A novel retinol-binding protein from rat. Purification and partial characterization.

A novel retinol-binding protein, resolved during purification into two essentially identical forms, has been discovered in the rat. It was purified to apparent homogeneity, using whole neonatal rat pups as source. The protein is distinct from other known retinol-binding proteins by behavior during purification, spectra of bound retinol, and immunochemical reactivity. It is a single polypeptide chain with molecular weight of about 16,000. The protein binds all-trans-retinol as an endogenous ligand. Retinol bound to the protein exhibited considerably altered absorbance and fluorescence excitation spectra compared to free retinol in organic solvent. The retinol-binding protein was found by radioimmunoassay in a number of tissues of the neonatal rat. However, liver and intestine had levels 100-fold higher than any other tissues examined. The intestine of the adult rat had levels 500-fold higher than any other tissue examined, with a decreasing gradient from jejenum to colon. The high levels in intestine suggest this protein may have a role in the absorption of retinol.