Renewal of visual pigment in photoreceptors of the blowfly

Summary Spectrophotometric measurements of photoreceptors 1–6 in the blowfly demonstrate that rhodopsin undergoes a continuous renewal. This involves, in the dark, the slow degradation of rhodopsin whereas metarhodopsin is degraded at a much faster rate. The effect of light is to reduce the rate at which metarhodopsin is degraded, i.e. the rate is inversely related to the intensity of the light. Rhodopsin synthesis is dependent on the presence of 11-cis retinal which is formed via a photoreaction from all-trans retinal resulting from the breakdown of rhodopsin and/or metarhodopsin: the biosynthesis of rhodopsin is therefore a light dependent process. Light of the blue/violet spectral range was found to mediate the isomerization of all-trans retinal into the 11-cis form. It is proposed that this stereospecificity is the result of all-trans retinal being bound to a protein. On the basis of the results a visual pigment cycle is proposed.