Relationship of major phosphorylation reactions and MgATPase activities to ATP-dependent shape change of human erythrocyte membranes

Human erythrocyte ghosts prepared by hemolysis and washing in hypotonic Tris are crenated by salt and divalent cations, but undergo shape change to smooth biconcave discs and stomatocytic forms when incubated with MgATP at 37 degrees C. This is normally accompanied by protein and lipid phosphorylations in which the major phosphate acceptors are the spectrin beta-chain and inositol phospholipids, respectively. The system was manipulated in several ways to demonstrate the independence of ATP-dependent shape change from the major phosphorylation reactions. Salt-extracted membranes incubated with adenosine, an inhibitor of spectrin and phosphatidylinositol kinases, underwent normal shape change despite reductions of greater than 90% in spectrin and phospholipid labeling by [gamma-32P]ATP. ATP-dependent shape change was blocked by vanadate at micromolar concentrations (half-maximal inhibition at less than 1 microM), but vanadate did not inhibit membrane autophosphorylation reactions or turnover of spectrin- or lipid-bound phosphate. Vanadate inhibited part of the ATP hydrolysis that accompanies shape change and is expressed in the presence of ouabain and EGTA. The vanadate-sensitive MgATPase activity was approximately 3 nmol Pi X min-1 X mg of protein-1. The results implicate it in ATP-dependent shape change.     

Ca2+- and Mg-ATP-dependent shape change of human erythrocyte ghosts and triton shells

Human erythrocyte membranes (ghosts) prepared from fresh blood changed in shape from spherical to crenated, when suspended in 10(-7)-10(-6) M Ca2+-EGTA buffers. Although the ghosts from long-stored ACD blood (10 weeks) were less sensitive to 10(-7)-10(-6) M Ca2+, the ghosts obtained from this blood after it had been preincubated with adenine and inosine for 3 h at 37 degrees C were highly sensitive to Ca2+. When these highly sensitive ghosts were incubated in 10 mM Tris-Cl buffer (pH 7.4) or 1 mM MgCl2 (pH 7.4) at 0 degrees C, they gradually lost Ca2+ sensitivity within 60 min, but they recovered Ca2+ sensitivity again after re-incubation with 2 mM Mg-ATP for 20 min at 37 degrees C followed by washing with 1 mM MgCl2 (pH 7.4). The shape of these highly Ca2+-sensitive ghosts immediately changed from crenate to disc on addition of 1 mM Mg-ATP even at 6 degrees C in the presence of 10(-7)-10(-6) M Ca2+. A similar shape change was also observed when ghosts treated with 0.5% Triton X-100 (Triton shells) were used. Triton shells from fresh blood ghosts or from long-stored blood ghosts which had been preincubated with 2 mM Mg-ATP for 20 min at 37 degrees C shrank immediately in the presence of 10(-6) M Ca2+ and then swelled on addition of 1 mM Mg-ATP. The specificity to ATP and the dependency on ATP concentration are in agreement with those of the ghost shape change at step 2 (Jinbu, Y. et al., Biochem biophys res commun 112 (1983) 384-390) [18]. These results suggest that cytoskeletal protein phosphorylation enhances sensitivity to Ca2+ and induces erythrocyte shape change in the presence of physiological concentrations of ATP and Ca2+.     

Extraction of membrane proteins in low ionic strengths]

Hemoglobin and the low molecular weight proteins 8 and 9 are extracted from ghosts during low ionic washing after the hypotonic hemolysis of erythrocytes. Furthermore, a loss of the proteins 4.5 and 7 was observed. The protein patterns of ghosts after isotonic hemolysis by freezing and thawing resemble the ghost protein patterns after hypotonic hemolysis and incomplete deprivation of Hb. Many if not all membrane proteins are eluted by repeated incubations of the ghosts in solutions of low ionic strength in the presence of EDTA. The spectrins, the proteins 5, 4.5, 7 and residual Hb are extracted preferentially. A selective extraction of the spectrins and the protein 5 is not detectable under these conditions. Often the spectrin bands are subdivided following low ionic incubation.     

