Rhodopsin phosphorylation suggests biochemical heterogeneities of retinal rod disks

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Abstract
Frogs (Rana pipiens) were injected subcutaneously with (3H)‐leucine and allowed to incorporate the radioactive amino acid into newly assembled disks in the retinal rod outer segment. The labeled disks served as a temporal market for following the turnover of rod outer segments. Animals were killed at different times after injection and outer segments were isolated and phosphorylated with ATP in the light. The visual pigment (as isorhodopsin) was regenerated with 9‐cis retinal, extracted, and chromatographed on epichlorohydrin triethanolamine cellulose so that phosphorylated pigment could be separated from unphosphorylated pigment. The ratio of (3H)‐radioactivity of phosphorylated pigment to that of unphosphorylated pigment was then plotted against the time after injection. The ratio was high when (3H)‐labeled disks were largely associated with the basal region of the rod and decreased as the labeled disks moved toward the rod apical region. The results were interpreted as suggesting that newer disks are phosphorylated preferentially to older disks. Papain digestion of (3H)‐labeled disks indicated that rhodopsin in newer disks is more susceptible to proteolysis than that in older disks.