Studies of the Metabolism of 5α-Cholesta-8,14-dien-3β-ol and 5α-Cholesta-7,14-dien-3β-ol in Rat Liver Homogenate Preparations

An algorithm for rapid computational construction of metabolic networks: a cholesterol biosynthesis example

Alternative pathways of metabolic networks represent the escape routes that can reduce drug efficacy and can cause severe adverse effects. In this paper we introduce a mathematical algorithm and a coding system for rapid computational construction of metabolic networks. The initial data for the algorithm are the source substrate code and the enzyme/metabolite interaction tables. The major strength of the algorithm is the adaptive coding system of the enzyme-substrate interactions. A reverse application of the algorithm is also possible, when optimisation algorithm is used to compute the enzyme/metabolite rules from the reference network structure. The coding system is user-defined and must be adapted to the studied problem. The algorithm is most effective for computation of networks that consist of metabolites with similar molecular structures. The computation of the cholesterol biosynthesis metabolic network suggests that 89 intermediates can theoretically be formed between lanosterol and cholesterol, only 20 are presently considered as cholesterol intermediates. Alternative metabolites may represent links with other metabolic networks both as precursors and metabolites of cholesterol. A possible cholesterol-by-pass pathway to bile acids metabolism through cholestanol is suggested.

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.

Cholesterol biosynthesis from lanosterol: regulation and purification of rat hepatic sterol 14-reductase

We have previously characterized the membrane-bound sterol 14-reductase (14-reductase) that catalyzes anaerobically NADPH-dependent reduction of the 14-double bond of delta 8,14-diene or delta 7,14-diene sterols that are sterol intermediates in cholesterol biosynthesis in mammals (Paik et al. (1984) J. Biol. Chem. 259, 13413-13423). To elucidate the regulatory mechanism as well as molecular characteristics of the 14-reductase, we extended our investigation on the consequences of alteration of the enzymic activity under various physiological conditions. The enzymic activity of rat hepatic sterol 14-reductase was induced more than 11-fold by feeding 5% cholestyramine plus 0.1% lovastatin (the CL-diet) for 7 days but was severely suppressed by feeding 5% cholesterol or 0.01% AY-9944 (an inhibitor of 14-reductase) for the same period. The increase or decrease in the 14-reductase activity also parallels the same change in the cholesterol synthetic rate in hepatocytes from rats that had been fed either the CL-diet or 0.01% AY-9944. In vitro inhibition studies revealed that AY-9944 acts as a competitive inhibitor of the 14-reductase (Ki = 0.26 microM). A diurnal variation was observed for the 14-reductase with peak activity near the middle of the dark cycle (10 p.m.), which was abolished by administration of cycloheximide. With induced enzyme conditions 14-reductase has been further purified with chromatographic procedures to near homogeneity. Purified 14-reductase appears to be a M(r) = 70,000 protein that is composed of two equally-sized subunits having a M(r) = 38,000. All properties of the purified 14-reductase suggest that the solubilized enzyme is the principal 14-reductase of microsomes. Taken together, our results provide the first evidence in support of a previously unknown regulatory role for the 14-reductase in the overall cholesterol synthetic pathway.

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.

Concerning the chemical synthesis of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one, a novel regulator of cholesterol metabolism

A four-step synthesis of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (I) from 7-dehydrocholesterol is described. This synthesis, which is efficient and suitable for kilogram scale work, was carried out in a 33% overall average yield (39% overall best yield). A major byproduct of the hydrolysis of 3 beta-benzoyloxy-14 alpha,15 alpha-epoxy-5 alpha-cholest-7-ene to I was found to be the ring C aromatic sterol 12-methyl-18-nor-5 alpha-cholesta-8,11,13-trien-3 beta-ol. Several other intermediates and byproducts of these reactions were also identified. All new sterols were characterized by 1H- and 13C-NMR.

