. "Cholesterol, NADPH + H+, and O2 react to form 24-hydroxycholesterol , NADP+, and H2O. This reaction is catalyzed by CYP46A1 in the endoplasmic reticulum membrane. In the body, this enzyme is expressed predominantly in the brain and is thought to play a major role in cholesterol turnover there (Javitt 2002)."^^ . . . . .