ATPase and phosphatase activities from human red cell membranes: I. The effects of N-ethylmaleimide

Probing the extracellular release site of the plasma membrane calcium pump

The plasma membrane Ca(2+) pump is known to mediate Ca(2+)/H(+) exchange. Extracellular protons activated (45)Ca(2+) efflux from human red blood cells with a half-maximal inhibition constant of 2 nM when the intracellular pH was fixed. An increase in pH from 7.2 to 8.2 decreased the IC(50) for extracellular Ca(2+) from approximately 33 to approximately 6 mM. Changing the membrane potential by >54 mV had no effect on the IC(50) for extracellular Ca(2+). This argues against Ca(2+) release through a high-field access channel. Extracellular Ni(2+) inhibited Ca(2+) efflux with an IC(50) of 11 mM. Extracellular Cd(2+) inhibited with an IC(50) of 1. 5 mM, >10 times better than Ca(2+). The Cd(2+) IC(50) also decreased when the pH was raised from 7.1 to 8.2, consistent with Ca(2+), Cd(2+), and H(+) competing for the same site. The higher affinity for inhibition by Ni(2+) and Cd(2+) is consistent with a histidine or cysteine as part of the release site. The cysteine reagent 2-(trimethylammonium)ethyl methanethiosulfonate did not inhibit Ca(2+) efflux. Our results are consistent with the notion that the release site contains a histidine.

ATPase activity of myosin in hair bundles of the bullfrog's sacculus

Mechanoelectrical transduction by a hair cell displays adaptation, which is thought to occur as myosin-based molecular motors within the mechanically sensitive hair bundle adjust the tension transmitted to transduction channels. To assess the enzymatic capabilities of the myosin isozymes in hair bundles, we examined the actin-dependent ATPase activity of bundles isolated from the bullfrog's sacculus. Separation of 32P-labeled inorganic phosphate from unreacted [gamma-32P]ATP by thin-layer chromatography enabled us to measure the liberation of as little as 0.1 fmol phosphate. To distinguish the Mg(2+)-ATPase activity of myosin isozymes from that of other hair-bundle enzymes, we inhibited the interaction of hair-bundle myosin with actin and determined the reduction in ATPase activity. N-ethylmaleimide (NEM) decreased neither physiologically measured adaptation nor the nucleotide-hydrolytic activity of a 120-kDa protein thought to be myosin 1 beta. The NEM-insensitive, actin-activated ATPase activity of myosin increased from 1.0 fmol x s-1 in 1 mM EGTA to 2.3 fmol x s-1 in 10 microM Ca2+. This activity was largely inhibited by calmidazolium, but was unaffected by the addition of exogenous calmodulin. These results, which indicate that hair bundles contain enzymatically active, Ca(2+)-sensitive myosin molecules, are consistent with the role of Ca2+ in adaptation and with the hypothesis that myosin forms the hair cell's adaptation motor.

Positive charge modifications alter the ability of XIP to inhibit the plasma membrane calcium pump

Exchange inhibitory peptide (XIP; RRLLFYKYVYKRYRAGKQRG) is the shortest peptide that inhibits the plasma membrane Ca pump at high Ca (A. Enyedi, T. Vorherr, P. James, D. J. McCormick, A. G. Filoteo, E. Carafoli, and J. T. Penniston, J. Biol. Chem. 264: 12313-12321, 1989). Sulfosuccinimidyl acetate (SNA)-modified XIP does not inhibit the Ca pump; SNA neutralizes the positive charge on Lys at positions 7, 11, and 17. Peptide 2CK-XIP (RRLLFYRYVYRCYCAGRQKG) inhibits the pump, but the iodoacetamido-modified peptide does not inhibit. Three peptide analogues, in which 7, 11, and 17 were Ala, Cys, or Lys, inhibited about as well as XIP. SNA modification of these analogues (each with 1 Lys) did not inhibit. SNA modification of 2CK-XIP results in a peptide that does not inhibit; thus position 19 is important. Our results suggest that it is critical that position 19 be positively charged, that positions 7, 11, and 17 are important contact points between XIP and the Ca pump (with at least one positively charged), and that, whereas it is not essential that residues 12 and 14 be positive, they cannot be negative.

