Chromatographic behavior of oxygenated derivatives of cholesterol

Oxygenated derivatives of cholesterol have important functions in many biochemical processes. These oxysterols are difficult to study because of their low physiological concentrations, the facile formation of cholesterol autoxidation artifacts, and lack of information on their chromatographic behavior. Focusing on metabolites and autoxidation products of cholesterol, we have documented the chromatographic mobilities of 35 oxysterols under a variety of conditions: eight solvent systems for thin-layer chromatography on silica gel, several mobile phases for reversed-phase high-performance liquid chromatography (HPLC), and two types of stationary phase for capillary gas chromatography (GC) using trimethylsilyl derivatives. Notable differences in selectivity could be obtained by modifying the stationary or mobile phases. Separations of oxysterol pairs isomeric at side-chain carbons or C-7 were achieved on normal-phase, reversed-phase, chiral, or silver-ion HPLC columns. Chromatographic behavior is also described for side-chain hexadeuterated and heptafluorinated oxysterols, which are useful as standards in isotope dilution analyses and autoxidation studies, respectively. The overall results are relevant to many problems of oxysterol analysis, including the initial separation of oxysterols from cholesterol, determination of highly polar and nonpolar oxysterols, separation of isomeric pairs, selection of derivatization conditions for GC analysis, and quantitation of the extent of cholesterol autoxidation.

Alternative pathways of sterol synthesis in yeast. Use of C(27) sterol tracers to study aberrant double-bond migrations and evaluate their relative importance

Yeast produce traces of aberrant sterols by minor alternative pathways, which can become significant when normal metabolism is blocked by inhibitors or mutations. We studied sterols generated in the absence of the delta(8)-delta(7) isomerase (Erg2p) or delta(5) desaturase (Erg3p) by incubating three mutant strains of Saccharomyces cerevisiae with 5 alpha-cholest-8-en-3beta-ol, 8-dehydrocholesterol (delta(5,8) sterol), or isodehydrocholesterol (delta(6,8) sterol), together with the corresponding 3 alpha-3H isotopomer. Nine different incubations gave altogether 16 sterol metabolites, including seven delta(22E) sterols formed by action of the yeast C-22 desaturase (Erg5p). These products were separated by silver-ion high performance liquid chromatography (Ag(+)-HPLC) and identified by gas chromatography-mass spectrometry, nuclear magnetic resonance spectroscopy, and radio-Ag(+)-HPLC. When delta(8)-delta(7) isomerization was blocked, exogenous delta(8) sterol underwent desaturation to delta(5,8), delta(6,8), and delta(8,14) sterols. Formation of delta(5,8) sterol was strongly favored over delta(6,8) sterol, but both pathways are essentially dormant under normal conditions of sterol synthesis. The delta(5,8) sterol was metabolically almost inert except for delta(22) desaturation, whereas the delta(6,8) sterol was readily converted to delta(5,7), delta(5,7,9(11)), and delta(7,9(11)) sterols. The combined results indicate aberrant metabolic pathways similar to those in mammalian systems. However, delta(5,7) sterol undergoes only slight isomerization or desaturation in yeast, an observation that accounts for the lower levels of delta(5,8) and delta(5,7,9(11)) sterols in wild-type yeast compared to Smith-Lemli-Opitz individuals.

A colorimetric assay for 7-dehydrocholesterol with potential application to screening for Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS; MIM 270400) is a genetic disorder characterized by hypocholesterolemia and elevated 7-dehydrocholesterol (7DHC) levels resulting from mutations affecting 7-dehydrocholesterol reductase. We describe a colorimetric assay for 7DHC with potential application to large-scale screening for SLOS. Reaction of 7DHC and its esters with the Liebermann-Burchard reagent resulted in a brief initial absorbance at 510 nm (pink color) followed by an absorbance at 620 nm (blue color) after 2 min, while cholesterol samples were essentially colorless. The assay could identify typical SLOS blood samples by their pink color and increased absorbance at 620 nm after 2 min. Colorimetric identification of mild SLOS cases requires monitoring of the transient absorbance at 510 nm, which must be detected immediately after rapid, consistent mixing of the reagents. The need for special mixing devices and rigorous validation precludes sporadic use of the assay for diagnosing suspected SLOS cases. We also studied the stability of 7DHC in dried SLOS blood spots on Guthrie cards, which are widely used for archiving neonatal blood. Decomposition of 7DHC was effectively retarded by storage at low temperature and by precoating of the cards with antioxidants. The combined results provide a foundation for development of a simple, automated test for SLOS screening.

Sterols in blood of normal and Smith-Lemli-Opitz subjects

Smith-Lemli-Opitz syndrome (SLOS) is a hereditary disorder in which a defective gene encoding 7-dehydrocholesterol reductase causes the accumulation of noncholesterol sterols, such as 7- and 8-dehydrocholesterol. Using rigorous analytical methods in conjunction with a large collection of authentic standards, we unequivocally identified numerous noncholesterol sterols in 6 normal and 17 SLOS blood samples. Plasma or erythrocytes were saponified under oxygen-free conditions, followed by multiple chromatographic separations. Individual sterols were identified and quantitated by high performance liquid chromatography (HPLC), Ag(+)-HPLC, gas chromatography (GC), GC-mass spectrometry, and nuclear magnetic resonance. As a percentage of total sterol content, the major C(27) sterols observed in the SLOS blood samples were cholesterol (12;-98%), 7-dehydrocholesterol (0.4;-44%), 8-dehydrocholesterol (0.5;-22%), and cholesta-5,7,9(11)-trien-3beta-ol (0.02;-5%), whereas the normal blood samples contained <0.03% each of the three noncholesterol sterols. SLOS and normal blood contained similar amounts of lathosterol (0.05;-0.6%) and cholestanol (0.1;-0.4%) and approximately 0.003;-0.1% each of the Delta(8), Delta(8(14)), Delta(5,8(14)), Delta(5,24), Delta(6,8), Delta(6,8(14)), and Delta(7,24) sterols. The results are consistent with the hypothesis that the Delta(8(14)) sterol is an intermediate of cholesterol synthesis and indicate the existence of undescribed aberrant pathways that may explain the formation of the Delta(5,7,9(11)) sterol. 19-Norcholesta-5,7,9-trien-3beta-ol was absent in both SLOS and normal blood, although it was routinely observed as a GC artifact in fractions containing 8-dehydrocholesterol. The overall findings advance the understanding of SLOS and provide a methodological model for studying other metabolic disorders of cholesterol synthesis.

