Weak or strong lights will activate visual receptor rod disk membrane (RDM) cyclic GMP phosphodiesterase (PDE) In the presence of GTP cofactor1,2. A similarly activated GTPase can exhaust small amounts of initially present GTP to deactivate the PDE. However, further additions of GTP reactivate PDE without more light, and deactivation by simple GTP depletion takes minutes or more, even at GTP concentrations 100 to 1,000 times lower than physiological levels1,2. A more rapid deactivation mechanism must exist if modulation of cytoplasmic cyclic GMP by light is to play a role on the tune scale (seconds) of events in vision3,4. We report here that ATP is essential to such rapid control and that its presence permits multiple cycles of activation-deactivation. The complete control mechanism seems to involve gamma phosphate transfer from both ATP and GTP.
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