Is Retinal-Phosphatidyl Ethanolamine the Chromophore of Rhodopsin?

Abstract

SEVERAL laboratories have reported the presence of the Schiff base of retinal and phosphatidyl ethanolamine (retinal-PE) in bovine rod outer segments^1–3. Poincelot et al.² claim that this compound is the chromophore of rhodopsin. Heller found only traces of phospholipid in purified bovine rhodopsin⁴, however, and presented evidence that retinal is attached directly to opsin in a substituted aldimine linkage⁵. Hall and Bacharach⁶ injected frogs with ³²P and found large amounts of radioactivity in rod phospholipids, but none in purified rhodopsin. The purified rhodopsins of Heller^4,5 and Hall and Bacharach⁶ had an absorption maximum at 500 nm and could be bleached by light. We recently determined the molar concentration of retinal-PE in bovine rods and compared it with the molar concentration of rhodopsin. We present here our preliminary findings.