Smith-Lemli-Opitz syndrome (SLOS) is a metabolic dis- order resulting from mutations in the gene encoding 7-de- hydrocholesterol reductase (DHCR7), the enzyme that catalyzes the reduction of 7-dehydrocholesterol (7-DHC)

This article is available online at http://www.jlr.org Smith-Lemli-Opitz syndrome (SLOS) is a metabolic disorder resulting from mutations in the gene encoding 7-dehydrocholesterol reductase (DHCR7), the enzyme that catalyzes the reduction of 7-dehydrocholesterol (7-DHC) to cholesterol (Chol) ( 1–7 ). Elevated levels of 7-DHC and reduced levels of Chol are observed in tissues and fl uids of patients with SLOS due to this enzymatic defect. SLOS is characterized by a broad spectrum of phenotypes including multiple congenital malformations and mental retardation ( 6, 8–10 ). We reported previously that 7-DHC has the highest measured propagation rate constant known for any lipid toward free radical chain oxidation ( 11 ). The rate constant determined for the propagation step in 7-DHC autoxidation (2260 M 1 s 1 ) is some 200 times that determined for Chol (11 M 1 s 1 ) and more than 10 times that of arachidonic acid (197 M 1 s 1 ), a polyunsaturated fatty acid that is considered to be highly susceptible to free radical oxidation. In a subsequent report, over a dozen oxysterols were isolated and characterized from 7-DHC free radical oxidation reactions and a reasonable mechanism involving abstraction of hydrogen atoms at C-9 and/or C-14 was proposed to account for the profi le of products formed ( 12 ). Due to its high oxidizability, substantially more oxidation products Abstract The level of 7-dehydrocholesterol (7-DHC) is elevated in tissues and fl uids of Smith-Lemli-Opitz syndrome (SLOS) patients due to defective 7-DHC reductase. Although over a dozen oxysterols have been identifi ed from 7-DHC free radical oxidation in solution, oxysterol profi les in SLOS cells and tissues have never been studied. We report here the identifi cation and complete characterization of a novel oxysterol, 3 ,5 -dihydroxycholest-7-en-6-one (DHCEO), as a biomarker for 7-DHC oxidation in fi broblasts from SLOS patients and brain tissue from a SLOS mouse model. Deuterated ( d 7 )-standards of 7-DHC and DHCEO were synthesized from d 7 -cholesterol. The presence of DHCEO in SLOS samples was supported by chemical derivatization in the presence of d 7 -DHCEO standard followed by HPLC-MS or GC-MS analysis. Quantifi cation of cholesterol, 7-DHC, and DHCEO was carried out by isotope dilution MS with the d 7 -standards. The level of DHCEO was high and correlated well with the level of 7-DHC in all samples examined ( R = 0.9851). Based on our in vitro studies in two different cell lines, the mechanism of formation of DHCEO that involves 5 ,6 -epoxycholest-7-en-3 -ol, a primary free radical oxidation product of 7-DHC, and 7-cholesten-3 ,5 ,6 -triol is proposed. In a preliminary test, a pyrimidinol antioxidant was found to effectively suppress the formation of DHCEO in SLOS fi broblasts. —Xu, L., Z. Korade, D. A. Rosado, Jr., W. Liu, C. R. Lamberson, and N. A. Porter. An oxysterol biomarker for 7-dehydrocholesterol oxidation in cell/mouse models for Smith-LemliOpitz syndrome. J. Lipid Res . 2011. 52: 1222–1233.