Acetylation with succinimidyl acetate affects both the catalytic site and the regulation of the erythrocyte Ca2+ pump

Acetylation of lysine residues of the erythrocyte Ca2+ pump using succinimidyl acetate (SA) led to its complete inactivation. In the absence of any of the major activators of the pump (namely calmodulin and acidic phospholipids), ATP fully protected the pump from inactivation by SA, with a K0.5 of 13 microM. This value is very close to the Km of the high-affinity site for ATP, thus suggesting that the residue(s) involved is(are) near or at the catalytic site of the Ca(2+)-ATPase. Furthermore, the presence of 500 microM ATP prevented the acetylation of about two residues per molecule of enzyme. Acetylation by SA also prevented the activation of the Ca2+ pump by calmodulin, acidic phospholipids or controlled trypsin proteolysis. This effect of SA treatment was not avoided by the presence of ATP in the preincubation medium, indicating a second set of modified residues. The fact that the three modes of activation were cancelled in a similar fashion by SA suggests that, although acting via different mechanisms, they share at least a common step in which SA-sensitive lysine residues may participate. Moreover, modification of the pump by SA plus ATP decreased the KCa when the activity was measured in both the absence and presence of calmodulin, suggesting that the residue(s) modified in this case is(are) involved directly in the regulation of the affinity for Ca2+.

Inhibition of the Ca pump of intact red blood cells by t-butyl hydroperoxide: importance of glutathione peroxidase

Incubation of human red blood cells (RBCs) with t-butyl hydroperoxide (tBHP) resulted in inhibition of the Ca-pump ATPase. This was demonstrated using an assay of the Ca-pump ATPase activity in intact RBCs. In this assay, activity of the Ca-pump ATPase is expressed as the rate constant of the initial loss of ATP in RBCs exposed to Ca and A23187. Pseudo-first-order rate constants (Ca-pump ATPase rate constants) were lower in the presence of tBHP versus controls. Incubation of RBCs with tBHP resulted in both a time- and concentration-dependent inhibition of the Ca-pump ATPase (IC50 approximately 1 mM). Incubation of RBCs with tBHP also resulted in decreased oxyhemoglobin, increased methemoglobin and increased thiobarbituric acid reactive substances (TBARS). GSH levels were significantly lower in the presence of tBHP. GSH fell from a control value of 2.2 mmol/l RBC to 0.46 mmol/l RBC after incubation with 0.25 mM tBHP for 15 min. Both butylated hydroxytoluene and stobadine prevented the formation of TBARS and were partially effective in protecting the Ca-pump ATPase from tBHP-induced inhibition. Dithiothreitol was completely effective in preventing the tBHP-induced formation of TBARS as well as inhibition of the Ca-pump ATPase. However, when added after exposure to tBHP, dithiothreitol was unable to restore Ca-pump ATPase activity completely. An activity of dithiothreitol independent of enzymic thiol group reduction was apparent. In the presence of mercaptosuccinate, a potent inhibitor of glutathione peroxidase, the ability of dithiothreitol to protect the Ca-pump ATPase from tBHP-induced inhibition was abolished. Therefore, protection by dithiothreitol may be afforded by its ability to replenish GSH from oxidized glutathione, thus allowing glutathione peroxidase to metabolize tBHP. These results may be interpreted to suggest that inhibition of the Ca-pump ATPase in intact RBCs occurs as a result of tBHP-induced oxidant stress and subsequent lipid peroxidation which can be prevented by certain antioxidants including butylated hydroxytoluene, stobadine, and thiol-containing compounds such as dithiothreitol. These findings provide further insight into the mode of action of hydroperoxides and certain reactive oxygen species that have been implicated in oxidative stress associated with various pathological conditions. The importance of the GSH/glutathione peroxidase system in metabolizing organic hydroperoxides is also demonstrated.

