Inhibitors of sterol synthesis. Studies of the metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one in Chinese hamster ovary cells and its effects on activities of early enzymes in cholesterol biosynthesis

A Facile Synthesis of 3β,25-dihydroxy-5α-Cholest-7-ene, the Precursor to HMG-CoA Reductase Inhibitor

An efficient four-step procedure is described for the synthesis of 3β,25-dihydroxy-5α-cholest-7-ene from 3β-hydroxy-5α-cholesta-7,24-diene in an overall yield of 34%. This product can serve as a precursor for the synthesis of an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase. The 3β-toluene- p-sulfonate ester of 5α-cholesta-7,24-diene was obtained from natural lanolin followed by treatment with toluene- p-sulfonyl chloride. The key step in the synthesis is a very mild method for the hydroxybromination of this ester with N-bromosuccinimide in water, followed by reduction with LiAlH4to obtain the 3β-toluene- p-sulfonate ester of 25-hydroxy-5α-cholesta-7-ene. The final product was obtained after desulfonation of 3β-toluene- p-sulfonyoxy-25-hydroxy-5α-cholesta-7-ene to afford 3β,25-dihydroxy-5α-cholest-7-ene in excellent yield. The reagents are all relatively cheap, non-toxic and stable.

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.

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.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Chemical synthesis of 15-ketosterols and their inhibitions of cholesteryl ester transfer protein

Described herein are the chemical syntheses of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one and 3 beta-hydroxy-5 alpha-cholest-8(14),16-dien-15-one from diosgenin and the examinations of their ability to inhibit the cholesteryl ester transfer protein (CETP). Clemmensen reduction of diosgenin gave cholest-5-ene-3 beta, 16 beta,26-triol. Tosylation of the latter compound gave cholest-5-ene-3 beta,16 beta,26-triol 26-tosylate which, upon reduction with LiAIH4, gave cholest-5-ene-3 beta,16 beta-diol. Hydrogenation-benzoylation of the latter to 5 alpha-cholest-3 beta,16 beta-diol 3 beta-benzoate followed by mesylation-elimination gave 5 alpha-cholest-16-ene-3 beta-ol 3 beta-benzoate. Controlled oxidation of the latter with CrO3-dimethylpyrazole gave 3 beta-hydroxy-5 alpha, 14 alpha-cholest-16-en-15-one 3 beta-benzoate. Oxidation of delta 16-15-one with SeO2 gave 3 beta-hydroxy-5 alpha-cholest-8(14),16-dien-15-one 3 beta-benzoate along with 3 beta-hydroxy-5 alpha, 14 beta-cholest-16-en-15-one 3 beta-benzoate. Selective hydrogenation of the delta 8(14),16-15-ketosteryl ester, followed by base hydrolysis gave 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one. Hydrolysis of 3 beta-hydroxy-5 alpha-cholest-8(14),16-dien-15-one 3 beta-benzoate in basic media gave 3 beta-hydroxy-5 alpha-cholest-8(14),16-dien-15-one. The effects of the 15-ketosterols on the CETP activity were studied in vitro by incubating cholesteryl ester donor (HDL), cholesteryl ester acceptor (LDL) and human plasma as a CETP source at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 26-aminocholesterol, 27-hydroxycholesterol, and 25-hydroxycholesterol on proliferation and cholesterol homeostasis in arterial myocytes

