title: Mechanism of isomerization of 11-cis-retinal in lipid dispersions by aromatic amines.
abstract: It has previously been shown that retinotoxic, primary aromatic amines catalyze the isomerization of 11-cis-retinal to its all-trans congener after Schiff base formation [Bernstein, P.S., Fulton, B.S., & Rando, R.R. (1986) Biochemistry 25, 3370-3377]. This process led to the short-circuiting of the visual cycle and the observed retinotoxicity when it occurred in vivo. The catalysis was also observed to occur in vitro in phosphatidylcholine-based vesicles but not in hydrocarbon solutions. The rate of isomerization of an aromatic amine Schiff base of 11-cis-retinal in the phospholipid vesicles was typically 10(3)-fold more rapid than in hydrocarbon solutions. In this article, the mechanistic basis of this apparently membrane-specific catalysis is described. It was found that the rate enhancement effect observed was independent of the lipid used. Moreover, a bilayer structure was not important because rate enhancements were also observed in micelles. The rapid isomerization rates observed in lipid dispersions appear not be free radical initiated because free radical quenching agents, such as alpha-tocopherol and beta-carotene, had little effect on the isomerization rates. It was further found that aliphatic amines, such as n-dodecylamine, could be substituted for the aromatic amines in phospholipid. Finally, and most importantly, it was found that the isomerization of the aromatic amine retinal Schiff bases in phospholipid vesicles was acid-catalyzed. It is concluded that the rate enhancements observed for the isomerization of 11-cis-retinal-aromatic amine Schiff bases in lipid dispersions over that in hydrocarbon solvents are due to the occurrence of acid-base catalysis in the former.
mesh_terms: D000588:Amines; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D008823:Micelles; D010713:Phosphatidylcholines; D012172:Retinaldehyde; D012176:Retinoids; D013329:Structure-Activity Relationship
chemical_list: D000588:Amines; D008823:Micelles; D010713:Phosphatidylcholines; D012176:Retinoids; D012172:Retinaldehyde