Catalyzes the side-chain cleavage reaction of cholesterol to pregnenolone, the precursor of most steroid hormones. cholesterol + 6 H(+) + 3 O2 + 6 reduced [adrenodoxin] = 4-methylpentanal + 4 H2O + 6 oxidized [adrenodoxin] + pregnenolone Lipid metabolism; C21-steroid hormone metabolism. Localizes to the matrix side of the mitochondrion inner membrane. In the ovary, not found in early vitellogenic follicles, barely detected in postvitellogenic follicles and abundant in post-ovulatory follicles. Belongs to the cytochrome P450 family.