The major relation existing between cell growth and cholesterol homeostasis prompted us to investigate the effect of 26-aminocholesterol (26-NH2), 27-hydroxycholesterol (27-OH), and 25-hydroxycholesterol (25-OH) on these cellular events. To test this relation, we incubated human and rat arterial myocytes with the sterols for 72 hours. All the tested compounds (0.5 to 7.5 mumol/L) inhibited rat and human myocyte proliferation and cholesterol biosynthesis in a dose-dependent manner. 26-NH2 was more potent than oxysterols in inhibiting human myocyte proliferation but equieffective in rat cells; 27-OH and 25-OH displayed similar activity in both cell lines. Inhibition of nuclear incorporation of thymidine in rat myocytes is consistent with decreased cell count. The antiproliferative effect of the tested sterols was reversible. The high inhibition (80%) of cholesterol biosynthesis necessary to induce a decrease in myocyte proliferation suggests a causal relation between the cholesterol synthetic pathway and these cellular processes. In addition, all the tested sterols were able to inhibit hydroxymethyl glutaryl-coenzyme A reductase activity in intact myocytes but not in cell-free extracts. The finding that 26-NH2 but not 27-OH or 25-OH does not suppress LDL receptor activity in either human or rat myocytes supports the achievement of selectivity over the coordinately regulated LDL receptor gene. The ability of 26-NH2 to interfere with myocyte proliferation and cholesterol synthesis without affecting the LDL receptor pathway confers at least in vitro a pharmacological interest on the compound in the process of atherogenesis.

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: synthesis and spectral properties of derivatives of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholest-8(14)-en-15-one fluorinated at carbon 7 or carbon 9 and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in cultured mammalian cells

As part of a program to prepare delta 8(14)-15-ketosterols that cannot readily be metabolized to cholesterol or side-chain oxygenated species, we have prepared 3 beta-hydroxy-7 alpha-fluoro-5 alpha-cholest-8(14)-en-15-one (VII) and the 9 alpha-hydroxy (IV), 9 alpha-fluoro (VI) and 7 alpha-fluoro (VIII) derivatives of 3 beta-hydroxy-25,26,26,26,27,27,27-heptafluoro-5 alpha-cholest-8(14)-en-15-one (II). Sterol IV was prepared by oxidation of the delta 8,14 dienol ethyl ether of the 3 beta-acetate of II with m-chloroperbenzoic acid, followed by mild alkaline hydrolysis of the 3 beta-acetate derivative of IV. Treatment of IV with hydrogen fluoride-pyridine gave VI. The 7 alpha-fluoro-15-ketosterols VII and VIII were synthesized by treating the 3 beta,15-bis-trimethylsilyl delta 7,14-dienol ether derivative of the appropriate delta 8(14)-15-ketosterol with N-fluoropyridinium triflate, followed by hydrolysis of residual trimethylsilyl ethers and purification by high-performance liquid chromatography. The combined results of 1H and 13C nuclear magnetic resonance (NMR) chemical shifts, 1H-1H coupling constants, 1H-19F long-range coupling constants and molecular modeling indicated that a 7 alpha-fluoro, 9 alpha-fluoro or 9 alpha-hydroxy substituent has negligible effect on the conformation of the 15-ketosterols. 1H and 13C-NMR data are also given for delta 6,8(14)- and delta 8(14),9(11)-15-ketosterols, synthetic byproducts that could not be detected readily in samples of the fluoro-15-ketosterols by chromatographic methods. Mass spectra of VI and of previously reported 9 alpha-fluoro and 9 alpha-hydroxy-delta 8(14)-15-ketosterols showed abundant M-62 or M-60 ions that appear to correspond to loss of ketene and HF or H2O. The 9 alpha-hydroxy-F7-15-ketosterol IV, the 7 alpha-fluoro-15-ketosterol VII and the 7 alpha-fluoro-F7-15-ketosterol VIII were of equivalent potency to the parent 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I) in lowering the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in CHO-K1 cells. The 9 alpha-fluoro-F7-15-ketosterol VI showed high potency but appeared to be slightly less active than I.

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.

