Studies on the Interaction Between Prealbumin, Retinol-binding Protein, and Vitamin A

Abstract

The dissociation of the human vitamin A-transporting protein complex into free prealbumin and free retinol-binding protein (RBP) was studied by gel chromatography under varying conditions of pH and ionic strength. The interaction between the two proteins was diminished at a low ionic strength and complete dissociation seemed to occur at an ionic strength of 0.002. Variations in pH did not appear to alter seriously the affinity between prealbumin and RBP as the prealbumin-RBP complex was the predominant component in the pH range of 8 to 3. The release of vitamin A from free RBP and from prealbumin-RBP was studied at various values of pH and ionic strength. A facilitated release of retinol from RBP was found at conditions of low ionic strength, whereas variations in pH gave rise to only moderate changes in the release velocity of vitamin A. The binding of prealbumin to RBP seemed to strengthen the interaction between RBP and retinol. These observations suggested a conformational change of one or both of the two proteins at low ionic strength. Sedimentation velocity analyses performed at various values of ionic strength indicated that RBP was amenable to conformational changes at a low ionic strength. Variations in ionic strength did not seem to alter the sedimentation behavior of prealbumin. Immunochemical analyses showed that the prealbumin-RBP complex dissociated in the presence of antibodies directed against RBP, whereas antibodies directed against prealbumin did not seem to affect the interaction between prealbumin and RBP. These data suggest that RBP is more liable to conformational changes than is prealbumin.