Adenosine study, hydrolysis

Phosphodiesterase Inhibitors. Because of the complexity of the biochemical processes involved in cardiac muscle contraction, investigators have looked at these pathways for other means of dmg intervention for CHF. One of the areas of investigation involves increased cycHc adenosine monophosphate [60-92-4] (cAMP) through inhibition of phosphodiesterase [9025-82-5] (PDE). This class of compounds includes amrinone, considered beneficial for CHF because of positive inotropic and vasodilator activity. The mechanism of inotropic action involves the inhibition of PDE, which in turn inhibits the intracellular hydrolysis of cAMP (130). In cascade fashion, cAMP-catalyzed phosphorylation of sarcolemmal calcium-channels follows, activating the calcium pump (131). A series of synthetic moieties including the bipyridines, amrinone and milrinone, piroximone and enoximone, [77671-31-9], C22H22N2O2S, all of which have been shown to improve cardiac contractiUty in short-term studies, were developed (132,133). These dmgs... 

Mechanical Work. All cells exhibit motile and contractile properties. The remarkable thing about these activities of cells is that they are based on the direct coupling of chemical to mechanical action, in contrast to the heat engines that we have developed to perform our work for us. The mechanisms by which this coupling of chemical to mechanical processes takes place is not well understood, but the hydrolysis of adenosine triphosphate is known to be an important part of the molecular pathway. Although thermodynamic studies cannot provide information about the molecular steps involved, any mechanism that is proposed must be consistent with thermodynamic data [4]. 

To date, five subtypes of these receptors have been cloned. However, initial studies relied on the pharmacological effects of the muscarinic antagonist pirenzepine which was shown to block the effect of several muscarinic agonists. These receptors were termed Mi receptors to distinguish them from those receptors for which pirenzepine had only a low affinity and therefore failed to block the pharmacological response. These were termed M2 receptors. More recently, M3, M4 and M5 receptors have been identified which, like the Mi and M2 receptors occur in the brain. Recent studies have shown that Mi and M3 are located posts)maptically in the brain whereas the M2 and M4 receptors occur pres)maptically where they act as inhibitory autoreceptors that inhibit the release of acetylcholine. The M2 and M4 receptors are coupled to the inhibitory Gi protein which reduces the formation of cyclic adenosine monophosphate (cyclic AMP) within the neuron. By contrast, the Mi, M3 and M5 receptors are coupled to the stimulatory Gs protein which stimulates the intracellular hydrolysis of the phosphoinositide messenger within the neuron (see Figure 2.8). 

Kinetic studies of the nucleotide analogs, y-phenylpropyl di- and triphosphate, have been undertaken to define the role of the adenosine residue in the chemical and enzymic reactions of adenosine triphosphate. A catalytic function associated with binding of metal ions at the adenine nitrogens has been ascribed to the adenosine moiety in phosphate transfer reactions in which adenosine di- or triphosphates function as the phosphate source109-"2. The pH-rate profile (Fig. 6) for the hydrolysis of -y-phenylpropyl diphosphate... 

These were differently affected by different procedures. For example, when the enzyme was activated at 55°, the increment in ki was slight, but k2 increased 3.5-fold. Similarly, in the presence of EDTA, fc, and k2 values decreased independently, suggesting that the sites for both activities were different. Center and Behai (5) found that with the P. mirabilis enzyme, cyclic 2, 3 -UMP competitively inhibited the hydrolysis of bis(p-nitrophenyl) phosphate. The Ki was 40 pAf very close to the Km for the cyclic nucleotide (Km, 75 yM) which indicated that the two compounds could serve as alternate substrates being hydrolyzed at the same active site. In contrast, 3 -AMP was a mixed inhibitor of cyclic 2, 3 -UMP and bis(p-nitrophenyl) phosphate hydrolysis. Adenosine was a mixed inhibitor of bis(p-nitrophenyl) phosphate hydrolysis but a competitive inhibitor of 3 -AMP hydrolysis. From such kinetic studies Center and Behai (5) suggested that two separate and adjacent sites A and B are involved in the hydrolysis of the diester and phos-phomonoester substrates. Site A serves as a binding site for hydrolysis of ribonucleoside 2, 3 -cyclic phosphates and together with site B catalyzes the hydrolysis of the diester bond. During this reaction 3 -... 

A few papers on phosphonate chemistry have appeared.235-237 The synthesis of the novel nucleoside bicyclic trisanhydrides (256 A = adenosin-f-yl) has been reported.235 A kinetic study of the alkaline hydrolysis of 4-substituted phenyl ethyl benzyl phosphonates (257) supports an associative A-E mechanism for the hydrolysis.236 The direct preparation of the esters of -nitrobenzyIpliosplionic acid from -nitrobenzyl halides has been reported.237... 

