3 ,5 -Cyclic adenylate

Nucleosides and nucleotides are found in places other than as part of the structure of DNA and RNA. We have seen, for example, that adenosine units are part of the structures of two important coenzymes, NADH and coenzyme A. The 5 -triphosphate of adenosine is, of course, the important energy source, ATP (Section 22. IB). The compound called 3, 5 -cyclic adenylic acid (or cyclic AMP) (Fig. 25.6) is an important regulator of hormone activity. Cells synthesize this compound from ATP through the action of an enzyme, adenylate cyclase. In the laboratory, 3, 5 -cyclic adenylic acid can be prepared through dehydration of 5 -adenylic acid with dicyclohexylcarbodiimide. 

FIGURE 25.6 3, 5 -Cyclic adenylic acid (cyclic AMP) and its biosynthesis and laboratory synthesis. 

When 3, 5 -cyclic adenylic acid is treated with aqueous sodium hydroxide, the major product that is obtained is 3 -adenylic acid (adenosine 3 -phosphate) rather than 5 -adenyhc acid. Suggest a mechanism that explains the course of this reaction. 

In muscle phosphorylase, the passage from the inactive to the active form is stimulated only by epinephrine. (It has been suggested that in the liver fluke. Fasciola hepatica, serotonin stimulates phosphorylase. In this fluke, serotonin also stimulates phosphofruc-tokinase, probably through the intermediate of 3, 5 -cyclic adenylic acid.) In liver, both epinephrine and glucagon stimulate the activity of the phosphorylase. In liver, the activation has been demonstrated in vivo in cell-free preparations epinephrine does not act directly on phosphorylase, but it stimulates the formation of 3, 5 -cyclic adenylic acid, which in turn catalyzes the conversion of inactive phosphorylase to active phosphorylase in the supernatant. The cyclic nucleotide is formed from ATP in the presence of a particulate enzyme (adenine cyclase), which is found in heart, muscle, liver, brain, and other organs. The reaction requires magnesium, and, as can be expected, 3, 5 -cyclic adenosine accumulates when the medium contains fluoride. 

Another cyclization of 3 -adenylic acid (3 -AMP) to 2, 3 -cyclic adenylic acid (2, 3 -cAMP) took place by condensation with carbiminodiimidazole (or iV-cyanoimidazole) in aqueous or anhydrous medium. It is supposed that the reaction of 3 -AMP probably proceeds via a phosphoric imidazolide [9]... 

It may be argued that morphogenetic factors regulate early embryo-genesis by interference with quite different processes. Binding of the factors to cytoplasmic membranes could lead to an activation of membrane-bound adenylcyclase and a release of cyclic adenylate as a second messenger (for reference see Sutherland et al., 1965 Jost and Rickenberg,... 

Another hypothesis describing the mode of action of ACTH has recently gained some support. It proposes that ACTH modifies cellular permeability through the intermediate of adenosine-3, 5 -mono-phosphate. The increased penetration of xylose and a-aminoisobutyric acid under the influence of ACTH has been established. It also has been proposed that ACTH stimulates the formation of cyclic adenylate, which in turn induces the synthesis of new mitochondrial proteins. The appearance of the new protein is associated with the development of activity of the enzyme involved in the conversion of cholesterol to pregnenolone. 

Under the influence of ACTH, free fatty acids and glycerol concentrations increase in adipose tissue. ACTH stimulates lipolysis in adipose tissue by activating a hormone-sensitive lipase. However, ACTH does not act directly on the lipase. There are at least two other intermediate messengers adenylate cyclase and cyclic adenylate. The molecular mechanism of action of ACTH on lipolysis will be discussed in more detail in the section devoted to adipose tissue metabolism. 

The third protein, called the edema factor, is an enzyme catalyzing the conversion of adenyl triphosphate (ATP) to cyclic adenyl monophosphate (cyclic-AMP). The enzyme is called adenyl cyclase. The ordinary adenyl cyclase is an endogenous enzyme involved in the signal transduction. The edema factor s catalytic portion sequesters calmodulin, another protein involved in the signal transduction, and hence makes it unavailable for a proper function. The edema factor is not catalyti-cally active until it binds calmodulin. Once it has bound cahnodulin, it starts making cyclic-AMP. In fact it makes cyclic-AMP in excess, and hence causes the death of the cell. The chemical details of these phenomena are yet to be studied. 

Park, C. R., Lewis, S. B., and Exton, J. H., 1972, Relationship of some hepatic actions of insulin to the intracellular level of cyclic adenylate. Diabetes 21(Suppl. 2) 439. 

Adenosine-3 -monophosphoric acid hydrate [3 -adenylic acid, 3 -AMP] [84-21-9] M 347.3, m 197°(dec, as 2H2O), 210°(dec), m 210°(dec), [a]s46 -50° (c 0.5, 0.5M Na2HP04), pK 3.65, pKz 6.05. It crystallises from large volumes of H2O in needles as the monohydrate, but is not very soluble in boiling H2O. Under acidic conditions it forms an equilibrium mixture of 2 and 3 adenylic acids via the 2, 3 -cyclic phosphate. When heated with 20% HCl it gives a quantitative yield of furfural after 3hours, unlike 5 -adenylic acid which only gives traces of furfural. The yellow monoacridine salt has m 175°(dec) and... 

FIGURE 2.6 Production of cyclic AMP from ATP by the enzyme adenylate cyclase. Cyclic AMP is a ubiquitous second messenger in cells activating numerous cellular pathways. The adenylate cyclase is activated by the a subunit of Gs-protein and inhibited by the a-subunit of Gj-protein. Cyclic AMP is degraded by phosphodiesterases in the cell. 

Group II assays consist of those monitoring cellular second messengers. Thus, activation of receptors to cause Gs-protein activation of adenylate cyclase will lead to elevation of cytosolic or extracellularly secreted cyclic AMP. This second messenger phosphorylates numerous cyclic AMP-dependent protein kinases, which go on to phosphorylate metabolic enzymes and transport and regulatory proteins (see Chapter 2). Cyclic AMP can be detected either radiometrically or with fluorescent probe technology. 

P2-Agonists cause airway smooth muscle relaxation by stimulating adenyl cyclase to increase the formation of cyclic adenosine monophosphate (cAMP). Other non-bronchodilator effects have been observed, such as improvement in mucociliary transport, but their significance is uncertain.11 P2-Agonists are available in inhalation, oral, and parenteral dosage forms the inhalation route is preferred because of fewer adverse effects. 

The effect of receptor stimulation is thus to catalyze a reaction cycle. This leads to considerable amplification of the initial signal. For example, in the process of visual excitation, the photoisomerization of one rhodopsin molecule leads to the activation of approximately 500 to 1000 transdudn (Gt) molecules, each of which in turn catalyzes the hydrolysis of many hundreds of cyclic guanosine monophosphate (cGMP) molecules by phosphodiesterase. Amplification in the adenylate cyclase cascade is less but still substantial each ligand-bound P-adrenoceptor activates approximately 10 to 20 Gs molecules, each of which in turn catalyzes the production of hundreds of cyclic adenosine monophosphate (cAMP) molecules by adenylate cyclase. 

Gs Noradrenaline and fi2 Dopamine Di and D5 Histamine H2 Serotonin 5-HT4 Stimulates adenylate cyclase increasing the concentration of cAMP (cyclic-adenosine-3, 5 -monophosphate)... 

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