Dark isomerization of retinals in the presence of phosphatidylethanolamine.

1. Dark incubation of retinoids (retinyl ester, retinol, retinal, retinaloxime) in suspensions of rod outer segment membranes leads to substantial isomerization (and partial degradation) in the case of retinals only. 2. All-trans, 13-cis and 9-cis-retinal all isomerize at the delta 13 double bond leading to an equilibrium with approximately 75% trans and 25% cis isomer at this bond (all-trans in equilibrium 13-cis and 9-cis in equilibrium 9,13-dicis). 11-cis-Retinal isomerizes irreversibly to a mixture of all-trans and 13-cis-retinal. 3. The active compound appears to be phosphatidylethanolamine present in the membrane. The amino group and the phosphate, as well as the hydrophobic part of the phospholipid are essential. 4. At least three factors are important for the phosphatidylethanolamine-catalyzed isomerization as studied with the 13-cis isomer: the concentration of phosphatidylethanolamine, the concentration of Schiff base between retinal and phosphatidylethanolamine and the presence of lipid aggregates. 5. Based on these observations a mechanism is proposed, which satisfactorily explains the specificity of the isomerization pattern. 6. It is suggested that reisomerization of all-trans to 11-cis retinal in vivo 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. 7. It is further concluded that retinal does not occur in vivo as a free intermediate. Direct transfer from one protein to another (opsin, retinol dehydrogenase, retinal binding proteins) seems to take place.