Linkage of Retinal to Opsin

RHODOPSIN, the visual pigment of vertebrate rods, has been shown to consist of a chromophore (11-cis retinal) bound to a protein (opsin)^1–2. It has been proposed that the linkage is a Schiff base between phosphatidyl ethanolaniine (PE) and retinal and that when exposed to light, the retinal migrates from PE to the ε-amino-group of a lysine residue in opsin^3–7. Most of the support for this theory comes from the observation that N-retinylidenephosphatidylethanolamine (N-RPE) can be extracted in the dark from rod outer segments (ROS)^3,4. Furthermore, N-retinylphosphatidylethanolamine (N-RH₂PE) has been extracted from ROS preparations after treating the visual pigment with acid and NaBH₄—conditions which are assumed fix retinal to its “native” binding site through a secondary amine linkage⁷. All these studies, however, were carried out on crude extracts of ROS in various detergents. These crude extracts contain large amounts of phospholipid and retinal which is not bound to opsin. Thus, the isolation of N-RPE from crude ROS extracts does not necessarily point to its involvement in the binding of retinal to opsin. In contradiction to these reports are findings that purified visual pigment contains no phospholipid^9,10 and that the molar concentration of N-RPE in bovine ROS is less than that of rhodopsin¹¹. We have taken advantage of the observation that visual pigment in the outer segment disks is continually being renewed¹² to label the rhodopsin with ³H-retinal and to show in yet another way that N-RPE does not exist in purified visual pigment.