The gain of rod phototransduction: reconciliation of biochemical and electrophysiological measurements

Neuron. 2000 Sep;27(3):525-37. doi: 10.1016/s0896-6273(00)00063-5.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Anura
  • Catalytic Domain
  • Cyclic GMP / metabolism
  • Dose-Response Relationship, Drug
  • Electrophysiology
  • Enzyme Activation / drug effects
  • Guanosine 5'-O-(3-Thiotriphosphate) / pharmacology
  • Light
  • Models, Biological
  • Phosphoric Diester Hydrolases / chemistry
  • Phosphoric Diester Hydrolases / metabolism
  • Retinal Rod Photoreceptor Cells / metabolism*
  • Rhodopsin / chemistry
  • Rhodopsin / metabolism
  • Rod Cell Outer Segment / metabolism
  • Rod Cell Outer Segment / ultrastructure
  • Transducin / chemistry
  • Transducin / metabolism
  • Transducin / pharmacology
  • Vision, Ocular / physiology*
  • Vision, Ocular / radiation effects


  • Guanosine 5'-O-(3-Thiotriphosphate)
  • Rhodopsin
  • Phosphoric Diester Hydrolases
  • Transducin
  • Cyclic GMP