Properties and subcellular localization of adenosine diphosphatase in rat heart

Ecto-hydrolysis of adenine nucleotides in rat blood platelets are altered by ovariectomy

There is evidence that estrogen is associated with a reduction on cardiovascular disease risk through inhibition of platelet aggregation and action on vascular function. The process of haemostasis can also be affected by adenine nucleotides and adenosine. Consequently, regulation of enzymes that hydrolyze these nucleotides in the bloodstream is essential in the modulation of the processes of platelet aggregation, vasodilatation and coronary flow. Ecto-ATP diphosphohydrolase and ecto-5'-nucleotidase from platelets are enzymes related to nucleotide hydrolysis. In the present study, we examined the effect of ovariectomy (OVX) and estradiol replacement therapy (ER) on the activity of the enzymes that degrade adenine nucleotides in platelets of female rats. The OVX group significantly decrease the hydrolysis of ATP, ADP and AMP by 42, 52 and 29.3%, respectively, when compared to a control group. ER did not reverse the inhibition of nucleotide hydrolysis observed in OVX rats. Our findings indicate that hormonal deprivation affects ATP, ADP and AMP hydrolysis by platelets and consequently the level of these nucleotides and adenosine in the circulation. Since, ADP is the most important platelet agonist and recruiting agent present in the microenvironment of the thrombus, our findings was contribute to a better comprehension of the cardiovascular complications described in alterations of sexual hormonal status.

Ovariectomy and estradiol replacement therapy alters the adenine nucleotide hydrolysis in rat blood serum

The low prevalence of coronary heart disease in premenopausal women and its increase after menopause are well established. Many studies have suggested that steroid hormones can inhibit platelet aggregation, reducing the cardiovascular risk. In addition, a number of studies have shown an effect of estrogen on vascular function. The process of haemostasis and thrombus formation can be also affected by adenine nucleotides and adenosine. Consequently, the regulation of enzymes that hydrolyze these nucleotides in the bloodstream is essential in the modulation of the processes of platelet aggregation, vasodilatation and coronary flow. Thus, in this study, we examined the effect of ovariectomy (OVX), estradiol replacement therapy and the in vitro administration of 17beta-estradiol, dehydroisoandrosterone 3-sulfate (DHEAS) and pregnenolone (PREG) on the activity of the enzymes that degrade adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) in the blood serum of female rats. OVX significantly increased the hydrolysis of ATP, ADP and AMP, whilst phosphodiesterase activity was unchanged. Estradiol replacement therapy significantly decreased the hydrolysis of the adenine nucleotides and of the substrate marker of phosphodiesterase. In vitro, the addition of steroid hormones did not have any effect on the nucleotide hydrolysis by rat serum. These results suggest the presence of a strong relation between these enzymes and the hormonal system. In addition, the alterations observed are important, because these enzymes control the nucleotides/nucleosides ratio in the circulation and thus the events related to haemostasis.

Characterization and localization of an ATP diphosphohydrolase activity (EC 3.6.1.5) in sarcolemmal membrane from rat heart

In the present report we describe an ATP diphosphohydrolase (apyrase EC 3.6.1.5) in rat cardiac sarcolemma. It is Ca2+ dependent and is insensitive to ouabain, orthovanadate, N-ethylmaleimide (NEM), lanthanum, and oligomycin that are classical ATPase inhibitors. Sodium azide that is a mitochondrial inhibitor at low concentrations, did not affect the enzyme activity at 5.0 mM or below. In contrast, at high concentrations (> 10 mM) sodium azide inhibited the enzyme. Levamisole, a specific inhibitor of alkaline phosphatase and P1, P5-di(adenosine 5'-)pentaphosphate (Ap5A), a specific inhibitor of adenylate kinase did not inhibit the enzyme. Mercury chloride showed a parallel inhibition of the hydrolysis of both substrates of apyrase. Similar inhibition profiles are powerful evidence for a common catalytic site for the hydrolysis of both substrates. The enzyme has an optimum pH range of 7.5-8.0 and catalyzes the hydrolysis of triphospho- and diphosphonucleosides other than ATP or ADP. The apparent Km (Michaelis constant) and Vmax (maximal velocity) are 62.1 +/- 5.2 microM and 1255.7 +/- 178 micromol inorganic phosphate liberated/min/mg with ATP and 59.4 +/- 4.3 microM and 269.2 +/- 39 micromol inorganic phosphate liberated/min/mg with ADP. Enzyme markers indicated that this apyrase is associated with the plasma membrane. A deposition of lead phosphate granules on the outer surface of the sarcolemmal vesicles was observed by electron microscopy in the presence of either ATP or ADP as substrate. It is suggested that the ATP diphosphohydrolase could regulate the concentration of extracellular adenosine, and thus is important in the control of vascular tone and coronary flow.