Inhibitors of sterol synthesis: effects of a 7 alpha-alkyl analog of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in cultured mammalian cells and on serum cholesterol levels and other parameters in rats

The 7 alpha-methyl analog (II) of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15- one (I) was prepared by chemical synthesis and evaluated with respect to its effects on HMG-CoA reductase activity in CHO-K1 cells and on serum cholesterol levels in rats. The 7 alpha-methyl substitution had no detectable effect on the potency of I in lowering HMG-CoA reductase activity in the cultured cells. In contrast, the 7 alpha-methyl substitution had a marked effect on the action of I in the suppression of food consumption in rats. Whereas II was less potent than I in lowering serum cholesterol levels in rats, it did so at dosage levels at which only slight or moderate effects on food consumption were observed. Full 1H and 13C-NMR assignments for II and intermediates in its synthesis have been presented. Conformational analysis, based on 1H-1H coupling constants, NMR shieldings and force-field calculations, indicated that the 7 alpha-methyl substitution had virtually no effect on the conformation of the 15-ketosterol apart from minor distortions of ring B.

Inhibitors of sterol synthesis. Chemical syntheses and spectral properties of 26-oxygenated derivatives of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in CHO-K1 cells

26-Oxygenated derivatives of delta 8(14)-15-ketosterols have been synthesized from (25R)-3 beta,26-diacetoxy-5 alpha-cholest-8(14)-en-15-one (IX) as part of a program to prepare potential metabolites and analogs of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I), a potent regulator of cholesterol metabolism. Partial hydrolysis of IX gave a mixture, from which the 3 beta,26-diol II and the 26-acetate (XI) and 3 beta-acetate (X) monoesters were isolated. Mitsunobu reaction of XI followed by hydrolysis gave (25R)-3 alpha,26-dihydroxy-5 alpha-cholest-8(14)-en-15-one (VI). Oxidation of XI with pyridinium chlorochromate followed by hydrolysis of the acetate gave (25R)-26-hydroxy-5 alpha-cholest-8(14)-ene-3,15-dione (VII). Oxidation of X with Jones reagent followed by hydrolysis of the acetate gave (25R)-3 beta-hydroxy-15-keto-5 alpha-cholest-8(14)-en-26-oic acid (IVa). Jones oxidation of II gave (25R)-3,15-diketo-5 alpha-cholest-8(14)-en-26-oic acid (VII). 1H and 13C nuclear magnetic resonance assignments and analyses of mass spectral fragmentation data are presented for each of the new compounds and their derivatives. The 3,15-diketone VII was found to be highly active in lowering the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in CHO-K1 cells, with a potency comparable to that of I. In contrast, 3 alpha,26-diol VI was less potent than I or VII. The two carboxylic acid analogs IVa and VIII were considerably less potent than VI in lowering the levels of HMG-CoA reductase activity.

Synergistic action of two oxysterols in the lowering of HMG-CoA reductase activity in CHO-K1 cells

3 beta-Hydroxy-5 alpha-cholest-8(14)-en-15-one (I) and (25R)-26-hydroxycholesterol (II), both potent regulators of sterol biosynthesis, have been found to show synergism in the reduction of the levels of HMG-CoA reductase activity in CHO-K1 cells. When equimolar concentrations of I and II were added in combination, synergistic reduction (p less than 0.0001) of enzyme activity was observed at total oxysterol concentrations of 0.1 microM, 0.2 microM, and 0.5 microM. Maximal synergistic effect in the lowering of reductase activity (28% greater than predicted) was observed at 0.1 microM total oxysterol concentration. Five additional experiments conducted with 50 nM oxysterols confirmed the synergistic effect at 0.1 microM total sterol concentration. These results suggest that the in vivo importance of I and II may be greater than that anticipated on the basis of the concentrations of the individual sterols.

