Purification and partial characterization of a cellular retinol-binding protein from human liver

A protein with binding specificity for retinol was purified from human liver. [3H]Retinol was added to liver extracts and the [3H]retinol-binding protein isolated by conventional chromatographic techniques including ion-exchange chromatography on DEAE-Sepharose, gel filtration on Sephadex G-75 and G-50 and preparative isoelectric focusing. The yield was 10-15% in different preparations and the degree of purification was about 3000-fold. The purified protein had a molecular weight of about 15,000 as estimated from both gel filtration and polyacrylamide gel electrophoresis in sodium dodecyl sulphate and homogeneous in several electrophoretic systems. Isoelectric focusing of the purified protein gave a doublet band. Only one fluorescent band at pH 4.70 was seen if the protein solution was incubated with excess retinol prior to isoelectric focusing. The isolated protein did not react with antiserum to the retinol-binding protein of plasma. The amino acid composition and the amino terminal amino acid sequence for the first sixteen amino acids of the purified protein differed significantly from that of the plasma retinol-binding protein.