Inhibitors of sterol synthesis. Studies of the distribution and metabolism of 5 alpha-[2,4-3H]cholest-8(14)-en-3 beta-ol-15-one after intragastric administration to rats

5 alpha-[2,4-3H]Cholest-8(14)-en-3 beta-ol-15-one was administered to a series of male Sprague-Dawley rats by intragastric intubation in the form of an emulsion in a mixture of triolein, sodium taurocholate, bovine serum albumin, and glucose. [4-14C]Cholesterol was similarly administered to a second series of rats. The distribution of 3H and 14C was studied at 12 and 48 h after the administration of the sterols. The results demonstrated that the 15-ketosterol is absorbed and metabolized to material with the chromatographic properties of fatty acid esters of the 15-ketosterol, to cholesterol, and to fatty acid esters of cholesterol. The [3H]cholesterol formed from the 15-ketosterol was characterized by its behavior on silicic acid-Super Cel column chromatography, by the chromatographic behavior of its acetate derivative on alumina-AgNO3 column chromatography, and by purification by way of its dibromide derivative without significant change in specific activity. The general distribution of 3H was similar to that of 14C. No unusual concentration of 3H in any of the organs studied was observed.

Inhibitors of cholesterol biosynthesis. Further studies of the metabolism of 5 alpha-cholest-8(14)-en-3 beta-ol-15-one in rat liver preparations

5 alpha-Cholest-8(14)-en-3 beta-ol-15-one is a potent inhibitor of sterol biosynthesis in mammalian cells in culture and has significant hypocholesterolemic activity upon oral administration to rodents and non-human primates. The conversion of the 15-ketosterol to cholesterol upon incubation with the 10,000 x g supernatant fraction of rat liver homogenate preparations under aerobic conditions has been reported (D.J. Monger, E.J. Parish and G.J. Schroepfer, Jr. (1980) J. Biol. Chem. 255, 11122-11129). Presented herein are results of studies of the metabolism of [2,4-3H]5 alpha-cholest-8(14)-en-3 beta-ol-15-one obtained upon incubation with the microsomal, cytosolic and the 10,000 x g supernatant fractions of liver homogenates of female rats under a variety of conditions. The results of these studies indicated metabolism of the 15-ketosterol to materials with the chromatographic properties of fatty acid esters of the 15-ketosterol, fatty acid esters of C27-monohydroxysterols, a component similar to the 15-ketosterol (possibly an isomer of the delta 8(14)-15-ketosterol), and a polar component. Detailed studies of the C27-monohydroxysterols obtained from incubation of the 15-ketosterol under anaerobic conditions indicated the formation of labeled 5 alpha-cholesta-8,14-dien-3 beta-ol and 5 alpha-cholest-7-en-3 beta-ol which were characterized by their behavior on silicic acid column chromatography, by the behavior of their acetate derivatives on medium pressure liquid chromatography on alumina-AgNO3 columns, and by co-crystallization of the labeled sterols with authentic unlabeled standards. The identification of 5 alpha-cholesta-8,14-dien-3 beta-ol and 5 alpha-cholest-7-en-3 beta-ol as metabolites of the 15-ketesterol, coupled with previous studies of the metabolism of 5 alpha-cholesta-8,14-dien-3 beta-ol and of 5 alpha-cholest-8(14)-ene-3 beta, 15 alpha-diol and 5 alpha-cholest-8(14)-ene-3 beta, 15 beta-diol has permitted the formulation of a scheme for the overall metabolism of the 15-ketosterol to cholesterol.

In vitro metabolism of mevalonic acid in the bovine retina

Bovine retinas were incubated with 3RS-[5-3H]-mevalonic acid under conditions similar to those previously shown to support opsin biosynthesis in vitro. TLC of the total lipids indicated the formation of numerous radiolabeled components, including sterols, hydrocarbons, and "fatty acid-like material." The nonsaponifiable lipids were analyzed by TLC, GLC, and chromatography on columns of silicic acid-Super Cel, silica gel G-Super Cel-silver nitrate, and alumina-Super Cel-silver nitrate. The major nonsaponifiable components had the chromatographic properties of squalene and "methylated sterols" (i.e., C30, C29, and C28 monohydroxy sterols). Cholesterol represented no more than 1% of the total radioactivity in the nonsaponifiable lipid fraction. The "fatty acid-like material" was derivatized with diazomethane, and the resulting methyl esters were analyzed by GLC before and after catalytic hydrogenation. The radioactivity did not correspond to the normal fatty acids endogenous to the retina, but rather had the chromatographic properties of C15 and C20 isoprenoid acids. These results obtained with intact retinas are consistent with our previous observations concerning mevalonic acid metabolism in cell-free homogenates of bovine retinas.

