Photophysiological functions of visual pigments.

In order to gain a fundamental understanding of functions of a visual pigment, i.e., photoreception and phototransduction, it is essential to elucidate the molecular structure of visual pigment, its photochemical behavior and connection of the pigment to the molecular physiological amplification mechanism for excitation of a visual cell. A rhodopsin, a rod visual pigment, is composed of an 11-cis-retinal bound with an apo-protein, opsin, through a protonated Schiff-base. Competitive inhibition of beta-ionone on regeneration of rhodopsin from an 11-cis-retinal and cattle opsin demonstrated the existence of a hydrophobic linkage between the beta-ionone ring of the retinal and the hydrophobic region of opsin. Owing to these two linkages, the 11-cis-retinal is fixed in an opsin cleft. As a result, it is endowed with new physiological functions as a chromophore of rhodopsin; the change of 11-cis-retinal (lambda max = 369 nm, epsilon = 26,400) to rhodopsin (lambda max = about 500 nm, epsilon = 40,600) brings not only a spectral shift from near ultraviolet to visible regions and an intensification of the molecular extinction coefficient, but also an increase of a quantum yield of 11-cis-retinal to all-trans form. High quantum yield of rhodopsin suggests rapid formation of the first photoproduct. Study of the first photoproduct was accelerated by the finding of bathorhodopsin, which was formed by irradiation at liquid nitrogen temperatures. The change of rhodopsin to bathorhodopsin has been inferred to be due to a photoisomerization of the chromophore from 11-cis to a twisted all-trans form. This isomerization hypothesis has been verified by the following experimental results. Irradiation of 7-cis- and 9-cis-rhodopsins at liquid nitrogen temperature produced the same bathorhodopsin as that from 11-cis-rhodopsin, indicating that the chromophore of bathorhodopsin should be in all-trans or transoid form. 7-Membered-rhodopsin, in which the rotation of 11-12 double bond of the retinylidene chromophore is locked, did not form bathorhodopsin by excitation of picosecond laser photolysis. This fact indicates that bathorhodopsin is a product formed by photoisomerization of the chromophore. Rhodopsin showed a positive circular dichroism (CD) in the visible while bathorhodopsin showed a remarkable negative CD. The reversal of the sign of CD indicates that not only large conformational change of the retinylidene chromophore occurs during the conversion of rhodopsin to bathorhodopsin, but also the direction of twist of the chromophore reverses. It is inferred that the chromophore of bathorhodopsin is a twisted trans form.(ABSTRACT TRUNCATED AT 400 WORDS)