Light adaption of the cyclic GMP phosphodiesterase of frog photoreceptor membranes mediated by ATP and calcium ions
The light-activated guanosine 3',5'-cyclic monophosphate (cyclic GMP) phosphodiesterase (PDE) of frog photoreceptor membranes has been assayed by measuring the evolution of protons that accompanies cyclic GMP hydrolysis. The validity of this assay has been confirmed by comparison with an isotope assay used in previous studies (Robinson et al. 1980. J. Gen. Physiol. 76: 631-645). The PDE activity elicited by either flash or continuous dim illumination is reduced if ATP is added to outer segment suspensions. This desensitization is most pronounced at low calcium levels. In 10(-9) M Ca/sup + +/, with 0.5 mM ATP and 0.5 mM GTP present, PDE activity remains almost constant as dim illumination and rhodopsin bleaching continue. At intermediate Ca/sup + +/ levels (10-7-10-5M) the activity slowly increases during illumination. Finally, in 10(-4) and PDE activity is more a reflection of the total number of rhodopsin molecules bleached than of the rate of the rhodopsin bleaching. At intermediate or low calcium levels a short-lived inhibitory process is revealed by observing a nonlinear summation of responses of the enzyme to closely spaced flashes of light. Each flash makes PDE activity less responsive to successive flashes, and a steady state is obtained in which activation and inactivation are balanced. It is suggested that calcium and ATP regulation of PDE play a role in the normal light adaption processes of frog photoreceptor membranes.
- OSTI ID:
- 5527682
- Journal Information:
- J. Gen. Physiol.; (United States), Vol. 77:5
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
MEMBRANES
PHOTOCHEMICAL REACTIONS
PHOSPHODIESTERASES
BIOCHEMICAL REACTION KINETICS
ATP
BIOASSAY
CALCIUM
FROGS
GUANOSINE
PHOTOCHEMISTRY
ALKALINE EARTH METALS
AMPHIBIANS
ANIMALS
AQUATIC ORGANISMS
CHEMICAL REACTIONS
CHEMISTRY
ELEMENTS
ENZYMES
ESTERASES
HETEROCYCLIC COMPOUNDS
HYDROLASES
KINETICS
METALS
NUCLEOSIDES
NUCLEOTIDES
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PURINES
REACTION KINETICS
RIBOSIDES
VERTEBRATES
550200* - Biochemistry
551000 - Physiological Systems