Structural changes in contractile proteins of muscle fibers studied by polarization ultraviolet fluorescence microscopy. X. The effect of ATP, Ca2+, pH change and ionic strength of the washing solution on the structural state of thick filaments

By the method of polarized ultraviolet fluorescence microscopy, effects of ATP, Ca2+, changes in pH and ionic strength of washing solution of the structural state of thick filaments in both actin-free muscle fibers of rabbit and anisotropic discs (A-discs) of glycerinated fibers of crab were studied. The dependence of tryptophan fluorescence anisotropy of thin filaments upon physico-chemical parameters (compounds) of washing solution has been found. The structural state of thick filaments was suggested to be influenced by ATP, Ca2+, changes in pH and ionic strength of washing solution.     

Osmotic hemolysis contrasted with freeze-thaw hemolysis

Red blood cell ghosts resulting from osmotic hemolysis in the presence of Mg ions (Mg ghosts) and ghosts resulting from slow freeze-thaw process (freeze-thaw ghosts) differ in many respects: (1) Mg ghosts spontaneously take on the disc shape immediately after hemolysis and resuspension in buffered salt solution; whereas freeze-thaw ghosts are spherical; (2) Mg ghosts appear to be less hemolyzed than freeze-thaw ghosts; (3) washed and cold-stored Mg ghosts contract or become biconcave discs when exposed to 30 μmoles of ATP/10⁹ ghosts at 37 °C; whereas freeze-thaw ghosts under similar conditions break up into microspheres and membrane filaments; (4) Mg ghosts become crenated discs and spheres when rehemolyzed and resuspended in buffered salt solution; whereas freeze-thaw ghosts tend to fragment; (5) the ATPase activity of Mg ghosts, particularly the nontransport ATPase activity, is considerably less than that of freeze-thaw ghosts.     

Unmasking of a potassium leak in resealed human red blood cell ghosts.

A selective potassium leak is observed in resealed, human red blood cell ghosts when hemolysis is performed with distilled water at pH 6.5, 0 degrees C. The leak, which has a maximum near pH 6.7, is suppressed when either magnesium or a chelating agent is present in the hemolysing medium. The potassium leak has the additional property that it can be suppressed after resealing by washing the ghost membranes in a medium containing a low concentration of ATP or EDTA. The data suggest that through the dilution of endogenous chelating agents at hemolysis a potassium leak may be unmasked.     

Unmasking of a potassium leak in resealed human red blood cell ghosts

A selective potassium leak is observed in resealed, human red blood cell ghosts when hemolysis is performed with distilled water at pH 6.5, 0° C. The leak, which has a maximum near pH 6.7, is suppressed when either magnesium or a chelating agent is present in the hemolysing medium. The potassium leak has the additional property that it can be suppressed after resealing by washing the ghost membranes in a medium containing a low concentration of ATP or EDTA. The data suggest that through the dilution of endogenous chelating agents at hemolysis a potassium leak may be unmasked.     

Nonparallel isometric tension response of rabbit soleus skinned muscle fibers to magnesium adenosine triphosphate and magnesium inosine triphosphate

The isometric tension response of single "skinned' rabbit soleus muscle fibers to MgATP and McITP in the absence of calcium was studied. [MgATP] or [MgITP] was varied in solutions of ionic strength 0.30 and temperature 20 degrees C. Steady-state tension that developed in MgATP or MgITP solutions was a biphasic bell-shaped function of log [MgATP] or log [MgITP] which increased from zero to maximum tension and then declined again to zero. Analysis of the data showed that, under comparable ionic conditions, percent tension vs. log [MgATP] and percent tension vs. log [MgITP] curves are not parallel. Instead, the percent tension vs. log [MgITP] curve is much broader. Additionally, under comparable ionic conditions maximum tension in MgITP solutions was higher than in MgATP solutions. In addition, in MgATP solutions, pH, [K+], and excess ATP were varied. Raising pH from 7 to 8, [K+] from 46 mM to 200 mM, or decreasing excess ATP from 2 to 0.5 mM all increased maximum tension. None of these factors, however, influenced the shape or position of the percent tension vs. log [MgATP] curve.     

