Characteristics of a Vitamin A-transporting Protein Complex Occurring in Human Serum

Abstract

A retinol-transporting protein complex has been isolated in good yield from normal human serum. The protein complex consists of prealbumin and the actual vitamin carrier, the human retinol-binding protein (RBP). The interaction between prealbumin and RBP is dependent on the salt concentration. Dissociation occurred at low ionic strength, which enabled each of the two proteins to be isolated. The purified proteins appeared homogeneous by Ouchterlony immunodiffusion, immunoelectrophoresis, and ultracentrifugation. The molecular weights determined by equilibrium ultracentrifugation and from sedimentation diffusion data were for prealbumin-RBP 85,000, for prealbumin 64,000, and for RBP 21,000. The frictional ratios (f/f0) were low for all three components, indicating a close to spherical appearance also for the prealbumin-RBP complex. On polyacrylamide gel electrophoresis the prealbumin was homogeneous, whereas the RBP separated into four molecular species. Amino acid analyses of the RBP indicated the presence of 181 residues, giving a calculated molecular weight of 20,600. Both prealbumin and RBP were devoid of carbohydrate. The only species of vitamin A bound to the RBP was the alcohol form (retinol). It is proposed that 1 mole of vitamin A is transported by 1 mole of RBP.