Membrane adenosine triphosphatase activities in rat pancreas

Crocin protects against cardiotoxicity induced by doxorubicin through TLR-2/NF-κB signal pathway in vivo and vitro

Doxorubicin (DOX) is widely used to treat multiple of tumors, but its clinical trials are allied with some serious adverse events mainly cardiac functional abnormalities. So the objective of our investigation is to identify the cardioprotective action of crocin (CRO), a natural compound derived from saffron, against DOX-induced cardiotoxicity. CRO was injected intraperitoneally (i.p.) to rats for sixconsecutive days and DOX (i.p.) was administered on the fourth day. H9c2 cells were treated with DOX for 24 h after being pre-treated by CRO for 2 h. CROreduced tachycardiaand J-point elevation,decreased the levelsof serum creatine kinase, lactate dehydrogenase,glutamic-oxalacetic transaminase and glutamic-pyruvic transaminase.CRO exerted positive effect on DOX-induced ROS productionand changes of oxidative stress biomarkers. CRO significantlydecreased intracellular Ca²⁺ concentration andincreased mitochondria membrane potentialin H9c2 cells. CRO also resisted the DOX-induced high expressionof tumor necrosis factor-αand interleukin-6, inhibitedapoptosisand improved the abnormal expression levels of Bcl-2, Bax and Caspase-3 proteins.CRO obviously restrained DOX-mediatedhigh expression of toll-like receptor-2 (TLR-2) and nuclear factor kappa-B (NF-κB) in ventricular tissue. Inbrief,CRO distinctly restrained DOX-mediated cardiotoxicity by inhibiting oxidative stress, inflammation, apoptoticandredressingcardiomyocyte calcium dyshomeostasis and mitochondria damage.These cardioprotective effects may berelated closely with the TLR2/NF-κB pathway.

Toxicity and effects of four insecticides on Na+, K+-ATPase of western flower thrips, Frankliniella occidentalis

Western flower thrips (WFT), Frankliniella occidentalis, has become an important pest of vegetables worldwide, due to its economic damage to crop production. In order to control WFT, chemical insecticides are widely used. However, WFT has developed a high resistance against many kinds of insecticides. Na⁺, K⁺-ATPase, playing an important role in the ionic transmission across the membrane, is commonly considered to be the target of several xenobiotic compounds. However, whether the Na⁺, K⁺-ATPase can be used as one of the target sites for controlling WFT is still unknown. In this study, resistance levels of WFT to four insecticides (chlorpyrifos, beta cypermethrin, abamectin, and thiamethoxam) were measured. It was found that all four insecticides exhibited significant inhibitory effects on WFT, especially on nymphs. The activity of Na⁺, K⁺-ATPase was estimated after the treatment of four insecticides. Additionally, mRNA expression levels of three Na⁺, K⁺-ATPase α-subunit isoforms (X1, X2 and X3) were detected using RT-qPCR. The transcription profile of three Na⁺, K⁺-ATPase α-subunit isoforms were diverse after treatment by these four insecticides, which indicated that these isoforms might play different roles in the tolerance to insecticides. The results suggested that Na⁺, K⁺-ATPase can obviously be inhibited by these four classes of insecticide, and may serve as the new target for controlling WFT.

Motility of Enzyme-Powered Vesicles

Autonomous nanovehicles powered by energy derived from chemical catalysis have potential applications as active delivery agents. For in vivo applications, it is necessary that the engine and its fuel, as well as the chassis itself, be biocompatible. Enzyme molecules have been shown to display enhanced motility through substrate turnover and are attractive candidates as engines; phospholipid vesicles are biocompatible and can serve as cargo containers. Herein, we describe the autonomous movement of vesicles with membrane-bound enzymes in the presence of the substrate. We find that the motility of the vesicles increases with increasing enzymatic turnover rate. The enhanced diffusion of these enzyme-powered systems was further substantiated in real time by tracking the motion of the vesicles using optical microscopy. The membrane-bound protocells that move by transducing chemical energy into mechanical motion serve as models for motile living cells and are key to the elucidation of the fundamental mechanisms governing active membrane dynamics and cellular movement.

