Effects of phloridzin, metavanadate and oligomycin on membrane bound (Na++ K++ Mg2+)ATPase activity in sugar beet roots

To clarify the reaction mechanism of a (Na⁺+ K⁺+ Mg²⁺)ATPase activity in sugar beet roots ( Beta vulgaris L. cv. Monohill) phloridzin, oligomycin (inhibitors of animal ATPases) and metavanadate (NH₄VO₃) have been used. Kinetic studies showed that: 1) Phloridzin inhibition is uncompetitive with respect to MgATP and not influenced by Na⁺ or K⁺. 2) This inhibition is only found in preparations made in the absence of sucrose. 3) Oligomycin and vanadate inhibit the ATPase in different ways. Omission of sucrose from the preparation medium favours vanadate inhibition but suppresses oligomycin inhibition. 4) The kinetic pattern of the Na⁺ activation of the ATPase differs in preparations made in the absence and presence of sucrose, but that of K⁺ activation is the same. – These results indicate that inclusion as against omission of sucrose from the preparation medium causes a conformational change of the membrane fragments/vesicles, which then expose different surfaces to the surrounding medium.     

Sucrose and ouabain effects on the kinetic properties of a membrane-bound (Na++ K++ Mg2+) ATPase in sugar beet roots

Kinetic studies of a microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase from sugar beet roots ( Beta vulgaris L. cv. Monohill) show that sucrose influences the MgATPase in different ways depending on the presence of K⁺ and/or Na⁺ 1) In the presence of the substrate MgATP and Na⁺ the effect of sucrose follows simple Michaelis-Menten kinetics. 2) In the presence of substrate together with K⁺ or (K⁺+ Na⁺), sucrose has little effect on the ATPase activity. 3) In the presence of Na⁺, onabain acts as an uncompetitive inhibitor with respect to MgATP. 4) In the presence of K⁺ or (K⁺+ Na⁺), the inhibition by ouabain is somewhat depressed and shows non-linearity when 1/v is plotted versus 1/MgATP. 5) Sucrose and Na⁺ activate in a competitive way, so that a successive increase of the Na⁺ level decreases the activation by sucrose. Both K_m and V-values are thereby changed. 6) The sucrose activation in the presence of Na⁺ is also influenced by ouabain. It is, therefore, suggested that Na⁺ may regulate the interference between the Na⁺/K⁺ pump and a sucrose sensitive system.     

Kinetic studies of a (Na++ K++ Mg2+)ATPase in sugar beet roots. III. A proposed model for the (Na++ K+) activation and its significance for field properties

A microsomal (Na⁺+ K⁺+ Mg²⁺)ATPase preparation from sugar beet roots was used. The activation by simultaneous addition of Na⁺ and K⁺ at different levels was examined in terms of steady state kinetics. The observed data can be summarized in the following way: 1. The apparent affinity between the enzyme and the substrate MgATP depends on the ratio between Na⁺ and K⁺. At low Na⁺ concentration (below 5 mM), the apparent K_m decreases with increasing concentrations of K⁺ (1–20 mM). At 5 mM Na⁺, the K⁺ level does not change the apparent K_m, while at Na⁺ levels above 10 mM, the apparent K_m between enzyme and substrate increases with increasing concentration of K⁺. 2. When the MgATP concentration is kept constant, homotropic cooperativity (concerning one type of ligand) and heterotropic cooperativity (concerning different types of ligands) exist in the activation by Na⁺ and K⁺. The Na⁺ binding is cooperative with different K_m values and Hill coefficients (n) in the presence of low and high concentration of K⁺. At low Na⁺ level (< 5 mM). a negative cooperativity exists for Na⁺ (n_Na < 1) which is more pronounced in the presence of high [K⁺]. When the concentration of Na⁺ is raised the negative cooperativity disappears and turns into a positive one (n_Na > 1). Only K⁺ binding in the presence of low [Na⁺] shows cooperativity with a Hill coefficient that reflects changes from negative to positive homotropic cooperativity with increasing concentrations of K⁺ (n_K < 1 → n_K > 1). In the presence of [Na⁺] > 10 mM, the changes in n_k are insignificant. 3. A model is proposed in which one or two different K sites and one or two Na sites control the catalytic activity, with multiple interactions between Na⁺, K⁺ and MgATP. 4. In the presence of Na⁺ (< 10 mM), K⁺ is probably bound to two K sites, one of which translocates K⁺ through the membrane by an antiport Na⁺/K⁺ mechanism. This could be connected with an elevated K⁺ uptake in the presence of Na⁺ and could therefore explain some field properties of sugar beets.     