Spectrin phosphorylation and shape change of human erythrocyte ghosts

Human erthrocyte membranes in isotonic medium change shape from crenated spheres to biconcave disks and cup-forms when incubated at 37 degrees C in the presence of MgATP (M. P. Sheetz and S. J. Singer, 1977, J. Cell Biol. 73:638-646). The postulated relationship between spectrin phosphorylation and shape change (W. Birchmeier and S. J. Singer, 1977, J. Cell Biol. 73:647-659) is examined in this report. Salt extraction of white ghosts reduced spectrin phosphorylation during shape changes by 85-95%. Salt extraction did not alter crenation, rate of MgATP-dependent shape change, or the fraction (greater than 80%) ultimately converted to disks and cup-forms after 1 h. Spectrin was partially dephosphorylated in intact cells by subjection to metabolic depletion in vitro. Membranes from depleted cells exhibited normal shape-change behavior. Shape-change behavior was influenced by the hemolysis buffer and temperature and by the time required for membrane preparation. Tris and phosphate ghosts lost the capacity to change shape after standing for 1-2 h at 0 degrees C. Hemolysis in HEPES or N- tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid yielded ghosts that were converted rapidly to disks in the absence of ATP and did not undergo further conversion to cup-forms. These effects could not be attributed to differential dephsphorylation of spectrin, because dephosphorylation during ghost preparation and incubation was negligible. These results suggest that spectrin phosphorylation is not required for MgATP-dependent shape change. It is proposed that other biochemical events induce membrane curvature changes and that the role of spectrin is passive.     

Fusion activity of influenza virus. A comparison between biological and artificial target membrane vesicles

We have investigated the pH-dependent fusion activity of influenza virus toward human erythrocyte ghosts, utilizing a recently developed fluorescence assay, which permits continuous monitoring of the fusion reaction. The rate of fusion is negligible at neutral pH but shows a sharp increase at pH values just below 5.5. This pH dependence profile closely corresponds to that of virus-induced hemolysis. Fusion is rapidly inactivated by a low-pH preincubation of the virus alone either at 37 or at 0 degrees C. The presence of ghosts during this low-pH preincubation, carried out at 0 degree C under which condition there is hardly any fusion, causes a significant protection of the viral fusion activity against inactivation. Fusion initiated at low pH can be arrested instantaneously by readjustment of the pH to neutral. The characteristics of fusion of influenza virus with ghosts deviate from those of fusion with cardiolipin liposomes (Stegmann, T., Hoekstra, D., Scherphof, G., and Wilschut, J. (1985) Biochemistry 24, 3107-3113). Fusion with ghosts is consistent with a requirement of the well-documented pH-dependent conformational change in the viral hemagglutinin, whereas fusion with cardiolipin liposomes does not exhibit a strict dependence on the conformational change. Rather, the negative surface charge on the liposomes plays a critical role, as zwitterionic liposomes containing gangliosides show fusion behavior similar to that of erythrocyte ghosts.     

Role of the bilayer in the shape of the isolated erythrocyte membrane

Summary The determinants of cell shape were explored in a study of the crenation (spiculation) of the isolated erythrocyte membrane. Standard ghosts prepared in 5mm NaP_i (pH 8) were plump, dimpled disks even when prepared from echinocytic (spiculated) red cells. These ghosts became crenated in the presence of isotonic saline, millimolar levels of divalent cations, 1mm 2,4-dinitrophenol or 0.1mm lysolecithin. Crenation was suppressed in ghosts generated under conditions of minimal osmotic stress, in ghosts from red cells partially depleted of cholesterol, and, paradoxically, in ghosts from red cells crenated by lysolecithin. The susceptibility of ghosts to crenation was lost with time; this process was potentiated by elevated temperature, low ionic strength, and traces of detergents or chlorpromazine.In that ghost shape was influenced by a variety of amphipaths, our results favor the premise that the bilayer and not the subjacent protein reticulum drives ghost crenation. The data also suggest that vigorous osmotic hemolysis induces a redistribution of lipids between the two leaflets of the bilayer which affects membrane contour through a bilayer couple mechanism. Subsequent relaxation of that metastable distribution could account for the observed loss of crenatability.     

