Kinetic evidence for a conformational transition in rhodopsin

AFTER light absorption, rhodopsin decays through a series of spontaneous reactions 1,10 , the kinetics of which have been the object of considerable study. In a variety of conditions the decay kinetics of bathorhodopsin (prelumirhodopsin), lumirhodopsin, and metarhodopsin I (meta I) have been found to be non-first order, but are consistent with a set of parallel, independent first-order reactions 2–10 . These kinetics have been assumed to result from the existence of multiple forms of each of these intermediates, and it has been proposed that rhodopsin itself exists in different forms or different environments in some conditions 7 . The kinetics of meta I→ meta II in both dodecyltrimethylammonium bromide- and digitonin-solubilised phospholipid-free rhodopsin are also consistent with two forms of meta I which decay independently (J. G. S. and E. O. Plante, unpublished). We show here that the effect of temperature changes on these kinetics is consistent with the hypothesis that rhodopsin exists in two forms in equilibrium resulting from the accessibility of two stable conformational states separated by a small difference in free energy.