ISOMERIZATION-REACTIONS OF RETINOIDS IN THE VISUAL-SYSTEM

Double-bond isomerization reactions in the retinoids provide a means t o initiate and then to terminate signal transduction. An apt example i s found in the visual system, where light causes the photoisomerizatio n of the 11-cis-retinal protonated Schiff base of rhodopsin to its all -trans congener. In order for vision to proceed, the reverse thermal i somerization must take place. Because 11-cis-retinoids are approximate ly 4 kcal/mol higher in energy than their all-trans counterparts, an e nergy source is required for this conversion. It has been established that the biosynthetic pathway for 11-cis-retinoids involves the direct processing of all-trans retinyl esters to 11-cis-retinol. In this pro cess, the negative free energy of ester hydrolysis provides the thermo dynamic driving force for the endergonic trans to cis isomerization. S ince retinyl esters are themselves generated by the transfer of acyl g roups from phospholipids, the energy for the isomerization reaction or iginates in the membrane phospholipids.