Adenylate metabolism in the heart. Regulatory properties of rabbit cardiac adenylate deaminase

Role of the interaction between troponin T and AMP deaminase by zinc bridge in modulating muscle contraction and ammonia production

The N-terminal region of troponin T (TnT) does not bind any protein of the contractile machinery and the role of its hypervariability remains uncertain. In this review we report the evidence of the interaction between TnT and AMP deaminase (AMPD), a regulated zinc enzyme localized on the myofibril. In periods of intense muscular activity, a decrease in the ATP/ADP ratio, together with a decrease in the tissue pH, is the stimulus for the activation of the enzyme that deaminating AMP to IMP and NH3 displaces the myokinase reaction towards the formation of ATP. In skeletal muscle subjected to strong tetanic contractions, a calpain-like proteolytic activity produces the removal in vivo of a 97-residue N-terminal fragment from the enzyme that becomes desensitized towards the inhibition by ATP, leading to an unrestrained production of NH3. When a 95-residue N-terminal fragment is removed from AMPD by trypsin, simulating in vitro the calpain action, rabbit fast TnT or its phosphorylated 50-residue N-terminal peptide binds AMPD restoring the inhibition by ATP. Taking in consideration that the N-terminus of TnT expressed in human as well as rabbit white muscle contains a zinc-binding motif, we suggest that TnT might mimic the regulatory action of the inhibitory N-terminal domain of AMPD due to the presence of a zinc ion connecting the N-terminal and C-terminal regions of the enzyme, indicating that the two proteins might physiologically associate to modulate muscle contraction and ammonia production in fast-twitching muscle under strenuous conditions.

Role of AMP deaminase in diabetic cardiomyopathy

Diabetes mellitus is one of the major causes of ischemic and nonischemic heart failure. While hypertension and coronary artery disease are frequent comorbidities in patients with diabetes, cardiac contractile dysfunction and remodeling occur in diabetic patients even without comorbidities, which is referred to as diabetic cardiomyopathy. Investigations in recent decades have demonstrated that the production of reactive oxygen species (ROS), impaired handling of intracellular Ca2+, and alterations in energy metabolism are involved in the development of diabetic cardiomyopathy. AMP deaminase (AMPD) directly regulates adenine nucleotide metabolism and energy transfer by adenylate kinase and indirectly modulates xanthine oxidoreductase-mediated pathways and AMP-activated protein kinase-mediated signaling. Upregulation of AMPD in diabetic hearts was first reported more than 30 years ago, and subsequent studies showed similar upregulation in the liver and skeletal muscle. Evidence for the roles of AMPD in diabetes-induced fatty liver, sarcopenia, and heart failure has been accumulating. A series of our recent studies showed that AMPD localizes in the mitochondria-associated endoplasmic reticulum membrane as well as the sarcoplasmic reticulum and cytosol and participates in the regulation of mitochondrial Ca2+ and suggested that upregulated AMPD contributes to contractile dysfunction in diabetic cardiomyopathy via increased generation of ROS, adenine nucleotide depletion, and impaired mitochondrial respiration. The detrimental effects of AMPD were manifested at times of increased cardiac workload by pressure loading. In this review, we briefly summarize the expression and functions of AMPD in the heart and discuss the roles of AMPD in diabetic cardiomyopathy, mainly focusing on contractile dysfunction caused by this disorder.

Regulation of skeletal muscle AMP deaminase. Carbethoxylation of His-51 belonging to the zinc coordination sphere of the rabbit enzyme promotes its desensitization towards the inhibition by ATP

BACKGROUND

Skeletal muscle AMP deaminase (AMPD1) regulates the concentration of adenine nucleotides during muscle contraction. We previously provided evidence that rabbit AMPD1 is composed by two HPRG 73 kDa subunits and two 85 kDa catalytic subunits with a dinuclear zinc site with an average of two histidine residues at each metal site. AMPD1 is mainly expressed in fast twitching fibers and is inhibited by ATP. The limited trypsinization of the 95-residue N-terminal domain of rabbit AMPD1 desensitizes the enzyme towards ATP inhibition at the optimal pH 6.5, but not at pH 7.1.

