Distribution of retinol isomerase in vertebrate eyes and its emergence during retinal development

Ocular tissue homogenates were incubated in darkness with [11,12-3H] all-trans retinol. Formation of radiolabeled 11-cis retinol was used as an index of isomerase activity and was determined by high-performance liquid chromatography. Isomerase was found in the eyes of cattle, human, rat, chicken, turtle, goldfish and frog, representing the mammals, birds, reptiles, bony fishes and amphibians. The enzyme was concentrated in the pigment epithelium (RPE). Variable activity was found in the retina, where the amount of radiolabeled 11-cis retinol formed under standard incubation conditions at protein concentrations of 0.03-1.08 mg/ml was 6.4 +/- 6.0% of that in the RPE-choroid. Using the same methodology, we could not detect isomerase in the retinas of three cephalopods (Octopus, Sepia and Loligo). In rats, isomerase was present at postnatal day 10 but not at postnatal days 0 and 4. Therefore, the expression in the RPE of retinol isomerase, which is essential for the formation of rhodopsin in the developing photoreceptors, is coordinated with the emergence of the rod outer segment in the retina. However, the continued expression of this enzyme in RCS rats does not depend on the presence of photoreceptors, because loss of photoreceptors was not associated with an absence of isomerase activity in RCS rats. Our findings suggest that a reciprocal flow of retinoids between the retina and the site of isomerase action in the RPE is a feature common to the visual cycle in all vertebrates.