UNBLEACHABLE RHODOPSIN WITH AN 11-CIS-LOCKED 8-MEMBERED RING RETINAL - THE VISUAL TRANSDUCTION PROCESS

Visual transduction occurs through photorhodopsin, the primary photopr oduct of rhodopsin, which relaxes to bathorhodopsin and a series of ot her intermediates until it reaches the metarhodopsin II stage, upon wh ich the enzymatic cascade leading to vision is activated. Despite adva nces in areas related to visual transduction, the triggering process i tself, a key problem in the chemistry of rhodopsin, has remained unsol ved. In order to clarify the extent of involvement of the chromophoric excited state versus the 11-cis to trans isomerization, and as an ext ension of past studies with 11-cis-locked seven-membered ring rhodopsi n (Rh7), 11-cis eight- and nine-membered ring retinal analogs, ret8 an d ret9, respectively, have been synthesized. The bulkiness of the tetr amethylene bridge in ret8 led to numerous unexpected obstacles in atte mpts to reconstitute a ret8-containing rhodopsin (Rh8) embedded in lip id bilayer membranes. These obstacles were solved by using methylated rhodopsin which gave MeRh8 containing 11-cis-ret8 as its chromophore. MeRh8 exhibited UV-vis and CD spectra very similar to those of native rhodopsin (Rh); furthermore, the quantum efficiency of photorhodopsin formation was comparable to that of Rh. Flash photolytic studies of Rh 8 and other ring analogs [Mizukami, T., Kandori, H., Shichida, Y., Che n, A.-H., Derguini, F., Caldwell, C. G., Bigge, C. F., Nakanishi, K., & Yoshizawa, T. (1993) Proc. Natl. Acad. Sci. U.S.A. 90,4072-4076] cou pled with the present enzymatic studies with MeRh8 and a series of dih ydro-rhodopsins have led to the conclusion that (i) charge translocati on in the excited state does occur; however, (ii) full cis-trans isome rization around 11-ene involving the entire polyene moiety is required for efficient transduction to occur. Repeated attempts to incorporate ret9 into opsin have as yet not been successful.