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Northup JK, Mansour TE. 
“Adenylate Cyclase from Fasciola Hepatica. 2. Role of Guanine Nucleotides in Coupling Adenylate Cyclase and Serotonin Receptors”. 
Molec. Pharmacol.. 1978;14:820-833.
Activation of fluke adenylate cyclase by serotonin (5-HT) or D-lysergic acid diethylamide (D-LSD) strongly depends upon guanine nucleotide. The activation by 5-HT in the absence of added GTP (4-fold over basal) is substantially reduced or abolished by washing the crude particulate fraction and by preparation of an enriched plasma membrane fraction from the crude particles. Full activation by 5-HT (26-fold over basal) could be obtained even in these preparations by addition of 100 EM GTP to the assay. GTP did not activate adenylate cyclase in the absence of 5-HT. In the presence of 5-HT, GTP activated through a single class of sites with a KA of 1.6 i 0.8 M. This site was highly specific for guanine nucleotide. Only inosine-5'-triphosphate effectively substituted for have GTP, while ATP, UTP, CTP, and xanthine triphosphate were completely ineffective at millmolar concentrations. Synthetic analogs of GTP, guanylyl imidophosphate, guanylyl methylenephosphate, Cr3+-GTP, and guanosine-5'-(3-O-thio)triphosphate, which are poorly hydrolyzed, all activated fluke adenylate cyclase in the absence of 5-HT. Activation by guanylyl imidophosphate and guanosine-5'-(3-O-thiotriphosphate exceeded by 2- to 3-fold the maximal activation by 6-HT and GTP, reaching activities of 3 to 5 nmoles cAMP/min*mg protein at 0.1 mM substrate. The rate, but not maximal extent of activation by guanylyl imidophosphate was increased by 5-HT and LSD. This activation persisted through extensive washing of the particles in the cold. However, at 25íC,. activation by guanylyl imidophosphate reversed with a half-time of 5.4 min. Decay of the activity after treatment with guanosine-5'-(3-O-thio)triphosphate was not first order and was considerably slower than reversal of guanylyl imidophosphate. These results are the consistent with the model that hydrolysis of guanine nucleotide is responsible for rapid reversal from the activated state and that hormonal ligands increase the rate of a rate-limiting step in the activation by GTP.
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