Ca(2+)-dependent heat production under basal and near-basal conditions in the mouse soleus muscle

1. The rate of energy expended for the clearance of sarcoplasmic Ca2+ by sarcoreticular Ca2+ uptake process(es), plus the concomitant metabolic reactions, was evaluated from measurements of resting heat production by mouse soleus muscle before and after indirect inhibition of Ca2+ uptake by sarcoplasmic reticulum (SR). 2. Direct inhibition of the Ca2+, Mg(2+)-ATPase of SR membrane in intact muscle preparations exposed to the specific inhibitor 2,5-di(tert-butyl-1,4-benzohydroquinone (tBuBHQ) slowly increased the rate of heat production (E). Indirect inhibition of SR Ca2+ uptake was obtained by reducing sarcoplasmic Ca2+ concentration (Ca2+i) as a consequence of reducing Ca2+ release from the SR using dantrolene sodium. This promptly decreased E by 12%. Exposure of the preparations to an Mg(2+)-enriched environment (high Mg2+) or to the chemical phosphatase 2,3-butanedione monoxime (BDM), two other procedures aimed at decreasing SR Ca2+ release, also acutely decreased E, by 20 and 24%, respectively. 3. Subthreshold-for-contracture depolarization of the sarcolemma achieved by increasing extracellular K+ concentration to 11.8 mM induced a biphasic increase of E: an initial peak to 290% of basal E, followed by a plateau phase at 140% of basal E during which resting muscle tension was increased by less than 3%. Most, if not all, of the plateau-phase metabolic response was quickly suppressed by dantrolene or high Mg2+ or BDM. Another means of increasing SR Ca2+ cycling was to partially remove the calmodulin-dependent control of SR Ca2+ release using the calmodulin inhibitor W-7. The progressive increase in E with 30 microM-W-7 was largely reduced by dantrolene or high Mg2+ or BDM. 4. In the presence of either dantrolene or BDM to prevent the effect of W-7 on SR Ca2+ release, exposure of the muscle to W-7 acutely suppressed about 3% of E. This and the above results confirm that the plasmalemmal, calmodulin-dependent Ca(2+)-ATPase, although a qualitatively essential part of the Ca2+i homeostatic system of the cell, can only be responsible for a very minor part of the energy expenditure devoted to the homeostasis of Ca2+i. Active Ca2+ uptake by SR which, at least in the submicromolar range of Ca2+i, is expected to be responsible for most of this Ca(2+)-dependent energy expenditure, might dissipate up to 25-40% of total metabolic energy in the intact mouse soleus under basal and near-basal conditions.

Inhibition of erythrocyte Ca2+-ATPase by activated oxygen through thiol- and lipid-dependent mechanisms

We have studied erythrocyte Ca2+-ATPase as a model target for elucidating effects of activated oxygen on the erythrocyte membrane. Either intracellular or extracellular generation of activated oxygen causes parallel decrements in Ca2+-ATPase activity and cytoplasmic GSH, oxidation of membrane protein thiols, and lipid peroxidation. Subsequent incubation with either dithiothreitol or glucose allows only partial recovery of Ca2+-ATPase, indicating both reversible and irreversible components which are modeled herein using diamide and t-butyl hydroperoxide. The reversible component reflects thiol oxidation, and its recovery depends upon GSH restoration. The irreversible component is largely due to lipid peroxidation, which appears to act through mechanisms involving neither malondialdehyde nor secondary thiol oxidation. However, some portion of the irreversible component could also reflect oxidation of thiols which are inaccessible for reduction by GSH, since we demonstrate existence of different classes of thiols relevant to Ca2+-ATPase activity. Activated oxygen has an exaggerated effect on Ca2+-ATPase of GSH-depleted cells. Sickle erythrocytes treated with dithiothreitol show a heterogeneous response of Ca2+-ATPase activity. These findings are potentially relevant to oxidant-induced hemolysis. They also may be pertinent to oxidative alteration of functional or structural membrane components in general, since many components share with Ca2+-ATPase both free thiols and close proximity to unsaturated lipid.

Compound 48/80 and calmodulin modify the interaction of ATP with the (Ca2+ + Mg2+)-ATPase of red cell membranes

Compound 48/80, an anti-calmodulin agent, reduces the maximum effect of ATP and does not affect the apparent affinity for ATP of the high-affinity site of the Ca2+-ATPase from calmodulin-bound membranes of human red cells. In the same preparation, 48/80 reduces more than 50-times the apparent affinity for ATP of the low-affinity site with little change in the maximum effect of the nucleotide at this site of the Ca2+-ATPase. The effects of compound 48/80 are independent of the concentration of Ca2+ between 30 and 200 microM. The apparent affinity of the low-affinity site of the Ca2+-ATPase for ATP is almost 100-fold less in calmodulin-stripped membranes than in calmodulin-bound membranes. In calmodulin-stripped membranes, exogenous calmodulin increases the apparent affinity for ATP up to the control values. These results indicate that apart from increasing the apparent affinity of the transport site for Ca2+, calmodulin also increases the apparent affinity of the regulatory site of the Ca2+-ATPase for ATP. Since this effect is exerted within the physiological ranges of ATP concentrations, it may participate in the physiological regulation of Ca2+ pumping by calmodulin.

