Human oxysterol-binding protein. I. Identification and characterization in liver

The function of cellular oxysterol-binding proteins is not known. Because of the proposed receptor-like role of these proteins in regulating cholesterol synthesis and the importance of the liver in the cholesterol metabolism of the body, we have studied the oxysterol-binding sites found in specimens of human liver. A protein that binds with high affinity to certain oxygenated derivatives of cholesterol has been identified and characterized in several ways. When 25-hydroxy-[3H]cholesterol is used as ligand, specific binding components can be identified in cytosolic liver extracts fractionated on sucrose density gradients in the fractions sedimenting at 4S and 7S. Cholesterol and steroid hormones do not compete for these binding components, but several oxysterols do. In cytosolic extracts, all binding appears to be a single class kinetically, with an apparent Kd of 28 x 10(-9) mol/L. We found the protein responsible to have a pI of about 4.8. Purification of the protein allowed preparation of an antiserum and subsequent immunoprecipitation of a photoaffinity-labeled component from crude cytosolic extracts. Electrophoresis of the immunoprecipitates revealed a single specifically labeled protein band, with a mol wt of about 57,000. The protein described here may have a regulatory role in the synthesis of cholesterol in the human.