Effect of hemoglobin A and S on human erythrocyte ghosts

The interaction of erythrocyte ghosts and vesicles with chromatographed hemoglobin (Hb) A and Hb S was studied under various conditions. Although no binding of either Hb A or Hb S to inside-out vesicles was detected, under conditions of physiological ionic strength and pH, several properties of white membrane ghosts were effected by the presence of Hb. Addition of Hb A and Hb S (2 g/dl) to membrane ghosts in 6 mM MgATP, 150 mM NaCl, 10 mM Tris-HCl buffer, pH 7.4, was found to effect the echinocyte-discocyte transition, the extent of endocytosis, the volume, and the sealing of ghosts. Our observations suggest that the structure of membrane ghosts is influenced by cytosol proteins and that the environment of the red cell membrane plays an important role in the definition and the control of the membrane structure and function.     

Vesicle release from rat red cell ghosts and increased association of cell membrane proteins with cytoskeletons induced by cadmium

When rat red cell ghosts were incubated with 0.1-0.5 mM CdCl2 in 10 mM Tris-HCl (pH 7.4) at 37 degrees C for 30 min, they became irregular in shape and released small vesicles. The release of vesicles was dependent on the incubation temperature and Cd2+ concentration. The maximum release occurred at 37 degrees C in the presence of 0.2 mM Cd2+. The protein composition of Cd2+-induced vesicles was similar to that of the vesicles released from ATP-depleted red cells. Upon incubation with 0.1-0.2 mM Cd2+, more than 90% of the Cd2+ added to the incubation buffer was recovered in ghosts and 15-20% of the ghost Cd2+ was located on the cytoskeletons prepared by washing ghosts with 0.5% Triton X-100 solution containing 0.1 M KCl and 10 mM Tris-HCl (pH 7.4). Moreover, the cytoskeletons prepared from Cd2+-treated ghosts markedly contained cell membrane proteins, bands 2.1, 3, 4.2 and 4.5, and glycophorins. The association of bands 3 and 4.2 with cytoskeletons increased with increasing concentrations of Cd2+ added to the incubation buffer and saturated at 0.2 mM Cd2+. The solubilization of cytoskeletal proteins, bands 1, 2 and 5, from ghosts at low ionic strength was almost completely suppressed by preincubation of ghosts with 0.1 mM Cd2+. HgCl2, PbCl2 and ZnCl2 at 0.2 mM each also produced an increased association of cell membrane proteins with cytoskeletons, whereas CaCl2 and MgCl2 did not.     

AMP deaminases of rat small intestine

Phosphocellulose column chromatography revealed the existence of two forms of AMP deaminase both in whole tissue and in the intestinal epithelium. AMP deaminase I, which eluted from the column as a first activity peak, exhibited hyperbolic, nonregulatory kinetics. The substrate half-saturation constants were determined to be 0.3 and 0.7 mM at pH 6.5 and 7.2, respectively, and did not change in the presence of ATP, GTP and Pi. AMP deaminase II, which eluted from the column as a second activity peak, was strongly activated by ATP and inhibited by GTP and Pi. The S0.5 constants were 3.5 and 7.1 at pH 6.5 and 7.2, respectively. At pH 7.2 ATP (1 mM) S0.5 decreased to 2.5 mM and caused the sigmoidicity to shift to hyperbolic. The ATP half-activation constant was increased 9-fold in the presence of GTP and was not affected by Pi. Mg2+ significantly altered the effects exerted by nucleotides. The S0.5 value was lowered 10-fold in the presence of MgATP and 5-fold in the presence of MgATP, MgGTP and Pi. When MgATP was present, AMP deaminase II from rat small intestine was less susceptible to inhibition by GTP and Pi. A comparison of the kinetic properties of the enzyme, in particular the greater than 100% increase in Vmax observed in the presence of MgCl2 at low (1 mM) substrate concentration, indicates that MgATP is the true physiological activator. GuoPP[NH]P at low concentrations, in contrast to GTP, did not affect the enzyme and even activated it at concentrations above 0.2 mM. We postulate that AMP deaminase II may have a function similar to that of the rat liver enzyme. The significance of the existence of an additional, non-regulatory form of AMP deaminase in rat small intestine is discussed.     

Use of ionic and zwitterionic (Tris/BisTris and HEPES) buffers in studies on hemoglobin function.

