title: The gain of rod phototransduction: reconciliation of biochemical and electrophysiological measurements.
abstract: We have resolved a central and long-standing paradox in understanding the amplification of rod phototransduction by making direct measurements of the gains of the underlying enzymatic amplifiers. We find that under optimized conditions a single photoisomerized rhodopsin activates transducin molecules and phosphodiesterase (PDE) catalytic subunits at rates of 120-150/s, much lower than indirect estimates from light-scattering experiments. Further, we measure the Michaelis constant, Km, of the rod PDE activated by transducin to be 10 microM, at least 10-fold lower than published estimates. Thus, the gain of cGMP hydrolysis (determined by kcat/Km) is at least 10-fold higher than reported in the literature. Accordingly, our results now provide a quantitative account of the overall gain of the rod cascade in terms of directly measured factors.
authors.affiliation: Department of Ophthalmology, Harvard Medical School, Boston, Massachusetts 02114, USA.
authors.forename: I B
authors.initials: IB
authors.lastname: Leskov
authors.forename: V A
authors.initials: VA
authors.lastname: Klenchin
authors.forename: J W
authors.initials: JW
authors.lastname: Handy
authors.forename: G G
authors.initials: GG
authors.lastname: Whitlock
authors.forename: V I
authors.initials: VI
authors.lastname: Govardovskii
authors.forename: M D
authors.initials: MD
authors.lastname: Bownds
authors.forename: T D
authors.initials: TD
authors.lastname: Lamb
authors.forename: E N
authors.initials: EN
authors.lastname: Pugh
authors.forename: V Y
authors.initials: VY
authors.lastname: Arshavsky
mesh_terms: D000818:Animals; D001001:Anura; D020134:Catalytic Domain; D006152:Cyclic GMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D004789:Enzyme Activation / Q000187:drug effects; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D008027:Light; D008954:Models, Biological; D010727:Phosphoric Diester Hydrolases / Q000737:chemistry / Q000378:metabolism; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism*; D012243:Rhodopsin / Q000737:chemistry / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000378:metabolism / Q000648:ultrastructure; D015293:Transducin / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D014785:Vision, Ocular / Q000502:physiology* / Q000528:radiation effects
chemical_list: D016244:Guanosine 5'-O-(3-Thiotriphosphate); D012243:Rhodopsin; D010727:Phosphoric Diester Hydrolases; D015293:Transducin; D006152:Cyclic GMP