Retinoids bound to interstitial retinol-binding protein during light and dark-adaptation

High-performance liquid chromatography was used to determine the types and amounts of retinoids bound to interstitial retinol-binding protein (IRBP) during light- and dark-adaptation in frogs. IRBP was separated from CRBP and CRA1BP by ion-exchange chromatography and quantitated by determining the amount of Serva Blue R dye bound to it in stained sodium dodecyl sulfate polyacrylamide gels. The amount of IRBP was not significantly different in light- and dark-adapted eyes (0.15 ± 0.05 nmol/ eye compared with 0.18 ± 0.08 nmol/ eye). In the dark-adapted state, RBP bound mainly 11- cis retinol and 11- cis retinal in quantities that summed to about 1 mol/mol RBP. After the onset of light-adaptation, all-trans retinol increased from its very low dark-adapted evel, peaked at 0.2 mol/mol IRBP and then declined to the dark-adapted level again. Concomitantly, the otal retinoid bound to IRBP fell, mainly because there was a drop in the amount of 11- cis retinal. During dark-adaptation, the amount of 11- cis retinal increased. No significant changes were seen in the amount of 11- cis retinol in light and darkness. These findings support the hypothesis that when rhodopsin is bleached IRBP transports all- trans retinol from the retina to the pigment epithelium and that it delivers 11- cis retinal to the rod outer segments for rhodopsin regeneration.