title: Studies on the properties of triphosphoinositide phosphomonoesterase and phosphodiesterase of rabbit iris smooth muscle.
abstract: The rabbit iris smooth muscle has been shown to contain triphosphoinositide phosphomonoesterase (phosphatidyl-myo-inositol-4,5-bisphosphate phosphohydrolase, EC 3.1.3.36) and phosphodiesterase (triphosphoinositide inositoltrisphosphohydrolase, EC 3.1.4.11) activities. Under our experimental conditions about 77% of the phosphomonoesterase and 61% of the phosphodiesterase activities were localized in the particulate fraction. The kinetic properties of the enzymes in the microsomal fraction were examined. The enzyme preparation was specific to polyphosphoinositides; it did not attack phosphatidylinositol under the present assay condition. The effects of Ca2+ and Mg2+ were also studied. Although the microsomal enzymes did not require added divalent cations for their activities, both the phosphomonoesterase and phosphodiesterase were appreciably inhibited by 1 mM EDTA. Phosphodiesterase and phosphomonoesterase were stimulated by Ca2+ and Mg2+, respectively. The demonstration of triphosphoinositide phosphodiesterase in the iris muscle, coupled with the findings that this enzyme is activated by Ca2+ and is not influenced by acetylcholine add further support to our previous conclusion (J. Pharmacol. Exp. Ther. (1978) 204, 655--668; J. Neurochem. (1978) 30, 517--525) that an increased Ca2+ influx, following the interaction between the neurotransmitter and its receptor, could act to stimulate the phosphodiesterase, thus leading to increased triphosphoinositide breakdown and increased phosphatidic acid via increased diacylglycerol.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology; D004492:Edetic Acid / Q000494:pharmacology; D007498:Iris / Q000201:enzymology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009130:Muscle, Smooth / Q000201:enzymology; D010716:Phosphatidylinositols; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D011817:Rabbits; D013347:Subcellular Fractions / Q000201:enzymology; D013379:Substrate Specificity; D013696:Temperature
chemical_list: D010716:Phosphatidylinositols; D004492:Edetic Acid; D010744:Phosphoric Monoester Hydrolases; D010727:Phosphoric Diester Hydrolases; D008274:Magnesium; D002118:Calcium