Ca2+-stimulated, Mg2+-dependent ATPase activity in neutrophil plasma membrane vesicles. Coupling to Ca2+ transport

Low concentrations of free Ca2+ stimulated the hydrolysis of ATP by plasma membrane vesicles purified from guinea pig neutrophils and incubated in 100 mM HEPES/triethanolamine, pH 7.25. In the absence of exogenous magnesium, apparent values obtained were 320 nM (EC50 for free Ca2+), 17.7 nmol of Pi/mg X min (Vmax), and 26 microM (Km for total ATP). Studies using trans- 1,2-diaminocyclohexane- N,N,N',N',-tetraacetic acid as a chelator showed this activity was dependent on 13 microM magnesium, endogenous to the medium plus membranes. Without added Mg2+, Ca2+ stimulated the hydrolysis of several other nucleotides: ATP congruent to GTP congruent to CTP congruent to ITP greater than UTP, but Ca2+-stimulated ATPase was not coupled to uptake of Ca2+, even in the presence of 5 mM oxalate. When 1 mM MgCl2 was added, the vesicles demonstrated oxalate and ATP-dependent calcium uptake at approximately 8 nmol of Ca2+/mg X min (based on total membrane protein). Ca2+ uptake increased to a maximum of approximately 17-20 nmol of Ca2+/mg X min when KCl replaced HEPES/triethanolamine in the buffer. In the presence of both KCl and MgCl2, Ca2+ stimulated the hydrolysis of ATP selectively over other nucleotides. Apparent values obtained for the Ca2+-stimulated ATPase were 440 nM (EC50 for free Ca2+), 17.5 nmol Pi/mg X min (Vmax) and 100 microM (Km for total ATP). Similar values were found for Ca2+ uptake which was coupled efficiently to Ca2+-stimulated ATPase with a molar ratio of 2.1 +/- 0.1. Exogenous calmodulin had no effect on the Vmax or EC50 for free Ca2+ of the Ca2+-stimulated ATPase, either in the presence or absence of added Mg2+, with or without an ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N',-tetraacetic acid pretreatment of the vesicles. The data demonstrate that calcium stimulates ATP hydrolysis by neutrophil plasma membranes that is coupled optimally to transport of Ca2+ in the presence of concentrations of K+ and Mg2+ that appear to mimic intracellular levels.     

Mitochondrial adenosine triphosphatase from human placenta--purification and catalytic properties

1. The purification of ATPase (EC 3.6.1.3) from human placental mitochondria is described. The yield based on mitochondrial enzyme activity was about 70% and the purification was 380-fold. 2. The rate of Mg-ATP hydrolysis was 85 mumole per min per mg of protein under optimum conditions. 3. Nucleoside triphosphates were hydrolyzed by the purified enzyme at decreasing rates in the following order: GTP greater than ITP greater than ATP greater than epsilon-ATP greater than UTP greater than CTP in Tris-HCl buffer (pH 8.0), and in the order: ATP greater than GTP greater than or equal to ITP greater than epsilon-ATP greater than UTP greater than CTP in Tris-bicarbonate buffer at pH 8.0. 4. The values of kinetic parameters are reported. The ATPase reaction deviated from typical Michaelis-Menten kinetics in Tris-HCl buffer but not in Tris-bicarbonate. Eadie-Hofstee plots for Mg-ATP hydrolysis were biphasic in Tris-HCl (Km = 0.2 mM, 0.09 mM) and monophastic in Tris-becarbonate medium (Km = 0.16 mM). 5. In the presence of Mg-ITP or Mg-GTP as substrates no curvature of the reciprocal plots was observed. 6. The results presented reflect the fact that multiple conformations of the enzyme molecule do exist and are probably involved in its regulatory functions. 7. The existence of two kinetically distinct classes of catalytic sites and of an anion-binding site on the placental ATPase is proposed.     