Characterization of an ATP diphosphohydrolase activity (APYRASE, EC 3.6.1.5) in rat blood platelets

In the present report we describe an apyrase (ATP diphosphohydrolase, EC 3.6.1.5) in rat blood platelets. The enzyme hydrolyses almost identically quite different nucleoside di- and triphosphates. The calcium dependence and pH requirement were the same for the hydrolysis of ATP and ADP and the apparent Km values were similar for both Ca(2+)-ATP and Ca(2+)-ADP as substrates. Ca(2+)-ATP and Ca(2+)-ADP hydrolysis could not be attributed to the combined action of different enzymes because adenylate kinase, inorganic pyrophosphatase and nonspecific phosphatases were not detected under our assay conditions. The Ca(2+)-ATPase and Ca(2+)-ADPase activity was insensitive to ATPase, adenylate kinase and alkaline phosphatase classical inhibitors, thus excluding these enzymes as contaminants. The results demonstrate that rat blood platelets contain an ATP diphosphohydrolase involved in the hydrolysis of ATP and ADP which are vasoactive and platelet active adenine nucleotides.

Subcellular localization and properties of adenosine diphosphatase in human placenta

It was found that mitochondria from human placenta exhibited an ADPase activity with the following characteristics. The enzyme responsible for this activity was associated with the inner mitochondrial membrane. It was not released by treatment of the submitochondrial particles with solutions of high ionic strength. Maximal ADP hydrolysis was reached at pH 8. Specific inhibitors for alkaline phosphatase (L-phenylalanine), myokinase (P1,P5-di(adenosine-5')pentaphosphate), or 5'-nucleotidase (concanavalin A) did not decrease ADP hydrolysis. ATP synthesis from ADP by myokinase was about 13 nmol/mg/min, whereas ADP hydrolysis reached values around 500 to 550 nmol/mg/min, indicating that a myokinase-H+ATPase combination could not account for the observed rates of ADP hydrolysis. The activity was stimulated by Mg2+, but high concentrations of this cation produced inhibition. High ADP concentrations did not inhibit ADPase activity. Kinetic measurements of the activity in the submitochondrial particles showed that the true substrate was ADP-Mg. The kinetic studies showed V(app) values of 476 and 270 nmol/mg/min, and Kmapp values of 416 and 8.7 microM.

Distribution of adenosine-5'-diphosphatase activity in the lactating bovine mammary gland

A Ca2+- and Mg(2+)-stimulated adenosine-5'-diphosphatase has been found in lactating bovine mammary glands. The enzyme is associated with membranes of mitochondrial, microsomal, and Golgi apparatus fractions. The pH activity curves for the Golgi apparatus and microsomal fractions display two distinct maxima, one at pH 5.8 and the other between pH 7.4 and 8.4. Studies with activators and inhibitors indicate the enzyme is similar to adenosine-5'-diphosphatase found in other tissues. Its occurrence in the Golgi apparatus fraction indicates a possible role for this enzyme in the milk secretory process, particularly in ATP cycling in vesicles. Its occurrence in the microsomal fraction suggests a role in plasma membrane functioning.

Distribution of ADPase activity in the lactating rat mammary gland and its possible role in an ATP cycle in the Golgi apparatus

A Ca2+/Mg(2+)-stimulated ADPase has been found to occur in the lactating rat mammary gland. The enzyme is membrane associated and occurs in mitochondrial, microsomal, and Golgi apparatus fractions. The pH activity curves for the Golgi apparatus and microsomal fractions display two distinct maxima, one at pH 6.3 and one at pH 7.4. Studies with inhibitors and activators indicate that the enzyme is similar to ADPases found in other tissues and is distinct from the uridine nucleoside diphosphatase previously reported in the mammary Golgi apparatus. The occurrence of ADPase in the Golgi apparatus indicates a possible role for this enzyme in the milk secretory process, while the microsomal enzyme could be involved in extracellular activities.

