Demonstration in Serum of two Physiological Forms of the Human Retinol Binding Protein*

Abstract. The contents of prealbumin, retinol binding protein, and retinol in normal human serum have been estimated. The amounts of prealbumin were, on average, 6 times larger than those of RBP, that is a two‐fold molar excess of prealbumin over RBP. Retinol and RBP were present in about the same molar amounts. Prealbumin occurred in two molecular forms in serum. About half of the prealbumin was bound to RBP (prealbumin‐RBP complex) and half was in free form. In spite of the molar excess of prealbumin, small amounts of free RBP were encountered. The free RBP was isolated from normal serum and displayed about the same characteristics as RBP isolated from the prealbumin‐RBP complex. Some significant differences were noted however. The free RBP had a lower content of retinol and one amino acid residue less of arginine. The contents of prealbumin, RBP, and retinol were also estimated in the sera of patients subjected to chronic haemodialysis. The amounts of prealbumin and retinol were normal or somewhat low whereas the values for RBP were considerably elevated. The size distribution of RBP and retinol in the serum of one of these patients showed that the amount of free RBP was considerably elevated whereas the amount of the prealbumin‐RBP complex was normal. Retinol was apparently associated only with the prealbumin‐RBP complex. Various analyses revealed that RBP, isolated in free form from serum, did not bind to prealbumin in contrast to RBP from the prealbumin‐RBP complex. It is suggested that the free species of RBP, present in serum, represents a transitory form which has fulfilled its physiological role and will rapidly be degraded by the kidney.