METHODS

The modified residues of rabbit AMPD1 after incubation with radioactive diethyl pyrocarbonate ([¹⁴C]DEP) causing the desensitization to inhibition by ATP at pH 7.1 have been identified by sequence analysis and MS analysis of the radioactive peptides liberated from the carbethoxylated enzyme by limited proteolysis with trypsin.

RESULTS

The study confirms the presence of a dinuclear zinc site in rabbit AMPD1 and shows that carbethoxylation of His-51 at the N-terminus of the catalytic subunit removes the inhibition of the enzyme by ATP at pH 7.1.

CONCLUSIONS

The desensitization to ATP is due to the modification of His-51 of the Zn₂ coordination sphere which is transduced in a conformational change of the enzyme C-terminus, where an ATP-binding site has been localized.

GENERAL SIGNIFICANCE

The progress in the study of the complex regulation of rabbit AMPD1 that shares an identical amino acid sequence with the human enzyme is important in relation to the role of the enzyme during mammalian evolution.

Excessive degradation of adenine nucleotides by up-regulated AMP deaminase underlies afterload-induced diastolic dysfunction in the type 2 diabetic heart

Type 2 diabetes mellitus (T2DM) is often complicated with diastolic heart failure, which decompensates under increased afterload. Focusing on cardiac metabolomes, we examined mechanisms by which T2DM augments ventricular diastolic stiffness in response to pressure overloading. Pressure-volume relationships (PVRs) and myocardial metabolomes were determined at baseline and during elevation of aortic pressure by phenylephrine infusion in a model of T2DM, OLETF, and its non-diabetic control, LETO. Pressure overloading augmented diastolic stiffness without change in systolic reserve in OLETF as indicated by a left-upward shift of end-diastolic PVR. In contrast, PVRs under cardioplegic arrest in buffer-perfused isolated hearts were similar in OLETF and LETO, indicating that extracellular matrix or titin remodeling does not contribute to the afterload-induced increase in stiffness of the beating ventricle of OLETF. Metabolome analyses revealed impaired glycolysis and facilitation of the pentose phosphate pathway in OLETF. Pressure overloading significantly reduced ATP and total adenine nucleotides by 34% and 40%, respectively, in OLETF but not in LETO, while NADH-to-NAD(+) ratios were similar in the two groups. The decline in ATP by pressure overloading in OLETF was associated with increased inosine 5-monophosphate and decreased adenosine levels, being consistent with the 2.5-times higher activity of cardiac AMP deaminase in OLETF. Tissue ATP level was negatively correlated with tau of LV pressure and LVEDP. These results suggest that ATP depletion due to excessive degradation of adenine nucleotides by up-regulated AMP deaminase underlies ventricular stiffening during acute pressure overloading in T2DM hearts.

The role of histidine-proline-rich glycoprotein as zinc chaperone for skeletal muscle AMP deaminase

Metallochaperones function as intracellular shuttles for metal ions. At present, no evidence for the existence of any eukaryotic zinc-chaperone has been provided although metallochaperones could be critical for the physiological functions of Zn²⁺ metalloenzymes. We propose that the complex formed in skeletal muscle by the Zn²⁺ metalloenzyme AMP deaminase (AMPD) and the metal binding protein histidine-proline-rich glycoprotein (HPRG) acts in this manner. HPRG is a major plasma protein. Recent investigations have reported that skeletal muscle cells do not synthesize HPRG but instead actively internalize plasma HPRG. X-ray absorption spectroscopy (XAS) performed on fresh preparations of rabbit skeletal muscle AMPD provided evidence for a dinuclear zinc site in the enzyme compatible with a (μ-aqua)(μ-carboxylato)dizinc(II) core with two histidine residues at each metal site. XAS on HPRG isolated from the AMPD complex showed that zinc is bound to the protein in a dinuclear cluster where each Zn²⁺ ion is coordinated by three histidine and one heavier ligand, likely sulfur from cysteine. We describe the existence in mammalian HPRG of a specific zinc binding site distinct from the His-Pro-rich region. The participation of HPRG in the assembly and maintenance of skeletal muscle AMPD by acting as a zinc chaperone is also demonstrated.

