title: The effects of octanol on the late photointermediates of rhodopsin.
abstract: Membrane suspensions of unperturbed rhodopsin and rhodopsin perturbed with 2.5 mM octanol were photolyzed with 477 nm laser pulses at 20 degrees C and 35 degrees C. Changes in absorbance were monitored at times ranging from 1 microsecond to 80 ms after excitation. The data were analyzed using singular value decomposition, global exponential fitting and kinetic modeling. A recently proposed model involving the photointermediate Meta-I380 (T. E. Thorgeirsson, J. W. Lewis, S. E. Wallace-Williams, and D. S. Kliger, Biochemistry 32, 13861-13872, 1993) fits data for samples with and without octanol. Comparison of the microscopic rates shows this alcohol accelerates the formation of Meta-II via Meta-I380. Activation and equilibrium thermodynamic parameters obtained from Arrhenius plots suggest that octanol reduces the entropy increase in forming both Meta-I380 and Meta-II. It also lowers the enthalpy of Meta-I380 relative to Lumi and of Meta-II relative to Meta-I480. To help determine whether octanol affects the protein directly or indirectly through the lipid bilayer, similar experiments were conducted using rhodopsin solubilized in 0.13% dodecyl maltoside with and without octanol. Spectral shifts in the presence of octanol suggest that a direct protein interaction exists in addition to previously reported effects dependent on membrane free volume.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Santa Cruz 95064, USA.
authors.forename: T L
authors.initials: TL
authors.lastname: Mah
authors.forename: I
authors.initials: I
authors.lastname: Szundi
authors.forename: J W
authors.initials: JW
authors.lastname: Lewis
authors.forename: S
authors.initials: S
authors.lastname: Jäger
authors.forename: D S
authors.initials: DS
authors.lastname: Kliger
mesh_terms: D000818:Animals; D002417:Cattle; D007700:Kinetics; D007834:Lasers; D008027:Light; D000442:Octanols / Q000494:pharmacology*; D010782:Photolysis; D012243:Rhodopsin / Q000737:chemistry* / Q000187:drug effects / Q000528:radiation effects; D012374:Rod Cell Outer Segment / Q000378:metabolism; D013053:Spectrophotometry
chemical_list: D000442:Octanols; D012243:Rhodopsin