Dark Isomerization of Retinals in the Presence of Phosphatidylethanolamine

Abstract

Dark incubation of retinoids (retinyl ester, retinol, retinal and retinaloxime) in suspensions of rod outer segment membranes leads to substantial isomerization (and partial degradation) only in retinals. All-trans, 13-cis and 9-cis-retinal all isomerize at the .DELTA.13 double blood leading to an equilibrium with .apprx. 75% trans and 25% cis isomer at this bond (all-trans .dblarw. 13-cis and 9-cis .dblarw. 9,13-dicis). 11-cis-Retinal isomerizes irreversibly to a mixture of all-trans and 13-cis-retinal. The active compound in the membrane is apparently phosphatidylethanolamine. The amino group, phosphate and the hydrophobic part of the phospholipid are essential. At least 3 factors are important for the phosphatidylethanolamine-catalyzed isomerization: the concentration of phosphatidylethanolamine, the concentration of Schiff base between retinal and phosphatidylethanolamine and the presence of lipid aggregates. A mechanism is proposed which satisfactorily explains the specificity of the isomerization pattern. Reisomerization of all-trans to 11-cis retinal in vivo apparently takes place by fixation of all-trans retinal on an adequate surface (e.g., opsin) and a localized nucleophilic attack on the C-11 atom, followed by trapping of the isomerized chromophore by opsin. Retinal apparently does not occur in vivo as a free intermediate. Direct transfer from 1 protein to another (opsin, retinol dehydrogenase and retinal binding proteins) seems to take place.