Evidence for a Noncatalytic Carrier Protein in Cholesterol Biosynthesis

Abstract

Abstract A requirement for the 105,000 x g supernatant of rat liver (S105) is demonstrated for five different reactions in cholesterol biosynthesis. This is best delinated using a buffer-washed acetone powder of liver microsomes. Incubation of S105 alone with each substrate yields chemically unchanged substrate. Evidence that S105 contains a protein responsible for this requirement includes the observations that it is nondialyzable, heat-labile, and destroyed by trypsin. Chromatography of S105 on Sephadex G-75 yields a protein peak which travels at the void volume and has associated with it both endogenous cholesterol and the ability to stimulate sterol synthesis as assayed in the conversion of Δ7-cholestenol to Δ5,7-cholestadienol. In addition, virtually all of the cholesterol present in the 105,000 x g supernatant of liver is bound to protein. These results plus other observations support the following hypothesis. S105 contains a noncatalytic carrier protein (STEROL CARRIER PROTEIN) 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 Sterol Carrier Protein (SCP) may be of key importance in the understanding of the enzymatic synthesis of cholesterol.