Metalloproteinase inhibitor counters high-energy phosphate depletion and AMP deaminase activity enhancing ventricular diastolic compliance in subacute heart failure

Cardiac matrix metalloproteinases (MMPs) stimulated by the sympathomimetic action of angiotensin II (AII) exacerbate chamber diastolic stiffening in models of subacute heart failure. Here we tested the hypothesis that MMP inhibition prevents such stiffening by favorably modulating high-energy phosphate (HEP) stores more than by effects on matrix remodeling. Dogs were administered AII i.v. for 1 week with tachypacing superimposed in the last two days (AII+P; n = 8). A second group (n = 9) underwent the same AII+P protocol but was preceded by oral treatment with an MMP inhibitor PD166793 [(S)-2-(4-bromo-biphenyl-4-sulfonylamino-3-methyl butyric acid] 1 week before and during the AII+P period. Pressure-volume analysis was performed in conscious animals, and myocardial tissue was subjected to in vitro and in situ zymography, collagen content, and HEP analysis (high-performance liquid chromatography). As reported previously, AII+P activated MMP9 and MMP2 and specifically exacerbated diastolic stiffening (+130% in chamber stiffness). PD166793 cotreatment prevented these changes, although myocardial collagen content, subtype, and cross-linking were unaltered. AII+P also reduced ATP, free energy of ATP hydrolysis (DeltaG(ATP)), and phosphocreatine while increasing free [ADP], AMP catabolites (nucleoside-total purines), and lactate. PD166793 reversed most of these changes, in part due to its inhibition of AMP deaminase. MMP activation may influence cardiac diastolic function by mechanisms beyond modulation of extracellular matrix. Interaction between MMP activation and HEP metabolism may play an important role in mediating diastolic dysfunction. Furthermore, these data highlight a potential major role for increased AMP deaminase activity in diastolic dysfunction.

Metabolic regulation of ATP breakdown and of adenosine production in rat brain extracts

ATP concentration is dramatically affected in ischemic injury. From previous studies on ATP mediated purine and pyrimidine salvage in CNS, we observed that when "post-mitochondrial" extracts of rat brain were incubated with ATP at 3.6 mM, a normoxic concentration, formation of IMP always preceded that of adenosine, a well known neuroactive nucleoside and a homeostatic cellular modulator. This observation prompted us to undertake a study aimed at assessing the precise pathways and kinetics of ATP breakdown, a process considered to be the major source of adenosine in rat brain. The results obtained using post-mitochondrial extracts strongly suggest that the breakdown of intracellular ATP at normoxic concentration follows almost exclusively the pathway ATP<=>ADP<=>AMP --> IMP --> inosine<=>hypoxanthine, with little, if any, intracellular adenosine production. At low ischemic concentration, intracellular ATP breakdown follows the pathway ATP<=>ADP<=>AMP --> adenosine --> inosine<=>hypoxanthine with little IMP formation. At the same time, extracellular ATP, whose concentration is known to be enhanced during ischemia, is actively broken down to adenosine through the pathway ATP --> ADP --> AMP --> adenosine, catalysed by the well characterized ecto-enzyme cascade system. Moreover, we show that during intracellular GTP catabolism, xanthosine, in addition to guanosine, is generated through the so called "ribose 1-phosphate recycling for nucleoside interconversion". These results considerably extend our knowledge on the long debated question of the extra or intracellular origin of adenosine in CNS, suggesting that at least in normoxic conditions, intracellular adenosine is of extracellular origin.

