Characterization of (Mg,Ca)-ATPase activity in rat pancreatic plasma membranes

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

The E-NTPDase family of ectonucleotidases: Structure function relationships and pathophysiological significance

Ectonucleotidases are ectoenzymes that hydrolyze extracellular nucleotides to the respective nucleosides. Within the past decade, ectonucleotidases belonging to several enzyme families have been discovered, cloned and characterized. In this article, we specifically address the cell surface-located members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase/CD39) family (NTPDase1,2,3, and 8). The molecular identification of individual NTPDase subtypes, genetic engineering, mutational analyses, and the generation of subtype-specific antibodies have resulted in considerable insights into enzyme structure and function. These advances also allow definition of physiological and patho-physiological implications of NTPDases in a considerable variety of tissues. Biological actions of NTPDases are a consequence (at least in part) of the regulated phosphohydrolytic activity on extracellular nucleotides and consequent effects on P2-receptor signaling. It further appears that the spatial and temporal expression of NTPDases by various cell types within the vasculature, the nervous tissues and other tissues impacts on several patho-physiological processes. Examples include acute effects on cellular metabolism, adhesion, activation and migration with other protracted impacts upon developmental responses, inclusive of cellular proliferation, differentiation and apoptosis, as seen with atherosclerosis, degenerative neurological diseases and immune rejection of transplanted organs and cells. Future clinical applications are expected to involve the development of new therapeutic strategies for transplantation and various inflammatory cardiovascular, gastrointestinal and neurological diseases.

Rat pancreas secretes particulate ecto-nucleotidase CD39

In exocrine pancreas, acini release ATP and the excurrent ducts express several types of purinergic P2 receptors. Thereby, ATP, or its hydrolytic products, might play a role as a paracrine regulator between acini and ducts. The aim of the present study was to elucidate whether this acinar-ductal signalling is regulated by nucleotidase(s), and to characterize and localize one of the nucleotidases within the rat pancreas. Using RT-PCR and Western blotting we show that pancreas expresses the full length ecto-nucleoside triphosphate diphosphohydrolase, CD39. Immunofluorescence shows CD39 localization on basolateral membranes of acini and intracellularly. In small intercalated/ interlobular ducts, CD39 immunofluorescence was localized on the luminal membranes, while in larger ducts it was localized on the basolateral membranes. Upon stimulation with cholecystokinin-octapeptide-8 (CCK-8), acinar CD39 relocalizes in clusters towards the lumen and is secreted. As a result, pancreatic juice collected from intact pancreas stimulated with CCK-8 contained nucleotidase activity, including that of CD39, and no detectable amounts of ATP. Anti-CD39 antibodies detected the full length (78 kDa) CD39 in pancreatic juice. This CD39 was confined only to the particulate and not to the soluble fraction of CCK-8-stimulated secretion. No CD39 activity was detected in secretion stimulated by secretin. The role of secreted particulate, possibly microsomal, CD39 would be to regulate intraluminal ATP concentrations within the ductal tree. In conclusion, we show a novel inducible release of full length particulate CD39, and propose its role in the physiological context of pancreatic secretion.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Relationship between Ca(2+)-transport and ATP hydrolytic activities in guinea-pig pancreatic acinar plasma membranes

Partially purified plasma membrane fractions were prepared from guinea-pig pancreatic acini. These membrane preparations were found to contain an ATP-dependent Ca(2+)-transporter as well as a heterogenous ATP-hydrolytic activity. The Ca(2+)-transporter showed high affinity for Ca2+ (KCa2+ = 0.04 +/- 0.01 microM), an apparent requirement for Mg2+ and high substrate specificity. The major component of ATPase activity could be stimulated by either Ca2+ or Mg2+ but showed a low affinity for these cations. At low concentrations, Mg2+ appeared to inhibit the Ca(2+)-dependent ATPase activity expressed by these membranes. However, in the presence of high Mg2+ concentration (0.5-1 mM), a high affinity Ca(2+)-dependent ATPase activity was observed (KCa2+ = 0.08 +/- 0.02 microM). The hydrolytic activity showed little specificity towards ATP. Neither the Ca(2+)-transport nor high affinity Ca(2+)-ATPase activity were stimulated by calmodulin. The results demonstrate, in addition to a low affinity Ca2+ (or Mg2+)-ATPase activity, the presence of both a high affinity Ca(2+)-pump and high affinity Ca(2+)-dependent ATPase. However, the high affinity Ca(2+)-ATPase activity does not appear to be the biochemical expression of the Ca(2+)-pump.