Kinetic Studies of a (Na++ K++ Mg2+) ATPase in Sugar Beet Roots

Kinetic studies of a microsomal, dithiotreitol treated, homogenate from sugar beet roots led to the following conclusions about its ATPase activity: (1) MgATP in complex appears to be the primary substrate for the reaction. The reciprocal equilibrium constant for the binding to the enzyme is estimated to be approximately 0.2 × 10^?3M. (2) Free ATP acts as a competitive inhibitor of the MgATP. The binding constant is about twice as high as for MgATP. Consequently the enzyme has less affinity for ATP than for MgATP. (3) Free Mg²⁺ has little influence on the velocity, as the binding affinity of the enzyme for Mg²⁺ is almost negligible.     

Kinetic Studies of a (Na++ K++ Mg2+) ATPase in Sugar Beet Roots

Kinetic studies of a dithiothreitol treated membrane ATPase fraction from sugar beet roots led to the following conclusions: 1) In the presence of MgATP, Na⁺ and K⁺ stimulate the ATPase activity in different ways following simple Michaelis-Menten kinetics. Thus separate sites for Na⁺ and K⁺ are suggested. 2) In the absence of K⁺, Na⁺ acts as an uncompetitive modifier raising the apparent K_m and V_max for MgATP. 3) In the absence of Na⁺, K⁺ activates non-competitively with respect to MgATP. Thus K⁺ increases V_max but does not affect the apparent affinity constant. 4) K⁺ and Na⁺ double the rate constants. 5) In the presence of Na⁺ or K⁺, Mg²⁺ in excess acts as a weak inhibitor to Na⁺ and/or K⁺ activity. 6) The temperature-activity dependence in the 5–40°C interval shows biphasic Arrhenius plots with the transition point between 15–18°C. The activation energy is lowered at temperatures > 18°C.     

Mechanism for Cd2+ inhibition of (K++ Mg2+)ATPase activity and K+ (86Rb+) uptake join roots of sugar beet (Beta vulgaris)

Steady state kinetics were used to examine the influence of Cd²⁺ both on K⁺ stimulation of a membrane-bound ATPase from sugar beet roots (Beta vulgaris L. cv. Monohill) and on K⁺(⁸⁶Rb⁺) uptake in intact or excised beet roots. The in vitro effect of Cd²⁺ was studied both on a 12000–25000 g root fraction of the (Na⁺+K⁺+Mg²⁺)ATPase and on the ATPase when further purified by an aqueous polymer two-phase system. The observed data can be summarized as follows: 1) Cd²⁺ at high concentrations (>100 μM) inhibits the MgATPase activity in a competitive way, probably by forming a complex with ATP. 2) Cd²⁺ at concentrations <100 μM inhibits the specific K⁺ activation at both high and low affinity sites for K⁺. The inhibition pattern appears to be the same in the two ATPase preparations of different purity. In the presence of the substrate MgATP, and at K⁺ <5 mM, the inhibition by Cd²⁺ with respect to K⁺ is uncompetitive. In the presence of MgATP and K⁺ >10 μM, the inhibition by Cd²⁺ is competitive. 3) At the low concentrations of K⁺, Cd²⁺ also inhibits the 2,4-dinitrophenol(DNP)-sensitive (metabolic) K⁺(⁸⁶Rb⁺) uptake uncompetitively both in excised roots and in roots of intact plants. 4) The DNP-insensitive (non metabolic) K⁺(⁸⁶Rb⁺) uptake is little influenced by Cd²⁺. As Cd²⁺ inhibits the metabolic uptake of K⁺(⁸⁶Rb⁺) and the K⁺ activation of the ATPase in the same way at low concentrations of K⁺, the same binding site is probably involved. Therefore, under field conditions, when the concentration of K⁺ is low, the presence of Cd²⁺ could be disadvantageous.     