A calpain-like proteolytic activity produces the limited cleavage at the N-terminal regulatory domain of rabbit skeletal muscle AMP deaminase: evidence of a protective molecular mechanism

On storage at 4 degrees C, rabbit skeletal muscle AMP deaminase undergoes limited proteolysis with the conversion of the native 85-kDa enzyme subunit to a 75-kDa core that is resistant to further proteolysis. Further studies have shown that limited proteolysis of AMP deaminase with trypsin, removing the 95-residue N-terminal fragment, converts the native enzyme to a species that exhibits hyperbolic kinetics even at low K+ concentration. The results of this report show that a 21-residue synthetic peptide, when incubated with the purified enzyme, is cleaved with a specificity identical to that reported for ubiquitous calpains. In addition, the cleavage of a specific fluorogenic peptide substrate by rabbit m-calpain is inhibited by a synthetic peptide that corresponds to residues 10-17 of rabbit skeletal muscle AMP deaminase; this peptide contains a sequence (K-E-L-D-D-A) that is present in the fourth subdomain A of rabbit calpastatin, suggesting that the N-terminus of AMP deaminase shares with calpastatin a regulatory sequence that might exert a protective role against the fragmentation-induced activation of AMP deaminase. These observations suggest that a calpain-like proteinase present in muscle removes from AMP deaminase a domain that holds the enzyme in an inactive conformation and which also contains a regulatory region that protects against unregulated proteolysis. We conclude that proteolysis of AMP deaminase is the basis of the large ammonia accumulation that occurs in skeletal muscle subjected to strong tetanic contraction or passing into rigor mortis.

Isolation and regulation of piglet cardiac AMP deaminase

The properties of piglet cardiac AMP deaminase were determined and its regulation by pH, phosphate, nucleotides and phosphorylation is described. AMP deaminase purified from the ventricles of newborn piglet hearts displayed hyperbolic kinetics with a Km of 2 mM for 5'-AMP. The enzyme had a pH optimum of 7.0 and was strongly inhibited by inorganic phosphate. ATP decreased the Km of the native enzyme 3-fold, but did not significantly block the inhibitory effects of phosphate. Kinetic parameters were not significantly altered in the presence of adenosine, cyclic AMP and NAD+, whereas, the Km was decreased by 50% in the presence of NADH. Piglet cardiac AMP deaminase was phosphorylated by protein kinase C, resulting in a 2-fold increase in Vmax with no change in Km. However, incubation with cAMP-dependent protein kinase did not affect enzyme kinetics. The 80-85 kD protein subunit of piglet cardiac AMP deaminase immunoreacted with antisera raised against human erythrocyte AMP deaminase, rabbit heart AMP deaminase and human recombinant AMP deaminase 3 (isoform E). These results are discussed in relation to in situ AMP deaminase activity in neonatal piglet heart myocytes.

AMP deaminase in piglet cardiac myocytes: effect on nucleotide metabolism during ischemia

The purpose of this study was to examine in situ regulation of AMP deaminase in newborn piglet cardiac myocytes and to determine its role in nucleotide metabolism during ischemia. When a rapid deenergization paradigm was used to assay AMP deaminase, enzyme activity depended on the hormonal and metabolic status of cells just before deenergization. Inosine 5'-monophosphate (IMP) formation was increased 150% in deenergized myocytes pretreated with phorbol 12-myristate 13-acetate (PMA; EC50 = 4.7 x 10(-8) M). This effect was 90% blocked with the protein kinase C (PKC) inhibitor staurosporine. In addition, the beta-adrenergic agonist isoproterenol stimulated AMP deaminase activity (EC50 = 1.5 x 10(-8) M), and IMP formation was directly correlated to intracellular cAMP levels (r2 = 0.9). Furthermore, adenosine increased IMP formation, whereas nonrespiring, glycolyzing piglet myocytes had reduced AMP deaminase activity. Pretreatment of perfused piglet hearts with adenosine, but not PMA, before exposure to global ischemia resulted in enhanced conversion of AMP to IMP during the ischemic period. Similar results were obtained in piglet myocytes preincubated with adenosine or PMA before exposure to simulated ischemia. These results may be relevant to the preconditioning phenomenon.

