52] Bleaching intermediate kinetics of rhodopsin, metarhodopsin I, and metarhodopsin II

Abstract

This chapter discusses the bleaching intermediate kinetics of rhodopsin, metarhodopsin I, and metarhodopsin II. When cattle rhodopsin absorbs light under near-physiological conditions, it is rapidly converted to metarhodopsin I (MI), which, in turn, decays to metarhodopsin II (MII). From the very earliest determination of the rate of MII accumulation, it has been reported that the kinetics are not simple first order. Rather, the rate has been accurately described by the sum of two exponentials. It appears that one fraction of the rhodopsin, which absorbs light and produces MII, does so at a slightly faster rate than the other. The ratio of rate constants is always about 5:1, but the relative amounts of the “fast-appearing MII” and “slow-appearing MII” depend on many experimental parameters. In terms of specific effects on MI→MII kinetics, temperature is a key parameter. The kinetic behavior of the transition of meta I to meta II that one observes is strongly dependent on two variables—namely, temperature and solvating environment.