Substrate-dependent modulation of ATPase activity by Na+ and K+ in roots of Plantago species

The equal rates of water vapour absorption by both bi- and trinucleate pollen indicate that their widely-differing rates of respiration have an intrinsic, biochemical basis. This was investigated with various metabolic inhibitors that were previously introduced into dry pollen via anhydrous acetone. The uncoupler, carbonyl cyanide m-chlorophenyl hydrazone, inhibited the O₂ uptake of rapidly respiring pollen and stimulated that of slowly respiring types to similar absolute values, that probably reflect the rates of substrate transport across the mitochondrial membranes. The extent of inhibition of the O₂ uptake by oligomycin, dicyclohexyl carbodiimide, antimycin A, and salicyl hydroxamic acid, alone and in combinations, indicates that hardly any oxidative phosphorylation and anabolic activities occur in slowly respiring, binucleate pollen species, having low-developed mitochondria and high energy charge values. The presence of the alternative pathway was insignificant. In other binucleate pollen species, characterized by recognizable mitochondria and low energy charge values, a limited ATP synthesis was established. The low energy charge values point to imbalance between phosphorylative and anabolic activities. In rapidly respiring, trinucleate pollen, containing well-developed mitochondria, a significant activity of the alternative oxidase was found. The energy charge values were high notwithstanding the large demand for ATP, mounting to 1.7 μmol h^?1 (mg pollen)^?1. In some pollen species, oligomycin highly stimulated the flow of electrons through the cytochrome pathway, which made an estimation of the ATP synthesis impossible.     

Effects of interrupted K+ supply on growth and uptake of K+, Ca2+, Mg2+ and Na+ in spring wheat

Effects of interrupted K⁺ supply on different parameters of growth and mineral cation nutrition were evaluated for spring wheat (Triticum aestivum L. cv. Svenno). K⁺ (2.0 mM) was supplied to the plants during different periods in an otherwise complete nutrient solution. Shoot growth was reduced before root growth after interruption in K⁺ supply. Root structure was greatly affected by the length of the period in K⁺ -free nutrient solution. Root length was minimal, and root branching was maximal within a narrow range of K⁺ status of the roots. This range corresponded to cultivation for the last 1 to 3 days, of 11 in total, in K⁺ -free nutrient solution, or to continuous cultivation in solution containing 0.5 to 2 mM K⁺. In comparison, both higher and lower internal/external K⁺ concentrations had inhibitory effects on root branching. However, the differing root morphology probably had no significant influence on the magnitude of Ca²⁺, Mg²⁺ and Na⁺ uptake. Uptake of Ca²⁺ and especially Mg²⁺ significantly increased after K⁺ interruption, while Na⁺ uptake was constant in the roots and slowly increased in the shoots. The two divalent cations could replace K⁺ in the cells and maintain electroneutrality down to a certain minimal range of K⁺ concentrations. This range was significantly higher in the shoot [110 to 140 μmol (g fresh weight)^?1] than in the root [20 to 30 μmol (g fresh weight)^?1]. It is suggested that the critical K⁺ values are a measure of the minimal amount of K⁺ that must be present for physiological activity in the cells. At the critical levels, K⁺ (⁸⁶Rb) influx and Ca²⁺ and Mg²⁺ concentrations were maximal. Below the critical K⁺ values, growth was reduced, and Ca²⁺ and Mg²⁺ could no longer substitute for K⁺ for electrostatic balance. In a short-term experiment, the ability of Ca²⁺ to compete with K⁺ in maintaining electroneutrality in the cells was studied in wheat seedlings with different K⁺ status. The results indicate that K⁺, which was taken up actively and fastest at the external K⁺ concentration used (2.0 mM), partly determines the size of Ca²⁺ influx.     

