Formation of cyclic AMP

The inhibitory GTP binding protein (G ) which has been demonstrated in many different cell types is also present in Leydig cells. Evidence for this was obtained from studies with cultured Leydig cells in which it was shown that the inhibitory effects of arginine vasotocin on steroidogenesis could be abolished by pertussis toxin, which inactivates Gj via ADP ribosylation [9]. However, effects of LH with or without vasotocin were not reported. Similar studies with forskolin [10] demonstrated that low concentrations of this compound inhibited hCG-stimulated cyclic AMP production and this inhibition was prevented by pertussis toxin. Recently, direct evidence for the presence of Gj in rat Leydig cells has been obtained by the demonstration of a 41000 protein (aj) after [32P]ADP ribosylation [11]. 

The level of cyclic AMP inside the cell is the balance between synthesis, breakdown and release. Increased intracellular levels of cyclic AMP can therefore also be obtained by inhibition of cyclic AMP metabolism with phosphodiesterase inhibitors. There are many reports of the use of these inhibitors to demonstrate increased steroidogenesis at submaximal (but not maximal) concentrations of LH, thus confirming the involvement of cyclic AMP. 

Effects of LH on other cyclic nucleotides have not been reported. Cyclic GMP production in mouse Leydig cells has recently been demonstrated but the stimulation of its synthesis could only be obtained with atriopeptins [12]. 

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Adenylate cyclase the enzyme that catalyzes the formation of cyclic AMP (cAMP). 

The action of adrenaline (or noradrenaline) involves binding to an extracellular receptor, of which there are two classes, the a- and p-receptor. When the hormone binds to the P-receptor, the hormone-receptor complex activates adenyl cyclase, which catalyses the formation of cyclic AMP from ATP. 

Protein kinases modulate the activity of many proteins, but what leads to the activation of a kinase Activation is often a multistep process initiated by hormones (Chapter 15). In some cases, the hormones trigger the formation of cyclic AMP, a molecule formed by cyclization of ATP. Cyclic AMP serves as an intracellular messenger in mediating the physiological actions of hormones, as will be discussed in Chapter 15. The striking finding is that most effects of cAMP... 

Figure 6.1. Adenylate cyclase cataly.ses the formation of cyclic AMP and inorganic phosphate (centre) from ATP (left) cyclic nucleotide phosphodiesterase catalyses the formation of 5 -AMP (right) from cyclic AMP (centre)...

While formation of cyclic AMP from ATP did not occur, the non-enzymatic formation of significant amounts of cyclic GMP from GTP in solutions heated for 3 min at 100°C at a pH near neutrality has been observed [96]. Guanylate cyclase reactions can be terminated by decreasing the pH of the mixture with or without heating. 

Precipitation with Ba(OH)2 and ZnS04 should not be carried out in the presence of high concentrations of ATP, as formation of cyclic AMP by a chemical reaction can occur [3]. Barium sulphate cannot be used for the purification of cyclic GMP solutions because it removes the nucleotide. 

This protein then acts as an enzyme for the formation of cyclic AMP (Fig. 12.44). Normally, the active site is open when the nerve is receiving pain messages, such that cyclic AMP acts as a secondary messenger and passes on the pain messages. However, when the A receptor is activated it probably changes shape and as a result leads to a change in the shape of the cyclase enzyme to close down the active site by which it can make cyclic AMP (see also Appendix 3). 

Cortisol secretion by isolated adrenal pouches in hypoxed dogs was stimulated by cyclic AMP, vasopressin and ACTH O. However, epinephrine and norepinephrine, which stimulate formation of cyclic AMP in fat pads, dichloroisoproterenol, which inhibits adenyl cyclase, or dihydroergota-mine, an inhibitor of action of cyclic AMP in the liver, failed to have any effect. 

Adenyi cyclase - The synthesis of cyclic AMP from ATP is catalyzed by the enzyme adenyi cyclase, which itself is responsive to hormone stimulation in various tissues . Catecholamines have been shown to stimulate the formation of cyclic AMP in a variety of tlssues (Table 1 ), and adenyi cyclase has been shown to be wide-spread in various tissues . Since procedures for the isolation and assay of this enzyme and measurement of the products of its reaction are available - , direct effects of analogues of ATP and/or cyclic AMP (or hormones) on the enzyme can be determined in an in vitro system. 

Epinephrine stimulated glucose oxidation while inhibiting P uptake into phospholipids in incubating dog thyroid slices (A8). Recently Pastan (P3) reported that fluoride, which stimulates the formation of cyclic AMP, increased the rate of glucose oxidation and phospholipid turnover in thyroid slices. However, it did not increase colloid droplet formation in the follicles. Colloid droplet formation is a characteristic action of TSH (D2). Although TSH may indeed stimulate the hexose monophosphate shunt, its specificity must involve other pathways as well. 

The first application of this 0 effect for determining the configuration of an oxygen chiral phosphate ester was the author s determination of the configuration of diastereomeric samples of cyclic [, 0]dAMP, the chiral substrate for studying the stereochemical consequences of the reverse reaction catalyzed by adenylate cyclase (formation of cyclic AMP from ATP), and of the hydrolysis reaction catalyzed by 3, 5 -cyclic nucleotide phosphodiesterase (25) (see Fig. 

Each adrenoceptor is coupled through a G protein to an effector mechanism. Effector mechanisms are proteins that are able to translate the conformational change caused by activation of the receptor into a biochemical event within the cell. All the p-adrenoceptors are coupled via specific G proteins (Gs) to the activation of adenylyl cyclase (14). Thus, when the receptor is stimulated by an agonist, adenylyl cyclase is activated to catalyze the formation of cyclic AMP (cAMP) from ATP. Called a second... 

Many data support the contention that the lipolytic action of prostaglandins is mediated via the adenyl cyclase system which catalyses the formation of cyclic AMP from ATP (Figure 7.8) [356, 357, 370-386], Thus under a variety of experimental conditions (for example, in the presence of... 

Functional receptors for melatonin have been localised and characterised on the pars tuberalis of a number of mammalian species (Morgan etal. 1991, Bockers et al. 1995, Bockmann et al. 1996). Using forskolin (1 [aM), as a non-specific stimulant of adenylate cyclase, melatonin (10 nM) was shown to inhibit the formation of cyclic AMP by 80-90 % in ovine pars tuberalis cells both before and after PercoU centrifugation (Morgan et al. 1991). 

One of the intracellular effectors that is activated by the (G-protein a-subunit)-GTP complex is adenylyl cyclase. This is an integral membrane protein which catalyses the formation of cyclic AMP (cAMP) from ATP (Figure 10.8). cAMP then acts as the second messenger in response to hormones such as glucagon and adrenaline. It is an allosteric activator of protein kinases. cAMP is also formed in the same way in response to a number of neurotransmitters. 

Figure 10.8 Adenylyl cyclase and the formation of cyclic AMP as an intracellular second messenger. (The structure of cyclic GMP is shown in the box.)...

In the uterus, the production of 3, S -cyclic AMP is induced by the activation of the /7-adrenergic receptors and followed by inhibition of the electrical and mechanical activities. The action of oxytocin may be partly due to its ability to prevent the formation of cyclic AMP. The role of the reactive disulfide groups in the peptide receptors has already been discussed. The complexity of the receptor structure is pointed out by the observation that metals potentiate specifically the action of S-S polypeptides on the rat uterus, in such a way that it becomes very sensitive to vasopressin when the perfusion fluid contains a metal The order of metal potency is... 

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