Characterization of the plasma membrane ATPase of Candida tropicalis

1) Plasma membrane vesicles from Candida tropicalis were isolated from protoplasts by differential centrifugation and purified in a continuous sucrose gradient. 2) The plasma membrane bound ATPase was characterized. It is highly specific for ATP and requires Mg2+. It is stimulated by K+, Na+ and NH4+. Lineweaver-Burk plots for ATPase activity are linear with a Vmax of 4.2 mumoles of ATP hydrolyzed min-1.mg-1 protein and a Km for ATP of 0.76 mM. The ATPase activity is inhibited competitively by ADP with a Ki of 1.7 mM and non competitively by vanadate with a Ki of 3 microM. The activity is unaffected by oligomycin or azide but is sensitive to DCCD.     

Characterization of the Ca2+- and Mg2+-Dependent ATPases in Electrophorus Electroplax Microsomes

Electrophorus electroplax microsomes were examined for Ca2+- and Mg2+-dependent ATPase activity. In addition to the previously reported low-affinity ATPase, a high-affinity (Ca2+,Mg2+)-ATPase was found. At low ATP and Mg2+ concentrations (200 microM or less), the high-affinity (Ca2+,Mg2+)-ATPase exhibits an activity of 18 nmol Pi mg-1 min-1 with 0.58 microM Ca2+. At higher ATP concentrations (3 mM), the low-affinity Ca2+-ATPase predominates, with an activity of 28 nmol Pi mg-1 min-1 with 1 mM Ca2+. In addition, Mg2+ can also activate the low-affinity ATPase (18 nmol Pi mg-1 min-1). The high-affinity ATPase hydrolyzes ATP at a greater rate than it does GTP, ITP, or UTP and is insensitive to ouabain, oligomycin, or dicyclohexylcarbodiimide inhibition. The high-affinity enzyme is inhibited by vanadate, trifluoperazine, and N-ethylmaleimide. Added calmodulin does not significantly stimulate enzyme activity; rinsing the microsomes with EGTA does not confer calmodulin sensitivity. Thus the high-affinity ATPase from electroplax microsomes is similar to the (Ca2+,Mg2+)-ATPase reported to be associated with Ca2+ transport, based on its affinity for calcium and its response to inhibitors. The low-affinity enzyme hydrolyzes all tested nucleoside triphosphates, as well as diphosphates, but not AMP. Vanadate and N-ethylmaleimide do not inhibit the low-affinity enzymes. The low-affinity enzyme reflects a nonspecific nucleoside triphosphatase, probably an ectoenzyme.     

Solubilization, purification, and characterization of a membrane-bound phospholipase A2 from the P388D1 macrophage-like cell line

The release of free arachidonic acid from membrane phospholipids is believed to be the rate-controlling step in the production of the prostaglandins, leukotrienes, and related metabolites in inflammatory cells such as the macrophage. We have previously identified several different phospholipases in the macrophage-like cell line P388D1 potentially capable of controlling arachidonic acid release. Among them, a membrane-bound, alkaline pH optimum, Ca2+-dependent phospholipase A2 is of particular interest because of the likelihood that the regulatory enzyme has these properties. This phospholipase A2 has now been solubilized from the membrane fraction with octyl glucoside and partially purified. The first two steps in this purification are butanol extractions that yield a lyophilized, stable preparation of phospholipase A2 lacking other phospholipase activities. This phospholipase A2 shows considerably more activity when assayed in the presence of glycerol, regardless of whether the substrate, dipalmitoylphosphatidylcholine, is in the form of sonicated vesicles or mixed micelles with the nonionic surfactant Triton X-100. Glycerol (70%) increases both the Vmax and the Km with both substrate forms, giving a Vmax of about 15 nmol min-1 mg-1 and an apparent Km of about 60 microM for vesicles and a Vmax of about 100 nmol min-1 mg-1 and an apparent Km of about 1 mM for mixed micelles. Vmax/Km is slightly greater for vesicles than for mixed micelles. The lyophilized preparation of the enzyme is routinely purified about 60-fold and is suitable for evaluating phospholipase A2 inhibitors such as manoalide analogues. Subsequent steps in the purification are acetonitrile extraction followed by high performance liquid chromatography on an Aquapore BU-300 column and a Superose 12 column. This yields a 2500-fold purification of the membrane-bound phospholipase A2 with a 25% recovery and a specific activity of about 800 nmol min-1 mg-1 toward 100 microM dipalmitoylphosphatidylcholine in mixed micelles. When this material was subjected to analysis on a Superose 12 sizing column, the molecular mass of the active fraction was approximately 18,000 daltons.     

