Activation of bovine retinal rod outer segment cGMP-specific phosphodiesterase by the transducin-GTP complex in a physiologically significant range of free calcium ion concentrations.

The kinetic behavior of cGMP-specific phosphodiesterase in a totally bleached bovine retinal rod outer segment suspension was studied by the pH-metric method at high and low concentrations of free calcium ions (≈100 μM and <10 nM, respectively). The phosphodiesterase was activated by low GTP concentrations (∼1-2 μM) that were comparable with the concentration of G-protein transducin, the GTP-binding alpha-subunit of which is the intrinsic activator of photoreceptor phosphodiesterase. The results allow the suggestion that, besides the earlier described system of RGS proteins participating in acceleration of GTP hydrolysis, rod outer segments also contain an additional Ca-dependent mechanism to inactivate so-called 'free' transducin, i.e. active transducin that has not managed to interact with phosphodiesterase within the time restricted by the duration of photoreceptor response. [ABSTRACT FROM AUTHOR]