Bile acid synthesis in the Smith-Lemli-Opitz syndrome: effects of dehydrocholesterols on cholesterol 7 a -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 b -hydroxysterol D 7 -reductase, the final enzyme in the cholesterol biosynthetic pathway. The patients have reduced plasma and tissue cholesterol concentrations with the accumulation of 7-dehydrocholesterol 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 production, the activities of key enzymes in classic and alternative bile acid biosynthetic pathways (microsomal cholesterol 7 a -hydroxylase and mitochondrial sterol 27-hydroxylase) were measured in liver biopsy specimens from two mildly affected SLOS patients. The effects of 7and 8-dehydrocholesterols on these two enzyme activities were studied by using liver from SLOS model rats that were treated with the D 7 -reductase inhibitor (BM15.766) for 4 months and were comparable with more severe SLOS phenotype in plasma and hepatic sterol compositions. In the SLOS patients, cholesterol 7 a -hydroxylase and sterol 27-hydroxylase were not defective. In BM15.766-treated rats, both enzyme activities were lower than those in control rats and they were competitively inhibited by 7and 8-dehydrocholesterols. Rat microsomal cholesterol 7 a -hydroxylase did not transform 7-dehydrocholesterol or 8-dehydrocholesterol into 7 a hydroxylated sterols. In contrast, rat mitochondrial sterol 27-hydroxylase catalyzed 27-hydroxylation of 7and 8-dehydrocholesterols, which were partially converted to 3 b hydroxycholestadienoic acids. Addition of microsomes to the mitochondrial 27-hydroxylase assay mixture reduced 27hydroxydehydrocholesterol concentrations, which suggested that 27-hydroxydehydrocholesterols were further metabolized by microsomal enzymes. These results suggest that reduced normal bile acid production is characteristic of severe SLOS phenotype and is caused not only by depletion of hepatic cholesterol but also by competitive inhibition of cholesterol 7 a -hydroxylase and sterol 27-hydroxylase activities by accumulated 7and 8-dehydrocholesterols. Unnatural bile acids are synthesized mainly by the alternative pathway via mitochondrial sterol 27-hydroxylase in SLOS. — Honda, A., G. Salen, S. Shefer, A. K. Batta, M. Honda, G. Xu, G. S. Tint, Y. Matsuzaki, J. Shoda, and N. Tanaka. Bile acid synthesis in the Smith-Lemli-Opitz syndrome: effects of dehydrocholesterols on cholesterol 7 a -hydroxylase and 27-hydroxylase activities in rat liver. J. Lipid Res. 1999. 40: 1520–1528. Supplementary key words 7-dehydrocholesterol • 8-dehydrocholesterol The Smith-Lemli-Opitz syndrome (SLOS) (1) is an autosomal recessive inherited birth defect. The syndrome is caused by deficient activity of 3 b -hydroxysteroid D 7 -reductase (2, 3), the final enzyme in the cholesterol biosynthetic pathway, and mutations in the D 7 -reductase gene have been identified in a number of SLOS patients (4–6). Patients are characterized clinically by mental retardation, failure to thrive, gastrointestinal malfunction, and multiple congenital anomalies including microcephaly, facial dysmorphism, limb anomalies, genital malformations, endocrine hypofunction, cataracts, and heart and kidney dysfunction (1, 7–10). Biochemically, the unusually low activity of 3 b -hydroxysteroid D 7 -reductase causes plasma and tissue cholesterol levels to be abnormally reduced and leads to the accumulation of the cholesterol precursor, 7dehydrocholesterol (cholesta-5,7-dien-3 b -ol), and its isomer, 8-dehydrocholesterol (cholesta-5,8-dien-3 b -ol) (11–14). According to current theory, cholesterol is the immediate precursor of bile acids exclusively synthesized in the liver that facilitate intestinal absorption of dietary lipids and fat-soluble vitamins. It is suggested that reduced cholesterol synthesis causes decreased formation of bile acids, which contributes to gastrointestinal malfunction and failure to thrive in SLOS. We previously reported (12) that fecal Abbreviations: DMES, dimethylethylsilyl; GC–MS, gas chromatography–mass spectrometry; GLC, gas–liquid chromatography; SIM, selected-ion monitoring; SLOS, Smith-Lemli-Opitz syndrome. 1 To whom correspondence should be addressed. by gest, on S etem er 0, 2017 w w w .j.org D ow nladed fom