A MODEL FOR THE RECOVERY KINETICS OF ROD PHOTOTRANSDUCTION, BASED ON THE ENZYMATIC DEACTIVATION OF RHODOPSIN

We propose a model for the recovery of the retinal rod photoresponse a fter a short stimulus. The approach describes the enzymatic deactivati on of the photoactivated receptor, rhodopsin, by simple enzyme kinetic s. An important feature of this description is that the R deactivatio n obeys different time laws, depending on the numbers of R formed per isce membrane and available enzyme molecules. If the enzyme works bel ow substrate saturation, the rate of deactivation depends linearly on the number of R, whereas for substrate saturation a hyperbolic relati on-the well-known Michaelis-Menten equation-applies. This dichotomy is used to explain experimental finding that the relation between the sa turation time of the photoresponse after short illumination and the fl ash strength has two sharply separated branches for low and high flash intensities (up to similar to 10% bleaching). By relating both branch es to properties of the enzymatic rhodopsin deactivation, the new mode l transcends the classical notion of a constant characteristic lifetim e of activated rhodopsin, With parameters that are plausible in the li ght of the available data and the additional information that the deac tivating enzyme, rhodopsin kinase, and the signaling G-protein, transd ucin, compete for the active receptor, the slopes of the saturation fu nction are correctly reproduced.