The functional characteristics of hemoglobin (Hb) depend on oxygenation-linked proton and anion binding and thus on solvent buffer groups and ionic composition. This study compares the oxygenation properties of human Hb in ionic [tris(hydroxymethyl)aminomethane (Tris) and BisTris] buffers with those in zwitterionic N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer under strictly controlled chloride concentrations at different pH values, two temperatures, and in the absence and presence of the erythrocytic cofactor, 2,3-diphosphoglycerate (DPG). In contrast to earlier studies (carried out at the same or different chloride concentrations) it shows only small buffer effects that are manifested at low chloride concentration and high pH. These observations suggest chloride binding to the Tris buffers, which reduces the interaction with specific chloride binding sites in the Hb. The findings indicate that HEPES allows for more accurate assessment of Hb-oxygen affinity and its anion and temperature sensitivities than ionic buffers and advocates standard use of HEPES in studies on Hb function. Precise oxygen affinities of Hb dissolved in both buffers are defined under standard conditions.     

Changes in membrane properties of rat red blood cells induced by cadmium accumulating in the membrane fraction

When rat red blood cells were incubated in a cadmium (Cd)-free medium following 1-h pretreatment with 0.5 mM CdCl2, incorporated Cd was retained in the cell during 14-h incubation and progressively accumulated in the membrane fraction, especially in the cytoskeleton fraction. In parallel to this accumulation, red cell filterability decreased and echinocytic cells increased, although intracellular ATP was maintained at the control level. The echinocytic shape was maintained in ghosts and cytoskeletons prepared from the Cd-loaded cells. In addition, the association of bands 2.1, 3, 4.2, and 4.5 with cytoskeletons increased and dissociation of cytoskeletal networks at low ionic strength decreased as the incubation time increased. Pretreatment of red blood cells with Cd also induced a release of small vesicles. These vesicles contained hemoglobin but were depleted of spectrin and actin, showing a phospholipid composition similar to that of red cell ghosts. These results suggest that the organization of cell membranes, especially cytoskeletal networks, is altered by Cd accumulation in the cytoskeleton fraction, which results in acceleration of age-related changes of red blood cells such as shape change and decreased filterability.     

Resealing to small solutes of white erythrocyte membranes after incubation with EDTA, Ca2+, salt, sucrose, phospholipase C

White, stable erythrocyte ghosts can recover their impermeability to small solutes after storage for several days in low-ionic-strength phosphate buffers at 0 °C. The accessibility, to their substrates, of the inner surface enzymes, glyceraldehyde-3-phosphate dehydrogenase, (G3PD), and NADH cytochrome c oxidoreductase, was used to assess resealing. The data from the two enzymes were confirmatory. None of the conditions used to investigate resealing altered the activity of the outer surface enzyme, acetylcholinesterase. Using G3PD activity, ghosts (freshly prepared by gentle stepwise hemolysis in hypotonic phosphate buffers and stored in 11 mm phosphate buffer, pH 7.4) were shown to be slightly sealed (33%). Incubation at 37 °C in the storage buffer with or without EDTA did not alter their permeability. Ionic strength rather than osmotic pressure appears to influence the sealing process since salt (286 mosm) elicited 91% sealing whereas sucrose (278 mosm) had little effect. Calcium in trace amounts caused resealing to 80%. Phospholipase C (C. welchii) completely abolished Ca²⁺-induced resealing. The data were highly reproducible although these ghosts were found to contain only 10 to 20% of the G3PD activity of the leaky ghosts prepared by shock hemolysis in 5 mm phosphate buffer, pH 8.0. The response to the resealing agents was similar regardless of the level of G3PD present. Neither calcium nor ETDA altered the chemical composition (sialic acid, cholesterol, phospholipid) of the membranes. The small amount (5%) of nonspecific loosely bound protein lost during incubation, could not be attributed to any of the test agents. The results suggest that calcium induced the recovery of impermeability by altering the association, distribution, and/or conformation of the proteins and phospholipids within the membrane.     