Insights from the proteome profile of Phytophthora capsici in response to the novel fungicide SYP-14288

Phytophthora capsica is a destructive oomycete plant pathogen that causes huge losses to crop production worldwide. However, the novel fungicide SYP-14288 has shown excellent activity against various stages of the oomycete life cycle as well against fungal plant pathogens. The current study utilized isobaric tags for relative and absolute quantitation technology to generate proteome profiles of P. capsici in the presence or absence of SYP-14288 in order to gain a greater understanding of the SYP-14288 mode of action. A total of 1,443 individual proteins were identified during the investigation, of which 599 were considered to have significantly altered expression. Further investigation using Cluster of Orthologous Groups of proteins analysis and Kyoto Encyclopedia of Genes and Genomes pathway analysis indicated most of the proteins with altered expression were associated with carbohydrate metabolism, energy metabolism and their downstream biological functions, especially with regard to oxidoreductase activity and subsequent adenosine triphosphate (ATP) production associated pathways. Quantitative expression analysis using qRT-PCR validated the proteomic data. These results seem to indicate that SYP-14288 treatment caused a shift in energy metabolism that resulted in the activation of compensatory mechanisms affecting carbohydrate and lipid metabolism. The study also found evidence that the up-regulation of transmembrane transporters and proteins associated with stress response might also be coopted to compensate for the disrupted proton gradient and other downstream effects. Taken together these results provide strong evidence that SYP-14288 has a similar mode of action to the oxidative phosphorylation uncoupler Fluazinam but further investigation, including molecular studies, is required to completely characterize the SYP-14288 mode of action in P. capsici. However, the proteomic data collected in the current study does provide important insight into the overall effect of SYP-14288 in P. capsici, which could be useful for the registration and application of this novel fungicide.

Effect of fermented Cordyceps sinensis on doxorubicin‑induced cardiotoxicity in rats

Cordyceps sinensis (CS) is a prominent medicinal herb in traditional Chinese medicine, and fermented CS is frequently used as a substitute for natural CS. Doxorubicin (DOX), an antitumor drug used in chemotherapy, is limited by its poor cardiotoxicity. The aim of the present study was to evaluate the protective effect of fermented CS against DOX-induced cardiotoxicity and the potential underlying mechanisms. Male Sprague-Dawley rats (180–200 g) were randomly assigned to seven different treatment groups: Normal control, DOX control, DOX+captopril (0.05 g/kg), 0.75, 1.5 and 3 g/kg DOX+CS, and the CS (1.5 g/kg) control. Histopathological changes, cardiac energy metabolism, cyclic adenosine monophosphate (cAMP) signaling and the associated mRNA expression of AMP-activated protein kinase (AMPK) were then evaluated. Fermented CS decreased the left ventricular weight index, heart weight index and mortality; however, it increased diastolic blood pressure and mean arterial pressure. In addition, it shortened the duration of the QRS complex and Sα-T segment, decreased serum creatine kinase (CK) and aspartate aminotransferase activity, inhibited histopathological changes and reduced brain natriuretic peptide content. Treatment with fermented CS also increased the activities of superoxide dismutase and glutathione peroxidase, reduced malondialdehyde content, increased the mitochondrial activities of Na⁺K⁺-adenosine 5′-triphosphate (ATP) ase, Ca²⁺Mg²⁺-ATPase and CK, and increased the creatine phosphate/ATP ratio and AMP/ATP ratio. Furthermore, it decreased the ATP/adenosine 5′-diphosphate (ADP) ratio, upregulated AMPKα2 expression, reduced the activity of serum phosphodiesterases (PDEs) and increased myocardial cAMP content. The results of the present study demonstrated that fermented CS attenuated DOX-induced cardiotoxicity by inhibiting myocardial hypertrophy and myocardial damage, ameliorating systolic function and the antioxidant enzyme system, improving cardiac energy metabolism, depressing the activities of PDEs, and by upregulating the cAMP and AMPK signaling pathways. Thus, fermented CS may be a candidate for the prevention of DOX-induced cardiotoxicity, cardiac energy impairment and against a number of cardiac diseases.

