Some properties of old and new rhodopsin in single Bufo rods

Rod photoreceptors renew the membranous disks of the outer segments (ROS). New disks are assembled at the proximal base and old disks are shed at the distal tip. Rhodopsin, the major protein of the disk, remains with the disk into which it was inserted. Thus, it is true that the oldest rhodopsin is at the tip and the newest at the base. A microspectrophotometer is used to examine the properties of rhodopsin in the two ends of the toad ROS. No differences between the two are found in absorption spectrum, concentration, dichroism, photoconversion rates, or lateral diffusion rates. Regeneration of rhodopsin from the bleached state is also studied but cannot be used to discriminate old from new rhodopsin because the point of entry of regeneration retinoids and/or their concentrations cannot be controlled. However, a new insight into pigment regeneration in the living toad eye is gained: regeneration is faster in the basal disks than in the distal.