Deactivation Kinetics of the Transduction Cascade of Vision

The response of the retinal rod cell to a dim flash lasts less than a second. This phototransduction is mediated by a guanine nucleotide-binding (G) protein cascade in which rhodopsin is the receptor, transducin is the G-protein, and the cGMP-specific phosphodiesterase (PDE) is the effector. Photoexcited rhodopsin activates transducin which in turn activates PDE. For this underlying biochemistry to be kinetically compatible with the photoresponse, both transducin and PDE must be deactivated in subsecond times. We report here direct measurements of their deactivation kinetics. The rate of heat release when transducin and PDE hydrolyze, respectively, GTP and cGMP was measured using time-resolved microcalorimetry. With only GTP present, the heat pulse comes from the activation of transducin and its subsequent deactivation by endogenous GTP hydrolysis. The nonhydrolyzable analog guanine 5'-[γ-thio]triphosphate was used to distinguish between these two processes: about 40% of the total heat is due to activation. From the time course of the deactivation heat, the active lifetime of transducin is