Doxorubicin toxicity changes myocardial energy metabolism in rats

BACKGROUND

Doxorubicin (DOX) is an antitumor antibiotics used against malignancies. But its toxicity limits the therapy of DOX.

OBJECTIVE

The purpose of this study was to evaluate DOX toxicity and the alteration of energy metabolism after short term and long term treatment.

METHODS

Male Sprague-Dawley rats were randomly assigned to four groups: Short term control group, short term DOX treatment group, long term control group and long term DOX treatment group. In short term treated group, rats were injected with DOX i.p. at a dose of 2.5 mg/kg every 48 h for six equal injections. In long term, treated group, rats were tail-intravenously injected with DOX at a dose of 3 mg/kg once a week for four weeks. At the end of the experiment, histopathological changes, general blood biomarkers, endogenous antioxidant enzymes, cardiac energy metabolism and related mRNA expression of AMPK signal pathway were determined.

RESULTS

DOX induced prominent oxidative stress, a higher mortality rate, histological and ECG changes, obvious cardiac hypertrophy, acute cardiac damage and cardiac energy impairment in short term treatment rats. In long term treatment rats, DOX caused serious nephropathy and systolic dysfunction, terrible cardiac energy impairment, clear alteration of substrate utilization and AMPK signal pathway.

CONCLUSION

DOX treatment can induce different damages after short term and long term treatment. In short term treatment group, rats experienced a terrible mortality rate about 40%, the acute cardiac damage, cardiac energy impairment and an early heart failure which are potential connected with reduction of glucose utilization. In the long term treatment group, serious nephropathy and obvious changes of mRNA expressions of AMPK signal pathway were observed. Meanwhile, the serious cardiac energy impairment and substrate utilization alteration denote an obviously heart failure. This study could be helpful to develop therapy strategies of DOX complications for clinical application.

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.

The metal coordination of sCD39 during ATP hydrolysis

BACKGROUND

The hydrolysis of ATP and ADP by ecto-nucleoside triphosphate diphosphohydrolase 1 (CD39) requires divalent cations, like Ca2+ and Mg2+. In spite of considerable work, it is not clear whether divalent cations bind to the enzyme in the absence of nucleotide or only as nucleotide-Me+2 complex. Here we study the protein ligands for Me+2.

RESULTS

When VO2+ was used as a substitute for Ca2+, the ATPase activity of soluble CD39 was 25% of that with Ca2+ as cofactor. Protein ligands of the VO2+-nucleotide complex bound to the catalytic site of soluble CD39 were characterized by electron paramagnetic resonance (EPR) spectroscopy. The EPR spectrum contained one species designated T with VO2+-AMPPNP as ligand. Two species D1 and D2 were observed when VO2+-AMPCP was bound to soluble CD39. The results suggest that species D1 and D2 represent the metal-ADP complexes at the catalytic site of soluble CD39 corresponding to the intermediate formed during ATP hydrolysis and the substrate for further hydrolysis, respectively.

CONCLUSIONS

VO2+ can functionally substitute for Ca2+ as a cofactor of sCD39, and it produces four different EPR features when bound in the presence of different nucleotides or in the absence of nucleotide. The metal coordination for each conformation corresponding to each EPR species is proposed, and the mechanism of sCD39 catalysis is discussed.

Tissue-specific distribution and modulatory role of the gamma subunit of the Na,K-ATPase

