endoplasmic reticulum membrane
4-Cholesten-7alpha,24(S)-diol-3-one, NADPH, and H+ react to form 5beta-cholestan-7alpha,24(S)-diol-3-one and NADP+. This reaction is catalyzed by AKR1D1 (3-oxo-5-beta-steroid 4-dehydrogenase). AKR1D1 is localized to the cytosol, and in the course of the reaction its steroid substrate moves from the endoplasmic reticulum membrane to the cytosol. It is unclear whether this translocation results simply from its increased hydrophilicity or is mediated by the enzyme or another transport protein (Russell 2003).