Polypeptide Activators of Cholesterol Side-Chain Cleavage

A rate-determining step in the cAMP-dependent action of ACTH on adrenal steroid biosynthesis is the interaction of cholesterol substrate with the cholesterol side-chain cleavage cytochrome P-450 in the mitochondrion. This interaction is rapidly and reversibly sensitive to inhibitors of protein synthesis. For this reason a hormone-dependent, labile protein activator of cholesterol side-chain cleavage has long been postulated as an obligatory intermediate in the tropic regulation of this reaction. Applying recent advances in liquid chromatography, two-dimensional gel electrophoresis, and enzyme reconstitution into liposomes, several laboratories have now reported the isolation and partial characterization of polypeptide candidates for the status of "labile protein."