Inhibitors of sterol synthesis. Chemical synthesis of 7 alpha-ethyl and 16 alpha-ethyl derivatives of delta 8(14)-15-oxygenated sterols and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase in CHO-K1 cells

The enolate of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (II), formed upon treatment of II with potassium tert-butoxide in tert-butanol, was alkylated with ethyl iodide. In addition to the major products, 3 beta-hydroxy-14 alpha-ethyl-5 alpha-cholest-7-en-15-one and its 3 beta-ethyl ether, small amounts of 3 beta-hydroxy-7 alpha-ethyl-5 alpha-cholest-8(14)-en-15-one (V), 3 beta-hydroxy-16 alpha-ethyl-5 alpha-cholest-8(14)-en-15-one (VI) and the 3 beta-ethyl ether of VI were isolated. When the enolate of II was formed by treatment with lithium diisopropylamide in tetrahydrofuran, the same alkylation furnished VI as the major product. Reduction of VI with lithium aluminum hydride gave 16 alpha-ethyl-5 alpha-cholest-8(14)-ene-3 beta, 15 alpha-diol (IX) and its 15 beta epimer X, which were separated by column chromatography. Full 1H and 13C nuclear magnetic resonance (NMR) assignments, augmented by nuclear Overhauser effect difference spectra for VI, established the stereochemistry of these diols at C-15 and C-16. The NMR results indicate that the 16 alpha-ethyl group affects the side-chain conformation. The effects of II, V, VI, IX and X on the levels of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were studied in CHO-K1 cells. With the exception of IX, each of the compounds reduced the levels of HMG-CoA reductase activity. The order of potency with respect to suppression of the elevated levels of HMG-CoA reductase activity induced by transfer of the cells to lipid-deficient medium, was II greater than V greater than VI greater than X.

Inhibitors of sterol synthesis. Chemical synthesis and spectral properties of (25R)-5 alpha-cholest-8(14)-ene-3 beta,15 beta,26-triol, a potential metabolite of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one and its effects on 3-hydroxy-3-methylglutaryl-coenzyme A reductase in CHO-K1 cells

(25R)-5 alpha-Cholest-8(14)-ene-3 beta,15 beta,26-triol (III) was prepared by reduction of (25R)-3 beta,26-diacetoxy-5 alpha-cholest-8(14)-en-15-one with sodium borohydride followed by treatment of the crude product with lithium aluminium hydride. The trihydroxysterol III, a potential metabolite of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one, was characterized by the results of mass spectral studies and by nuclear magnetic resonance (NMR) spectroscopy. Full 1H and 13C NMR assignments for III and 5 alpha-cholest-8(14)-ene-3 beta,15 beta-diol are given and used to establish the structure of III. The triol was found to be very potent in lowering the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in Chinese hamster ovary cells.

Inhibitors of sterol synthesis. Synthesis and spectral properties of 3 beta-hydroxy-24-dimethylamino-5 alpha-chol-8(14)-en-15-one and its effects on HMG-CoA reductase activity in CHO-K1 cells

A simple, three-step synthesis of the 25-aza analog of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I) is described. Treatment of 3 beta-acetoxy-24-hydroxy-5 alpha-chol-8(14)-en-15-one (III) with 1.75 equivalents of tosyl chloride in pyridine for 24 h at 5 degrees C gave 3 beta-acetoxy-24-tosyloxy-5 alpha-chol-8(14)-en-15-one (IV). In contrast, treatment of III with 3.95 equivalents of tosyl chloride in pyridine for 12 h at 48 degrees C gave 3 beta-acetoxy-24-chloro-5 alpha-chol-8(14)-en-15-one (V). Treatment of IV with dimethylamine in dioxane yielded 3 beta-acetoxy-24-dimethylamino-5 alpha-chol-8(14)-en-15-one (VI). Hydrolysis of VI with LiOH.H2O in methanol gave 3 beta-hydroxy-24-dimethylamino-5 alpha-chol-8(14)-en-15-one (VII). 1H- and 13C-NMR assignments are presented for compounds IV-VII. The 25-aza analog (VII) of the 15-ketosterol (I), at a concentration of 1.0 microM, caused a 47% lowering of the level of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in Chinese hamster ovary cells.