Purification and characterisation of bovine spleen ADPase

ADPase has been purified from bovine spleen. Electrophoresis revealed a minor contaminent in the preparation that may represent ADPase that has been decreased in size by limited proteolysis during extraction and purification. The native enzyme behaves on SDS/PAGE as a 100-kDa protein but ADPase is a glycoprotein and so its electrophoretic behaviour may be anomalous. The apparent molecular mass is decreased to 70 kDa after removal of carbohydrate by treatment with a glycosidase. The use of a cross-linking reagent followed by electrophoresis suggests that the enzyme is composed of a single subunit. The specific activity of the purified material was 115 U/mg protein. The enzyme catalyses the hydrolysis of nucleoside di- and tri-phosphates but nucleoside monophosphates are not acted upon. The Km for ADP (approx. 10-15 microM) is unaffected by the state of purification of the enzyme, but catalytic activity of the purified material is stimulated by inclusion of detergent in the assay system and by calcium ions. Maximum activity is seen at pH 8.0-8.5 with ADP as substrate but the optimum shifts to 7.5-8.0 for ATP hydrolysis.

Production of AMP and adenosine in the interstitial fluid compartment of the isolated perfused normoxic guinea pig heart

The pathway of production of AMP and adenosine in the myocardial interstitial fluid compartment was studied in the isolated perfused normoxic guinea pig heart by collecting the transmyocardial effluent (t.m.e.) with the method of De Deckere and Ten Hoor (1977). Besides adenosine and inosine, AMP was found in t.m.e. Infusion of alpha,beta-methylene adenosine 5'-diphosphate (AOPCP), a specific inhibitor of the ecto 5'-nucleotidase, resulted in increases in t.m.e. AMP and inosine and a decrease in adenosine. Infusion of acetate producing a nearly twofold increase in myocardial AMP content did not increase the t.m.e. AMP even in the presence of AOPCP. In preparations made from 6-OH dopamine treated animals, the t.m.e. adenosine and inosine were reduced and AOPCP produced smaller increases in AMP and inosine, indicating that most if not all of the t.m.e. AMP originated from the sympathetic nerve terminals. Infusions of beta,gamma-imidoadenosine and beta,gamma-methylene adenosine 5'-triphosphate (AMPPNP and AMPPCP), non-hydrolysable analogs of ATP, resulted in dose-dependent increases in the t.m.e. AMP, which were much augmented in the presence of AOPCP. AMPPNP produced similar effects in 6-OH dopamine-treated preparations. As AMPPNP and AMPPCP are good substrates of ATP pyrophosphohydrolase, these findings indicate the presence of ATP pyrophosphohydrolase in the myocardial interstitial space.

Purification of ATP diphosphohydrolase from bovine aorta microsomes

ATP diphosphohydrolase (EC 3.6.1.5) hydrolyzes pyrophosphate bonds of nucleoside di- and triphosphates in the presence of divalent cations. We purified the enzyme from the vessel wall of bovine aortas. The procedure gave a homogeneous preparation of ATP diphosphohydrolase for the first time from an animal source. Bovine aorta microsomes were treated with 50 mM bicarbonate buffer (pH 10.0) containing 0.025% Triton X-100. The enzyme was then solubilized from the microsomes with 0.5% Triton X-100 and purified to homogeneity by DEAE-Sepharose CL-6B chromatography and 5'AMP-Sepharose 4B affinity chromatography. The apparent molecular mass of the pure enzyme was 110 kDa. The activity recovered was 6% of that of the microsomes. The enzyme was more active with Ca2+ than Mg2+. The sensitivity of ADPase activity to divalent cations was higher than that of ATPase activity. The enzyme had broad substrate specificity to nucleoside di- and triphosphates.

Demonstration of plasma-membrane adenosine diphosphatase activity in rat lung

The adenosine diphosphatase (ADPase) activity of rat lung has been investigated. Subcellular fractionation of lung tissue homogenates by sucrose density gradient centrifugation has shown the ADPase activity to be associated with the plasma membrane. ADPase was solubilised from the membranes and fractionated by ammonium sulphate precipitation to separate a specific, low-Km ADPase from non-specific alkaline phosphatase activity. The solubilised ADPase has a Km of 50 microM at pH 7.5 and appears to be distinct from ATPase.

An ADPase from sarcoplasmic reticulum of rabbit muscle cleaves ADP bound to F-actin

We have found that sarcoplasmic reticulum from rabbit muscle contains an ADPase which cleaves ADP bound to F-actin. The interaction is not of the simple Michaelis-Menten type, the order of the reaction being larger than the first. A possible explanation of this behaviour could be that ADPase binds to two adjacent actin monomers with a preferred orientation thus cleaving preferentially the nucleotide of one of the two monomers.

Enzymatic synthesis of [beta-32P]ADP using adenylate kinase and [gamma-32P]ATP

An enzymatic method for the synthesis of [beta-32P]ADP from [gamma-32P]ATP is described. This substrate is required for the assay of ADPase and is not commercially available. The method described results in a preparation of [beta-32P]ADP of high purity with a yield of approximately 40% the theoretical obtainable.