REDUCED LIGHT-DEPENDENT PHOSPHORYLATION OF AN ANALOG VISUAL PIGMENT CONTAINING 9-DEMETHYLRETINAL AS ITS CHROMOPHORE

9-Demethyl rhodopsin (9dR), an analog of vertebrate rhodopsin, consists of opsin and a covalently attached chromophore of 11-cis 9-demthylretinal. Electrophysiological evidence that photoactivated 9dR (9dR*) undergoes abnormally slow deactivation in salamander rods (Corson, D. W., Cornwall, M. C., and Pepperberg, D. R. (1994) Visual Neurosci, 11, 91-98) raises the possibility that opsin phosphorylation, a reaction involved in visual pigment deactivation, operates abnormally on 9dR*. This possibility was tested by measuring the light-dependent phosphorylation of 9dR in preparations obtained from bovine rod outer segments. Outer segment membranes containing 9dR or regenerated rhodopsin were flash-illuminated in the presence of [gamma-P-32]ATP and rhodopsin kinase, further incubated in darkness, and then analyzed for opsin-bound [P-32]P-i. [P-32]P-i incorporation by 9dR* increased with both incubation period and bleaching extent but, under all conditions tested, was less than that measured in rhodopsin controls. Results obtained with 30-s incubation periods indicated that the maximal initial rate of incorporation by 9dR* is about 25% of that by photoactivated rhodopsin, The results imply that the low incorporation of P-i by 9dR* results from a reduced rate of phosphorylation by rhodopsin kinase and are consistent with the prolonged lifetime of 9dR* determined electrophysiologically.