Progestins block cholesterol synthesis to produce meiosis-activating sterols

The resumption of meiosis is regulated by meiosis-preventing and meiosis-activating substances in testes and ovaries. Certain C29 precursors of cholesterol are present at elevated levels in gonadal tissue, but the mechanism by which these meiosis-activating sterols (MAS) accumulate has remained an unresolved question. Here we report that progestins alter cholesterol synthesis in HepG2 cells and rat testes to increase levels of major MAS (FF-MAS and T-MAS). These C29 sterols accumulated as a result of inhibition of Delta24-reduction and 4alpha-demethylation. Progesterone, pregnenolone, and 17alpha-OH-pregnenolone were potent inhibitors of Delta24-reduction in an in vitro cell assay and led to the accumulation of desmosterol, a Delta5,24 sterol precursor of cholesterol. A markedly different effect was observed for 17alpha-OH-progesterone, which caused the accumulation of sterols associated with inhibition of 4alpha-demethylation. The flux of 13C-acetate into lathosterol and cholesterol was decreased by progestins as measured by isotopomer spectral analysis, whereas newly synthesized MAS accumulated. The combined evidence that MAS concentrations can be regulated by physiological levels of progestins and their specific combination provides a plausible explanation for the elevated concentration of MAS in gonads and suggests a new role for progestins in fertility.

Meiosis-activating sterol and the maturation of isolated mouse oocytes

This study was carried out to examine the effects of the meiosis-activating C(29) sterol, 4,4-dimethyl-5 alpha-cholesta-8,14, 24-trien-3 beta-ol (FF-MAS), on mouse oocyte maturation in vitro. Cumulus cell-enclosed oocytes (CEO) and denuded oocytes (DO) from hormonally primed, immature mice were cultured 17-18 h in minimum essential medium (MEM) containing 4 mM hypoxanthine plus increasing concentrations of FF-MAS. The sterol induced maturation in DO with an optimal concentration of 3 microg/ml but was without effect in CEO, even at concentrations as high as 10 microg/ml. Some stimulation of maturation in hypoxanthine-arrested CEO was observed when MEM was replaced by MEMalpha. Interestingly, the sterol suppressed the maturation of hypoxanthine-arrested CEO in MEM upon removal of glucose from the medium. FF-MAS also failed to induce maturation in DO when meiotic arrest was maintained with dibutyryl cAMP (dbcAMP). The rate of maturation in FF-MAS-stimulated, hypoxanthine-arrested DO was slow, as more than 6 h of culture elapsed before significant meiotic induction was observed, and this response required the continued presence of the sterol. Although the oocyte took up radiolabeled lanosterol, such accumulation was restricted by the presence of cumulus cells. In addition, lanosterol failed to augment FSH-induced maturation and was even inhibitory at a high concentration. Moreover, the downstream metabolite, cholesterol, augmented the inhibitory action of dbcAMP on maturation in both CEO and DO. Two inhibitors of 14 alpha-demethylase, ketoconazole, and 14 alpha-ethyl-5 alpha-cholest-7-ene-3 beta, 15 alpha-diol that can suppress FF-MAS production from lanosterol failed to block consistently FSH-induced maturation. These results confirm the stimulatory action of FF-MAS on hypoxanthine-arrested DO but do not support a universal meiosis-inducing function for this sterol.

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 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.

Kinetics and plasma concentrations of 26-hydroxycholesterol in baboons

26-Hydroxycholesterol (26OHC), a major oxysterol in human blood, is believed to play an important role in reverse cholesterol transport, bile acid formation, and regulation of various cellular processes. Using isotope dilution mass spectrometry, we measured plasma 26OHC concentrations in baboons fed either a high cholesterol/saturated fat (HC-SF) or normal chow diet. Plasma 26OHC levels in baboons were comparable to those reported for humans and were positively correlated with plasma cholesterol concentrations. Animals on the HC-SF diet had significantly higher 26OHC levels (0.274+/-0.058 microM, mean+/-S.D.) than those on the chow diet (0.156+/-0.046 microM). In separate experiments, [(3)H]26OHC was injected into four tethered baboons, and multiple blood samples drawn over a 1-h period were analyzed for [(3)H]26OHC and 26OHC. Fitting the specific radioactivity data to a two-pool compartmental model indicated a rapidly turning over plasma compartment (t(1/2) 2.9-6.0 min) and a second compartment with slow turnover (t(1/2) 76-333 min). The calculated 26OHC production rate was 2.5 micromol/kg body weight/day. Assuming all 26OHC is converted to bile acids, the 26OHC production rate corresponds to about 10% of total bile acid production in adult baboons. These results indicate that rapid turnover of plasma 26OHC at submicromolar concentrations could significantly contribute to bile acid synthesis.

