Vitamin A deficiency reduces the concentration of visual pigment protein within blowfly photoreceptor membranes

Visual pigment extracts prepared from rhabdomeric membranes of vitamin A deficient blowflies contain a 5-10 times lower concentration of rhodopsin than extracts from flies which were raised on a vitamin A rich diet. Spectrophotometry showed that digitonin-solubilized rhodopsin from blowfly photoreceptors R1-6 has an absorbance maximum at about 490 nm, but no unusually enhanced beta-band in the ultraviolet. The extracts did not contain detectable concentrations of other visual pigments nor was there any evidence for the presence of photostable vitamin A derivatives. Sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that the concentration of opsin in the rhabdomeric membrane is significantly reduced in vitamin A deficient flies compared to normal flies. The results indicate that the synthesis of opsin or its incorporation into the photoreceptor membrane is regulated by the chromophore concentration in the receptor cell. Furthermore, our findings open up the possibility that differences in the spectral absorption and excitability of photoreceptors from normal and vitamin A deficient flies result from the differing opsin content of the rhabdomeres.