Stimulation of calcium transport of sarcoplasmic reticulum vesicles by the calcium complex of ethylene glycol bis(beta-aminoethyl ether)-N,N',-tetraacetic acid

The calcium ion dependence of calcium transport by isolated sarcoplasmic reticulum vesicles from rabbit skeletal muscle has been investigated by means of the Calcium-stat method, in which transport may be measured in the micromolar free calcium ion concentration range, in the absence of calcium buffers. At pH 7.2 and 20 degrees C, ATP, in the range 1 to 10 mM, decreased [Ca2+]0.5 from 2.0 microM to 0.3 microM and decreased Vmax of oxalate-supported transport from 0.5 to 1.3 mumol min-1 mg-1. Simultaneous measurements of transport and of ATPase activity in the range 0.8 to 10 microM free Ca2+ showed a ratio of 2.1 calcium ions translocated/molecule of ATP hydrolyzed. Transport, in the presence of 5 mM ATP, ceased when calcium ion concentration fell to 0.6 to 1.2 microM, whilst ATPase activity of 90 nmol of ATP hydrolyzed min-1 mg-1 persisted. The data obtained by the Calcium-stat method differed from those described previously using calcium buffers, in that they showed lower apparent affinities of the transport site for calcium ions, more marked sigmoidal behavior, an effect of ATP concentration on Ca2+ concentration dependence and lower ATPase activity in the absence of transport. The calcium complex of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (CaEGTA) had no effect when transport was stimulated maximally at saturating free Ca2+ concentrations. However, at calcium ion levels below [Ca2+]0.5, 70 microM CaEGTA stimulated transport to rates of 20 to 45% of Vmax. Half-maximal stimulation of transport occurred at 19 microM CaEGTA. CaEGTA, 50 microM, decreased [Ca2+]0.5, determined at 5 mM ATP, from 1.3 microM to 0.45 microM. It is proposed that a ternary complex, E . Ca2+ . EGTA4-, is formed as an intermediate species during CaEGTA-stimulated calcium transport by sarcoplasmic reticulum membranes and stimulates the calcium pump at limiting free Ca2+ ion concentration.     

Characterization of calcium transport by basal plasma membranes from human placental syncytiotrophoblast.

We have studied the mechanisms involved in calcium (Ca2+) transport through the basal plasma membranes (BPM) of the syncytiotrophoblast cells from full-term human placenta. These purified membranes were enriched 25-fold in Na+/K(+)-adenosine triphosphate (ATPase), 37-fold in [3H] dihydroalprenolol binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg2+, a basal Ca2+ uptake was observed, which followed Michaelis-Menten kinetics, with a Km Ca2+ of 0.18 +/- 0.05 microM and Vmax of 0.93 +/- 0.11 nmol/mg/min. The addition of Mg2+ to the incubation medium significantly decreased this uptake in a concentration-dependent manner, with a maximal inhibition at 3 mM Mg2+ and above. The Lineweaver-Burk plots of Ca2+ uptake in the absence and in the presence of 1 mM Mg2+ suggest a noncompetitive type of inhibition. Preloading the BPM vesicles with 5 mM Mg2+ had no significant effect on Ca2+ uptake, eliminating the hypothesis of a Ca2+/Mg2+ exchange mechanism. This ATP-independent Ca2+ uptake was not sensitive to 10(-6) M nitrendipine nor to 10(-4) M verapamil. An ATP-dependent Ca2+ transport was also detected in these BPM, whose Km Ca2+ was 0.09 +/- 0.02 microM and Vmax 3.4 +/- 0.2 nmoles/mg/3 min. This Ca2+ transport requires Mg2+, the optimal concentration of Mg2+ being approximately 1 mM. Preincubation of the membrane with 10(-6) M calmodulin strongly enhanced the initial ATP-dependent Ca2+ uptake. Finally, no Na+/Ca2+ exchange process could be demonstrated.     

