Can metarhodopsin I activate rod outer segment phosphodiesterase?
The rod photoreceptors of vertebrate retinas contain a cGMP phosphodiesterase (PDE) that is activated by light. The light is absorbed by rhodopsin that activates an intermediate GTP-binding protein; this species then activates the PDE. Photo-excited rhodopsin passes through a series of transient states, and the purpose of this study is to identify the earliest state that interacts with the GTP-binding protein and thus activate the PDE. The majority of evidence points to this state being metarhodopsin II (MII), but PDE activation is seen at low temperatures where the rhodopsin reaction sequence is not expected to pass beyond the metarhodopsin I (MI) stage. Light thresholds for PDE activation have been determined under conditions where little MII is generated, and these are compared with the concentration of MII. The conclusion is that for a criterion threshold of PDE activity, the MII concentration is constant, irrespective of the amount of MI present, which suggests that MI cannot activate the PDE system.
Knowles
A
A
Department of Biochemistry, University of Bristol, England.
Pepe
I M
IM
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism*; D000818:Animals; D002417:Cattle; D006152:Cyclic GMP / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects / Q000528:radiation effects; D019204:GTP-Binding Proteins / Q000378:metabolism; D008027:Light; D010786:Photoreceptor Cells / Q000201:enzymology*; D012168:Retinal Pigments / Q000494:pharmacology*; D012243:Rhodopsin / Q000031:analogs & derivatives / Q000494:pharmacology*; D012374:Rod Cell Outer Segment / Q000201:enzymology*; D013053:Spectrophotometry; D013696:Temperature
D012168:Retinal Pigments; C013110:metarhodopsins; D012243:Rhodopsin; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D019204:GTP-Binding Proteins; D006152:Cyclic GMP