Photolysis Intermediates of Human Rhodopsin

Photochemical studies were conducted on human rhodopsin at 20 °C to characterize the intermediates which precede the formation of metarhodopsin II, the trigger for the enzyme cascade mechanism of visual transduction. Human rhodopsin was prepared from eyes which had previously been used for corneal donations. Time resolved absorption spectra collected from 10⁻⁸ to 10⁻⁶ s after photolysis of human rhodopsin in detergent suspensions displayed biexponential decay kinetics. The apparent lifetimes obtained from the data are 65 ± 20 and 292 ± 25 ns, almost a factor of 2 slower than the corresponding rates in bovine rhodopsin. The spectra can be fit well using a model in which human bathorhodopsin decays toward equilibrium with a blue-shifted intermediate (BSI) which then decays to lumirhodopsin. Spectra and kinetic rate constants were determined for all these intermediates using a global analysis which showed that the spectra of the human intermediates are remarkably similar to bovine intermediates. Microscopic rate constants derived from this model are 7.4 × 10⁶ s⁻¹ for bathorhodopsin decay and 7.5 × 10⁶ s⁻¹ and 4.6 × 10⁶ s⁻¹ for the forward and reverse reactions of BSI, respectively. Decay of lumirhodopsin to later intermediates was studied from 10⁻⁶ to 10⁻¹ s after photolysis of rhodopsin in human disk membrane suspensions. The human metarhodopsin I ⇄ metarhodopsin II equilibrium appears to be more forward shifted than in comparable bovine studies.