Kinetic studies on the formation and decay of metarhodopsins from bovine retinas

The kinetics of the equilibration of MRI in equilibrium MRII were studied over a range of temperature and pH. Two methods were used to evaluate the relaxation time of this process. The rate of disappearance of MRI or the rate of appearance of MRII were measured after a light flash. The established equilibrium between these two intermediates at low temperatures was subjected to pressure perturbations. This latter technique was used to show that the equilibrium was truly reversible and to determine the relaxation time. From these experiments the equilibrium constants and forward and reverse rate constants were evaluated under different conditions. The simplest interpretation of our results is that the reaction can be described by a single kinetic step. This step involves a conformation change which is accompanied by a change in pK of approx. 3 units of an ionizing group of rhodopsin, resulting in the uptake of a proton during the formation of MRII.