Cardiac adenylate deaminase: molecular, kinetic and regulatory properties under phosphate-free conditions

Adenylate deaminase (EC 3.5.4.6) may help to regulate the adenine nucleotide catabolism characteristic of such disease states as myocardial ischaemia. We report analysis of the molecular, kinetic and allosteric properties of rabbit heart adenylate deaminase when extracted and purified under phosphate-free conditions (i.e., with Hepes/KOH). The enzyme's subunit molecular mass (approximately 81 kDa), pI (6.5), substrate specificity for 5'-AMP, and activation by K+ were identical in the absence or presence of phosphate. At each chromatographic step during isolation without phosphate, cardiac adenylate deaminase showed a lower apparent activity as compared with the enzyme prepared with phosphate present. Kinetic constants for the phosphate-free rabbit heart adenylate deaminase preparation (Km 0.54 mM AMP; Vmax. 1.4 mumol/min per mg of protein) were approximately 10-fold lower than those of the enzyme isolated with phosphate. The same irreversible decrease in kinetic constants could be achieved by dialysing phosphate from the phosphate-containing enzyme preparation. The relationship between enzyme activity and substrate concentration was sigmoidal in the presence of phosphate, but hyperbolic in its absence. Cardiac adenylate deaminase under phosphate-free conditions was no longer allosterically activated by ATP and ADP, yet remained inhibitable by GTP. Enzyme inhibition by the transition-state mimic coformycin was not influenced by phosphate status. The phosphate-free preparation of rabbit heart adenylate deaminase was markedly labile and extremely susceptible to proteolysis by trypsin or chymotrypsin. The inactivation kinetics and fragmentation pattern in response to controlled proteolysis depended on whether the enzyme had been isolated with or without phosphate present, suggesting a conformational difference between the two enzyme preparations. These data constitute direct evidence that the absence of phosphate irreversibly converts cardiac adenylate deaminase into a pseudo-isoenzyme with distinct kinetic, regulatory and stability properties.

Comparative enzymology of AMP deaminase, adenylate kinase, and creatine kinase in vertebrate heart and skeletal muscle: the characteristic AMP deaminase levels of skeletal versus cardiac muscle are reversed in the North American toad

The specific activity of three characteristic enzymes, adenylate deaminase, adenylate kinase, and creatine kinase, in the skeletal muscles and heart of a variety of vertebrate land animals, including the human, are surveyed. Data from this study and available studies in the literature suggest that adenosine monophosphate deaminase in land vertebrates is quite high in white skeletal muscle, usually somewhat lower in red muscle, and 15- to 500-fold lower in cardiac muscle. Adenosine monophosphate deaminase is active primarily under ischemic or hypoxic conditions which occur frequently in white muscle, only occasionally in red muscle, and ought never occur in heart muscle, and this may therefore account for observed enzyme levels. The common North American toad, Bufo americanus, provides a striking exception to the rule with cardiac adenosine monophosphate deaminase as high as in mammalian skeletal muscle, whereas its skeletal muscle level of adenosine monophosphate deaminase is several times lower. The exceptional levels in the toad are not due to a change in substrate binding and are not accompanied by comparable change in the level of adenylate or creatine kinase. Nor do they signal any major change in isozyme composition, since a human muscle adenosine monophosphate deaminase-specific antiserum reacts with toad muscle adenosine monophosphate deaminase, but not with toad heart adenosine monophosphate deaminase. They do not represent any general anuran evolutionary strategy, since the bullfrog (Rana catesbeiana) and the giant tropic toad (Bufo marinus) have the usual vertebrate pattern of adenosine monophosphate deaminase distribution.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of mammalian cardiac AMP deaminase by protein kinase C-mediated phosphorylation