Rapid release of 42K and 86Rb from an occluded state of the Na,K-pump in the presence of ATP or ADP

We have measured the time course of release of 42K and 86Rb from an occluded state of the Na,K-pump using a rapid filtration apparatus. We have found that at 20 degrees C and in the presence of ATP, 42K is released with a rate constant of approximately 45 s-1 and 86Rb with a rate constant of approximately 20 s-1; both ATP and ADP are effective at a low affinity site (Kd approximately 0.3 and 1 mM, respectively) with the rate of deocclusion being only half as great in ADP as in ATP. Mg2+ stimulates 2-fold at low concentrations probably by forming MgATP, and free Mg2+ is strongly inhibitory at high concentrations (Kd approximately 10 mM). Mg2+ also decreases the affinity for ATP, and the data are consistent with mixed type inhibition; from the analysis the dissociation constant is approximately 1 mM for the inhibitory Mg2+ and the Rb+-occluded form without ATP. The rate of 42K or 86Rb release increases monotonically with pH while ATPase activity decreases above pH 8, so that deocclusion is not rate-limiting in the overall cycle at high pH. This is reflected by a convergence of the rate of Na,K-ATPase and Na,Rb-ATPase activities at high pH and by a decrease in the observed steady-state level of the occluded 86Rb intermediate at high pH. K+, Rb+, Na+, and Cs+, but not Li+, increase the rate of 42K and 86Rb release at constant ionic strength, presumably at sites other than the transport sites. The spontaneous rate of deocclusion is only approximately 0.1 s-1 at low ionic strength in the absence of nucleotides, and it is increased markedly by all cations tested except Li+. Overall the data are consistent with deocclusion as a rate-limiting step in the Na,K-pump cycle.     

A study of the mechanism of ferritin formation. The effect of pH, ionic strength and temperature, inhibition by imidazole and kinetic analysis

The rate of ferritin formation in the buffers 4-morpholinepropanesulphonic acid (Mops), 4-morpholineethanesulphonic acid (Mes) and imidazole at pH values from 5.0 to 6.5 is quite similar. However, the rate of iron deposition is much greater in Mops and Mes at pH values above 6.5 than in imidazole. Increasing the concentration of imidazole inhibits ferritin formation and also leads to a transformation in the shape of the kinetic curves observed. This inhibiton is also observed at constant ionic strength but is not found for non-complexing buffers such as Mops. An inhibition of ferritin formation in imidazole and in Mops buffers is also observed with increasing ionic strength. We conclude that the unprotonated form of imidazole inhibits iron deposition, possibly by binding to the active site of the apoferritin molecule. The temperature dependence of iron deposition was examined. An optimum temperature of 50 degrees C was found but the Arrhenius plots were non-linear. On the basis of these and previous results, a kinetic model is developed which accounts well for ferritin formation at pH values below 6.5 and above 7.0 in non-complexing buffers. The model does not account for the kinetics observed at pH values close to neutrality.     

Discrimination of assembled and disassembled forms of gizzard myosin by papain

Chicken gizzard myosin in 0.15 M or 0.5 M NaCl was cleaved at two sites of heavy chain with 2-10 micrograms/ml papain. MgATP inhibited these cleavages of myosin in 0.15 M NaCl but not in 0.5 M NaCl. The protective effect of ATP was observed at concentrations as low as 10 microM and increased in proportion to ATP concentration to a maximum at 1 mM. ADP was as effective as ATP, while adenosine 5'-[beta, gamma-imido]triphosphate, an unhydrolyzable ATP analogue, was less effective than ATP or ADP. AMP had no protective effect on the digestion of myosin and GTP inhibited slightly the digestion. When the papain-insensitive myosin in 0.15 M NaCl and 2.5 mM MgATP was phosphorylated by Ca2+/calmodulin-dependent myosin light-chain kinase, the myosin restored the vulnerability to papain. However, the two papain-susceptible forms, nonphosphorylated form in the absence of MgATP and phosphorylated form in the presence of MgATP, yielded very similar but distinct proteolytic fragments upon the digestion. When the extent of myosin assembly was estimated by the turbidimetry of myosin suspension in 0.15 M NaCl, nonphosphorylated myosin in the absence and presence of MgATP was assembled and disassembled, respectively, and phosphorylated myosin in the presence of MgATP was assembled. These results suggest that, at physiological ionic strength, papain as a probe distinguishes disassembled myosin and assembled myosin as papain-insensitive and papain-sensitive forms, respectively.