The interaction of retinol-binding protein with its plasma-membrane receptor.

125I-labelled retinol-binding protein (RBP) bound to specific receptors in human placental brush-border membranes. Binding at 22 degrees C reached equilibrium within 15 min, but prolonged incubation caused a subsequent decline. Scatchard analysis of the equilibrium binding data at 22 degrees C and 15 min showed high-(3.0 +/- 2.7 x 10(-9) M) and low-(9.5 +/- 3.5 x 10(-8) M) affinity binding components. 125I-RBP, bound to membranes at 22 degrees C for 15 min and subsequently dissociated with excess unlabelled RBP, exhibited biphasic dissociation kinetics consisting of fast and slow components of release. In contrast, Scatchard analysis and dissociation kinetics of the binding that had taken place at 37 degrees C for 1 h showed the fast-dissociating/low-affinity binding component, but little of the slow-dissociating/higher-affinity binding component. When 125I-RBP, after incubation with membranes at 37 degrees C for 1 h, was re-isolated and subjected to dissociation kinetic analysis using a fresh batch of membranes, the fast-dissociating phase was unchanged, but the slow phase was almost absent. The complex kinetics were interpreted in terms of a heterogeneity in RBP consisting of high- and low-affinity binding forms. The higher-affinity-binding form is thought to be converted into the lower-affinity state on binding to the receptor. Transthyretin inhibited 125I-RBP binding to the membrane, suggesting that free, rather than transthyretin-associated, RBP bound to the receptor. The RBP receptor was trypsin-, heat- and thiol-group-specific-reagent sensitive and was highly specific for RBP.