Inhibition of Ca2+ release channel (ryanodine receptor) activity by sphingolipid bases: mechanism of action

Sphingosine inhibits the activity of the skeletal muscle Ca2+ release channel (ryanodine receptor) and is a noncompetitive inhibitor of [3H]ryanodine binding (Needleman et al., Am. J. Physiol. 272, C1465-1474, 1997). To determine the contribution of other sphingolipids to the regulation of ryanodine receptor activity, several sphingolipid bases were assessed for their ability to alter [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes and to modulate the activity of the Ca2+ release channel. Three lipids, N,N-dimethylsphingosine, dihydrosphingosine, and phytosphingosine, inhibited [3H]ryanodine binding to both skeletal and cardiac SR membranes. However, the potency of these three lipids and sphingosine was lower in rabbit cardiac membranes when compared to rabbit skeletal muscle membranes and when compared to sphingosine. Like sphingosine, the lipids inhibited [3H]ryanodine binding by greatly increasing the rate of dissociation of bound [3H]ryanodine from SR membranes, indicating that these three sphingolipid bases were noncompetitive inhibitors of [3H]ryanodine binding. These bases also decreased the activity of the Ca2+ release channel incorporated into planar lipid bilayers by stabilizing a long closed state. Sphingosine-1-PO4 and C6 to C18 ceramides of sphingosine had no significant effect on [3H]ryanodine binding to cardiac or skeletal muscle SR membranes. Saturation of the double bond at positions 4-5 decreased the ability of the sphingolipid bases to inhibit [3H]ryanodine binding 2-3 fold compared to sphingosine. In summary, our data indicate that other endogenous sphingolipid bases are capable of modulating the activity of the Ca2+ release channel and as a class possess a common mechanism of inhibition.

Synthesis of [3alpha-3H]cholesta-5,8-dien-3beta-ol and tritium-labeled forms of other sterols of potential importance in the Smith-Lemli-Optiz syndrome

Five unsaturated sterols relevant to the Smith-Lemli-Opitz syndrome have been prepared in high radiochemical purity with a tritium label at the 3alpha position. Swern oxidation of cholesta-5,8-dien-3beta-ol and other unlabeled C27 sterols afforded the corresponding 3-ketosteroids, and reduction with tritiated NaBH4 gave the desired 3alpha-3H sterols, with double bonds at the delta(5,8), delta(5,8(14)), delta(6,8), delta(6,8(14)), and delta8 positions. High radiochemical purity of the tritiated sterols was demonstrated by normal phase, reversed phase, and silver-ion (Ag+) high-performance liquid chromatography (HPLC). In the course of this work, we developed a medium-pressure variant of Ag+-HPLC for purifying radiolabeled samples, documented significant isotopic fractionation of the 3alpha-tritiated sterols and their acetates on Ag+-HPLC, and discovered unexpected effects of a delta(8(14)) bond on the conformation of 3-keto-delta5-steroids. The synthetic and analytical methodologies described herein should provide a sound basis for investigating the origin and metabolism of sterols involved in the Smith-Lemli-Opitz syndrome and in late stages of cholesterol biosynthesis.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

An improved synthesis of (20R,22R)-cholest-5-ene-3beta,20,22-triol, an intermediate in steroid hormone formation and an activator of nuclear orphan receptor LXR alpha

Asymmetric dihydroxylation of (20(22)E)-cholesta-5,20(22)-dien-3beta-ol acetate (2a), prepared from pregnenolone, gave a 1:1 mixture (67% yield) of (20R,22R)-cholest-5-ene-3beta,20,22-triol 3-acetate (3a) and its 20S,22S isomer 3b. Highly purified 3a and 3b were obtained by semipreparative silver ion high performance liquid chromatography. Saponification of 3a and 3b gave (20R,22R)-cholest-5-ene-3beta,20,22-triol (4a) and its 20S,22S isomer 4b. This simple approach provided the natural isomer 4a more efficiently than previously described chemical or enzymatic syntheses. Full 1H and 13C nuclear magnetic resonance data were presented for triols 4a and 4b and their synthetic precursors. Side-chain conformations of 2a, its 20(22)Z isomer, 4a, and 4b were studied by molecular mechanics and nuclear Overhauser effect difference spectroscopy.

Sterol synthesis. Preparation and characterization of fluorinated and deuterated analogs of oxygenated derivatives of cholesterol

Oxygenated sterols, including both autoxidation products and sterol metabolites, have many important biological activities. Identification and quantitation of oxysterols by chromatographic and spectroscopic methods is greatly facilitated by the availability of authentic standards, and deuterated and fluorinated analogs are valuable as internal standards for quantitation. We describe the preparation, purification and characterization of 43 oxygenated sterols, including the 4 beta-hydroxy, 7 alpha-hydroxy, 7 beta-hydroxy, 7-keto, and 19-hydroxy derivatives of cholesterol and their analogs with 25,26,26,26,27,27,27-heptafluoro (F7) and 26,26,26,27,27,27-hexadeuterio (d6) substitution. The 7 alpha-hydroxy, 7 beta-hydroxy, and 7-keto derivatives of (25R)-cholest-5-ene-3 beta, 26-diol (1d) and their 16,16-dideuterio analogs were also prepared. These d2-26-hydroxysterols and [16,16-2H2]-(25R)-cholest-5-ene-3 beta, 26-diol (1e) were synthesized from [16,16-2H2]-(25R)-cholest-5-ene-3 beta, 26-diol diacetate (2e), which can be prepared from diosgenin. The highly specific deuterium incorporation at C-16 in 1e and 2e should be useful in mass spectral analysis of 26-hydroxycholesterol samples by isotope dilution methods. The delta 5-3 beta, 7 alpha, 26- and delta 5-3 beta, 7 beta, 26-triols were regioselectively oxidized/isomerized to the corresponding delta 4-3-ketosteroids with cholesterol oxidase. Also described are 5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol, its 5 beta,6 beta-isomer, cholestane-3 beta, 5 alpha,6 beta-triol, their F7 and d6 derivatives, and d3-25-hydroxycholesterol, which was prepared from 3 beta-acetoxy-27-norcholest-5-en-25-one (30). The 43 oxysterols and most synthetic intermediates were isolated in high purity and characterized by chromatographic and spectroscopic methods, including mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Detailed mass spectral assignments are presented, and 1H NMR stereochemical assignments are derived for the C-19 protons of 19-hydroxysterols and for the side-chain protons of 30.

