Diurnal variations in amino acid incorporation into inner segment opsin

Polyacrylamide gel and immunoelectrophoresis were used to measure the specific activity of nascent inner segment opsin following injections of labeled amino acids into frogs at different times during the diurnal cycle. Animals injected just after light onset showed the greatest incorporation of label into opsin, while animals injected during the night period had the lowest levels of isotope incorporation. As determined by rocket immunoelectrophoresis, there was a 13-fold decline in specific activity in animals injected during the late night hours in comparison with animals injected just after light onset. In contrast, rod outer segment rhodopsin-specific activity did not vary as greatly. Animals subjected to a 12-hour light-12-hour dark diurnal cycle incorporated only twice the radiolabeled amino acids into their rhodopsin compared with animals injected and maintained in constant dark for 24 hours. Taken together, these experiments suggest that adult Rana pipiens accumulate opsin within their inner segments during the dark phase of the diurnal cycle and that alterations of this pool effect changes in inner segment opsin specific activity. Differences in outer segment rhodopsin specific activity are probably due to averaged metabolic differences between constant dark and light-dark animals.