Effect of Na+, K+, Mg2+ and ATP on the Relative Electrophoretic Mobility of Sugar Beet Membrane Particles with NaK ATPase Activity

Electrophoretic measurements on membrane coated particles were performed with a Zytopherometer. Tris-HCl buffer 0.2 M pH 7.0 at 37°C with addition of different combinations of Na⁺, K⁺, Mg²⁺ and ATP was used as test medium. The membranes were of two types, an untreated preparation with low NaK ATPase activity and a deoxycholate treated preparation with high NaK ATPase activity. There was no marked difference in reaction between the two types of membranes. To both types of membranes Mg²⁺ gave a strong positive and ATP a slight negative addition to the membrane charge. In the presence of ATP Na⁺ gave a higher charge contribution than did K⁺ or a combination of Na⁺ and K⁺. This implies that K⁺ gives a higher affinity for ATP than Na⁺ does and or that ATP mediates a higher affinity for Na⁺ than for K⁺.     

Properties of Mg2+-Stimulated and (Na++K+)-Activated Adenosine-5'-Triphosphatase from Sugar Beet Cotyledons

Sugar beet leaf homogenate contains Mg²⁺-stimulated ATPase activity with the highest specific activity in the 25,000–30,000 ×g-fraction. This fraction also has (Na⁺+ K⁺)-activated ATPase activity. Both activities have two pH optima, one stable at pH 7.9 and one variable at lower pH. When optimal conditions of Na⁺ and K⁺ were tested with 64 combinations of these ions, at least two mountains of activity were revealed. The (Na⁺+ K⁺)-ATPase had a high specificity for ATP. It had lost about 50% of its original activity after 56 days of storage at ?85°C. The activity drop was most pronounced at high ionic concentrations in the test medium. The (Na⁺+ K⁺)-ATPase shows four peaks of activity when tested at constant ionic strength. The idea is put forward that the four peaks reflect two ATPases, one in the tonoplast and one in the plasmalemma, which undergo conformational changes in relation to the ionic milieu.     

Aluminium interactions with K+(86Rb+) and 45Ca2+ fluxes in three cultivars of sugar beet (Beta vulgaris)

Three cultivars of sugar beet (Beta vulgaris L.), which are sensitive to aluminium (Al) in the order Primahill > Monohill > Regina, were grown in water culture for 2 weeks. Nutrients were supplied at 15% increase of amounts daily, corresponding to the nutrient demand for maximal growth. The 2.4-dinitrophenol (DNP)-sensitive (metabolic) and DNP-insensitive (non-metabolic) uptake of aluminium, phosphate. ⁴⁵Ca²⁺ and K⁺(⁸⁶Rb⁺) in roots were measured as well as transport to shoots of intact plants. All 3 cultivars absorbed more aluminium if DNP was present during the aluminium treatment than in its absence. It is suggested that sugar beets are able to extrude aluminium activity or that they possess an active mechanism to keep Al outside the cell. The presence of Al in the medium during the 1-h experiment affected the metabolic and non-metabolic fluxes of ⁴⁵Ca²⁺ and K⁺(⁸⁶Rb⁺) in different ways. In the presence of DNP, the influx of both ⁴⁵Ca²⁺ and K⁺(⁸⁶Rb⁺) and the efflux of ⁴⁵Ca²⁺ were inhibited by Al in a competitive way. At inhibition of ⁴⁵Ca²⁺ influx, 2 Al ions are probably bound per Ca²⁺ uptake site in cv. Regina (Al-tolerant), but in cvs Primahill and Monohill only one Al ion is bound (more Al sensitive). Aluminium competitively inhibited the active efflux of ⁴⁵Ca²⁺ (absence of DNP) in almost the same way in the 3 cultivars. In contrast, aluminium stimulated the influx of K⁺(⁸⁶Rb⁺) in cvs Primahill, Monohill and Regina in the absence of DNP. Thus, the Al effects on active and passive K⁺(⁸⁶Rb⁺) influx are different. The total influx of K⁺(⁸⁶Rb⁺) increased in the presence of Al and might be connected to an active exclusion of Al. Regina is the least Al-sensitive cultivar, probably because Al interferes less with the Ca²⁺ fluxes and because this cultivar actively excludes phosphate in the presence of Al. Thus Al-phosphate precipitation within the plant could be avoided.     

