Synthesis of retinoids by human retinal epithelium and transfer to rod outer segments

The synthesis and release of 11-cis-retinoids by primary cultures of human retinal pigment epithelium (RPE) and the transfer of these retinoids to co-incubated human rod outer segments (ROS) were studied. Monolayers of 2-3-week-old cultured RPE incorporate tritiated all-trans-retinol, esterify it to the corresponding retinyl palmitate, form 11-cis-retinol and 11-cis-retinaldehyde and release retinaldehyde into the culture medium. The ratio of 11-cis to all-trans isomers of retinol, retinyl palmitate and retinaldehyde formed in the cells along with retinaldehyde released and incorporated into the ROS progressively increases, indicating a progressive increase in the concentration of 11-cis isomer from the time it is formed in RPE cells until its transfer to ROS. Incorporation of 11-cis-retinaldehyde into the ROS is directly related to the amount of albumin present in the media, suggesting the transfer of retinoids from RPE to photoreceptor to be a protein-mediated process. Events leading to isomerization, esterification, oxidation and release of retinoids by human RPE and incorporation of retinoids into ROS can therefore be examined in vitro.