Phosphodiesterase and GTPase in rod outer segments. Kinetics in vitro.
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.
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
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