Conversion of retinol to 3,4-didehydroretinol in the tadpole

The conversion of retinol to 3,4-didehydroretinol in bullfrog tadpoles was studied by injecting [3H] all-trans retinol into the peritoneal cavity. The specific activities of retinoids in the eye and the rest of the body at various time intervals after the injection were then determined by HPLC (high-performance liquid chromatography). Radioactivity was observed in ocular 3,4-didehydroretinyl esters after 2 days and their specific activity increased throughout the 2 weeks of experiment. This demonstrates that tadpoles can convert retinol to its 3,4-didehydro derivative. In vitro experiments performed on isolated eye cups also suggested that the ocular tissues could convert retinol to 3,4-didehydroretinol. In the eye, the specific activity of porphyropsin or all-trans 3,4-didehydroretinal (extracted by the denaturing solvent acetone) exceeded that of the all-trans 3,4-didehydroretinyl esters in storage. This suggests that the main ocular store of 3,4-didehydroretinyl esters does not constitute a precursor pool for porphyropsin synthesis.