title: Localization and properties of bovine photoreceptor 5'-nucleotidase.
abstract: The main light-activated enzyme of the vertebrate photoreceptor is cGMP phosphodiesterase, whose product is GMP. GMP would be broken down to guanosine by the enzyme 5'-nucleotidase on the cytoplasmic (extradiscal) surface of the disks. The presence of 5'-nucleotidase on the cytoplasmic surface was verified by using sucrose continuous gradients to show its association with the photoreceptors and by using disk preparation and concanavalin A binding to demonstrate its presence on the extradiscal surface. Further studies using detergents and freeze-thaw showed that an even higher 5'-nucleotidase activity is present on the intradiscal surface; however, it is the smaller cytoplasmic surface activity that is potentially relevant to the physiology. The 5'-nucleotidase on the extradiscal surface is light insensitive, has a broad optimal pH range, shows a divalent cation dependence, and is competitively inhibited by nucleoside di- and triphosphates. When the data determined experimentally were extrapolated to physiological conditions, we obtained a decay time constant for GMP breakdown by 5'-nucleotidase in the range of 0.4 to 1.06 s. This time constant is in the range of the time constants of the fall of rod cell receptor potential, suggesting a possible role for GMP level in visual transduction.
mesh_terms: D015720:5'-Nucleotidase; D000818:Animals; D002417:Cattle; D002462:Cell Membrane / Q000201:enzymology; D002499:Centrifugation, Density Gradient; D003208:Concanavalin A / Q000494:pharmacology; D003902:Detergents / Q000494:pharmacology; D004355:Drug Stability; D007700:Kinetics; D009708:Nucleotidases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010786:Photoreceptor Cells / Q000201:enzymology*; D012243:Rhodopsin / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000201:enzymology; D012995:Solubility; D013696:Temperature
chemical_list: D003902:Detergents; D003208:Concanavalin A; D012243:Rhodopsin; D009708:Nucleotidases; D015720:5'-Nucleotidase