Effects of salinity and replacement of K+ by Na+ on lipid composition in two sugar beet inbred lines

The effects of NaCl and replacement of K⁺ by Na⁺ on the lipid composition of the two sugar beet inbred lines FIA and ADA were studied (a) with increasing additions of NaCl to the basal medium, and (b) with increasing replacement of K⁺ by Na⁺ at the same total concentration as in the basal medium. Direct relations were noted between NaCl concentration of the nutrient solution and the phospholipid concentration in the roots of FIA, the genotype characterized by a low K⁺/Na⁺ ratio, as well as between NaCl in the medium and the phospholipid concentration in the shoots of ADA, the genotype with a high K ⁺/Na ⁺ ratio. The sulfolipid level in the roots of FIA was maintained at higher NaCl concentrations, while it was decreased in ADA. The glycolipid concentration in the shoots of ADA and the degree of unsaturation of the fatty acids of the total lipid fraction were decreased by salinity, indicating reduced biosynthesis of chloroplast glycolipids and/or accelerated oxidation of these lipids in the presence of NaCl.
In the Na⁺ for K⁺ replacement experiment a low content of K⁺ in the medium resulted in decreased levels of total lipids, phospholipids and sulfolipid in the roots of both genotypes, which did not relate to root growth. K⁺-leakage from the roots at low K⁺-level in the medium may be reduced by the increase in saturation of the lipids. In the shoots of ADA increased levels of total lipids, phospholipids and Sulfolipid were noted at a low K⁺-concentration of the nutrient solution.     

Modification of the Rate of Ouabain Binding to (Na++ K+)ATPase by Lithium Ions

We report on the interactions of Li+, a congener of K+ with the (Na+ + K+)-ATPase from E Electricus as measured by their effects on the rate of [3H]-ouabain binding to this enzyme. Like K+, Li+ slows ouabain binding under both Type I (Na+ + ATP) and Type II (P1) conditions, but with lower affinity. In contrast to K+, the Li+ inhibition curve is hyperbolic, suggesting interaction at an uncoupled site. Also differing from the complete inhibition by high K+, a residual ouabain-binding rate persists at high Li+. The interactions of Li+ and K+ are synergistic: the apparent K+ affinity increases 3 to 4-fold in presence of Li+. These results are consistent with the conclusion that Li+ interacts with only one of the two K+ sites and may be of interest in interpreting lithium pharmacology.     