The stepwise hydrolysis of adenine nucleotides by ectoenzymes of rat renal brush-border membranes

Evidence is presented for the existence of ectoenzymes in rat renal cortical brush-border membrane vesicles that produce adenosine as a final product using either ATP, ADP or AMP as substrate. The enzymes are insensitive to levamisole, ouabain, oligomycin and N-ethylmaleimide, and have absolute requirement for divalent cations with following order of activation Mg2+ greater than Ca2+ greater than Mn2+ greater than Ba2+ greater than Zn2+. At least two separate enzymes can be distinguished. One is capable of hydrolyzing ATP, other nucleoside triphosphates and ADP, but not AMP. The enzyme is insensitive to concanavalin A. The other enzyme hydrolyzes AMP and is strongly inhibited by this lectin. Mg2(+)-stimulated ATP hydrolysis displays saturation kinetics which is not of the simple Michaelis-Menten type, but is biphasic with a high-affinity (K'm = 0.16 mM) and low-affinity site (K'm = 9.0 mM), respectively. The low-affinity site hydrolyzes ATP, ITP and GTP to a similar extent, whereas CTP and UTP with about 40% lower rate. The high-affinity site splits ATP much better than other nucleoside triphosphates. Hydrolysis of ADP follows simple Michaelis-Menten saturation kinetic with apparent Km = 0.38 +/- 0.06 mM. Inhibition, activation and substrate specificity studies indicate that nucleoside triphosphatase and nucleoside diphosphatase may reside on the same protein. Kinetics of the AMP hydrolysis is hyperbolic with apparent Km = 76 +/- 9 microM. The cascade of ectonucleotidases in the brush-border membrane of the proximal tubule may catalyze the degradation of filtered nucleotides into adenosine and phosphate, the compounds which are thereafter probably reabsorbed by separate transport systems.

Relation of ATPases in rat renal brush-border membranes to ATP-driven H+ secretion

In the presence of inhibitors for mitochondrial H+-ATPase, (Na+ + K+)- and Ca2+-ATPases, and alkaline phosphatase, sealed brush-border membrane vesicles hydrolyse externally added ATP demonstrating the existence of ATPases at the outside of the membrane ("ecto-ATPases"). These ATPases accept several nucleotides, are stimulated by Ca2+ and Mg2+, and are inhibited by N.N'-dicyclohexylcarbodiimide (DCCD), but not by N-ethylmaleimide (NEM). They occur in both brush-border and basolateral membranes. Opening of brush-border membrane vesicles with Triton X-100 exposes ATPases located at the inside (cytosolic side) of the membrane. These detergent-exposed ATPases prefer ATP, are activated by Mg2+ and Mn2+, but not by Ca2+, and are inhibited by DCCD as well as by NEM. They are present in brush-border, but not in basolateral membranes. As measured by an intravesicularly trapped pH indicator. ATP-loaded brush-border membrane vesicles extrude protons by a DCCD- and NEM-sensitive pump. ATP-driven H+ secretion is electrogenic and requires either exit of a permeant anion (Cl-) or entry of a cation, e.g., Na+ via electrogenic Na+/D-glucose and Na+/L-phenylalanine uptake. In the presence of Na+, ATP-driven H+ efflux is stimulated by blocking the Na+/H+ exchanger with amiloride. These data prove the coexistence of Na+-coupled substrate transporters, Na+/H+ exchanger, and an ATP-driven H+ pump in brush-border membrane vesicles. Similar location and inhibitor sensitivity reveal the identity of ATP-driven H+ pumps with (a part of) the DCCD- and NEM- sensitive ATPases at the cytosolic side of the brush-border membrane.

