Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:4.6.1.1 (adenylate cyclase)
 19,190 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Mechanisms of stimulus-response coupling in platelets are as complex and varied as the compounds that elicit the responses. The complexities are compounded by feedback mechanisms from substances released or synthesized by platelets as well as by "cross-talk" between signal transduction pathways. Examples of cross-talk include the ability of epinephrine to inhibit platelet adenylate cyclase through a G protein-mediated mechanism while causing platelet aggregation by some other mechanism and the ability of cAMP to inhibit thrombin-stimulated diacylglycerol formation. Despite the complexities, certain common threads are beginning to emerge, such as the involvement of G proteins in transducing many receptor-mediated processes, the involvement of relatively few second messenger pathways and the role of calcium in many of events leading to platelet responses, and the common involvement of protein kinases in carrying out second messenger function. The latter offers a useful assay for the effect of many agonists because they lead to the phosphorylation of specific proteins that can readily be detected by radioautography. Indeed, the emphasis has shifted in the past 10 years from relatively crude measurements of platelet function such as aggregation to precise, quantifiable measurement of processes such as protein phosphorylation and calcium release, which are indicators of the fundamental mechanisms involved in platelet function and thus serve as assays of these processes. On the other hand, there are other pathways and regulators yet to be discovered, notably regarding the action of epinephrine and the regulation of phospholipase A2. In addition, certain receptors remain elusive, including those for ADP and eicosanoids. The mechanisms of action of thrombin and cathepsin G, which involve their proteolytic activities, also remain an enigma. The combination of new insights into second messenger function and the techniques of molecular biology will allow many of these problems to be resolved, providing new approaches to therapy of thromboembolic disorders.
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PMID:Mechanisms of platelet activation and inhibition. 215 2

Hyperthyroidism or increased thyroid function has been reported in many patients with trophoblastic tumors. In these cases, greatly increased human chorionic gonadotropin (hCG) levels and suppressed TSH levels suggest that hCG has thyrotropic activity. Recent investigations have clarified the structural homology not only in the hCG and TSH molecules but also in their receptors, and this homology suggests the basis for the reactivity of hCG with the TSH receptor. The clinical significance of the thyrotropic action of hCG is now also recognized in normal pregnancy and hyperemesis gravidarum. Highly purified hLH binds to recombinant hTSH receptor and is about 10 times as potent as purified hCG in increasing cAMP. The beta-subunits of hCG and hLH share 85% sequence identity in their first 114 amino acids but differ in the carboxy-terminal peptide because hCG beta contains a 31-amino acid extension (beta-CTP). A recombinant mutant hCG that lacks beta-CTP showed almost identical potency to LH on stimulation of recombinant hTSH receptor. If intact hCG were as potent as hLH in regard to its thyrotropic activity, most pregnant women would become thyrotoxic. One of the roles of the beta-CTP may be to prevent overt hyperthyroidism in the first trimester of pregnancy when a large amount of hCG is produced by the placenta. Nicked hCG preparations, obtained from patients with trophoblastic disease or by enzymatic digestion of intact hCG, showed approximately 1.5- to 2-fold stimulation of recombinant hTSH receptor compared with intact hCG. This suggests that the thyrotropic activity of hCG may be influenced by the metabolism of the hCG molecule itself. Deglycosylation and/or desialylation of hCG enhances its thyrotropic potency. Basic hCG isoforms with lower sialic acid content extracted from hydatidiform moles were more potent in activating adenylate cyclase, and showed high bioactivity/immunoactivity (B/I) ratio in CHO cells expressing human TSH receptors. This is consistent with the finding that the beta-CTP truncated hCG with higher thyrotropic potency is substantially deglycosylated and desialylated in the beta-subunit relative to intact hCG because all four O-linked glycosylation sites occur within the missing C-terminal extension. The desialylated hCG variant also interacts directly with recombinant hTSH receptors transfected into human thyroid cancer cells. There is thyroid-stimulating activity in sera of normal pregnant women, and this correlates with serum hCG levels. The thyroid gland of normal pregnant women may be stimulated by hCG to secrete slightly excessive quantities of T4 and induce a slight suppression of TSH, perhaps being about 1 mU/L less than nongravid levels, but not high enough to induce overt hyperthyroidism. Maternal thyroid glands may secrete more thyroid hormone during early pregnancy in response to the thyrotropic activity of hCG that overrides the normal operation of the hypothalamic-pituitary-thyroid feedback system. Biochemical hyperthyroidism associated with hyperemesis gravidarum has been attributed to hCG. In patients with hyperemesis gravidarum, thyrotropic in serum correlated with hCG immunoreactivity, and the severity of vomiting as indicated by clinical and biochemical parameters correlated with the degree of thyroid stimulation. To understand the thyrotropic action of hCG, it is necessary to know whether hCG activates the same domain of the TSH receptor as does TSH. The identification of the molecular structure of the hCG isoform with the highest thyrotropic potency will resolve the enigma of gestational thyrotoxicosis and the hyperthyroidism associated with trophoblastic disease and hCG-producing tumors.
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PMID:Thyrotropic action of human chorionic gonadotropin. 856 83

We have recently characterized a novel angiotensin II/vasopressin (Ang II/AVP) dual receptor coupled to adenylate cyclase and responding with equal sensitivity to Ang II and AVP. To gain insight into putative renal physiological roles of the dual Ang II/AVP receptor, we determined its pharmacological binding properties and renal immunocytochemical distribution. The effective displacement of [3H]AVP by [1-deamino-Val14,D-Arg8]-vasopressin (DVDAVP), a specific antidiuretic AVP analogue, supports a V2-type AVP receptor characteristic of the Ang II/AVP receptor. Displacement of 125I-Ang II by losartan but not by PD 123319 defines the Ang II/AVP receptor as a novel AT1 receptor isoform coupled to adenylate cyclase, in contrast to prototype Ca(2+)-mobilizing AT1 receptors. Neither Ang II nor AVP displace each other, corroborating the predicted discrete binding domains for Ang II and AVP but presenting an enigma for the dissection of putative Ang II- and AVP-specific hierarchical roles of the dual Ang II/AVP receptor. The renal cytolocalization of the Ang II/AVP receptor to the outer medullary thick ascending limb tubules and inner medullary collecting ducts is consistent with the well-established AVP stimulation of sodium and water reabsorption in these tubules. These data suggest that the Ang II/AVP receptor might provide the molecular basis for the observed similar stimulatory effects of Ang II and AVP on renal tubular sodium and fluid reabsorption at physiological hormone concentrations.
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PMID:Renal immunocytochemical distribution and pharmacological properties of the dual angiotensin II/AVP receptor. 909 83