Activation of (Na+, K+)-ATPase by Nanomolar Concentrations of GM1 Ganglioside

GM1 ganglioside binding to the crude mitochondrial fraction of rat brain and its effect on (Na+, K+)-ATPase were studied, the following results being obtained: (a) the binding process followed a biphasic kinetics with a break at 50 nM-GM1; GM1 at concentrations below the break was stably associated, while over the break it was loosely associated; (b) stably bound GM1 activated (Na+, K+)-ATPase up to a maximum of 43%; (c) the activation was dependent upon the amount of bound GM1 and was highest at the critical concentration of 20 pmol bound GM1 X mg protein-1; (d) loosely bound GM1 suppressed the activating effect on (Na+, K+)-ATPase elicited by firmly bound GM1; (e) GM1-activated (Na+, K+)-ATPase had the same pH optimum and apparent Km (for ATP) as normal (Na+, K+)-ATPase but a greater apparent Vmax; (f) under identical binding conditions (2 h, 37 degrees C, with 40 nM substance) all tested gangliosides (GM1, GD1a, GD1b, GT1b) activated (Na+, K+)-ATPase (from 26-43%); NeuNAc, sodium dodecylsulphate, sulphatide and cerebroside had only a very slight effect. It is suggested that the ganglioside activation of (Na+-K+)-ATPase is a specific phenomenon not related to the amphiphilic and ionic properties of gangliosides, but due to modifications of the membrane lipid environment surrounding the enzyme.     

Heterogeneous Na+ Sensitivity of Na+,K+-ATPase Isoenzymes in Whole Brain Membranes

Abstract: The Na⁺ sensitivity of whole brain membrane Na⁺,K⁺-ATPase isoenzymes was studied using the differential inhibitory effect of ouabain (α1, low affinity for ouabain; α2, high affinity; and α3, very high affinity). At 100 m M Na⁺, we found that the proportion of isoforms with low, high, and very high ouabain affinity was 21, 38, and 41%, respectively. Using two ouabain concentrations (10⁻⁵ and 10⁻⁷ M ), we were able to discriminate Na⁺ sensitivity of Na⁺, K⁺-ATPase isoenzymes using nonlinear regression. The ouabain low-affinity isoform, α1, exhibited high Na⁺ sensitivity [ K _a of 3.88 ± 0.25 m M Na⁺ and a Hill coefficient ( n ) of 1.98 ± 0.13]; the ouabain high-affinity isoform, α2, had two Na⁺ sensitivities, a high ( K _a of 4.98 ± 0.2 m M Na⁺ and n of 1.34 ± 0.10) and a low ( K _a of 28 ± 0.5 m M Na⁺ and an n of 1.92 ± 0.18) Na⁺ sensitivity activated above a thresh old (22 ± 0.3 m M Na⁺); and the ouabain very-high-affinity isoform, α3, was resolved by two processes and appears to have two Na⁺ sensitivities (apparent K _a values of 3.5 and 20 m M Na⁺). We show that Na⁺ dependence in the absence of ouabain is the result of at least of five Na⁺ reactivities. This molecular functional characteristic of isoenzymes in membranes could explain the diversity of physiological roles attributed to isoenzymes.     

Na+,K+-ATPase and Mg2+-ATPase Activities in Different Regions of Rat Brain During Alloxan Diabetes

The effect of alloxan diabetes on the activities of Na+,K+-ATPase and Mg2+-ATPase was studied in three regions of rat brain at various time intervals after the onset of diabetes. It was observed that Na+,K+-ATPase activity increased at early time intervals after diabetes, followed by a recovery to near control levels in all three regions of the brain. There was an overall increase in Mg2+-ATPase activity in all the regions. A reversal of the effect was observed with insulin administration to the diabetic rats.     

Kinetic Studies of (Mg2+)ATPase in Oat Roots

Kinetic studies on a microsomal fraction from oat roots (Arena sativa) indicate that the MgATP complex is the primary substrate for the ATPase. Free ATP was a weak competitive inhibitor. Free Mg²⁺ appeared to be either a competitive or noncompetitive inhibitor.     

Immunocytochemical Localization of (Na+,K+)-ATPase in the Goldfish Optic Nerve

Antiserum to the catalytic subunit of goldfish brain (Na+, K+)-ATPase has been employed at the electron microscopic level by means of the peroxidase-antiperoxidase immunohistochemical method. In optic nerve, antigenic sites are restricted to the nodes of Ranvier. No reaction product is detected in underlying internodal neurolemma. Outgrowing neurites for cultured retinal explants devoid of glial ensheathment exhibit a continuous distribution of the enzyme subunit. Antibodies against eel electroplax (Na+, K+)-ATPase cross-react with the goldfish brain enzyme and show a similar immunocytochemical distribution pattern.     