New methods for determining the enantiomeric purity of erythro -sphingosine

The enantiomeric purity of erythro -sphingosine samples can be determined simply, reliably, and accurately from 1H or 19F nuclear magnetic resonance spectra of the alpha-methoxy-alpha-(trifluoromethyl)phenylacetate (MTPA) derivative. As little as 0.1% of the minor enantiomer could be observed in a 1-mg sample, and detection limits of 1% and 5% were estimated for samples of 100 microg and 10 microg. The two threo -sphingosine enantiomers and four dihydrosphingosine stereoisomers were also differentiated by this technique, which served as an effective method for assessing the purity of sphingosine and dihydrosphingosine samples. Enantiomeric and diastereomeric purities could also be determined by normal-phase high performance liquid chromatographic analysis of the MTPA derivatives.

Chemical synthesis of D-ribo-phytosphingosine-1-phosphate, a potential modulator of cellular processes

d-erythro -Sphingosine-1-phosphate (2), an intermediate in sphingosine metabolism, shows a diversity of biological activities. Comparable roles might be anticipated for d-ribo -phytosphingosine-1-phosphate (1). We describe an efficient three-step chemical synthesis of 1 from d-ribo -phytosphingosine. Our approach is based on standard phosphoramidite methodology and on the finding of Boumendjel and Miller ( J. Lipid Res. 1994. 35: 2305-2311) that sphingosine can be monophosphorylated at the 1-hydroxyl without protection of the 3-hydroxyl. However, we were unable to duplicate their reported synthesis of 2 without important modifications in reagents and reaction conditions. Under the reported conditions for preparing 2, we obtained a cyclic carbamate (14), which we have isolated and identified. The structures of 1 and the cyclic carbamate 14 were elucidated by a combination of mass spectrometry and 1D and 2D nuclear magnetic resonance spectroscopy.

Sterols affecting meiosis: novel chemical syntheses and the biological activity and spectral properties of the synthetic sterols

4,4-Dimethyl-5alpha-cholesta-8,14,24-trien-3beta-ol (I) from human follicular fluid and 4,4-dimethyl-5alpha-cholesta-8,24-dien-3beta-ol (II) from bull testes have been reported to activate meiosis in mouse oocytes (Byskov et al., 1995. Nature. 374: 559-562). Described herein are new chemical syntheses of I, II, and the delta8(14),24 analog XXII. A critical step in these syntheses was a remarkably high yield side chain oxidation of 3beta-acetoxy-4,4-dimethyl-5alpha-cholest-8(14)-en-15-one to the corresponding C24 24-hydroxy compound VI. Oxidation of VI to the aldehyde, followed by Wittig olefination gave 3beta -acetoxy4,4-dimethyl-5alpha-cholesta-8(14),24-dien-15- one. Reduction with sodium borohydride to the 15beta-hydroxysteryl ester, dehydration with sulfuric acid in CHCl3, and saponification furnished I in high purity. Reduction of VI with sodium borohydride to the 15-hydroxysteroid followed by dehydration gave 3beta-acetoxy-4,4-dimethyl-5alpha-chola-8,14-dien-24-o l. Hydrogenation over Raney nickel gave the monounsaturated delta8(14) and delta8 compounds. Oxidation to the corresponding aldehydes followed by Wittig olefination and saponification gave II and XXII. Chromatographic, mass spectral, and 1H and 13C nuclear magnetic resonance spectral data have been presented for the synthetic sterols and their derivatives. I, II, XXII, and their delta8,14 and delta7,14 analogs, at 3 microg per ml, caused a resumption of meiosis in mouse oocytes in the presence of hypoxanthine (3.5 mM). Under the same conditions, delta5 and delta5,7 sterols were inactive.

Sterol synthesis. Synthesis of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluorocholest-5-en-7-one and its effects on HMG-CoA reductase activity in Chinese hamster ovary cells, on ACAT activity in rat jejunal microsomes, and serum cholesterol levels in rats