Erythrocyte (Ca+2 + Mg+2)-ATPase activity: increased sensitivity to oxidative stress in glucose-6-phosphate dehydrogenase deficiency

The effect of the thiol-oxidizing agent diamide on erythrocyte (Ca+2 + Mg+2)-ATPase activity was measured in normal and glucose-6-phosphate-dehydrogenase-deficient (G6PD-) cells. Although the enzyme activity before the oxidative stress was similar in both groups, diamide induced a markedly greater inhibition in the enzyme activity in the G6PD- cells than in the normal controls. These data indicate dependence of erythrocyte (Ca+2 + Mg+2)-ATPase, in part, on the redox status of the cell. The increased vulnerability of (Ca+2 + Mg+2)-ATPase to oxidative stress in G6PD- may be of pathophysiological relevance to their premature destruction in oxidant-induced hemolysis.

The effects of alkali metal ions on active Ca2+ transport in reconstituted ghosts from human red cells

1. K+, Rb+ or Na+ increase from 30 to 90% the maximum rate of Ca2+ transport from resealed ghosts, leaving unaltered the apparent affinity of the Ca2+ pump for Ca2+. Li+ is ineffective as activator of Ca2+ transport. 2. K+ does not change the temperature dependence of Ca2+ transport. 3. The effects of K+ and Na+ are half-maximal at 4.6 mM and 24 mM, respectively. 4. The site(s) at which K+ or Na+ combine are accessible only from the cytoplasmic surface of the cell membrane. 5. Under conditions in which Na+ activates the Ca2+ pump no Na+ efflux coupled to Ca2+ efflux is apparent.

Vanadate inhibition of active Ca2+ transport across human red cell membranes

(1) Vanadate (pentavalent vanadium) inhibits with high affinity (K0.5 = 3 microM) the ATP-dependent Ca2+ efflux in reconstituted ghosts from human red cell. (2) To inhibit Ca2+ efflux vanadate has to have access to the inner surface of the cell membrane (3) The inhibitory effect of vanadate is potentiated by intracellular Mg2+ and by intracellular K+. (4) Ca2+ in the external medium antagonizes the inhibitory effect of vanadate.

Phosphorylation and dephosphorylation of the Ca2+ pump of human red cells in the presence of monovalent cations

(1) In the presence of calcium ions, K+ increases the rate and the steady state level of phosphorylation of human red cell membranes by [gamma-32P)ATP. The effect of K+ is mimicked by Rb+, NH4+ and Cs+. Electrophoresis experiments suggest that the phosphorus taken up by the membranes in the presence of K+ is bound to the phosphoenzyme of the Ca2+-ATPase. (2) (Ca2+ + K+)-dependent phosphorylation requires Ca2+ and ATP with the same apparent affinity as the phosphorylation of the Ca2+ pump and the effect of K+ on phosphorylation is exerted with the same apparent affinity as that for the activation of the Ca2+-ATPase by K+. (3) The rate of hydrolysis of phosphoenzyme made in the presence of K+ is higher than that made in its absence and K+ increases the ratio Ca2+-ATPase activity/Ca2+-dependent phosphoenzyme concentration. (4) Results suggest that monovalent cations activate the Ca2+ pump because they increase the level and the turnover of the phosphoenzyme of the Ca2+-ATPase.