Ionic and substrate requirements of the high affinity calcium pumping ATPase in endoplasmic reticulum of pancreas

1. Calcium transport and ATPase activities were determined in microsomal vesicles from pancreatic tissue enriched in endoplasmic reticulum membranes. 2. Calcium transport and ATPase share the following properties: (i) magnesium was required with a K0.5 of 0.7 mM and maximal pumping ATPase activity at 5 mM Mg-ATP; (ii) at saturating magnesium concentrations, calcium increased ATP splitting activity up to three times with an apparent K0.5 close to 0.3 microM calcium; (iii) potassium stimulated the high calcium affinity Mg2+-dependent ATPase and calcium transport. 3. The properties of the calcium pumping system fulfil the cationic and substrate requirements from a physiological point of view.

The prostasome: its secretion and function in man

An intact organelle, the prostasome, is secreted by the acinar epithelial cell of the human prostate gland. The ultrastructural location of the prostasome is within membrane-bound storage vesicles in the epithelial cells. Prostasomes are delivered into the glandular lumen by an exocytotic event, which is preceded by fusion of adjacent membranes belonging to the storage vesicle and the epithelial cell. Alternatively, the storage vesicle can be translocated in toto from the cell interior into the acinar lumen through the plasma membrane. This latter event has been designated diacytosis. Both phenomena seem to occur with approximately equal frequency in the human prostate gland. An ATPase system that is Mg2+ and Ca2+-dependent is firmly linked to the membranes encasing the prostasomes. The ATPase system may be the molecular basis for vectorial transport of calcium into these organelles. Also a protein kinase activity is located in the membranes. An increase in membrane thickness was observed on phosphorylation. The physiologic function of the prostasomes is not known. They may be important for promoting forward motility of spermatozoa.

Electrogenic calcium transport in plasma membrane of rat pancreatic acinar cells

ATP-dependent 45Ca2+ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl2 and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na+,K+)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca2+ uptake was maximal with 10 and 2 mumol/liter free Ca2+, and half-maximal with 0.9 and 0.5 mumol/liter free Ca2+. It was maximal at 3 and 0.2 mmol/liter free Mg2+ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg2+ was replaced by Mn2+ or Zn2+ ATP-dependent Ca2+ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn2+ could replace Mg2+ for Ca2+ uptake by 20%. Other divalent cations such as Ba2+ and Sr2+ could not replace Mg2+ in Ca2+ uptake. Ca2+ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca2+ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca2+ uptake. The sequence was K+ greater than Rb+ greater than Na+ greater than Li+ greater than choline+ in plasma membranes and Rb+ greater than or equal to K+ greater than or equal to Na+ greater than Li+ greater than choline+ for rough endoplasmic reticulum. The anion sequence was Cl greater than or equal to Br greater than or equal to 1 greater than SCN greater than NO3 greater than isethionate greater than cyclamate greater than gluconate greater than SO2(4) greater than or equal to glutarate and Cl- greater than Br greater than gluconate greater than SO2(4) greater than NO3 greater than 1 greater than cyclamate greater than or equal to SCN, respectively. Ca2+ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K+ and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca2+ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca2+ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca2+ from the cell that might be involved in the regulation of the cytosolic Ca2+ level.

A Ca2+-stimulated adenosine triphosphatase in Golgi-enriched membranes of lactating murine mammary tissue

A membrane fraction isolated from lactating murine mammary tissue and enriched for the Golgi membrane marker enzyme galactosyltransferase exhibited Ca2+-stimulated ATPase activity (Ca-ATPase) in 20 microM-free Mg2+ and 10 microM-MgATP, with an apparent Km for Ca2+ of 0.8 microM. Exogenous calmodulin did not enhance Ca2+ stimulation, nor could Ca-ATPase activities be detected in millimolar total Mg2+ and ATP. When assayed with micromolar Mg2+ and MgATP the Ca-ATPases of skeletal-muscle sarcoplasmic reticulum and of calmodulin-enriched red blood cell plasma membranes were half-maximally activated by 0.1 microM- and 0.6 microM-Ca2+ respectively. All three Ca-ATPases were inhibited by similar micromolar concentrations of trifluoperazine, but the Golgi activity was unaffected by quercetin in concentrations which completely inhibited both the sarcoplasmic-reticulum and red-blood-cell enzymes. The results are consistent with the hypothesis that the high-affinity Ca-ATPase is responsible for the ATP-dependent Ca2+ transport exhibited by Golgi-enriched vesicles derived from lactating mammary gland [Neville, Selker, Semple & Watters (1981) J. Membr. Biol. 61, 97-105; West (1981) Biochim. Biophys. Acta 673, 374-386].