Using AMP deaminase (AMP aminohydrolase; EC 3.5.4.6) purified from rabbit left-ventricular heart tissue, we report direct investigation of the potential for cardiac AMP deaminase activity to be regulated by kinase-mediated phosphorylation. Rabbit heart AMP deaminase served as a substrate for Ca2+/phospholipid-dependent protein kinase (protein kinase C; PKC) exclusively; no other mammalian protein kinase phosphorylated the enzyme. PKC-dependent AMP deaminase phosphorylation was rapid, linear with respect to time and the concentrations of PKC and AMP deaminase in the reaction, and inhibitable by staurosporine. Upon phosphorylation, the apparent Km of cardiac AMP deaminase decreased from 5.6 mM to 1.2 mM, without effect on the Vmax. Whether phosphorylated or not, rabbit heart AMP deaminase was inhibited by 1.0 mM GTP, which decreased the Vmax. by approximately 50% in each case. PKC-dependent phosphorylation of cardiac AMP deaminase did not alter the enzyme's allosterism toward millimolar ATP or ADP: both nucleotides at 1.0 mM concentration decreased the apparent Km to approximately 0.5 mM. Treatment of cardiac phospho-AMP deaminase with either the protein phosphatase calcineurin or alkaline phosphatase generated a dephosphorylated form which displayed molecular and kinetic properties identical with those of the originally isolated enzyme. These data raise the possibility that a phosphorylation-dephosphorylation mechanism may regulate flux through AMP deaminase in the heart under pathological conditions, such as myocardial ischaemia, characterized by PKC activation and adenylate depletion.

Isolation and characterization of AMP deaminase from mammalian (rabbit) myocardium

AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) is a ubiquitous enzyme in eukaryotes, which may play a role in ATP catabolism during myocardial ischaemia. We report isolation of AMP deaminase from rabbit myocardium with a 19% recovery and a 650-fold enrichment, using a newly devised protocol involving sequential cation-exchange, gel-permeation and affinity chromatographies. The cardiac AMP deaminase preparation described was electrophoretically and chromatographically homogeneous and contained one unique N-terminal residue (leucine). The isolated enzyme was sensitive to various cations (K+, Mg2+, Ca2+). The pH optimum of purified cardiac AMP deaminase was 6.8, its pI was 6.5, and it displayed substrate-specificity toward 5'-AMP. The subunit molecular mass of rabbit heart AMP deaminase on SDS/PAGE (81 kDa) and the holoenzyme molecular mass as estimated by non-denaturing size-exclusion h.p.l.c. (330 kDa) indicated that the native enzyme was a tetramer. Cardiac AMP deaminase displayed a sigmoidal substrate-saturation curve in the presence of 100 mM KCl. Apparent Michaelis constants were a Km of 5.8 mM AMP and a Vmax. of 11.1 mumol/min per mg of protein. ATP and ADP were positive allosteric effectors of cardiac AMP deaminase: the apparent Km was decreased to 1.7 mM by 1.0 mM ATP. The enzyme was inhibited by GTP, coformycin, coformycin 5'-phosphate, palmitoyl-CoA, inorganic phosphate compounds, and the metal chelator o-phenanthroline. No inhibition either by product nucleotide (IMP) or by nicotinamide nucleotides was detected when these agents were examined at concentrations up to 2.5 mM. We conclude that this enzyme preparation offers a means by which the kinetic mechanism and regulation of mammalian cardiac AMP deaminase may be directly investigated.

Phorbol esters and cyclic AMP activate AMP deaminase in adult rat cardiac myocytes

Using rapid deenergization as a probe for adenylate deaminase activity in intact adult rat cardiac myocytes, we have previously established that IMP formation is enhanced by alpha-adrenergic agonists. In the present study, the effect of adrenergic agents on adenylate deaminase was further characterized. Phenylephrine (PE)3 increased IMP production in a dose-dependent fashion with an EC50 of 8 x 10(-7) M. The response to PE was reversed within 10 min by the alpha 1-antagonist, prazosin. Likewise, adenylate deaminase was also activated in ventricular myocytes challenged with phorbol 12-myristate 13-acetate (PMA, EC50 = 5 nM); cardiac cells presented with 100 nM PMA increased IMP production from 4.4 +/- 0.5 (control) to 15.7 +/- 0.9 nmol/mg protein when subsequently deenergized. The effects of PMA and PE were attenuated 85 +/- 5% and 96 +/- 4%, respectively, by pretreatment of cells with 150 nM staurosporine, an inhibitor of protein kinase C. Furthermore, incubation of cardiac cells with 1 microM PMA for 24 h blunted the response to both PMA and phenylephrine 85-90%. Elevating cyclic AMP (cAMP) content to greater than 15 pmol/mg by treatment with forskolin or isoproterenol plus isobutylmethylxanthine also resulted in enhanced adenylate deaminase activity, but this stimulatory effect was not abolished by 24 h incubation with 5 microM PMA. Forskolin and PMA-induced increases in IMP production appeared to be additive. However, 0.5 microM isoproterenol inhibited the cellular response to phenylephrine by about 30% but did not affect PMA-stimulated adenylate deaminase activity. We conclude that both cAMP and protein kinase C stimulate adenylate deaminase, perhaps through selective activation of different isoforms. However, cAMP also exerts partial inhibition on alpha-adrenoreceptor-mediated increases in IMP production.