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

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

Differential effect of L-thyroxine on phospholipid biosynthesis in mitochondria and microsomal fraction

1. The action of L-thyroxine on the incorporation of radioactive choline or CDP-choline into phosphatidylcholine in vitro was explored in liver and brain microsomal fraction and mitochondria obtained from young adult rats. 2. In liver mitochondria isolated from animals treated with L-thyroxine (40 mg/kg body wt. during 6 days), the incorporation of both radioactive precursors into phosphatidylcholine was significantly decreased compared with normal controls, whereas in the total homogenate and in the microsomal fraction the incorporation was similar in the experimental and control groups. In subcellular fractions isolated from brain, the incorporation of precursors was similar in L-thyroxine-treated and normal animals. 3. Liver mitochondria isolated from normal animals incubated in vitro with CDP-choline, in the presence of different concentrations of L-thyroxine, showed also a marked decrease in the incorporation of label into phosphatidylcholine, whereas no significant changes were found in the total homogenate and in the microsomal fraction compared with control experiments. 4. The differential effect of L-thyroxine on the incorporation of radioactive precursors into phosphatidylcholine of isolated liver subcellular fractions gives further support to the hypothesis that liver mitochondria can independently synthesize part of their own phospholipids. 5. Possible mechanisms of the action of the hormone at the mitochondrial level are discussed.

Studies on lithium transport across the red cell membrane. VI. Properties of a sulfhydryl group involved in ouabain-resistant Na+-Li+ (and Na+-Na+) exchange in human and bovine erythrocytes

The reactivity of the SH-group essential for ouabain-resistant Na+-Li+ (and Na+-Na+) exchange and its location within the membrane are studied on human and beef erythrocytes and beef red cell ghosts. N-ethylmaleimide (NEM), 1,6-hexane dimaleimide, and iodoacetamide can induce an irreversible, partial inhibition of Na+-Li+ exchange in erythrocytes of the two species. The development of the inhibition due to the alkylating agents is greatly accelerated by external Na+ and Li+. The inhibition takes 3 min (NEM) and 60 min (iodoacetamide) to come to completion in isotonic Na+ media, but is hardly detectable in choline+, K+ or Mg2+ media. The transport site of the exchange system and the site promoting NEM binding exhibit similar affinities for external Na+. The impermeable, monofunctional glutathione derivative of 1,6-hexane dimaleimide does not inhibit Na+-Li+ exchange. The mercurials PCMBS, PCMB, and Hg2+ inhibit Na+-Li+ exchange in beef, but not in human erythrocytes. The inhibitory action of PCMBS, being slightly accelerated by external Na+, is fully reversed by penetrating thiols such as 2-mercaptoethanol, whilst glutathione, an impermeable thiol, is ineffective. Pretreatment with PCMBS affords partial protection from the irreversible inhibition caused by NEM. Oxidation with copper orthophenanthroline inhibits Na+-Li+ exchange only when performed in the presence of penetrating thiols such as 2-mercaptoethanol. It is concluded that the SH-reagents studied inhibit Na+-Li+ exchange by modifying an essential SH-group of a membrane protein in such a way that the turnover number of the exchange system is reduced. This SH-group is separated from both the red cell exterior and interior by a penetration barrier and seems to be distinct from the cation binding site. The action of external Na+ and Li+ in promoting the reaction of alkylating inhibitors is interpreted to result from a conformational change of the transport protein induced by the binding of external Na+ or Li+.

Mechanisms of inhibition of active transport ATPases by mercurials

Inhibition by methylmercury and mercuric chloride of Mg,Ca ATPase and Na,K ATPase activities in human erythrocyte ghosts was correlated with the binding capacity of ghosts for the mercurial. Full inhibition was always reached below saturation of binding capacity, and half-inhibition at levels as low as 10% saturation. Under such conditions, concentrations of free inhibitor were negligibly low, and existing mathematical models of inhibition were not applicable. New inhibitor partition equations were introduced to model the mechanisms of action of mercurials. Up to 7 methylmercury groups were calculated to bind to one Na,K ATPase molecule at non-inhibitory sites, while only one reacted with the inhibitory site. Mg,Ca ATPase showed simple one-hit inhibition (one mercurial per enzyme); further washing of ghosts, however, unmasked a second binding site (cooperative two-hit inhibition). Affinities of mercurials to sites of inhibition were calculated relative to other ligands in erythrocyte membranes: the ratios ranged from 3 : 1 to 50 : 1. The results demonstrated the use of binding capacity assays and inhibitor partition equations to measure and compare the susceptibilities of membrane-bound enzymes to poisoning by mercurials.