Characterization of calcium uptake into rough endoplasmic reticulum of rat pancreas

ATP-dependent Ca2+ uptake into isolated pancreatic acinar cells with permeabilized plasma membranes, as well as into isolated endoplasmic reticulum prepared from these cells, was measured using a Ca2+ -specific electrode and 45Ca2+. Endoplasmic reticulum was purified on an isopycnic Percoll gradient and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the rough endoplasmic reticulum RNA was enriched threefold and the typical marker for the plasma membrane Na+,K+(Mg2+)ATPase was decreased 20-fold. When different fractions of the Percoll gradient were compared, 45Ca2+ uptake correlated with the RNA content and not with the Na+,K+(Mg2+)ATPase activity. The characteristics of nonmitochondrial Ca2+ uptake into leaky isolated cells and 45Ca2+ uptake into isolated endoplasmic reticulum were very similar: Calcium uptake was maximal at 0.3 and 0.2 mmol/liter free Mg2+, at 1 and 1 mmol/liter ATP, at pH 6.0 and 6.5, and free Ca2+ concentration of 2 and 2 mumol/liter, respectively. Calcium uptake decreased at higher free Ca2+ concentration. 45Ca2+ uptake was dependent on monovalent cations (Rb+ greater than K+ greater than Na+ greater than Li+ greater than choline+) and different anions (Cl- greater than Br- greater than SO4(2-) greater than NO3- greater than I- greater than cyclamate- greater than SCN-) in both preparations. Twenty mmol/liter oxalate enhanced 45Ca2+ uptake in permeabilized cells 10-fold and in vesicles of endoplasmic reticulum, fivefold. Calcium oxalate precipitates in the endoplasmic reticulum of both preparations could be demonstrated by electron microscopy. The nonmitochondrial Ca2+ pool in permeabilized cells characterized in this study has been previously shown to regulate the cytosolic free Ca2+ concentration to 0.4 mumol/liter. Our results provide firm evidence that the endoplasmic reticulum plays an important role in the regulation of the cytosolic free Ca2+ concentration in pancreatic acinar cells.

The common occurrence of ATP diphosphohydrolase in mammalian plasma membranes

In the plasma membranes from several mammalian tissues (including normal and tumor tissues), a Mg2+ (or Ca2+)-dependent ATP phosphohydrolase activity is present in much greater amount than the (Na+ + K+)-ATPase. The ouabain-insensitive activity can be attributed to at least two enzymes, an ATPase (EC 3.6.1.3) and an ATP diphosphohydrolase (EC 3.6.1.5). The ATPase hydrolyzes ATP and other nucleoside triphosphates and is not inhibited by azide. The ATP diphosphohydrolase hydrolyzes both ATP and ADP (and other nucleoside tri- and diphosphates) and the hydrolysis of adenine nucleotides is strongly inhibited by 10 mM azide. The ratios of these two enzymes in the various membranes (as determined by the extent of azide inhibition) vary widely. The ATP diphosphohydrolase accounts for most of the Mg2+ (or Ca2+)-dependent ATP hydrolysis activity of the plasma membranes of liver (mouse), kidney (dog), two mouse sarcomas, and a human astrocytoma (xenograft in athymic mice). The ATPase is more dominant in the plasma membranes from mouse brain and human oat cell carcinoma. The widespread presence of the ATP diphosphohydrolase in plasma membrane from various types of tissues is demonstrated for the first time and is of particular interest in view of its relatively high activity in the plasma membranes of two sarcomas. The membrane-bound ATP diphosphohydrolase is characterized with respect to its metal ion activators, substrates, and inhibitors. These results should facilitate the distinction of this enzyme from other ATP hydrolyzing enzymes of plasma membranes in future investigations.