Inhibitors of sterol synthesis. Metabolism of [2,4-3H]5 alpha-cholest-8(14)-en-3 beta-ol-15-one after oral administration to a nonhuman primate

5 alpha-Cholest-8(14)-en-3 beta-ol-15-one is a potent inhibitor of cholesterol biosynthesis which has significant hypocholesterolemic activity upon oral administration to rodents and nonhuman primates. In the present study the metabolism of the 15-ketosterol has been investigated after the oral administration of a mixture of [2,4-3H]5 alpha-cholest-8(14)-en-3 beta-ol-15-one and [4-14C]cholesterol to 8 baboons. Blood samples were obtained at 4, 8, 12, 16, and 24 h after administration of the labeled sterols. Clear differences in the time courses of the levels of 3H and 14C in plasma were observed. 3H in plasma showed maximum values at 4 to 8 h, whereas maximum values for the levels of 14C were observed much later. 3H in plasma was shown to be primarily in the form of its metabolites, i.e. esters of the 15-ketosterol, cholesterol, and cholesteryl esters. The levels of the 15-ketosterol and of each of these metabolites showed different changes with time. The labeled cholesterol (and the cholesterol moiety of the cholesteryl esters), formed from the [2,4-3H]-15-ketosterol, was characterized by chromatography and by purification by way of its dibromide derivative. At 24 h after the administration of the labeled sterols, the distribution of 3H in plasma lipoprotein fractions paralleled that of 14C, with most of the 3H and 14C in high density lipoprotiens (HDL) and low density lipoproteins (LDL). Almost all of the 3H in HDL and in LDL was found as cholesterol, cholesteryl esters and esters of the 15-ketosterol. The distribution of 3H in HDL and in LDL of the free 15-ketosterol, esters of the 15-ketosterol, cholesterol, and cholesteryl esters was similar to that of plasma, thereby indicating no unusual concentration of any of the 3H labeled components in HDL or LDL.

Inhibitors of sterol synthesis. 15-oxygenated steryl ester hydrolase activity of rat liver at neutral and acid pH

5 alpha-Cholest-8(14)-en-3 beta-ol-15-one (15 ketosterol) is a potent inhibitor of cholesterol biosynthesis with significant hypocholesterolemic activity. The results of a recent study (Schroepfer, G.J., Jr., Christophe, A., Chu, A.J., Izumi, A., Kisic, A. and Sherrill, B.C. (1988) Chem. Phys. Lipids 48, 29-58) have indicated that, after intragastric administration of the 15-ketosterol in triolein to rats, most of the compound in intestinal lymph occurs in the form of the oleate ester, which is associated with chylomicrons. Moreover, after intravenous administration of chylomicrons containing the oleate ester of 15-[2,4-3H]ketosterol, rapid and selective uptake of 3H by liver was observed, which was associated with the rapid and substantial appearance of labeled free 15-ketosterol in liver. The present study concerns the capabilities of rat liver fractions to catalyze the hydrolysis of 15-ketosteryl oleate. Efficient hydrolysis was observed at acid pH with a digitonin-solubilized extract of rat liver, with a rate similar to that for the hydrolysis of cholesteryl oleate. The distribution of acid 15-ketosteryl oleate hydrolase of whole liver homogenate on a metrizamide isopycnic density gradient was similar to that of acid cholesteryl oleate hydrolase and acid phosphatase, suggesting that the lysosomal acid lipase is the enzyme responsible for the hydrolysis of the 15-ketosteryl oleate at acid pH. At neutral pH, 15-ketosteryl oleate and cholesteryl oleate was hydrolyzed at similar rates by the microsomal fraction of liver homogenate, whereas the 15-ketosteryl oleate was hydrolyzed at a much lower rate than cholesteryl oleate by the cytosolic fraction. The distribution of neutral 15-ketosteryl oleate hydrolase activity of whole liver homogenate on a metrizamide isopycnic density gradient was most correlated to a microsomal esterase, whereas cholesteryl oleate hydrolase activity was most correlated to a cytosolic enzyme. Both 15-ketosteryl oleate and cholesteryl oleate hydrolase activities were correlated to a mitochondrial marker enzyme.