Binding of squalene, lanosterol, desmosterol, and cholesterol to proteins in brain and liver 105,000 g supernatant fractions: Evidence for specific binding sites

Abstract

The binding of squalene, lanosterol, desmosterol, and cholesterol to proteins in 105,000 g supernatant fraction (S₁₀₅) from brain and liver of rats was investigated. The S₁₀₅ fractions from both tissues contain specific binding sites for sterols, which are sensitive to trypsin. The dissociation constants for squalene and sterol protein complexes were in the range of 10⁻⁶ M and were not appreciably different for proteins in brain and liver S₁₀₅. Competition studies revealed that both brain and liver S₁₀₅ contain one receptor protein which binds lanosterol and is specific for methyl sterols, and a second receptor which binds both desmosterol and cholesterol. Binding of 7-dehydrocholesterol reported by others must occur at a third independent site since this compound does not interfere with the binding of lanosterol, desmosterol, or cholesterol. Although binding of squalene to proteins in brain and liver S₁₀₅ does occur, we were unable to show the specificity of squalene binding. The concentration of desmosterol and cholesterol binding sites, which ranged from 6 to 10 nmol/mg protein, was 3- to 5-fold higher than the concentration of squalene and lanosterol binding sites (1.6–2.3 nmol/mg protein). The brain S₁₀₅ from suckling rats contained fewer binding sites for desmosterol and cholesterol than the brain S₁₀₅ from weaned rats. However, the concentration of lanosterol binding sites in brain S₁₀₅ did not show an age-dependent change. The receptor proteins in brain and liver appear to be identical.