Cytosolic proteins that bind oxygenated sterols. Cellular distribution, specificity, and some properties.

The differential binding of cholesterol and 25-hydroxycholesterol to cytosolic proteins in various types of cells was investigated. 25-Hydroxycholesterol taken up by six different established cell culture lines and by mouse spleen cells in primary culture was bound to cytosolic components which, during velocity gradient centrifugation, displayed sedimentation coefficients of approximately 5 S and 8 S. In contrast, cholesterol taken up by the cells was concentrated near the bottom (approximately 20 S) of the sucrose density gradient but was distributed throughout. Results with primary cultures of mouse fetal liver differed from thos obtained with other cell cultures in that both sterols appeared principally in a 5 S band. Further characterization of the binding components from intact L cells indicated that binding of 25-hydroxycholesterol to the 8 S fraction was saturable and reversible, whereas binding to the 5 S band was not saturable. The 8 S 25-hydroxycholesterol complex involved a protein with a relatively long half-life. The complex was essentially stable at 0 degrees C but dissociated slowly at 25 degrees C. Sulfhydryl functions were not required for sterol binding, and formation of the complex was not dependent upon cAMP. Competition studies with intact cells and with isolated cytosol indicated that a number of other oxygenated sterols bind to 8 S sites occupated by 25-hydroxycholesterol. Those sterols which are potent suppressors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) activity competed for these binding sites, while those which do not suppress the reductase did not compete for them. These studies suggest that the binding of oxygenated sterols to the 8 S cytosolic component may be in some sense specific, while that to the 5 S component is nonspecific. The kinetics of formation and dissociation of the 8 S 25-hydroxy-cholesterol.protein complex and of the suppression of 3-hydroxy-3-methylglutaryl-CoA reductase in the presence of the diol was also consistent with a postulated role for the complex in the regulation of the enzyme.