Bile acid synthesis in the Smith-Lemli-Opitz syndrome: effects of dehydrocholesterols on cholesterol 7 alpha-hydroxylase and 27-hydroxylase activities in rat liver

The Smith-Lemli-Opitz syndrome (SLOS) is a congenital birth defect syndrome caused by a deficiency of 3 beta-hydroxysterol Delta(7)-reductase, the fin al enzyme in the cholesterol biosynthetic pathway. The patients have reduce d plasma and tissue cholesterol concentrations with the accumulation of 7-d ehydrocholesterol and 8-dehydrocholesterol. Bile acid synthesis is reduced and unnatural cholenoic and cholestenoic acids have been identified in some SLOS patients. To explore the mechanism of the abnormal bile acid producti on, the activities of key enzymes in classic and alternative bile acid bios ynthetic pathways (microsomal cholesterol 7 alpha-hydroxylase and mitochond rial sterol 27-hydroxylase) were measured in liver biopsy specimens from tw o mildly affected SLOS patients, The effects of 7- and 8-dehydrocholesterol s on these two enzyme activities were studied by using liver from SLOS mode l rats that were treated with the Delta(7)-reductase inhibitor (BM15.766) f or 4 months and were comparable with more severe SLOS phenotype in plasma a nd hepatic sterol compositions. In the SLOS patients, cholesterol 7 alpha-h ydroxylase and sterol 27-hydroxylase were not defective. In BM15.766-treate d rats, both enzyme activities were lower than those in control rats and th ey were competitively inhibited by 7- and 8-dehydrocholesterols. Rat micros omal cholesterol 7 alpha-hydroxylase did not transform 7-dehydrocholesterol or 8-dehydrocholesterol into 7 alpha-hydroxylated sterols, In contrast, ra t mitochondrial sterol 27-hydroxylase catalyzed 27-hydroxylation of 7- and 8-dehydrocholesterols, which were partially converted to 3 beta-hydroxychol estadienoic acids. Addition of microsomes to the mitochondrial 27-hydroxyla se assay mixture reduced 27-hydroxydehydrocholesterol concentrations, which suggested that 27-hydroxydehydrocholesterols were further metabolized by m icrosomal enzymes. These results suggest that reduced normal bile acid prod uction is characteristic of severe SLOS phenotype and is caused not only by depletion of hepatic cholesterol but also by competitive inhibition of cho lesterol 7 alpha-hydroxylase and sterol 27-hydroxylase activities by accumu lated 7- and 8-dehydrocholesterols. Unnatural bile acids are synthesized ma inly by the alternative pathway via mitochondrial sterol 27-hydroxylase in SLOS.