Linkage of Retinal to Opsin and Absence of Phospholipids in Purified Frog Visual Pigment₅₀₀

Abstract

VISUAL pigments are components of the membranous disks of the rod and cone outer segments of vertebrates¹. The traditional picture of visual pigment₅₀₀ (rhodopsin) is a chromophore (retinal) bound to opsin through a Schiff base linkage^2,3. A role has long been sought for the large quantities of phospholipids present in outer segment disks, however, and it has been proposed that, in native visual pigment, retinal is bound by a Schiff base linkage to phosphatidyl ethanolamine (PE) rather than to opsin^4–6. When exposed to light, retinal is considered to migrate from PE to the ε-amino group of a lysine residue in opsin, but Heller⁷ has found that purified visual pigment contains little or no phospholipid (<2 per cent). The following experiments are intended to resolve this contradiction; we have used the recent observation that visual pigment in the outer segment disks is continually being renewed^8,9, to show that purified visual pigment contains no phospholipid.