Regeneration of rhodopsin and isorhodopsin in rod outer segment preparations: absence of effect of solvent parameters

Regeneration of rhodopsin and isorhodopsin from bleached bovine rod outer segment suspensions and 11- or 9-cis retinal occurs at similar rates (second-order rate constants of 1000 M-1s-1 and 900 M-1s-1, respectively) and to a similar extent (maximal regeneration at pH 5.5-7.0). Formation of both pigments follows pseudo-first order kinetics in an excess of retinal at pH 7.0. These results suggest that the mechanisms of regeneration are similar for the 11-cis and 9-cis retinal isomers. Solvent ionic strength, buffer concentration, viscosity, and polarity have no significant effect on the rate of pigment formation, providing further evidence that the retinal binding site in bleached bovine rod outer segments is well protected from the hydrophilic environment.