Two forms of intermediates of frog rhodopsin in rod outer segments

Abstract

Using frog rod outer segments, we measured changes of the absorption spectrum during the conversion of rhodopsin to a photosteady-state mixture composed of rhodopsin, isorhodopsin and bathorhodopsin by irradiation with blue light (440 nm) at − 190°C and during the reversion of bathorhodopsin to a mixture of rhodopsin and isorhodopsin by irradiation with red light (718 nm) at − 190°C. The reaction kinetics was expressed by one exponential in the former case and by two exponentials in the latter. These results suggest that rhodopsin is composed of a single molecular species, while bathorhodopsin is composed of two kinds of molecular species designated as batho 1-rhodopsin and batho 2-rhodopsin. On warming the two forms of bathorhodopsin, each bathorhodopsin converted to its own lumirhodopsin, metarhodopsin I and finally a free all- trans-retinal plus opsin. The absorption spectra of the two forms of bathorhodopsin, lumirhodopsin and metarhodopsin I were measured at − 190°C. We infer that a rhodopsin molecule in the excited state relaxes to either batho 1-rhodopsin or batho 2-rhodopsin, and then converts to its own intermediates through one of the two parallel pathways.