Formation of 7-cis- and 13-cis-retinal pigments by irradiating squid rhodopsin

Abstract

Squid [Todarodes pacificus] rhodopsin was irradiated with orange light (> 530 nm) at various temperatures from -190 to 10.degree. C until a photo steadystate mixture was formed. The chromophoric retinals were extracted from the photo steadystate mixtures and their isomer composition was analyzed by high-performance liquid chromatography. In the case of a photo steadystate mixture formed at -85.degree. C, large peaks in the chromatogram were found at the positions 7-cis- and 13-cis-retinals. Each peak was further identified by synthesizing the pigments from these retinals with cattle opsin or apobacteriorhodopsin. Both 7-cis- and 13-cis-retinals were extracted from a photo steadystate mixture formed by irradiation at -40, at 0 or at 10.degree. C. These isomers were scarcely detected in a photo steadystate mixture formed by irradiation at -190.degree. C, though 9-cis-retinal was a major constituent in this mixture. Irradiation of lumirhodopsin at -190.degree. C, produced 7-cis-retinal pigment. Bathorhodopsin may have a conformation to prevent the formation of 7-cis-retinal from the all-trans form and this particular conformation may be relaxed by the conversion of bathorhodopsin to lumirhodopsin.