The Na,K-ATPase comprises a catalytic alpha subunit and a glycosylated beta subunit. Another membrane polypeptide, gamma, first described by Forbush et al. (Forbush, B., III, Kaplan, J. H., and Hoffman, J. F. (1978) Biochemistry 17, 3667-3676) associates with alpha and beta in purified kidney enzyme preparations. In this study, we have used a polyclonal anti-gamma antiserum to define the tissue specificity and topology of gamma and to address the question of whether gamma has a functional role. The trypsin sensitivity of the amino terminus of the gamma subunit in intact right-side-out pig kidney microsomes has confirmed that it is a type I membrane protein with an extracellular amino terminus. Western blot analysis shows that gamma subunit protein is present only in membranes from kidney tubules (rat, dog, pig) and not those from axolemma, heart, red blood cells, kidney glomeruli, cultured glomerular cells, alpha1-transfected HeLa cells, all derived from the same (rat) species, nor from three cultured cell lines derived from tubules of the kidney, namely NRK-52E (rat), LLC-PK (pig), or MDCK (dog). To gain insight into gamma function, the effects of the anti-gamma serum on the kinetic behavior of rat kidney sodium pumps was examined. The following evidence suggests that gamma stabilizes E1 conformation(s) of the enzyme and that anti-gamma counteracts this effect: (i) anti-gamma inhibits Na,K-ATPase, and the inhibition increases at acidic pH under which condition the E2(K) --> E1 phase of the reaction sequence becomes more rate-limiting, (ii) the oligomycin-stimulated increase in the level of phosphoenzyme was greater in the presence of anti-gamma indicating that the antibody shifts the E1 left and right arrow left and right arrow E2P equilibria toward E2P, and (iii) when the Na+-ATPase reaction is assayed with the Na+ concentration reduced to levels (</=2 mM) which limit the rate of the E1 --> --> E2P transition, anti-gamma is stimulatory. These observations taken together with evidence that the pig gamma subunit, which migrates as a doublet on polyacrylamide gels, is sensitive to digestion by trypsin, and that Rb+ ions partially protect it against this effect, indicate that the gamma subunit is a tissue-specific regulator which shifts the steady-state equilibria toward E1. Accordingly, binding of anti-gamma disrupts alphabeta-gamma interactions and counteracts these modulatory effects of the gamma subunit.

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.

Effect of carbodiimides on the activity of Mg(2+)-ATPase of slow-twitch muscle microsomal membranes

1. The hydrophobic N,N'-dicyclohexylcarbodiimide (DCCD) inhibits the activity of Mg(2+)-ATPase of slow-twitch muscle microsomal fraction. 2. The inhibition is dependent on time and concentration, with half-maximal inhibition occurring at 0.4 mM concentration of carbodiimide after a 0.5 hr incubation at room temperature. 3. ATP does not protect against the inhibition. 4. Two water-soluble carbodiimides, 1-cyclohexyl-3-(2-morpholinoethyl)-carbodiimide (CMCD) and 1-ethyl-3(3-dimethylaminopropyl)-carbodiimide (EDCD), are not inhibitory. 5. Inhibition of Mg(2+)-ATPase activity by DCCD is accompanied by covalent incorporation of the radioactive agent into the partially purified enzyme preparation.

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.

The localization of (Ca2+ or Mg2+)-ATPase in plasma membranes of renal proximal tubular cells

Basolateral and brush-border vesicles from pig kidney cortex were prepared by differential centrifugation followed by free-flow electrophoresis. A low-affinity (Ca2+ or Mg2+)-ATPase which co-migrated with alkaline phosphatase was demonstrated. A considerable enrichment (by a factor of 10) of this ATPase activity was only observed in the brush-border and not in the basolateral membrane fractions. Maximal stimulation of this brush-border enzyme by Ca2+ was achieved when the ratio of Ca2+ to ATP reached a value between 1 and 2. The enzyme was not inhibited by excess Ca2+ or Mg2+. A kinetic analysis of the azide-insensitive (Ca2+ or Mg2+)-ATPase gave a Km of 0.43 mM for Ca-ATP and of 0.14 mM for Mg-ATP.

Gastric (H+,K+)-ATPase

Gastric acid secretion results from the activity of a specific ATPase, the (H+,K+)-ATPase. This enzyme, discovered in 1973, exchanges H+ for K+. It has two ATP binding sites, both involved in enzyme activity, whose affinities vary as a function of the H+ and K+ concentrations. Hydrolysis of ATP at the highest affinity site leads to the synthesis of a covalent aspartyl phosphate which accumulates in the absence of K+. The presence of this cation accelerates dephosphorylation resulting in the stimulation of ATPase (and PNPPase) activity. The structure of membranous (H+,K+)-ATPase is poorly defined. n-Octylglucoside solubilizes an active enzyme of 390-420 kDa which can be partly depolymerized using cholate. The monomer, characterized in SDS has a 95 kDa molecular mass and is inactive. In the presence of magnesium, (H+,K+)-ATPase catalyzes the active and neutral exchange of H+ for K+ at the expense of ATP. In the absence of ATP, (H+,K+)-ATPase acts as a passive transporter exchanging K+ for K+ at maximal rate and H+ for K+ at a 20 times slower rate.

