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. Belongs to the cytochrome P450 family.