Secretion of calcium into milk: review

Milk calcium exists in bound and ionized forms. Bound calcium is associated both with casein micelles and complexed to citrate and phosphate. Ionized calcium in milk is 1 to 4 millimolar, at least 1000 times its postulated concentration in the mammary alveolar cell. For this reason active transport mechanisms are necessary for transfer of this nutrient to the lumen of the mammary alveolus. Evidence that the major active transport system is a calcium adenosine triphosphatase residing in the membrane of the Golgi secretory vesicle is summarized. This adenosine triphosphatase appears to be activated by calcium concentrations in the micromolar range, to require magnesium ions, and to operate by phosphorylation of a 100,000 dalton enzyme intermediate. Metabolic processes are required to maintain a low concentration of calcium within the cytosol of the mammary alveolar cell. Because no evidence for sodium/calcium exchange could be found in the mammary gland of the lactating mouse, we suggest that these processes operate through a calcium adenosine triphosphatase in the basolateral membrane of the cell. Decreased calcium in the alveolar lumina decreased the integrity of the barrier between blood and milk. It is postulated from observations in other secretory systems that an increase in cystolic activity calcium may play a role in lactogenesis.

Sequential hydrolysis of the gamma- and beta-phosphate groups of ATP by the ATP diphosphohydrolase from pig pancreas

The ATP diphosphohydrolase (EC 3.6.1.5) from pig pancreas hydrolyzes triphospho- and diphosphonucleosides. The reaction products of ATP hydrolysis are ADP, AMP and orthophosphate, but AMP accumulates at a faster rate than ADP. A time-course study showed a simultaneous breakdown of ATP and ADP with initial rates for ATP and ADP hydrolysis of 2.1 and 3.8 mumol/min per mg protein, respectively. However, the rates reached similar values toward the end of the incubation period. According to double reciprocal plots and Dixon plots, the Km values for ATP and ADP are similar, Vmax for ADP hydrolysis is twice the Vmax for ATP hydrolysis and both nucleotides are competitive inhibitors of the other with their Ki values similar to their Km. These results are consistent with a sequential hydrolysis of the two diphosphoester bonds of ATP: ATP first binds to the enzyme, its gamma-phosphate group is hydrolyzed and released, resulting in an enzyme-ADP complex which either breaks down to free enzyme and ADP or is further processed via hydrolysis of the beta-phosphate group, releasing free enzyme, AMP and Pi. The experimental data showed that the processing step is favored.

Calcium uptake into acini from rat pancreas: evidence for intracellular ATP-dependent calcium sequestration

Intracellular ATP-dependent Ca2+-sequestration mechanisms were studied in isolated dispersed rat pancreatic acini following treatment with saponin or digitonin to disrupt their plasma membranes. In the presence of 45Ca2+ concentrations less than 10(-6) mol/liter, addition of 5 mmol/liter ATP caused a rapid increase in 45Ca2+ uptake exceeding the control by fivefold. ADP mimicked the ATP effect by 50 to 60%, whereas other nucleotides such as AMP-PNP, AMP-PCP, CTP, UTP, ITP, GTP, cAMP and cGMP did not. Maximal ATP-promoted Ca2+ uptake was obtained at 10(-5) mol/liter Ca2+. Inhibition of Ca2+ uptake by mitochondrial inhibitors was dependent on the Ca2+ concentration, indicating the presence of different Ca2+ storage systems. Whereas the apparent half-saturation constant found for mitochondrial Ca2+ uptake was approximately 4.5 X 10(-7) mol/liter, in the presence of antimycin and oligomycin (nonmitochondrial uptake) it was approximately 1.4 X 10(-8) mol/liter. In the absence of Mg2+ both ATP- and ADP-promoted Ca2+ uptake was nearly abolished. The Ca2+ ionophore and mersalyl blocked Ca2+ uptake, Electron microscopy showed electron-dense precipitates in the rough endoplasmic reticulum of saponin-treated cells in the presence of Ca2+, oxalate and ATP, which were absent in intact cells and in saponin-cells without ATP or pretreated with A23187. The data suggest the presence of mitochondrial and nonmitochondrial ATP-dependent C2+ storage systems in pancreatic acini. The latter is likely to be located in the rough endoplasmic reticulum.

