Supernatant protein factor facilitates intermembrane transfer of squalene

Squalene epoxidation of microsome-associated squalene is stimulated by a soluble protein termed "supernatant protein factor" (SPF) (Saat, Y. A., and Bloch, K. E. (1976)J. Biol. Chem. 251, 5155-5160). In the absence of SPF, the initial rate for microsome-bound squalene epoxidation is rapid for 5 to 10 min but falls off sharply thereafter. SPF does not affect the rapid initial epoxidation rate of reaction but maintains it for longer periods. This SPF effect on enzyme kinetics indicates that SPF facilitates the otherwise rate-limiting access of squalene to the epoxidse site. Trypsin treatment of microsomes totally inactivates squalene epoxidase. When such trypsin-treated squalene-containing microsomes are incubated with normal, squalene-free, enzymatically active microsomes, formation of squalene epoxide is not observed. However, if SPF is included in this system, conversion of squalene to 2,3-oxidosqualene occurs rapidly. Lowering the temperature from 37 degrees to 22 degrees C abolishes the SPF effect in assay systems containing either normal or trypsin-treated plus normal microsomes. These findings show that SPF promotes the transfer of squalene from one microsome population to another, i.e. intermembrane transfer of substrate.