 nmol/(mg X h). These results suggest that the cytoskeletal ATPase is actin ATPase and the actin ATPase is activated by spectrin and band 4.1.     

A minor sialoglycoprotein of the human erythrocyte membrane

The sialoglycoprotein periodate fuchsin sulfite 2 has about 8% of the sialic acid contained in the sialoglycoproteins of the human erythrocyte membrane. This polypeptide appears to have an apparent monomeric molecular weight of 35,000, somewhat smaller than the monomer of the major sialoglycoprotein (periodate fuchsin sulfite 1) as judged by sodium dodecyl sulfate-polyacry lamide gel electrophoresis, and has frequently been confused with the monomer of the major sialoglycoprotein. Periodate fuchsin sulfite 2 is not labeled by the lactoperoxidase procedure in the intact cell, although it is accessible to neuraminidase and other hydrolases. On the other hand, this component can be labeled by lactoperoxidase on the cytoplasmic surface of open membranes or resealed ghosts. Thus, it is a trans membrane protein. Although most of the other transmembrane proteins of the human erythrocyte membrane are extracted from the membrane by 0.1% Triton X-100 in 7 mm phosphate buffer, pH 7.4, this component is not removed and may be a cytoskeletal component. Trypsin, chymotrypsin, and thermolysin peptides, as well as cyanogen bromide fragments, clearly indicate that the primary sequence of this polypeptide can be distinguished from dimeric or monomeric forms of the major sialoglycoprotein (periodate fuchsin sulfite 1).     

Kinetic properties of the purified Ca2+-translocating ATPase from human erythrocyte plasma membrane

The basic kinetic properties of the solubilized and purified Ca2+-translocating ATPase from human erythrocyte membranes were studied. A complex interaction between the major ligands (i.e., Ca2+, Mg2+, H+, calmodulin and ATP) and the enzyme was found. The apparent affinity of the enzyme for Ca2+ was inversely proportional to the concentration of free Mg2+ and H+, both in the presence or absence of calmodulin. In addition, the apparent affinity of the enzyme for Ca2+ was significantly increased by the presence of calmodulin at high concentrations of MgCl2 (5 mM), while it was hardly affected at low concentrations of MgCl2 (2 mM or less). In addition, the ATPase activity was inhibited by free Mg2+ in the millimolar concentration range. Evidence for a high degree of positive cooperativity for Ca2+ activation of the enzyme (Hill coefficient near to 4) was found in the presence of calmodulin in the slightly alkaline pH range. The degree of cooperativity induced by Ca2+ in the presence of calmodulin was decreased strongly as the pH decreased to acid values (Hill coefficient below 2). In the absence of calmodulin, the Hill coefficient was 2 or slightly below over the whole pH range tested. Two binding affinities of the enzyme for ATP were found. The apparent affinity of the enzyme for calmodulin was around 6 nM and independent of the Mg2+ concentration. The degree of stimulation of the ATPase activity by calmodulin was dependent on the concentrations of both Ca2+ and Mg2+ in the assay system.     

Evidence of voltage-induced channel opening in Na/K ATPase of human erythrocyte membrane

Summary Previous studies have shown that human erythrocytes when subjected to a high voltage pulsation, in the microsecond time range, lysed in an isotonic medium. The hemolysis was the result of the colloid osmotic swelling, which, in turn, was caused by the voltage perforation of the red cell membranes. In this work we demonstrate that in a low ionic medium at least 35% of the pores was related to the opening of Na⁺/K⁺ ATPase channels. The membrane conductance generated by the externally applied electric field could be partially blocked by a specific inhibitor, ouabain, or by a specific cross-linking reagent, Cu⁺⁺-phenanthroline, of the ATPase. The effect of ouabain was saturable and had a mid-point of saturation at 0.15 m. This value agrees with the physiological inhibition constant of the drug. K⁺ ion in the external medium suppressed the effect of ouabain, as has also been demonstrated in physiological studies. Experiment presented in this communication also suggests that the Na⁺/K⁺ ATPase was not perforable in a high ionic medium, and that a large fraction of the voltage-induced pores occurred at as yet unidentified sites.     

Activation of Mg-ATPase (spectrin-dependent ATPase) by Ca2+.

The dependence of saponin-stimulated Mg-ATPase activity in the erythrocyte membrane on Ca2+ concentration was studied. In the membrane of freshly sampled human erythrocytes we found for this enzyme and Ca2+ an apparent dissociation constant of 0.611 mumol/l (SE +/- 0.106 mumol/l) and Hill coefficient of 0.93 (SE +/- 0.05). The enzyme is in most probability identical with Ca,Mg-ATPase of high affinity to Ca2+ described also as spectrin-dependent Ca,Mg-ATPase.     

