Isomerization of all-trans-9-and 13-desmethylretinol by retinal pigment epithelial cells

Photoisomerization of 11-cis-retinal to all-trans-retinal triggers phototra nsduction in the retinal photoreceptor cells and causes ultimately the sens ation of vision. 11-cis-Retinal is enzymatically regenerated through a comp lex set of reactions in adjacent retinal pigment epithelial cells (RPE). In this study using all-trans-9-desmethylretinol (lacking the C-19 methyl gro up) and all-trans-13-desmethylretinol (lacking the C-20 methyl group), we e xplored the effects of C-19 and C-20 methyl group removals on isomerization of these retinols in RPE microsomes. The C-19 methyl group may be involved in the substrate activation, whereas the C-20 methyl group causes steric h indrance with a proton in position C-10 of 11-cis-retinol; thus, removal of this group could accelerate isomerization. We found that all-trans-9-desme thylretinol and all-trans-13-desmethylretinol are isomerized to their corre sponding 11-cis-alcohols, although with lower efficiencies than isomerizati on of all-trans-retinol to 11-cis-retinol. These findings make the mechanis m of isomerization through the C-19 methyl group unlikely, because in the c ase of 9-desmethylretinol, the isomerization would have to progress by prot on abstraction from electron-rich olefinic C-9. The differences between all -trans-retinol, all-trans-9-desmethylretinol, and all-trans-13-desmethylret inol appear to be a consequence of the enzymatic properties, and binding af finities of the isomerization system, rather than differences in the chemic al or thermodynamic properties of these compounds. This observation is also supported by quantum chemical calculations. It appears that both methyl gr oups are not essential for the isomerization reaction and are not likely in volved in formation of a transition stage during the isomerization process.