A P-450 heme-thiolate protein. The enzyme can also produce 25-hydroxycholesterol. In addition, it can further hydroxylate the product to 24,25-dihydroxycholesterol and 24,27-dihydroxycholesterol . This reaction is the first step in the enzymic degradation of cholesterol in the brain as hydroxycholesterol can pass the blood---brain barrier whereas cholesterol cannot . The direct electron donor to the enzyme is EC 188.8.131.52, NADPH---hemoprotein reductase .