Puccinellia tenuiflora maintains a low Na+ level under salinity by limiting unidirectional Na+ influx resulting in a high selectivity for K+ over Na+

Puccinellia tenuiflora is a useful monocotyledonous halophyte that might be used for improving salt tolerance of cereals. This current work has shown that P. tenuiflora has stronger selectivity for K⁺ over Na⁺ allowing it to maintain significantly lower tissue Na⁺ and higher K⁺ concentration than that of wheat under short- or long-term NaCl treatments. To assess the relative contribution of Na⁺ efflux and influx to net Na⁺ accumulation, unidirectional ²²Na⁺ fluxes in roots were carried out. It was firstly found that unidirectional ²²Na⁺ influx into root of P. tenuiflora was significantly lower (by 31–37%) than in wheat under 100 and 150 m m NaCl. P. tenuiflora had lower unidirectional Na⁺ efflux than wheat; the ratio of efflux to influx was similar between the two species. Leaf secretion of P. tenuiflora was also estimated, and found the loss of Na⁺ content from leaves to account for only 0.0006% of the whole plant Na⁺ content over 33 d of NaCl treatments. Therefore, it is proposed that neither unidirectional Na⁺ efflux of roots nor salt secretion by leaves, but restricting unidirectional Na⁺ influx into roots with a strong selectivity for K⁺ over Na⁺ seems likely to contribute to the salt tolerance of P. tenuiflora .     

Effects of Cations on (Ca2++ Mg2+)-Activated ATPase from Rat Brain

Abstract: With a partially purified, membrane-bound (Ca + Mg)-activated ATPase preparation from rat brain, the K_0.5 for activation by Ca²⁺ was 0.8 p μm in the presence of 3 mm -ATP, 6 mm -MgCl₂, 100 m_M-KCI, and a calcium EGTA buffer system. Optimal ATPase activity under these circumstances was with 6-100 μm -Ca²⁺, but marked inhibition occurred at higher concentrations. Free Mg²⁺ increased ATPase activity, with an estimated K_0.5, in the presence of 100 μm -CaCl₂, of 2.5 mm ; raising the MgCl₂ concentration diminished the inhibition due to millimolar concentrations of CaCl₂, but antagonized activation by submicromolar concentrations of Ca²⁺. Dimethylsulfoxide (10%, v/v) had no effect on the K_0.5 for activation by Ca²⁺, but decreased activation by free Mg²⁺ and increased the inhibition by millimolar CaCl₂. The monovalent cations K⁺, Na⁺, and TI⁺ stimulated ATPase activity; for K⁺ the K_0.5 was 8 mm , which was increased to 15 mm in the presence of dimethylsulfoxide. KCI did not affect the apparent affinity for Ca²⁺ as either activator or inhibitor. The preparation can be phosphorylated at 0°C by [γ-³²P]-ATP; on subsequent addition of a large excess of unlabeled ATP the calcium dependent level of phosphorylation declined, with a first-order rate constant of 0.12 s^?1. Adding 10 mm -KCI with the unlabeled ATP increased the rate constant to 0.20 s^?1, whereas adding 10 mm -NaCl did not affect it measurably. On the other hand, adding dimethyl-sulfoxide slowed the rate of loss, the constant decreasing to 0.06 s^?1. Orthovanadate was a potent inhibitor of this enzyme, and inhibition with 1 μm -vanadate was increased by both KCI and dimethylsulfoxide. Properties of the enzyme are thus reminiscent of the plasma membrane (Na + K)-ATPase and the sarcoplasmic reticulum (Ca + Mg)-ATPase, most notably in the K⁺ stimulation of both dephosphorylation and inhibition by vanadate.