title: Phosphodiesterase and GTPase in rod outer segments. Kinetics in vitro.
abstract: The hydrolysis of cyclic guanosine monophosphate (cyclic GMP) and of guanosine triphosphate (GTP) by the broken rods of the frog retina after a flash of light have been studied in vitro with a constant perfusion method. The activation has an onset apparently instantaneous as observed with the existing possible time resolution of 3 s. The activation is followed by a partial inactivation that does not bring the activity back to the pre-flash level. GTP or the non-hydrolysable guanyl-5'-ylimidodiphosphate (GMP-PNP) is required for the normal light-activation of the phosphodiesterase and in its absence both the speed of activation and the sensitivity are greatly reduced. The activation speed, the sensitivity (threshold at approx. 0.00004% bleaching), and the kinetic constants do not exclude a direct role in the process of excitation for the phosphodiesterase and suggest a subsidiary but as yet undefined role for the GTPase.
mesh_terms: D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism*; D000818:Animals; D001001:Anura; D006152:Cyclic GMP / Q000378:metabolism; D003624:Darkness; D004307:Dose-Response Relationship, Radiation; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D010727:Phosphoric Diester Hydrolases / Q000528:radiation effects; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D010786:Photoreceptor Cells / Q000378:metabolism* / Q000528:radiation effects; D011893:Rana esculenta
chemical_list: D006165:Guanylyl Imidodiphosphate; D006160:Guanosine Triphosphate; D010744:Phosphoric Monoester Hydrolases; D010727:Phosphoric Diester Hydrolases; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D020558:GTP Phosphohydrolases; D006152:Cyclic GMP