Purification and partial characterization of a cellular carotenoid- binding protein from ferret liver

Abstract

A cellular carotenoid-binding protein was purified to homogeneity from β-carotene-fed ferret liver utilizing the following steps: ammonium sulfate precipitation, ion exchange, gel filtration, and affinity chromatography. The final purification was 607-fold. [14C]β-Carotene co-purified with the binding protein throughout the purification procedures. SDS-PAGE of the purified protein showed a single band with an apparent molecular mass of 67 kDa. Scatchard analysis of the specific binding of the purified protein to β-carotene showed two classes of binding sites, a high affinity site with an apparent K(d) of 56 x 10-9 M and a low affinity site with a K(d) of 32 x 10-8 M. The B(max) for β-carotene binding to the high affinity site was 1 mol/mol, while that for the low affinity site was 145 mol/mol. The absorption spectrum of the complex showed a 32-nm bathochromic shift in λ(max) with minor peaks at 460 and 516 nm. Except for α-carotene and cryptoxanthin, none of the model carotenoids or retinol competed with β-carotene binding to the protein. Thus, a specific carotenoid-binding protein of 67 kDa has been characterized in mammalian liver with a high degree of specificity for binding only carotenoids with at least one unsubstituted β-ionone ring.