Purification and Partial Characterization of a Lutein-binding Protein from the Midgut of the Silkworm

A lutein-binding protein was purified from fifth instar larval midgut of Bombyx mori by a combination of ammonium sulfate fractionation and three chromatographic procedures, gel filtration, chromatofocusing, and anion exchange chromatography. The protein has a pI of 5.4 and an apparent molecular mass of 35,000 Da, as determined by a linear gradient SDS-polyacrylamide gel electrophoresis. The lutein-protein complex is water-soluble and more stable than the carotenoid or protein alone. The carotenoid moiety was identified by thin layer chromatography, light absorption spectroscopy, and high performance liquid chromatography as all-trans-lutein. Lutein is specifically and stoichiometrically bound to the protein, with a ratio of 3 mol of lutein per mol of protein. Binding of lutein (absorption maximum in hexane at 454 nm) to the apoprotein results in a marked red spectral shift of about 38 nm, giving rise to absorption maxima at 432, 462, and 492 nm in 20 mM Tris-HCl, pH 7.0. The lutein-protein complex is characterized by fine spectral structure indicating that lutein is in a relatively rigid environment. This protein is distributed in equal amounts throughout the midgut and in all developmental stages of the larval B. mori.