Two distinct rhodopsin molecules within the disc membrane of vertebrate rod outer segments

The kinetics of the metarhodopsin I-II reaction have been measured over a wide range of temperatures (1-37C ) and pH values (4.5-8) with suspensions containing fragments of bovine rod outer segments. It was found that for all conditions the occurrence of metarhodopsin II could be described by two independent first-order processes. The fast component: slow component amplitude ratio depends upon pH and temperature. The kinetics of the lumi-metarhodopsin I reaction show the same pH dependence for the fast component: slow component amplitude ratio as the one observed for the metarhodopsin II signals. All the results observed could be described with the assumption that rhodopsin itself exists in two conformational states before bleaching which are in a pH and temperature-dependent equilibrium. This hypothesis is confirmed by its ability to explain some apparently anomalous observations in the literature.