Open AccessDark Isomerization of Retinals in the Presence of Phosphatidylethanolamine
Author(s) -
GROENENDIJK Gerard W. T.,
JACOBS Cor W. M.,
BONTING Sjoerd L.,
DAEMEN Frans J. M.
Publication year - 1980
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1980.tb06002.x
Subject(s) - isomerization , retinaldehyde , chemistry , retinal , opsin , phosphatidylethanolamine , stereochemistry , double bond , cis–trans isomerism , photochemistry , rhodopsin , biochemistry , phospholipid , membrane , phosphatidylcholine , organic chemistry , catalysis
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 Δ 13 double bond leading to an equilibrium with approximately 75% trans and 25% cis isomer at this bond (all‐ trans ⇔ 13‐ cis and 9‐ cis ⇔ 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.