Effect of parathyroid hormone and dietary phosphate on phosphate transport in renal outer cortical and outer medullary brush-border membrane vesicles

Two distinctive sodium-dependent phosphate transport systems have been identified in early and late proximal tubules; a high-capacity process located only in outer cortical tissue, and a high affinity present in both outer cortical and outer medullary brush-border membranes (Km 0.1-0.25 mM). A third, sodium-independent, pH gradient-stimulated system (Vmax 4.7 +/- 0.3 nmol.mg-1.min-1, Km 0.15 +/- 0.002 mM) is present in the outer medulla, but absent in outer cortex. Brush-border vesicles were prepared from outer cortical and outer medullary tissue of pigs maintained on low (less than 0.05%), normal (0.4%), or high (4%) phosphate diets. Sodium-dependent phosphate uptake of the high-capacity system decreased (Vmax, 9.4 to 2.2 nmol.mg-1.min-1) from low to high phosphate diet, whereas uptake rates decreased about 50% in the high-affinity system. There were no changes in the respective Km values. The pH gradient-stimulated uptake also decreased (Vmax, 6.9 to 3.0 nmol.mg-1.min-1) with no change in mean Km value (0.15 +/- 0.001 mM) with dietary manipulation. Administration of 1 U parathyroid hormone prior to study resulted in a decrease in sodium-dependent uptake by 40-50% and in pH-dependent uptake (36%) with no change in the respective Km values. In conclusion, the antecedent dietary phosphate intake and parathyroid hormone administration appropriately alters phosphate uptake across the brush-border membrane of all three systems, sodium-dependent and pH gradient-stimulated phosphate transport.     

Substrate regulation of the sarcoplasmic reticulum ATPase. Transient kinetic studies.

The rate of phosphorylation of the Ca2+-dependent ATPase of sarcoplasmic reticulum vesicles by ITP and ATP was studied using a millisecond mixing and quenching device. The rate of phosphorylation was slower when the vesicles were preincubated in a Ca2+-free medium than when preincubated with Ca2+, regardless of the substrate used and of the pH of the medium. When the vesicles were preincubated with Ca2+ at pH 7.4 an overshoot of phosphorylation was observed in the presence of ITP. The overshoot was abolished when the pH of the medium was decreased to 6.0 or when the vesicles were preincubated in a Ca2+-free medium. Using vesicles preincubated with Ca2+ the apparent Km for ITP found was 2.5 mM at pH 6.0 and 1.0 mM at pH 7.4. The Vmax observed (77 mumol g-1 s-1) did not change with the pH of the medium. Both at pH 6.0 and 7.4 the apparent Km for ATP was 3 microM when preincubated in a Ca2+-free medium. At pH 6.0 the Vmax for ATP varied from 96 to 33 mumol g-1 s-1 depending on whether the vesicles were preincubated in the presence or absence of Ca2+. At pH 7.4 the Vmax for ATP was 90 mumol g-1 s-1 in both conditions. The rate of phosphorylation of the vesicles was dependent on the relative Ca2+ and Mg2+ concentrations of the reaction medium regardless of the substrate used.     

