Minor aberrant pathways of
cholesterol biosynthesis normally produce only trace levels of abnormal
sterol metabolites but may assume major importance when an essential biosynthetic step is blocked. Cholesta-5,8-dien-3beta-ol, its Delta(5,7) isomer, and other noncholesterol
sterols accumulate in subjects with the
Smith-Lemli-Opitz syndrome (SLOS), a severe developmental disorder caused by a defective Delta(7)
sterol reductase gene. We have explored the formation and metabolism of unsaturated
sterols relevant to SLOS by incubating
tritium-labeled Delta(5,8), Delta(6, 8), Delta(6,8(14)), Delta(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)
isomerase inhibitor), and/or
AY-9944 (a Delta(7)
reductase inhibitor). The
tritium-labeled metabolites from each incubation were identified by
silver ion high 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 via the Delta(5,7)
sterol, which also slowly isomerized back to the Delta(5,8)
sterol. The Delta(6,8)
sterol was metabolized rapidly to
cholesterol by an
oxygen-requiring 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 isomerization to the Delta(5,7)
sterol. The Delta(8)
sterol was partially metabolized 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 results were used to formulate a scheme of normal and aberrant biosynthetic pathways that illuminate the origin and metabolic fate of abnormal
sterols observed in SLOS and
chondrodysplasia punctata.