A comparative study of plasma membrane Mg2+ -ATPase activities in normal, regenerating and malignant cells

Plasma membranes have been prepared from rat normal liver cells, regenerating liver cells and Yoshida ascites hepatoma 66 cells after intact cells were first bound to polylysine-coated polyacrylamide beads, and the membrane-associated Mg2+ -ATPase activity was assayed directly on beads with membrane attached. With plasma membranes from normal liver cells, Km for ATP and V were found to be higher than those in regenerating liver cells and hepatoma cells. Vanadate caused a different sensitivity of the activity, without an effect in normal liver cells and with an inhibition in regenerating liver cells and hepatoma cells. The activity in normal and regenerating liver cells decreased with increasing temperature above 24-30 degrees C, while the activity in hepatoma cells continued to increase linearly to 37 degrees C. Unlike the enzyme in normal and regenerating liver cells, the hepatoma enzyme was shown to have a higher phase transition temperature and lower activation energies. In all three kinds of cells the activity was increased by the dephosphorylation of plasma membranes and unaffected by the phosphorylation. By means of histochemical Mg2+ -ATPase staining applied on polyacrylamide gels, at least three major bands which show the enzymic activity were visible in normal and regenerating liver and a single band was detected in hepatoma cells.

Evidence for the existence of actomyosin ATPase in the rat pancreas. Isolation and biochemical characterization

In a crude extract of rat pancreas, myosin was associated with a protein having the same electrophoretic mobility as actin. This myosin was purified after dissociation of the actomyosin complex with KI-ATP. On sodium dodecylsulfate/acrylamide gel electrophoresis, the isolated pancreatic myosin showed a major component of approximately 200 kDa, and two smaller components with apparent molecular weight of 22 and 15 kDa, respectively. This purified myosin exhibited high ATPase activity in the presence of K+ + EDTA or Ca2+ and very little activity in the presence of Mg2+. (K+ + EDTA)-ATPase activity showed one pH optimum at 8.0, while Ca2+-ATPase activity showed two pH optima at 6.0 and 9.0, respectively. (K+ + EDTA)-stimulated enzyme activity was specific for ATP whereas Ca2+-stimulated activity showed low specificity for nucleoside triphosphates.

Membrane adenosine triphosphatase activities in rat pancreas

The membrane ATPase activities present in rat pancreas were studied to investigate the possible role of ATPase enzymes in HCO3(-) secretion in the pancreas. It was found that all the HCO3(-)-sensitive (anion-sensitive) ATPase activity was accountable as pancreatic mitochondrial ATPase, thus supporting the view that a distinct plasma membrane 'bicarbonate-ATPase' is not involved in HCO3(-) secretion in pancreas. A remarkably high Mg+- and CA2+-requiring ATPase activity (30 mumol ATP hydrolysed/min per mg) was found in the plasma membrane fraction (rho = 1.10-1.13). This activity has been characterized in some detail. It is inhibited by p-fluorosulfonylbenzoyladenosine, an affinity label analogue of ATP and the analogue appears to label covalently a protein of Mr approximately 35 000. The (Ca2+ + Mg2+)-ATPase activity did not form a 'phosphorylated-intermediate' and was vanadate-insensitive. These and other tests have served to demonstrate that the (Ca2+ + Mg2+)-ATPase activity is different in properties from (Na+ + K+)-ATPase, Ca2+-ATPase, (H+ + K+)-ATPase or mitochondrial H+-ATPase. Apart from the (Ca2+ + Mg2+)-ATPase of plasma membrane and mitochondrial ATPase, the only other membrane ATPase activities noted were (Na+ + K+)-ATPase, which occurred in the same fractions as the (Ca2+ + Mg2+)-AtPase at rho = 1.10-1.13 and was of surprisingly low activity, and an ATPase activity in light membrane fractions (rho - 1.08-1.09) derived from zymogen granule membranes. At this time, therefore, there is no obvious candidate for an ATPase activity at the luminal surface of pancreatic cells which is directly involved in ion transport, but the results presented here direct attention to the high activity (Ca2+ + Mg2+)-ATPase in the plasma membrane fraction.