The effect of ruthenium red and NEM on lithium efflux from human erythrocytes

The lithium pump in intact human erythrocytes which has previously been shown to be repressed following lithium carbonate ingestion by manic-depressive subjects is now shown to be inhibited selectively by N-ethylmaleimide and ruthenium red, both of which are known to inhibit the erythrocyte Ca pump. It is proposed that active lithium efflux is linked to the concurrent operation of the calcium pump.

Reversal of the calcium pump in human red cells

Human red cells containing low ATP and high Pi concentrations were suspended in media with and without 2 mM Ca2+, and the incorporation of (32P)Pi into ATP was measured. There was some incorporation whatever the medium, but in every experiment there was an extra incorporation when the cells were in the Ca2+-containing medium. This extra incorporation was abolished by the ionophore A23187, which collapses the Ca2+ concentration gradient across the membranes, or by LaCl3, which blocks the Ca2+ pump. Starved and phosphate-loaded cells also show an uptake of Ca2+ which is not apparent in fresh cells. Results are consistent with the idea that Ca2+-dependent incorporation of Pi into ATP is catalyzed by the Ca2+ pump using energy derived from the Ca2+ concentration gradient.

Two classes of site for ATP in the Ca2+-ATPase from human red cell membranes

(1) The response of the Ca2+-ATPase activity from human red cell membranes to ATP concentrations can be represented by the sum of two Michaelis-like curves: one with a Km of 2.5 micrometer and the other with a Km of 145 micrometer. (2) The maximum Ca2+-ATPase activity elicited by occupation of the site with lower Km represents about 10% of the activity attainable at non-limiting ATP concentrations. (3) 30--50% of the Ca2+-ATPase activity with lower Km remains in the absence of Mg2+ . Mg2+ increases V and the maximum effect of Ca2+, having no effect on the apparent affinities for ATP and Ca2+. (4) The large increase in Ca2+-ATPase activity which results from the occupation of the site with higher Km only takes place when Mg2+ is present. (5) Results are compatible with the idea that the Ca2+-ATPase from human red cell membranes has two classes of site for ATP binding, both of which are occupied when the enzyme catalyzes the hydrolysis of ATP at maximum rate. (6) The properties of the high affinity site suggest that this is the catalytic site of the Ca2+-ATPase. It is proposed that binding of ATP at the low affinity site regulates the turnover of the system.

ATPase and phosphatase activities from human red cell membranes: II. The effects of phospholipases on Ca2+-dependent enzymic activities

Treatment of human red cell membranes with pure phospholipase A2 results in a progressive inactivation of both Ca2+-dependent and (Ca2+ + K+)-dependent ATPase and phosphatase activities. When phospholipase C replaces phospholipase A2, Ca2+-dependent ATPase activity and Ca2+-dependent phosphorylation of red cell membranes are lost, while Ca2+-dependent phosphatase activity is enhanced and its apparent affinity for Ca2+ is increased about 20-fold. Activation of Ca2+-dependent phosphatase following phospholipase C treatment was not observed in sarcoplasmic reticulum preparation. Phospholipase C increases the sensitivity of the phosphatase to N-ethylmaleimide but has little effect on the kinetic parameters relating the phosphatase activity to substrate and cofactors, suggesting that no extensive structural disarrangement of the Ca2+-ATPase system has occurred after incubation with phospholipase C.

ATPase and phosphatase activities from human red cell membranes. III. Stimulation of K+-activated phosphatase by phospholipase C

Treatment of red cell membranes with pure phospholipase C inactivates (Na+ + K+)-ATPase activity and Na+-dependent phosphorylation but increases K+-dependent phosphatase activity. When phospholipase A2 replaces phospholipase C, all activities are lost. Activation of K+-dependent phosphatase by treatment with phospholipase C is caused by an increase in the maximum rate of hydrolysis of p-nitrophenylphosphate and in the maximum activating effect of K+, the apparent affinities for substrate and cofactors being little affected. After phospholipase C treatment K+-dependent phosphatase is no longer sensitive to ouabain but becomes more sensitive to N-ethylmaleimide. In treated membranes Na+ partially replaces K+ as an activator of the phosphatase. Although ATP still inhibits phosphatase activity, neither ATP, nor ATP+Na+ are able to modify the apparent affinity for K+ of K+-dependent phosphatase in these membranes.