Studies on the enzymatic synthesis of cholesterol: Use of a liver acetone powder

Abstract

We have defined special conditions for the preparation of an acetone powder of rat liver microsomes which is capable of converting squalene to cholesterol in high yield. This preparation is also useful for the demonstration of cofactor requirements for certain reactions in sterol biosynthesis. Buffer washed acetone powders are virtually completely dependent upon the 105,000 ×g supernatant of rat liver (S₁₀₅) for activity, yet S₁₀₅ by itself is inert in sterol synthesis. The ability of S₁₀₅ to stimulate sterol synthesis is heat liable, nondialyzable, trypsin sensitive, and has been partially purified by ammonium sulfate precipitation and chromatography on Sephadex G-200. These results plus other experiments support the following hypothesis: the 105,000 ×g supernatant of rat liver (S₁₀₅) contains a noncatalytic carrier protein (Sterol Carrier Protein or SCP) which originates from the endoplasmic reticulum, binds the substrate, and makes the substrate reactive to the sterol synthesizing enzymes present in the acetone powder of liver microsomes. The participation of SCP may be an important general mechanism in the biological synthesis of cholesterol.