Fully active Fe-protein of the nitrogenase from Azotobacter vinelandii contains at least eight iron atoms and eight sulphide atoms per molecule

A purified preparation of kidney basolateral membrane vesicles is capable of ATP-dependent Ca2+ uptake. The reaction has high affinity for Ca2+ (Km about 0.1 microM) and a V of 5.8 nmol Ca2+ X mg-1 protein X min-1 in the predominantly right-side-out vesicular preparation used. It is inhibited by vanadate (K0.5 about 5 microM) and by anti-calmodulin drugs. A stimulatory effect of calmodulin is visible in membranes depleted of the activator. Exposure of basolateral membranes to 125I-azido-modified calmodulin results in the specific labeling of a membrane protein of Mr 141 000, which is tentatively suggested to be the Ca2+-pumping ATPase.     

Interaction of FliI, a component of the flagellar export apparatus, with flagellin and hook protein.

FliI is a key component of the flagellar export apparatus in Salmonella typhimurium. It catalyzes the hydrolysis of ATP which is necessary for flagellar assembly. Affinity blotting experiments showed that purified flagellin and hook protein, two flagellar axial proteins, interact specifically with FliI. The interaction of either of the two proteins with FliI, increases the intrinsic ATPase activity. The presence of either flagellin or hook protein stimulates ATPase activity in a specific and reversible manner. A Vmax of 0.12 nmol Pi min-1 microgram-1 and a Km for MgATP of 0.35 mM was determined for the unstimulated FliI; the presence of flagellin increased the Vmax to 0.35 nmol Pi min-1 microgram-1 and the Km for MgATP to 1.1 mM. The stimulation induced by the axial proteins was fully reversible suggesting a direct link between the catalytic activity of FliI and the export process.     

Purification and properties of the F1-ATPase from liver mitochondria of Gallus gallus.

1. This paper is the first detailed report of the purification of a mitochondrial ATPase from an avian species. 2. The Gallus gallus liver mitochondrial F1-ATPase was purified by chloroform extraction and ion-exchange chromatography. 3. The enzyme shows the five alpha, beta, tau, delta, and epsilon subunits characteristic of mitochondrial F1-ATPases. 4. The Km for ATP is 1 mM and for Mg 0.5 mM with a specific activity of 25.2 mu moles of ATP hydrolyzed x min-1 x mg-1. 5. Unlike mammals enzymes the chicken mitochondrial ATPase shows maximal activity with ITP as substrate, and is strongly inhibited by Cu.     

Energy-linked transhydrogenase. Effects of valinomycin and nigericin on the ATP-driven transhydrogenase reaction catalyzed by reconstituted transhydrogenase-ATPase vesicles

Reconstituted transhydrogenase-ATPase vesicles obtained with purified beef heart transhydrogenase and oligomycin-sensitive ATPase were investigated with respect to the mode of interaction between the two proton pumps, with special reference to the relative contributions of the membrane potential and proton gradient using valinomycin and nigericin in the presence of potassium. In the absence of ionophores and at low ATP concentrations, below 20 microM, the ATPase generated a proton motive force which was predominantly due to a membrane potential, whereas at saturating concentrations of ATP the proton gradient was the predominant component. The ATP-dependence of the rate of the ATP-driven transhydrogenase reaction showed apparent Km values in the low and high ATP concentration range of about 3 and 56 microM, respectively, with a corresponding difference in Vmax of about 3-fold. It is concluded that the reconstituted transhydrogenase can utilize both a membrane potential and a proton gradient, separately or combined, where the relative contributions of these components depend on the activity of the ATPase. In the reconstituted vesicles, the maximally active transhydrogenase is apparently driven by an electrochemical proton gradient where the membrane potential and the proton gradient contribute one-third and two-thirds, respectively. The rate-dependent relative generation of a membrane potential and pH gradient presumably reflects the proton pump characteristics of the ATPase and/or buffering/permeability characteristics of the vesicles rather than the properties of the transhydrogenase per se. These results are discussed in relation to current models for transhydrogenase-linked proton translocation.     

