THE FORMATION OF TWO FORMS OF BATHORHODOPSIN AND THEIR OPTICAL PROPERTIES

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Abstract
Abstract— Using two kinds of rhodopsin preparations (digitonin extract and rod outer segments suspension), we measured changes in absorption spectra during the conversion of rhodopsin or isorhodopsin to a photosteady state mixture composed of rhodopsin, isorhodopsin and bathorhodopsin by irradiation with blue light (437 nm) at 77 K and during the reversion of bathorhodopsin to a mixture of rhodopsin and isorhodopsin by irradiation with red light (> 650 nm) at 77 K. The reaction kinetics could be expressed with only one exponential in the former case and with two exponentials in the latter case. These data suggest that both rhodopsin and isorhodopsin are composed of a single molecular species, while bathorhodopsin is composed of two molecular species, designated as bathorhodopsin1 and bathorhodopsin2. The absorption spectra of these bathorhodopsin were calculated by two different methods (kinetic method and warming‐cooling method). The former was based on the kinetics of the conversion of two forms of bathorhodopsin by irradiation with the red light. The spectra obtained by this method were consistent with those obtained by the warming‐cooling method. Bathorhodopsin1 and bathorhodopsin2 have Λmax at 555 and 538 nm, respectively. The two forms of bathorhodopsin are interconvertible in the light, but not in the dark. Thus, we suggest that a rhodopsin molecule in the excited state relaxes to either bathorhodopsin1 or bathorhodopsin2 through one of the two parallel pathways.