In mammals, the synthesis of pregnenolone from cholesterol is the first step in the process of steroid hormone synthesis and also a rate limiting step. This reaction is carried out in the mitochondria of the adrenal cortex, catalyzed by the cytochrome CYP11A1 (also known as P450scc), and works together with CYP11A1, including Adx and AdR. CYP11A1, Adx, and AdR belong to the cholesterol hydroxylase/lyase system (CH/L), which catalyzes the initial step of steroid synthesis in mammals, namely the process of producing pregnenolone from cholesterol. Pregnenolone is an important precursor of steroid hormones. This reaction process includes three consecutive single oxygen addition reactions, namely the generation of 22R hydroxycholesterol (22HC), the generation of 20R, 22R dihydroxycholesterol, and the cleavage of C20-C22 bonds. In each single oxygen addition reaction process, two electrons and one molecular oxygen are required to participate. The electrons are provided by NADPH and transmitted to P450scc through NADPH AdR and Adx. Adx forms a complex with P450scc and can serve as a mobile electron transporter.