Aberrant pathways in the late stages of cholesterol biosynthesis in the rat: origin and metabolic fate of unsaturated sterols relevant to the Smith-Lemli-Opitz syndrome

Minor aberrant pathways of cholesterol biosynthesis normally produce only t race levels of abnormal sterol metabolites but may assume major importance when an essential biosynthetic step is blocked. Cholesta-5,8-dien-3 beta -o l, its Delta (5,7) isomer, and other noncholesterol sterols accumulate in s ubjects with the Smith-Lemli-Opitz syndrome (SLOS), a severe developmental disorder caused by a defective Delta (7) sterol reductase gene. We have exp lored the formation and metabolism of unsaturated sterols relevant to SLOS by incubating tritium-labeled Delta (5,8), Delta (6,8), Delta (6,8(14)), De lta (5,8(14)), and Delta (8) sterols with rat liver preparations, More than 60 different incubations were carried out with washed microsomes or the 10 ,000 g supernatant under aerobic or anaerobic conditions; some experiments included addition of cofactors, fenpropimorph (a Delta (8)-Delta (7) isomer ase inhibitor), and/or AY-9944 (a Delta (7) reductase inhibitor). The triti um-labeled metabolites from each incubation were identified by silver ion h igh performance liquid chromatography on the basis of their coelution with unlabeled authentic standards, as free sterols and: or acetate derivatives. The Delta (5,8) sterol was converted slowly to cholesterol,ia the Delta (5 ,7) sterol, which also slowly isomerized back to the Delta (5,8) sterol, Th e Delta (6,8) sterol was metabolized rapidly to cholesterol by an oxygen-re quiring pathway via the Delta (7,9(11)), Delta (8), Delta (7), and Delta (5 ,7) sterols as well as by an oxygen-independent route involving initial iso merization to the Delta (5,7) sterol. The Delta (8) sterol was partially me tabolized to Delta (5,8), Delta (6,8), Delta (7,9(11)), and Delta (5,7,9(11 )) sterols when isomerization to Delta (7) was blocked. The combined result s were used to formulate a scheme of normal and aberrant biosynthetic pathw ays that illuminate the origin and metabolic fate of abnormal sterols obser ved in SLOS and chondrodysplasia punctata.