ATP-dependent S-(2,4-dinitrophenyl)glutathione transport in canalicular plasma membrane vesicles from rat liver

Uptake of the thioether S-(2,4-dinitrophenyl)glutathione (DNPSG) in canalicular plasma membrane vesicles from rat liver is enhanced in the presence of ATP and exhibits an overshoot with a transient 5.5-fold accumulation of DNPSG. Stimulation by ATP is not caused by the generation of a membrane potential, based on responses of the indicator dye oxonol V. ATP-dependent uptake has an apparent Km of 71 microM for DNPSG and a Vmax of 0.34 nmol.min-1.mg of vesicle protein-1. Protein thiol groups are essential for transport activity as indicated by the sensitivity of DNPSG transport to sulfhydryl reagents. There is competitive inhibition with other thioethers, S-hexylglutathione (Ki = 66 microM), the photoaffinity label S-(4-azidophenacyl)glutathione (Ki = 56 microM), as well as with glutathione disulfide (Ki = 0.44 mM) and with the bile acid taurocholate (Ki = 0.61 mM). GSH (2 mM) or cholate (0.4 mM) does not inhibit. Both glutathione disulfide and taurocholate show ATP-dependent transport in the canalicular membrane vesicles which is inhibited by DNPSG. No ATP-dependent transport is found for GSH. Transport of DNPSG is also inhibited competitively by alpha-naphthyl-beta-D-glucuronide (Ki = 0.42 mM) but not by alpha-naphthylsulfate (2 mM), and there is substantial inhibition with the glucuronides from ebselen and p-nitrophenol. The results indicate that the canalicular transport system for DNPSG is directly driven by ATP and that the biliary transport of other classes of compounds may also proceed via this system.     

Ca2+ transport and Ca2+-dependent ATP hydrolysis by Golgi vesicles from lactating rat mammary glands.

Ca2+ transport across mammary-gland Golgi membranes was measured after centrifugation of the membrane vesicles through silicone oil. In the presence of 2.3 microM free Ca2+ the vesicles accumulated 5.8 nmol of Ca2+/mg of protein without added ATP, and this uptake was complete within 0.5 min. In the presence of 1 mM-ATP, Ca2+ was accumulated at a linear rate for 10 min after the precipitation of intravesicular Ca2+ with 10 mM-potassium oxalate. ATP-dependent Ca2+ uptake exhibited a Km of 0.14 microM for Ca2+ and a Vmax. of 3.1 nmol of Ca2+/min per mg of protein. Ca2+-dependent ATP hydrolysis exhibited a Km of 0.16 microM for Ca2+ and a Vmax. of 10.1 nmol of Pi/min per mg of protein. The stoichiometry between ATP-dependent Ca2+ uptake and Ca2+-stimulated ATPase varied between 0.3 and 0.7 over the range 0.03-8.6 microM-Ca2+. Both Ca2+ uptake and Ca2+-stimulated ATPase were strongly inhibited by orthovanadate, which suggests that the major mechanism by which Golgi vesicles accumulate Ca2+ is through the action of the Ca2+-stimulated ATPase. However, Ca2+ uptake was also decreased by the protonophore CCCP (carbonyl cyanide m-chlorophenylhydrazone), indicating that it may occur by other mechanisms too. The effect of CCCP may be related to the existence of transmembrane pH gradients (delta pH) in these vesicles: the addition of 30 microM-CCCP reduced delta pH from a control value of 1.06 to 0.73 pH unit. Golgi vesicles also possess a Ca2+-efflux pathway which operated at an initial rate of 0.5-0.57 nmol/min per mg of protein.     

Comparison between ATP-supported and GTP-supported phosphate turnover of the calcium-transporting sarcoplasmic reticulum membranes.