3 beta-Hydroxycholest-5-en-7-one (I; 7-ketocholesterol) is an oxysterol of continuing interest in biology and medicine. In the present study, we have prepared a side-chain fluorinated analog, 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluorocholest-5-en-7-one (VI), with the anticipation that the F7 substitution would block major metabolism of the 7-ketosterol, and thereby enhance its potential in vivo effects on serum cholesterol levels and other parameters. Chromium trioxide/dimethyl pyrazole oxidation of the acetate derivative of the previously described 25,26,26,26,27,27,27-heptafluorocholest-5-en-3 beta-ol (Swaminathan et al., 1993. J. Lipid Res. 34, 1805-1823) followed by mild alkaline hydrolysis gave VI. The effects of VI on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity in Chinese hamster ovary (CHO-K1) cells, on acyl coenzyme A-cholesterol acyltransferase (ACAT) activity in rat jejunal microsomes, and on serum cholesterol levels and other parameters in male Sprague-Dawley rats were determined and compared with those obtained with I and with another alpha, beta-unsaturated ketosterol, i.e. 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (II). I and VI showed essentially the same potency, considerably less than that of II, in lowering the levels of HMG-CoA reductase activity in CHO-K1 cells. Whereas addition of II to rat jejunal microsomes inhibited ACAT activity (IC50 approximately 3 microM), I and VI had no effect under the conditions studied (from 1 to 16 microM). Dietary administration of I, at levels of 0.1 and 0.15%, had no effect on food consumption, gain in body weight, or serum cholesterol levels. At 0.2%, I caused a modest decrease in body weight gain and a slight decrease in serum cholesterol levels (relative to ad libitum but not pair-fed control animals). The F7-7-ketosterol VI, at 0.26% in diet (the molar equivalent of 0.2% I), had no effect on food consumption, body weight, or serum cholesterol levels. Administration of I (0.1, 0.15 or 0.2% in diet) caused increases in the weight of small intestine. In contrast, no effect of VI (0.26% in diet) on small intestinal weight was observed.

An alternative synthesis of 4,4-dimethyl-5 alpha-cholesta-8,14,24-trien-3 beta-ol, an intermediate in sterol biosynthesis and a reported activator of meiosis and of nuclear orphan receptor LXR alpha

4,4-dimethyl-5 alpha-cholesta-8,14,24-trien-3 beta-ol, a sterol of current biological interest, has been synthesized in six steps from 3 beta-acetoxy-4,4-dimethyl-5 alpha-cholest-8(14)-en-15-one.

Sterol synthesis. A timely look at the capabilities of conventional and silver ion high performance liquid chromatography for the separation of C27 sterols related to cholesterol biosynthesis

Sterol intermediates in the biosynthesis of cholesterol have recently assumed a very prominent position in a number of important problems in medicine and biology. In studies of these matters, the separation and identification of the sterol intermediates present formidable challenges, a situation which does not appear to be generally appreciated. High performance liquid chromatography (HPLC) is a simple and rapid approach for the separation of the concerned compounds. Reversed phase HPLC is very commonly used for this purpose. In the present studies, we have evaluated the capabilities of reversed phase, normal phase, and silver ion HPLC for the separation of sterols. Using an extensive collection of authentic sterols, our studies indicate very limited capabilities of reversed phase and normal phase HPLC for the separation of C27 sterols differing in the number and location of olefinic double bonds. In contrast, silver ion HPLC provided remarkable separations of the same compounds, either as the free sterols or their acetate derivatives. These findings, coupled with the results of recent studies of the properties of the same compounds by gas chromatography and by nuclear magnetic resonance and mass spectroscopy, have important implications regarding current application of methodologies for the separation, identification, and quantitation of sterol intermediates in cholesterol biosynthesis as critical portions of investigations on a number of current and emerging problems in biology and medicine.

The orphan nuclear receptor LXRalpha is positively and negatively regulated by distinct products of mevalonate metabolism

LXRalpha is an orphan member of the nuclear hormone receptor superfamily that displays constitutive transcriptional activity. We reasoned that this activity may result from the production of an endogenous activator that is a component of intermediary metabolism. The use of metabolic inhibitors revealed that mevalonic acid biosynthesis is required for LXRalpha activity. Mevalonic acid is a common metabolite used by virtually all eukaryotic cells. It serves as a precursor to a large number of important molecules including farnesyl pyrophosphate, geranylgeranyl pyrophosphate, cholesterol, and oxysterols. Inhibition of LXRalpha could be reversed by addition of mevalonic acid and certain oxysterols but not by other products of mevalonic acid metabolism. Surprisingly, the constitutive activity of LXRalpha was inhibited by geranylgeraniol, a metabolite of mevalonic acid. These findings suggest that LXRalpha may represent a central component of a signaling pathway that is both positively and negatively regulated by multiple products of mevalonate metabolism.

An updated look at the analysis of unsaturated C27 sterols by gas chromatography and mass spectrometry

Gas chromatography-mass spectrometry (GC-MS) and GC are commonly used methods for the identification and quantitation of sterols from samples of biological origin. To investigate the utility and limitations of these methods, we have determined gas chromatographic mobilities and mass spectral properties of 5alpha-cholestan-3beta-ol and 26 unsaturated C27 sterols as their acetate and trimethylsilyl (TMS) ether derivatives by GC and GC-MS. The GC retention data showed that numerous sterols were essentially coeluted on capillary GC columns coated with either 5% phenyl-95% methyl polysiloxane or polyethylene glycol, although the peaks were more widely dispersed on the latter column. Mass spectra of many groups of sterol isomers were also quite similar. Sterol mixtures of any complexity are likely to contain coeluting components, and attempts to establish structures based on mass spectra that may represent a mixture of sterol isomers could easily lead to errors. Our results demonstrate that GC and GC-MS alone cannot generally be used for rigorous structure determinations of individual components in mixtures of unsaturated sterols. However, all but a few of the 26 sterols could be distinguished by their combined chromatographic mobilities on the two GC columns coupled with critical examination of their mass spectra. GC-MS analysis of appropriate sterol subclasses or preferably individual sterol components obtained by prior purification by other methods may provide valuable supporting evidence for the identification of sterol structures. Reliability of identification is dependent upon careful attention to GC and MS conditions, calibration of GC and MS data with authentic sterol standards, and consideration of possible decomposition under GC conditions and of the effect of overloading on GC retention times.