Partial purification of a nucleoside triphosphatase from the inner membrane of the chloroplast envelope of pea

A Mg2+-NTPase has been partially purified from the inner membrane of the pea chloroplast envelope. Isolated envelope membranes were solubilized with Triton X-100 and fractionated by DEAE-Sephadex chromatography, followed by ultrafiltration and sucrose density gradient centrifugation. An approximate 35-fold increase in the specific activity of the vanadate and sodium fluoride sensitive NTPase was obtained. Analysis of the partially purified NTPase by sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed a single 37-kDa polypeptide that appeared to be associated with the activity. In support of this identification, it is demonstrated that the 37-kDa polypeptide can be photolabeled with 8-azido-ATP.

Cation-stimulated ATPase activity in purified plasma membranes from tobacco hornworm midgut

Purified goblet cell apical membranes from Manduca sexta larval midgut exhibit a specific ATPase activity approx. 20-fold higher than that in the 100 000 X g pellet of a midgut homogenate. The already substantial ATPase activity in this plasma membrane segment is doubled in the presence of 20-50 mM KCl. At ATP concentrations ranging from 0.1 to 3.0 mM, the presence of 20 mM KCl leads to a 10-fold increase in the enzyme's affinity for ATP. ATPase activity is greatest at a pH of approx. 8. In addition to ATP, GTP serves as a substrate, but CTP, ADP, AMP and p-nitrophenyl phosphate do not. Either Mg2+ or Mn2+ is required for activity and cannot be replaced by Ca2+ or Zn2+. The ATPase activity of goblet cell apical membranes is inhibited by neither the typical (Na+ + K+)-ATPase inhibitors, ouabain and orthovanadate, nor by the typical mitochondrial F1F0-ATPase inhibitors, azide and oligomycin. Although 1.5 microM DCCD is ineffective, 150 microM DCCD leads to total inhibition of ATPase activity. The ATPase activity of goblet cell apical membranes is stimulated not only by K+, but also, in order of decreasing effectiveness, by Rb+, Li+, Na+ and even Mg2+. Replacement of Cl- by Br-, F- and HCO3- has less influence than variation of the cations. However, replacement of Cl- by NO3- inhibits strongly this ATPase activity. The ATPase activity described above is characteristic of the alkali metal ion pump containing apical membranes of goblet cells and is not enhanced to a similar degree in other purified midgut epithelial cell plasma membrane segments. Its localization, its broad cation specificity and its insensitivity to ouabain all mimic properties of active ion transport by the lepidopteran midgut and suggest this ATPase as a possible key component of the lepidopteran electrogenic alkali metal ion pump.

Comparison of the rat microsomal Mg-ATPase of various tissues

The microsomal Mg-ATPase from various rat tissues was compared. After fractionating the microsomal vesicles by sucrose gradient centrifugation, the highest specific activity of the Mg-ATPase was found in the low-density vesicles which contained plasma membrane. A large fraction (25-90%) of the microsomal Ca-independent Mg-ATPase found in each tissue had the following properties: (1) the Km for ATP was 0.2 mM; (2) the rate of ATP hydrolysis by the Mg-ATPase was nonlinear due to an ATP-stimulated inactivation of the enzyme; (3) wheat germ agglutinin, concanavalin A, glutaraldehyde, and antiserum prevented inactivation induced by ATP or AdoPP[NH]P; (4) detergents at relatively low detergent:protein ratios increased the rate of inactivation with little change in the initial rate of ATP hydrolysis; (5) the Mg-ATPase was inactivated by irradiation in the presence of 8-azido ATP. (6) in addition to ATP, the Mg-ATPase was able to hydrolyze CTP, GTP, UTP, ITP, and GTP but was unable to hydrolyze any of the 10 nonnucleotide phosphocompounds which were tested; (7) the bivalent cation requirement of the Mg-ATPase could be provided by Mg2+, Ca2+, Mn2+, Zn2+, or Co2+ but the enzyme was inactive in the presence of Cu2+, Sr2+, Ba2+, or Be2+; (8) the Mg-ATPase activity was not altered by ionophores or inhibitors of the Na,K-ATPase, the Ca,Mg-ATPase or the mitochondrial F1ATPase. These data suggest that a major portion of the microsomal, basal Mg-ATPase activity is due to one unique enzyme found in most if not all tissues.