Study of the hydrolysis of adenosine triphosphate acid (ATP) showed that the temperature gradient during the reaction under microwave conditions can reach ca. 30°C, therefore reliable temperature measurement is the key factor for interpretation and comparison of rate of any reaction under conventional and microwave conditions. In this case, the Arrhenius parameters were estimated for the reaction under conventional conditions. Then taking into account the temperature gradient in investigated samples, the concentrations of the reagents were calculated for the experiments carried out under microwave conditions to show that both theoretical and experimental values fell within the range of the experimental error [23]. 

SM2/AM1 and SM3/PM3 models were used to study the hydrolysis of pyrophosphate, which is coupled to virtually all biosynthetic reactions. However, the authors concluded that extreme care must be taken when applying semiempirical methods to compounds containing second-row atoms, since they may produce anomalously high atomic charges [98]. On the other hand, a study on syn and anti conformations of solvated cyclic 3 ,5 -adenosine monophosphate indicated that SM3/PM3 and SM2/AM1 models are inexpensive yet accurate approaches... 

The activation mechanism of phosphosulfate linkages (P—O —S)has been studied to understand the chemistry of biological sulfate-transfer reactions of phosphosul-fates of adenosine (APS and PAPS). Several phosphosul-fates were prepared and subjected to several nucleophilic reactions including hydrolysis. In general, phosphosulfates are stable in neutral aqueous mediay but become labile under acidic conditions, resulting in selective S—O fission. This S—O fission appears to occur by unimolecular elimination of sulfur trioxide, which can react with a nucleophilic acceptor, leading to a sulfate-transfer reaction. This process can be accelerated by Mg2+ ion when the solvent is of low water content. Under neutral conditions, divalent metal ions also were found to catalyze nucleophilic reactions, but these occurred on phosphorus to result in exclusive P-O fission. 

The NAD glycohydrolase (EC 3.2.2.5) in this study catalyzes the hydrolysis of NAD+ to form nicotinamide, adenosine diphosphate ribose (ADPR), and H+. The assay developed for this activity follows the disappearance of the substrate NAD+ and the production of nicotinamide. 

It would be of interest to halt the hydrolysis when liberation of guanosine and adenosine approaches a maximum, and determine whether the pyrimidine nucleotides are present as a dinucleotide or as the two mononucleotides. It is not clear whether the action of the non-specific phosphatase on an artificially-prepared, equimolecular mixture of the four mononucleotides has been studied (although the individual mononucleotides have been so examined by Bredereck, Beuchelt and Richter ), but Kobayashi has found that guanylic acid is hydrolyzed more readily than adenylic acid which, in turn, is hydrolyzed more readily than the pyrimidine nucleotides. Furthermore, Bredereck, et oZ. have shown that mild chemical hydrolysis of ribosenucleic acid with aqueous pyridine at 100° gives guanylic acid (G) plus a trinucleotide composed of adenylic (A), cytidylic (C), and uridylic (U) acids. On further hydrolysis in aqueous pyridine, adenylic acid is split off. Hence, in ribosenucleic acid, (G) is at one end of the molecule and, in the trinucleotide, (A) is at one end of the molecule. Possible formulas for the tetranucleotide are therefore... 

The Cr and Co complexes of ATP (adenosine 5 -triphosphate) have been shown to be very good analogues for MgATP. The hydrolysis and decomposition of these complexes have now been studied in some detail. The breakdown of the tridentate [Co(NH3)3ATP] is rapid, producing high levels of free ATP and lesser amounts of ADP. Rate constants for hydrolysis are 100-5000 times greater than those for uncomplexed ATP. 

The hydrolysis of adenosine 5 -triphosphate (ATP) in the presence of various cobalt(III) complexes has been studied. Complexes such as [Co(en)3] which have no available sites for coordination of the substrate display no catalytic activity. Complexes having one site or two sites in a trans configuration such as tetraethylenepentaminecobalt(III) or bis(dimethylgly-oximato)cobalt III) slightly enhance ATP hydrolysis. However, complexes with two available sites in a cis configuration such as cis-a- or crs-jS-Co(trien) exhibit considerable activity. Both the reactions ATP+HjO- ADP+Pj and ATP+H20- AMP+PPi occur with these systems. The complex [Co(dien)] effectively enhances the hydrolysis of ATP to ADP+Pj. At pH 4.0 the uncatalyzed hydrolysis rate constant for ATP hydrolysis is 1.18xl0 s at 50 °C. For ATP UxlO M) and [Co (dien)] "" (2xlO M) at pH 4.0, = 1.75X 10 at 50°C, a rate... 

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