Modulation of skeletal muscle Ca2(+)-release channel activity by sphingosine

The effect of D-erythro-C18-sphingosine (sphingosine) and related compounds on the Ca(2+)-release channel (ryanodine binding protein) was examined on rabbit skeletal muscle membranes, on the purified ryanodine binding protein, and on the channel reconstituted into planar lipid bilayers. Sphingosine inhibited [3H]ryanodine binding to sarcoplasmic reticulum (SR) membranes in a dose-dependent manner similar to published results (R. A. Sabbadini, R. Betto, A. Teresi, G. Fachechi-Cassano, and G. Salviati. J. Biol. Chem. 267: 15475-15484, 1992). The sphingolipid also inhibited [3H]ryanodine binding to the purified ryanodine binding protein. Our results demonstrate that the inhibition of [3H]ryanodine binding by sphingosine is due to an increased rate of dissociation of bound [3H]ryanodine from SR membranes and a decreased rate of association of [3H]ryanodine to the high-affinity site. Unlike other modulators of the Ca(2+)-release channel, sphingosine can remove bound [3H]ryanodine from the high-affinity site within minutes. Sphingosine increased the rate of dissociation of [3H]ryanodine bound to a solubilized proteolytic fragment derived from the carboxy terminus of the ryanodine binding protein (cleavage at Arg4475). Sphingosine also inhibited the activity of the Ca(2+)-release channel incorporated into planar lipid bilayers. Taken together, the data provide evidence for a direct effect of sphingosine on the Ca(2+)-release channel. Sphingosine is a noncompetitive inhibitor at the high-affinity ryanodine binding site, and it interacts with a site between Arg4475 and the carboxy terminus of the Ca(2+)-release channel.

Inhibitors of sterol synthesis. Synthesis and spectral properties of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholestan-15-one

3 beta-Hydroxy-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholestan-15-one (4) has been prepared as part of a program to synthesize 15-ketosterols that are not readily metabolized to cholesterol or side-chain oxygenated species. Saponification of 3 beta-acetoxy-5 alpha-chola-8(14),23-dien-15-one (5) followed by lithium-ammonia reduction with a bromobenzene quench gave 3 beta-hydroxy-5 alpha-chol-23-en-15-one (6). Addition of (CF3)2CFI to 6 in the presence of triethylborane gave an iodide preparation, which was reduced to 4 with tributyltin hydride (71% overall yield of 4 from 5). The 23-iodide preparations consisted of 6:1 mixtures of (23R)-3 beta-hydroxy-23-iodo-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholestan-15-one (9a) and its C-23 epimer 9b with variable amounts of 4. Compound 4 was also prepared by lithium-ammonia reduction of the delta 8(14) analogs of 4 and iodides 9a and 9b. The presence of small amounts of 6 in the latter product suggested a side reaction involving cleavage of the C24-C25 bond with loss of a (CF3)2CF radical. Also prepared were 25,26,26,26,27,27,27-heptafluoro-5 alpha-cholestane-3 beta, 15 alpha-diol, its 15 beta epimer, the 7 alpha-methyl analog of 4, 3 beta-hydroxy-7 alpha-methyl-5 alpha-cholestan-15-one (16), and (25R)-3 beta,26-dihydroxy-5 alpha-cholestan-15-one. Full 1H and 13C-NMR data of high precision with complete signal assignments are given for all new compounds. Definitive 1H-NMR stereochemical assignments of the C-24 protons were established for most sterols with a C8H17 side chain based on analysis of the downfield H-24 resonance in a 750-MHz spectrum of 16. Detailed electron-impact mass spectral data are presented together with a summary of major fragmentation patterns for 15-hydroxy- and 15-ketosteroids with and without a delta 8(14) bond.

Silver ion high pressure liquid chromatography provides unprecedented separation of sterols: application to the enzymatic formation of cholesta-5,8-dien-3 beta-ol

We report that silver ion HPLC provides remarkable separations of C27 sterols differing only in the number or location of olefinic double bonds. This technique has been extended to LC-MS, analysis of purified components by GC, GC-MS, and 1H NMR, and to its use on a semipreparative scale. The application of this methodology for the demonstration of the catalysis, by rat liver microsomes, of the conversion of 7-dehydrocholesterol to cholesta-5,8-dien-3 beta-ol is also presented.

Analysis of unsaturated C27 sterols by nuclear magnetic resonance spectroscopy

Complete 1H and 13C nuclear magnetic resonance (NMR) signal assignments have been established for 5 alpha-cholestan-3 beta-ol, 22 unsaturated C27 sterols, and their acetate derivatives. Assignments were made from a combination of 1D and 2D spectra and include stereochemical 1H assignments for the C-22 and C-23 protons of 5 alpha-cholesta-8,24-dien-3 beta-ol and other delta 24 sterols with a C8 side chain. At the temperature and concentration range described, chemical shifts were generally reproducible to +/- 0.01 ppm in 13C spectra and +/- 0.001 ppm in 1H spectra. Except for some overlapped or strongly coupled 1H resonances, chemical shifts are given to these precisions, which are an order of magnitude better than for most data given previously. Full 1H NMR data have been reported previously for only three of the 46 compounds, and 13C data were unavailable for many, including the previously undescribed cholesta-5,8(14)-dien-3 beta-ol. An extensive set of 1H-1H coupling constants for the unsaturated sterols indicated considerable conformational diversity, which was confirmed by molecular modeling. The conformational diversity together with other factors led to a complex pattern of 13C substituent-induced chemical shifts (SCS) that appeared to elude any simple empirical correlations with structure. By contrast, the 1H SCS correlated reasonably well with simple structural features. The high precision of the SCS revealed small but measurable effects of a double bond on 1H resonances up to 12 bonds away. Also discussed are the utility and limitations of NMR for the identification of unsaturated sterols, estimation of purity, and analysis of mixtures, with an emphasis on special problems encountered at a microgram level.

