Enzymatic Formation, Degradation, and Action of Cyclic Nucleotides

Abstract

The enzyme, adenylate cyclase catalyzes the conversion of ATP to cyclic AMP and pyrophosphate. The enzyme in most cells appears associated with plasma membrane, with its activity regulated by a receptor(s) exposed to extracellular space (cf. review by Perkins, 1973). Receptors include those for the following hormones: catecholamines, histamine, serotonin, and polypeptides. In addition, the activity of adenylate cyclase is dependent on a variety of other control mechanisms. Magnesium ions are required for enzyme activity. Calcium ions stimulate cyclase activity but are inhibitory at high concentrations. High concentrations of fluoride ions markedly enhance cyclase activity in cell-free preparations, but not in intact cells. GTP appears to play an important regulatory role, enhancing cyclase activity in cell-free preparations and potentiating or permitting hormone responses. Adenosine inhibits adenylate cyclase from certain cell types, such as fat cells, and stimulates cyclases from platelets and neuroblastoma cells. Finally, macromolecular factors which activate or inhibit cyclases have been reported. Adenylate cyclase in homogenates is usually assayed in the presence of a phosphodiesterase inhibitor. When radioactive ATP is used as substrate, often a trapping pool of cyclic AMP is added in addition to the phosphodiesterase inhibitor. The activity of cyclase will depend on the concentration of ATP and the ratio of ATP and magnesium ions. Often, low concentrations of ATP are employed along with an ATP-regenerating system.