The study deals with the interrelationship of the phosphate-transferring activities of the calcium-transporting sarcoplasmic reticulum membrane vesicles: the phosphate exchange between nucleoside triphosphate (NTP) and nucleoside diphosphate (NDP) (NTP-NDP exchange), the calcium-dependent NTase, and the phosphorylation of NDP by inorganic phosphate in the presence of NTP (NTP-Pi exchange). Different nucleotides were used as phosphate donors and acceptors. It is demonstrated for the phosphate transfer from ITP to GDP that the NTP-NDP exchange exhibits ping-pong kinetics with Mg-ITP and unliganded GDP as substrates. The apparent affinities of the enzyme for the nucleoside diphosphate and triphosphate species are deduced according to this mechanism. The enzyme's affinity for the nucleoside triphosphates and diphosphates depends on its functional state being considerably lower under conditions of NTP-NDP exchange than during NTP splitting or NTP synthesis. ATP and GTP are split with the same low rates when calcium-activated NTPase is inhibited by high internal calcium concentrations after calcium transport has reached steady state. The rates of the NTP-NDP exchange reactions, however, differ by a factor of about 10 being approximately equal to 3 mumol . mg-1 . min-1 for ATP-ADP and only approximately equal to 0.3 mumol . mg-1 . min-1 (22 degrees C) for GTP-GDP. When the sarcoplasmic reticulum vesicles are made calcium-permeable, the calcium transport ATPase is turned on and the rates of GTP and ATP splitting increase about tenfold. Yet, while the rate of ATP-ADP exchange is little reduced, the rate of GTP-GDP exchange drops by approximately 50%. The persisting exchange activity of calcium-permeable vesicles demonstrates that high internal calcium concentrations are not required for the transfer of the protein-bound phosphoryl group to NDP during NTP-NDP exchange.     

Interaction of acetyl phosphate and carbamyl phosphate with plant phosphoenolpyruvate carboxylase.

Acetyl phosphate produced an increase in the maximum velocity (Vmax. for the carboxylation of phosphoenolpyruvate catalysed by phosphoenolpyruvate carboxylase. The limiting Vmax. was 22.2 mumol X min-1 X mg-1 (185% of the value without acetyl phosphate). This compound also decreased the Km for phosphoenolpyruvate to 0.18 mM. The apparent activation constants for acetyl phosphate were 1.6 mM and 0.62 mM in the presence of 0.5 and 4 mM-phosphoenolpyruvate respectively. Carbamyl phosphate produced an increase in Vmax. and Km for phosphoenolpyruvate. The variation of Vmax./Km with carbamyl phosphate concentration could be described by a model in which this compound interacts with the carboxylase at two different types of sites: an allosteric activator site(s) and the substrate-binding site(s). Carbamyl phosphate was hydrolysed by the action of phosphoenolpyruvate carboxylase. The hydrolysis produced Pi and NH4+ in a 1:1 relationship. Values of Vmax. and Km were 0.11 +/- 0.01 mumol of Pi X min-1 X mg-1 and 1.4 +/- 0.1 mM, respectively, in the presence of 10 mM-NaHCO3. If HCO3- was not added, these values were 0.075 +/- 0.014 mumol of Pi X min-1 X mg-1 and 0.76 +/- 0.06 mM. Vmax./Km showed no variation between pH 6.5 and 8.5. The reaction required Mg2+; the activation constants were 0.77 and 0.31 mM at pH 6.5 and 8.5 respectively. Presumably, carbamyl phosphate is hydrolysed by phosphoenolpyruvate carboxylase by a reaction the mechanism of which is related to that of the carboxylation of phosphoenolpyruvate.     