Sphingolipid bases. A revisitation of the O-methyl derivatives of sphingosine. Isolation and characterization of diacetate derivatives, with revised 13C nuclear magnetic resonance assignments for D-erythro-sphingosine

As described by Carter et al. (J. Biol. Chem. 1951. 192: 197-207), O-methyl derivatives of sphingosine are formed upon acid hydrolysis of sphingolipids in the presence of methanol. In the present study, we have isolated four O-methyl ethers of C18-sphingosine by medium pressure liquid chromatography of their diacetate derivatives, i.e., (2S,3R,4E)-1-acetoxy-2-acetamido-3-methoxy-4-octadecene, its (2S,3S) epimer, (2R,3E,5R)-1-acetoxy-2-acetamido-5-methoxy-3-octadecene, and its (2R,5S) epimer. Structures were determined by physical, chromatographic, and spectral properties. The 5-O-methyl ethers, which were the predominant byproducts of sphingolipid hydrolysis, were easily distinguished from the 3-O-methyl ethers by chromatography, and all four isomers could be differentiated by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. NMR analysis of the original N-acetate and diacetate samples of O-methylsphingosines I and II of Carter et al. demonstrated that they correspond to the 5-O-methyl ethers (2R,5R and 2R,5S, respectively), with purities of approximately 90-99%. Resolution enhancement of the 126-MHz 13C NMR spectra of the O-methyl ethers and D-erythro-C18-sphingosine (Ia) afforded distinct signals for nearly all carbon atoms. 13C NMR assignments of carbons 7-15 were made from their lanthanide-induced shifts, and revised assignments for olefinic carbons of Ia were established based upon 1H-13C shift correlation experiments.

Inhibitors of sterol synthesis. Metabolism-based design and construction of a new analog of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one and its effects in cultured mammalian cells and in rats

3 beta-Hydroxy-5 alpha-cholest-8(14)-en-15-one (I) is a potent regulator of cholesterol metabolism. In the present study, the 7 alpha-methyl-25,26,26,26,27,27,27-heptafluoro analog (X) of I has been synthesized with the goal of blocking not only the side chain oxidation of I but also its conversion to cholesterol. X was prepared in seven steps from the known 7 alpha-methyl analog (IX) of I. Treatment of the acetate of IX with a mixture of trifluoroacetic anhydride, hydrogen peroxide, and sulfuric acid gave 3 beta-acetoxy-7 alpha-methyl-24-hydroxy-5 alpha-chol-8(14)-en-15-one (XII) in remarkably high (68%) yield. Dehydration of XII via the orthonitrophenylselenide to the 23-ene, followed by addition of (CF3)2CFI gave (23R)-3 beta-acetoxy-7 alpha-methyl-23-iodo-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholest-8(14)-en-15-one (XV). Reductive deiodination of XV with tributyltin hydride, followed by hydrolysis of the acetate gave 3 beta-hydroxy-7 alpha-methyl-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholest-8(14)-en-15-one (X). The F7-7 alpha-methyl-15-ketosterol X lowered the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in CHO-K1 cells with a potency equivalent to that of I. X showed significant hypocholesterolemic action upon oral administration to rats, with a potency far in excess of the 7 alpha-methyl-15-ketosterol IX lacking the F7 substitution. In marked contrast to I, X showed little or no suppression of food consumption in rats. Upon oral administration of X to rats, low levels of X (relative to cholesterol), characterized by chromatographic and gas chromatography-mass spectrometric methodologies, were observed in serum, liver, and small intestine. No material was observed with the expected properties of F7-7-methylcholesterol (or potential intermediates in its possible formation from X). In contrast to I, X lowered serum cholesterol levels at dosages at which no effect on food consumption was observed.

Inhibitors of sterol synthesis. Synthesis and spectral properties of 3 beta-hydroxy-5 alpha-cholestan-15-one and its 17 beta-epimer and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity

3 beta-Hydroxy-5 alpha-cholestan-15-one (2a) and its 14 beta-epimer 2b were prepared from 3 beta-acetoxy-5 alpha-cholest-8(14)-ene (3). Hydroboration of 3 at 45-50 degrees C gave a mixture of 5 alpha,14 alpha-cholestane-3 beta,15 alpha-diol and 5 alpha,14 beta-cholestane-3 beta,15 beta-diol, which were separated on silica gel as their 3 beta-tert-butyldimethylsilyl ethers 5a and 5b. Oxidation of 5a with pyridinium chlorochromate, followed by desilylation with tetrabutylammonium fluoride gave 2a. Analogous transformations of 5b gave 2b contaminated with 2a. Desilylation of 5b followed by oxidation with pyridinium chlorochromate resulted in a mixture composed mainly of 5 alpha,14 beta-cholestane-3,15-dione and 2b. Successive chromatographic separations on silica gel and reversed phase media gave 2b of high purity. Compound 2a was also prepared by lithium-ammonia reduction of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (96% yield) and by selective reduction of 5 alpha-cholestane-3,15-dione with lithium tri-tert-butoxyaluminum hydride (90% yield). Isomers 2a and 2b were readily epimerized under acidic or basic conditions or under conditions used for gas chromatographic analysis. The purities of 2a and 2b were measured from nuclear magnetic resonance (NMR) spectra; chromatographic methods gave less reliable estimates of purity. NMR data also showed that ring C of the 14 beta sterols is predominantly in a chair conformation. The effects of 2a and 2b on the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase have been studied in Chinese hamster ovary cells.

