cGMP phosphodiesterase of retinal rods is regulated by two inhibitory subunits.

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Abstract
The cGMP phosphodiesterase (PDE) of cattle retinal rod outer segments comprises three types of subunits: the two heavy catalytic ones, PDE.alpha. and PDE.beta., each around 85 kDa, and the light inhibitory one, PDE.gamma. or I (11 kDa). The relative stoichiometry is usually assumed to be 1:1:1. PDE activation in the visual transduction cascade results from removal of the inhibitor by the .alpha. subunit of transducin (T.alpha.). The stoichiometric complex T.alpha.-I, separated from activated PDE, has been isolated and characterized. Analyzing now the activated PDE, we find that it still contains some inhibitor content of the native enzyme and the other totally devoid of it. The same two species are observed upon activation of PDE by very short tryptic proteolysis, which specifically degrades the inhibitor. This leads us to conclude that the composition of the native enzyme is PDE.alpha..beta.-I2. The two inhibitory subunits are differentially bound, sequentially removable, and exchangeable between the native complex PDE.alpha..beta.-I2 and the fully active PDE.alpha..beta.. The possibility of this exchange precludes as yet an unambiguous estimate of the actual activity of the intermediate complex PDE.alpha..beta.-I. The differential binding the exchangeability of the inhibitors raises the possibility of a fast, diffusion controlled, switch-off mechanism of PDE activity after a flash, which would shortcut the inactivation resulting from the slow GTPase rate of transducin.

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