Cardiac sarcolemmal and sarcoplasmic reticulum membrane vesicles exhibit distinctive (Ca-Mg)-ATPase substrate specificities

The nucleoside 5'-triphosphate (NTP) substrate specificities for Ca-stimulated ATPase and ATP-dependent Ca2+ uptake activities have been examined in cardiac sarcolemma (SL) and sarcoplasmic (SR) membrane vesicles. The results indicate that SL membrane vesicles exhibit a much narrower range of NTP substrate specificities than SR membranes. In SR membrane vesicles, the Ca-stimulated Mg-dependent hydrolysis of ATP and dATP occurred at nearly equivalent rates, whereas the rates of hydrolysis of GTP, ITP, CTP, and UTP ranged from 16-33% of that for ATP. All of the above nucleotides also supported Ca2+ transport into SR vesicles; dATP was somewhat more effective than ATP while GTP, ITP, CTP, and UTP ranged from 28-30% of the activity for ATP. In the presence of oxalate, the initial rate of Ca accumulation with dATP was 4-fold higher than for ATP, whereas the activity for GTP, ITP, CTP, and UTP ranged from 35-45% of that for ATP. For the SL membranes, Ca-activated dATP hydrolysis occurred at 60% of the rate for ATP; GTP, ITP, CTP, and UTP were hydrolyzed by the SL preparations at only 7-9% of the rate for ATP. NTP-dependent Ca2+ uptake in SL membranes was supported only by ATP and dATP, with dATP 60% as effective as ATP. GTP, ITP, CTP, and UTP did not support the transport of Ca2+ by SL vesicles. The results indicate that the SL and SR membranes contain distinctly different ATP-dependent Ca2+ transport systems.     

An ATP-driven Cl- pump in the brain

EDTA-treated microsomes prepared from rat brain mainly consisted of sealed membrane vesicles 200-500 nm in diameter and were rich in both Cl- -ATPase and Na+,K+-ATPase activities. Such Cl- -ATPase-rich membrane vesicles accumulated Cl- in an ATP-dependent and osmotically reactive manner in the presence of 1 nM ouabain. The Cl- uptake was maximally stimulated by ATP with a Km value of 1.5 mM; GTP, ITP, and UTP partially stimulated Cl- uptake, but CTP, beta, gamma-methylene ATP, ADP, and AMP did not. The ATP-dependent Cl- uptake was accelerated by an increase in the medium Cl- concentration with a Km value of 7.4 mM. Such stimulation of Cl- uptake by ATP was dependent on the pH of the medium, with an optimal pH of 7.4, and also on the temperature of the medium, with an optimal range of 37-42 degrees C. Ethacrynic acid dose dependently inhibited the ATP-dependent Cl- uptake with a concentration for half-maximal inhibition at 57 microM. N-ethylmaleimide (0.1 mM) completely inhibited and sodium vanadate (1 mM) partially inhibited the ATP-dependent Cl- uptake. The membrane vesicles did not accumulate H+ in the Cl- uptake assay medium. The ATP-dependent Cl- uptake profile agreed with that of Cl- -ATPase activity reported previously (Inagaki, C., Tanaka, T., Hara, M., and Ishiko, J. (1985) Biochem. Pharmacol. 34, 1705-1712), and this strongly supports the idea that Cl- -ATPase in the brain actively transports Cl-.     

Ca2+-dependent ATPases in the basolateral membrane of rat kidney cortex

The basolateral segment of the rat renal tubular plasma membrane possesses Ca2+-dependent ATPase activity which was independent of Mg2+. Two kinetic forms were found: one, was a high affinity (apparent Km for free Ca2+ of 172 nM) low capacity (Vmax of 144 nmol of Pi X min-1 mg-1 protein) type; the other, had low affinity (apparent Km of 25 microM) and high capacity (896 nmol of Pi X min-1 X mg-1 protein). Mg2+ inhibited both Ca2+-ATPases. The high affinity enzyme exhibited positive cooperativity with respect to ATP, with a n value of 1.6. Ca2+-ATPase activity was not affected by calmodulin and was not inhibited by vanadate. On the other hand, both high and low affinity Ca2+-ATPase activities were increased when 1,25-dihydroxycholecalciferol was given to vitamin D-deficient rats. Kinetically, the enhanced activities were due to an increase in the Vmax values; the apparent affinities for free Ca2+ were not changed. The physiological function of the vitamin D-sensitive, Mg+-independent, Ca2+-ATPase activities remains to be established.