Inhibitors of sterol synthesis. Submicromolar 14 alpha-ethyl-5 alpha-cholest-7-ene-3 beta, 15 alpha-diol causes a major modification of the sterol composition of CHO-K1 cells and a marked change in cell morphology

Incubation of Chinese hamster ovary cells (CHO-K1) with 14 alpha-ethyl-5 alpha-cholest-7-ene-3 beta,15 alpha-diol (0.1 microM) in lipid-deficient medium led to a major change in cellular sterol composition, which was characterized by a very marked accumulation of C30 sterols (lanosterol and 24,25-dihydrolanosterol). The accumulation of C30 sterols was associated with a striking change in cell morphology. The change in cell shape (elongation) was similar to that described previously (A. W. Hsie and T. T. Puck, 1971. Proc. Natl. Acad. Sci. USA. 68: 358-361; and confirmed herein) for CHO-K1 cells incubated in the presence of dibutyryl cAMP (1 mM). This change in morphology, induced by dibutyryl cAMP, was not accompanied by a change in cellular sterol composition. The cell elongation and accumulation of C30 sterols, induced by the 14 alpha-ethyl diol, were prevented by the addition of cholesterol (10 microM or 100 microM) and were reversed by removal of the 14 alpha-ethyl diol from the incubation medium. Incubation of the cells with the 14 alpha-ethyl diol had no effect on the levels of cAMP under the conditions studied. Incubation of the cells with miconazole (10 microM) or with lanosterol (10 microM) was also associated with the accumulation of C30 sterols and an elongation of the cells. 24,25-Dihydrolanosterol (10 microM) also induced similar changes in cellular morphology. The results presented herein demonstrate that marked changes in the sterol composition of CHO-K1 cells can be effected by incubation of the cells with 14 alpha-ethyl-5 alpha-cholest-7-ene-3 beta, 15 alpha-diol, miconazole, or lanosterol. In addition, the findings reported herein indicate an important role of sterols in the control of the shape of these cells.

Inhibitors of sterol synthesis. Effects of a new fluorinated analog of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one in rats

3 beta-Hydroxy-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholest-8(14)-en-15-one (VII), an analog of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I) in which conversion to 26- and 25-oxygenated metabolites is blocked by the F7-substitution, was administered to male Sprague-Dawley rats at levels of from 0.025 to 0.15% by weight in a ground chow diet. Administration of VII resulted in lowering of the levels of serum cholesterol at dosages as low as 0.025% by weight in diet. In marked contrast to I, VII had little or no effect on food consumption. Whereas administration of I at a level of 0.1% by weight in diet resulted in a cessation of growth, VII, at approximately the same molar concentration in diet, had only slight or no effect on changes in total body weight. Significant levels of 25,26,26,26,27,27,27-heptafluorocholesterol (VIII) were observed in serum and liver, indicating the conversion of VII to VIII. Characterization of VIII in liver was based upon the results of gas chromatography, low and high resolution mass spectral studies, infrared spectroscopy, and 1H and 13C nuclear magnetic resonance spectroscopy. The levels of VIII in serum appeared to be related to dosage and duration of administration of VII.

Inhibitors of sterol synthesis: 3 beta-hydroxy-25,26,26,26,27,27,27- heptafluoro-5 alpha-cholestan-15-one, an analog of a potent hypocholesterolemic agent in which its major metabolism is blocked

The chemical synthesis of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluoro- 5 alpha-cholestan-15-one (IV) has been pursued to provide an analog of the potent hypocholesterolemic agent 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I) in which its major metabolism is blocked. Reduction of 3 beta-acetoxy-5 alpha-chola-8(14),23-dien-15-one with lithium in liquid ammonia gave 3 beta-hydroxy-5 alpha-chol-23-en-15-one (VI). Addition of (CF3)2CFI to VI in the presence of triethylborane gave 3 beta-hydroxy-23R-iodo-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholestan- 15-one, which was reduced to IV with tributyltin hydride. IV was found to be highly active in lowering the levels of HMG-CoA reductase activity in CHO-K1 cells, in lowering acyl coenzyme A:cholesterol acyltransferase activity in jejunal microsomes, and in lowering serum cholesterol levels in rats.

Inhibitors of sterol synthesis. An improved chemical synthesis of 26-oxygenated delta 8(14)-15-ketosterols having the 25R configuration

(25R)-3 beta,26-Dihydroxy-5 alpha-cholest-8(14)-en-15-one (I) was synthesized in four steps from (25R)-3 beta,26-diacetoxycholesta-5,7-diene (III) in 30% overall yield. Isomerization of III with HCl in chloroform-dichloromethane at -60 degrees C gave (25R)-3 beta,26-diacetoxy-5 alpha-cholesta-7,14-diene together with the 5 alpha-delta 8,14 and 5 beta-delta 8,14 isomers in a 5:1:1 ratio. Epoxidation of the crude diene mixture with m-chloroperbenzoic acid, followed by hydrolysis in acetone containing concentrated HClO4 (0.1%) gave (25R)-3 beta,26-diacetoxy-5 alpha-cholest-8(14)-en-15-one (VIII), accompanied by numerous minor byproducts, including the 5 alpha,14 beta-delta 7, 5 alpha, 14 beta-delta 8 and 5 beta,14 beta-delta 8 isomers of VIII. All four 15-ketosterol esters were isolated by chromatography and fully characterized by mass spectrometry and 1H and 13C nuclear magnetic resonance. Treatment of VIII with potassium carbonate in degassed methanol gave I.