Molecular forms of human heart muscle AMP deaminase

Chromatography on phosphocellulose revealed the presence of two, kinetically different forms of human heart AMP deaminase. The main portion of the activity eluted from the column at about 1.1 M KCl, and the enzyme present in this eluate manifested a sigmoidal type of substrate saturation kinetics. The results from gel filtration indicate that human heart AMP deaminase is a tetrameric protein capable of aggregating in more complex active structures.

Purification and properties of AMP-deaminase from human uterine smooth muscle

AMP-deaminase from human uterine smooth muscle has been isolated, and properties of the enzyme were characterized. At pH 7.0, and in the presence of 100 mM potassium chloride the enzyme manifests a distinctly sigmoidal type of kinetics, with S0.5 parameter value about 12 mM. 1 mM ATP strongly activates the enzyme, and diminishes the value of S0.5 to 1.2 mM. In contrast to that 2.5 mM orthophosphate slightly inhibits the activity of AMP-deaminase studied and increases the S0.5 to about 14 mM. Similarly to ATP, orthophosphate does not influence the maximum velocity of the reaction. Electrophoresis in the presence of sodium dodecyl sulphate revealed that the molecular weight of human smooth muscle AMP-deaminase subunit is close to 37 kDa.

Regulatory properties of AMP deaminase isoenzymes from rabbit red muscle

We examined the kinetic and regulatory properties of the two isoenzymes of red muscle AMP deaminase, forms A and B, corresponding respectively to the single isoenzymes present in the heart and white skeletal muscle. At the optimal pH value, 6.5, both enzymes show hyperbolic substrate-velocity curves and are inhibited by GTP, inducing sigmoid kinetics. An effect similar to that of GTP is exerted on form B by ATP, whereas form A is almost insensitive to this nucleotide. At pH 7.1 both enzymes follow sigmoid kinetics. ATP enhances the sigmoidicity of the substrate-velocity curve of form B, but it stimulates form A, reverting sigmoidal to hyperbolic kinetics shown by the enzyme at optimal pH. At pH 7.1, form A is also less sensitive to the inhibitory action of Pi and GTP. These results suggest that, owing to the presence of form A, AMP deamination occurs in red muscle also at moderate work intensity. A possible role of this process in counteracting the production of adenosine by 5'-nucleotidase is hypothesized.

Regulatory properties of pigeon heart muscle AMP deaminase

The kinetic and regulatory properties of purified pigeon heart muscle AMP deaminase were investigated. In the presence of 100 mM potassium chloride, the enzyme exhibited a slightly sigmoidal type of kinetics. Addition of ATP to the incubation medium changed the reaction rate versus substrate concentration plot into a hyperbolic one, and caused a decrease of the half-saturation constant (S0.5). ADP presence caused the change of both the S0.5 and Vmax parameters, exerting either an activating or inhibitory effect, depending upon the substrate concentration. Orthophosphate inhibited the enzyme at all substrate concentrations, increasing the value of the S0.5 parameter. In the presence of ATP, ADP and orthophosphate, added to the incubation medium at approximately physiological concentrations, pigeon heart AMP deaminase still seems to preserve its activated form. Active long chain fatty acids clearly inhibited enzyme activity even at micromolar concentrations. Interpretation of the kinetic data in terms of the allosteric theory of Monod et al. (1965, J. Mol. Biol. 12, 88-118) indicates that heart muscle AMP deaminase may operate as a functionally active dimer.