A monomeric form of human erythrocyte membrane acetylcholinesterase

Purified detergent solubilized dimeric human erythrocyte acetylcholinesterase (6.3 S form) was converted to a stable monomeric 3.9 S species when treated with 2-mercaptoethanol and iodoacetic acid. More than 60% of the enzymatic activity were recovered after this treatment. A decreased susceptibility to reduction and alkylation was observed with purified, detergent depleted acetylcholinesterase aggregates. When erythrocyte membranes (ghosts) were subjected to the same treatment, acetylcholinesterase could subsequently be solubilized as monomeric 3.9 S form and and more than 90% of the activity were recovered. Monomeric acetylcholinesterase was less reactive towards antibodies raised against (dimeric) human erythrocyte membrane acetylcholinesterase and towards antibodies against human erythrocyte membranes. The results suggest that acetylcholinesterase is present as dimeric species in human erythrocyte membranes despite the fact that fully active monomers can be obtained.     

Relationship between membrane ATPase and shape changes in the dog erythrocyte

The effects of heating blood to 57 degrees C on intraerythrocytic calcium, membrane ATPase activity and cell shape have been studied in canine blood. Intraerythrocytic calcium was determined by use of arsenazo III, membrane ATPase activity was determined by inorganic phosphorous formation and erythrocyte shape was determined by scanning electron microscopy. The results of this study showed that this degree of thermal trauma would cause a 27% increase in intraerythrocytic calcium and a 38% decrease in ATPase activity. During these changes in calcium and ATPase activity the erythrocyte changed form from biconcave to spherical. Addition of catalase (3,200 U/ml) to the blood prior to heating prevented the changes observed in intraerythrocytic calcium, membrane ATPase activity and shape. The addition of the free-radical generating combination of hypoxanthine-xanthine oxidase to blood produced a 20% decrease in membrane ATPase activity and a change in erythrocyte shape, but did not alter intraerythrocytic calcium. These results suggest that free-radicals are responsible for the changes in membrane ATPase activity. The observation that shape change occurs when ATPase activity has been decreased, but calcium has not been increased, suggests that membrane ATPase activity levels are more important in producing changes in erythrocyte shape than are intraerythrocytic calcium levels.     

Method for the preparation of human erythrocyte membrane with low basal calcium ATPase, responsive to stimulation

A simple procedure for preparing erythrocyte membranes with low basal Ca2+ ATPase activity is described, which is stimulated several-fold by the addition of hemolysate in the incubation mixture. The cells are hemolyzed in hypotonic imidazole buffer and resulting membranes are washed with hypotonic phosphate buffer (pH 8.0) and the hemolyzing medium. The membrane preparations also have Mg2+-stimulated and Na+-K+-stimulated ATPase activities. The method allows the comparison of basal Ca2+ ATPase as well as hemolysate- or calmodulin-stimulated Ca2+ ATPase activities and thus may be useful in studying Ca2+ ATPase activity in various physiopathological conditions.     

The effect of supplying rats with vitamin K and administration of pelentane on Na,K- and spectrin-dependent ATPase in erythrocyte ghosts]

It was found that the activity of spectrin-dependent ATPase of erythrocyte ghosts isolated from rats with alimentary deficiency of vitamin K was significantly increased as compared with control animals, whereas in rats kept on a vicasol-rich diet this parameter was unchanged. In vitamin K-deficient rats the amount of proteins loosely bound to erythrocyte membranes was significantly reduced. At the same time, the activity of the integral enzyme (Na, K-ATPase) did not depend on the vitamin K provision despite the fact that in vitamin K-deficient animals kept on a vicasol-rich diet the enzyme affinity for ouabain was strongly decreased as compared with control. It was suggested that this effect might be due to the changes in the lipid and protein environment of the membrane-bound enzyme. Administration of the antivitamin K, pelentane, did not induce any conspicuous changes in the enzyme activities. It was concluded that antivitamin K does not induce any modification of the properties of erythrocyte-linked enzymes observed under conditions of vitamin K deficiency.     

Phosphorylation of the isolated high-affinity (Ca2+ + Mg2+) ATPase of the human erythrocyte membrane

Solubilized and purified high-affinity (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) of the human erythrocyte membrane (Wolf, H.U., Dieckvoss, G. and Lichtner, R. (1977) Acta Biol. Ger. 36, 847) has been phosphorylated and dephosphorylated under various conditions with respect to Ca2+ and Mg2+ concentrations. In the range, 0.001--100 mM, the rate of phosphorylation was dependent on Ca2+ concentration, showing a maximum at 10 mM. The phosphorylation rate was nearly independent of the Mg2+ concentration within the range 0.01-1 mM. This enzyme has at least three Ca2+ binding sites with different affinities and regulatory functions: (1) binding to the high-affinity site yields phosphorylation of the enzyme; (2) binding to a low-affinity site (Ca2+ concentrations higher than 40 microM) inhibits dephosphorylation or the conformational change which is necessary for dephosphorylation; (3) by binding to an additional low-affinity site, Ca2+ at concentrations higher than 1 mM abolishes negative cooperative behaviour (shown below 1 mM Ca2+) and causes weak positive cooperativity between at least two catalytic subunits in the phosphorylation reaction. The phosphoprotein obtained at Ca2+ concentrations above 1 mM dephosphorylates spontaneously after removal of the divalent metal ions. Addition of Mg2+ accelerates the dephosphorylation rate. Affinities of the inhibitory Ca2+ binding sites are reduced by the binding of substrate or K+.