Retinoid specificity of interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP), the predominant protein in the interphotoreceptor matrix of retina, has been implicated in transfer of retinoids between retinal pigment epithelium and photoreceptor cells. In this work, the interactions of several retinoids with IRBP were studied in order to clarify whether the protein displays specificity toward particular forms of these ligands. The equilibrium dissociation constants of complexes of 11-cis- and all-trans-retinols and retinaldehydes with IRBP were measured. It was found that IRBP contains two binding sites for 11-cis-retinaldehyde and for all-trans-retinaldehyde and retinol. Binding affinities followed the order: 11-cis-retinaldehyde > all-trans-retinol > all-trans-retinaldehyde > 11-cis-retinol. The kinetic parameters of the dissociation of these retinoids from binding sites on IRBP were measured by monitoring the rate of transfer of the retinoids from IRBP to synthetic unilamellar vesicles. 11-cis-Retinaldehyde and all-trans-retinol were found to dissociate from the strong binding site of IRBP 3-4-fold slower than all-trans-retinaldehyde and 11-cis-retinol. The higher binding affinities and the slower rates of dissociation from IRBP displayed by 11-cis-retinaldehyde and by all-trans-retinol correspond to the physiological need to shuttle these particular retinoids between pigment epithelium and photoreceptor cells across the interphotoreceptor matrix as part of the visual cycle.