ABSTRACT
It INTRODUCTION The central question of interest here is how 11-cis-retinoids are generated nonphotochemically in the eye subsequent to the bleaching of rhodopsin. A chemical energy source is required to carry out the endothennic biosynthesis of 11-cisretinoids. The source of this chemical energy has been identified. It will be shown how phospholipids can act as group transfer substrates by donating the acyl group at the sn-1 position to vitamin A to produce an all-trans-retinyl ester. The alltrans-retinyl ester is further processed with isomerization and hydrolysis to produce 11-cis-retinol, the precursor to 11-cis-retinal, the visual chromophore (1,2). The necessary free energy to drive the endergonic isomerization process comes from the free energy of hydrolysis of the retinyl ester (1,2). In this overall process, membrane phospholipids provide the ultimate source of energy needed to drive the isomerization reaction, which is a component of the vertebrate visual cycle. This process reveals a new role for membrane phospholipids: they can serve as an energy source.