Partial purification and characterization of rabbit-kidney brush-border (Ca2+ or Mg2+)-dependent adenosine triphosphatase

The ATPase activity of rabbit-kidney brush border can be activated almost equally well by Ca2+ and Mg2+ and, therefore, should be called (Ca2+ or Mg2+)-ATPase. This enzyme was solubilized and enriched 14-fold by the following steps: pretreatment with papain removed 69% of alkaline phosphatase without attacking a significant portion of the ATPase activity. Addition of 1% cholate removed 65% of the protein but no ATPase activity. The combination of cholate (0.5%) and deoxycholate (0.4%) solubilized most of the ATPase activity and most of the remaining protein. A column chromatography of the extract on Sepharose CL-2B resulted in an 6.5-fold increase of specific ATPase activity. A precipitation by ammonium sulfate (40% saturation) produced an additional 1.9-fold increase. The yield of this partial purification was 16%. Towards the nucleotides UTP and GTP the enzyme showed an activity slightly higher, and towards ITP and CTP an activity slightly lower than that with ATP. ADP was split about half as fast as ATP. AMP was not accepted by the enzyme. Replacing MgCl2 by CaCl2 resulted in an ATPase activity of 92% of that with MgCl2. Using calcium- and magnesium-ATP as substrates, apparent Km values of 0.22 and 0.33 mM, respectively, were obtained. The gel electrophoresis revealed the enrichment of a protein with an apparent Mr of 95000 and also that of microvillus actin.

Characterization of the ouabain-insensitive ATPase activity of rat liver plasma membranes

Very little is currently known about the ouabain-insensitive ATPase activity of the liver plasma membrane; we have therefore characterized it in plasma membranes from rat liver prepared using two different isolation techniques. Greater than 85% of ATPase activity in both preparations was ouabain-insensitive. Based on the effects of multiple inhibitors, including dicyclohexylcarbodiimide (DCCD), N-ethylmaleimide (NEM), vanadate, and oligomycin, the ouabain-insensitive ATPase activity of rat liver plasma membranes consists of a family of ATP-hydrolysing enzymes. Ouabain-insensitive ATPase activity was stimulated by HCO3-in both plasma membranes (13%) and mitochondria (69%) over the range of 7.5 to 9.0, and HCO3--stimulation was similarly inhibited by oligomycin in both preparations. A fraction of the ouabain-insensitive ATPase of liver plasma membranes is inhibited by DCCD and is resistent to inhibition by oligomycin; these characteristics are similar to those of the non-mitochondrial H+-ATPases recently described in lysosomes, endosomes, clathrin-coated vesicles, and Golgi from liver and other cell types.

Evidence that Mg2+- or Ca2+-activated adenosine triphosphatase in rat pancreas is a plasma-membrane ecto-enzyme

Preparations of enzymically dispersed rat pancreatic cells hydrolyse externally added nucleoside triphosphates and diphosphates at high rates in the presence of Mg2+ or Ca2+. The lack of response to specific inhibitors and activators differentiates this hydrolytic activity from that of other well-characterized ion-transporting ATPases. Studies based on inactivation of this hydrolytic activity by the covalently reacting, slowly permeating probe diazotized sulphanilic acid indicated that this nucleoside tri- and di-phosphatase is primarily a plasma-membrane ecto-enzyme. It is the major ATPase activity associated with intact cells, homogenates and isolated plasma-membrane fractions. Concanavalin A stimulates this ATPase activity of intact cells and isolated plasma-membrane fractions. The insensitivity of this ATPase activity to univalent ions and inhibitors of pancreatic electrolyte secretion, taken together with the evidence that the active site is externally located, suggests that this enzyme is not directly involved in HCO3- secretion in the pancreas. Its actual function remains unknown.

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.

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.