Involvement of NADPH-cytochrome c reductase in the rat liver squalene epoxidase system

Abstract Microsomal squalene epoxidase has previously been solubilized with Triton X-100 and resolved into fractions, F A and F B , by DEAE-cellulose chromatography (Ono T. and Bloch K. (1975) J. Biol. Chem. 250, 1571–1579). It has now been found that F B is identical with NADPH-cytochrome c reductase (denoted F PT , EC 1.6.2.3). Although both NADPH and NADH served as electron donors, the former was preferred for squalene epoxidase activity in the reconstituted system of F A and F B . F B is characterized by its ability to reduce cytochrome c by NADPH. In place of F B , partially purified F PT was tested for its ability to support squalene epoxidation in the presence of F A . A stepwise purification of the deoxycholate-solubilized F PT yielded an increase in specific F PT activity with a parallel increase in squalene epoxidase activity. Bromelain-solubilized F PT was less effective. Rabbit antisera preparations to the purified F PT solubilized with trypsin were shown to inhibit concomitantly F PT . activity and squalene epoxidase activity. These observations support the concept that squalene epoxidation is primarily mediated via a flavoprotein, NADPH-cytochrome c reductase, and a terminal oxidase, squalene epoxidase, which is distinct from cytochrome P -450.