Purification and properties of a soluble protein activator of rat liver squalene epoxidase.

A soluble rat liver protein, termed "supernatant protein factor" (SPF), that stimulates microsomal squalene epoxidase has been purified approximately 11,000-fold. The most highly purified preparation obtained by isoelectric focusing shows a single coincident peak for activity and protein (the isoelectric point, pI, was 6.74). SPF is about 95% pure, judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and it migrates to a position corresponding to an apparent molecular weight of 47,000. An amino acid analysis of SPF is presented, and the properties of SPF and of the various soluble protein activators of microsomal sterol biosynthesis described by other laboratories are compared.