Alterations of purine salvage pathways during differentiation of rat heart myoblasts towards myocytes

Enzyme activities of purine catabolism and salvage, the concentrations of high-energy phosphates and the reutilisation of purine bases and purine nucleosides were studied in rat heart myoblasts and myocytes. Rat heart myoblasts H9c2(2-1) were grown in Dulbecco's modified Eagle's minimum essential medium supplemented with 10% fetal calf serum. Reduction of fetal calf serum to 2% for 1 week resulted in a differentiation into myocytes with respect to their morphological features and their enzyme pattern. In differentiated myocytes, activity of 5'-nucleotidase was increased more than 2-fold, and AMP deaminase and creatine kinase activities were more than 10-fold elevated. The concentration of creatine phosphate in differentiated myocytes was doubled compared to that in myoblasts. The uptake into myoblasts and myocytes and the incorporation into adenine nucleotides was highest using adenosine, inosine and adenine uptake rates were intermediate, and hypoxanthine was utilised least. Differentiation of myoblasts into myocytes resulted in a slightly lower overall uptake of adenosine and adenine, whereas about 40% more inosine and hypoxanthine were utilised by myocytes. Increasing the phosphate concentration in the incubation medium up to 50 mmol/l resulted in a stimulation of uptake of all purine compounds tested. This stimulation was more pronounced in myoblasts.

Regulatory properties of 14-day embryo and adult hen heart AMP-deaminase; the influence of pH on the enzyme activity

The variation of kinetic parameters with pH for the reaction catalysed by the purified 14-day embryo and adult hen heart AMP-deaminase was shown to be similar but not identical. The pH-dependence of the half-saturation constant (K0.5) is well pronounced, and the plot of pK0.5 vs pH is manifested as a bell-shaped curve for both developmental forms of the enzyme. In contrast to that, the maximum velocity of the reaction (Vmax) catalyzed by these enzymes does not change significantly in the range pH 5.6-7.4.

Regulatory properties of 14 day embryo and adult hen heart AMP-deaminase

Chromatography on phosphocellulose column revealed changes in the elution profile of chicken heart AMP-deaminase during ontogenesis. The extracts from the heart of adult hen and 14 day-old embryo displayed a single peak of the enzyme activity at a slightly different elution volume, whereas in the heart extract of 1 day-old chicken two molecular forms of adenylate deaminase have been eluted. The kinetic and regulatory properties of the purified adult hen heart AMP-deaminase were studied and compared with those of the corresponding enzyme from 14 day-old embryo heart. Both enzymes exhibited a slightly sigmoid-shaped plot of the reaction rate versus substrate concentration, which shifted to hyperbolic form when ATP or ADP were added into the incubation medium. The enzymes were strongly activated by ATP, less efficiently by ADP and the activatory effect was enhanced at low substrate concentration. Orthophosphate inhibited both enzymes but this inhibition was more potent for the embryo heart enzyme. Palmitoyl-CoA inhibited adult hen but not the embryo heart AMP-deaminase. The data presented indicate that the differences also in the regulatory properties of the molecular forms studied do exist and correspond with the ontogenetic differences observed previously (Kaletha and Skladanowski (1981) Experientia 37, 232-234) concerning the effect of temperature on the chicken heart adenylate deaminase.

Hen heart AMP-deaminase--the combined effect of ATP, ADP and orthophosphate on the enzyme activity

Interpretation of the kinetic data in terms of concerted transition theory indicated that in the presence of 100 mM potassium chloride hen heart AMP-deaminase may be active as a dimer. The presence of ATP, but not of the ADP in the incubation medium shifts completely the allosteric equilibrium towards the active, accessible to the substrate form of the enzyme. In the joint presence of main enzyme effectors (ATP, ADP and orthophosphate) added to the incubation medium at physiological concentrations, the plot of the reaction rate versus substrate concentration manifested hyperbolic dependence and the value of half-saturation constant (K0.5) did not differ from the value of this parameter obtained for ATP(alone)-activated enzyme.