A cytochrome P450 monooxygenase that catalyzes the side-chain hydroxylation and cleavage of cholesterol to pregnenolone, the precursor of most steroid hormones (PubMed:11412116). Catalyzes three sequential oxidation reactions of cholesterol, namely the hydroxylation at C22 followed with the hydroxylation at C20 to yield 20R,22R-hydroxycholesterol that is further cleaved between C20 and C22 to yield the C21-steroid pregnenolone and 4-methylpentanal (PubMed:11412116). Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate and reducing the second into a water molecule. Two electrons are provided by NADPH via a two-protein mitochondrial transfer system comprising flavoprotein FDXR (adrenodoxin/ferredoxin reductase) and nonheme iron-sulfur protein FDX1 or FDX2 (adrenodoxin/ferredoxin) (PubMed:11412116). cholesterol + 6 H(+) + 3 O2 + 6 reduced [adrenodoxin] = 4-methylpentanal + 4 H2O + 6 oxidized [adrenodoxin] + pregnenolone cholesterol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = (22R)-hydroxycholesterol + H2O + 2 oxidized [adrenodoxin] (22R)-hydroxycholesterol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = (20R,22R)-20,22-dihydroxycholesterol + H2O + 2 oxidized [adrenodoxin] (20R,22R)-20,22-dihydroxycholesterol + 2 H(+) + O2 + 2 reduced [adrenodoxin] = 4-methylpentanal + 2 H2O + 2 oxidized [adrenodoxin] + pregnenolone Lipid metabolism; C21-steroid hormone metabolism. Steroid metabolism; cholesterol metabolism. Interacts with FDX1/adrenodoxin. Localizes to the matrix side of the mitochondrion inner membrane. Detected in adrenal cortex and corpus luteum (at protein level). Belongs to the cytochrome P450 family.