Sterol composition of bovine retinal rod outer segment membranes and whole retinas

The sterol composition of bovine retinal rod outer segment membranes and whole retinas was studied by detailed chromatographic analyses. Cholesterol represented at least 98% of the total 3 beta-monohydroxy sterols of rod outer segment membranes, accounting for 1.68 +/- 0.15% of the dry weight. Whole retinas contained 1.76 +/- 0.29% cholesterol by dry weight, representing at least 99% of the total 3 beta-monohydroxy sterols. Trace amounts of a component having the chromatographic properties of 5 alpha-cholestan-3 beta-ol were found in rod outer segment membranes and whole retinas. Very small amounts of a component having the chromatographic properties of 5 alpha-cholest-7-en-3 beta-ol were found in whole retinas, but not in rod outer segment membranes. The molar ratio of cholesterol to rhodopsin in bovine rod outer segment membranes was approximately 4.7. Cholesterol accounted for only 5-7 mol% of total rod outer segment membrane lipids.

Synthesis, properties and reactions of 3 beta-benzoyloxy-7 alpha-15 beta-dichloro-5 alpha-cholest-8(14)-ene

Treatment of 3 beta-benzoyloxy-14 alpha,15 alpha-epoxy-5 alpha-cholest-7-ene (I) with gaseous HCl in chloroform at -40 degrees C gave, in 87% yield, 3 beta-benzoyloxy-7 alpha,15 beta-dichloro-5 alpha cholest-8(14)-ene (III). Reduction of the latter compound with lithium aluminum hydride in ether at room temperature for 20 min gave, in 86% yield, 7 alpha-15 beta-dichloro-5 alpha-cholest-8(14)-en-3 beta-ol (IV). The latter compound was fully characterized and assignments of the individual carbon peaks in the 13C nuclear magnetic resonance spectra of this sterol have been completed. Reduction of III with excess lithium aluminum hydride in refluxing ether for 4 days gave, in 74% yield, 5 alpha-cholesta-7,14-dien-3 beta-ol (VI). Reduction of the dichloro-steryl benzoate III with lithium triethylborohydride in tetrahydrofuran gave, in 88% yield, 5 alpha-cholest-8(14)-en-3 beta-ol (VII). A similar reduction using lithium triethylborodeuteride led to the formation of [7 beta, 15 xi-2 H2]-VIIa. Treatment of III with concentrated HCl in a mixture of chloroform and methanol gave, in 79% yield, 3 beta-benzoyloxy-5 alpha-cholest-8(14)-en-15-one (II) which was characterized as such and as the corresponding free sterol.

Sterol synthesis. Chemical synthesis of 3beta-benzoyloxy -14alpha, 15alpha-epoxy-5alpha-cholest-7-ene, a key intermediate in the synthesis of 15-oxygenated sterols

3BETA-Benzoyloxy-14alpha, 15alpha-cholest-7-ene was obtained in 96% yield upon treatment of 3beta-benzoyloxy-5alpha-cholesta-7, 14-diene with m-chloroperbenzoic acid. The delta7-14alpha, 15alpha-epoxy-steryl ester provides a useful intermediate for the syntheses of sterols with an oxygen function at carbon atom 15. For example, treatment of 3beta-benzoyloxy-14alpha, 15alpha-epoxy-5alpha-cholest-7-ene with methanolic hydrochloric acid gave 3beta-benzoyloxy-5alpha-cholest-8(14)-en-15-one in 82% yield.

Enzymatic conversion of 5alpha-cholesta-7, 14-dien-3beta-ol to 5alpha-cholesta-8, 14-dien-3beta-ol

5alpha-Cholesta-7, 14-dien-3beta-ol, previously shown to be efficiently converted to cholesterol upon incubation with rat liver homogenate preparations under aerobic conditions, has been studied as to its possible conversion to 5alpha-cholesta-8, 14-dien-3beta-ol. Efficient conversion was observed upon incubation in the presence of washed mictosomes of rat liver under anaerobic conditions. This observation is of importance in consideration of possible metabolic pathways in the biosythesis of cholesterol.

Inhibition of enzymatic reduction of delta 14-double bond of 5 alpha-cholesta-8,14-dien-3 beta-ol and 5 alpha-cholesta-7,14-dien-3 beta-ol by AY-9944

AY-994 (trans-1,4-bis-(2-chlorobenzyl-aminomethyl) cyclohexane dihydrochloride), a potent inhibitor of the enzymatic reduction of the delta 7-double bond of delta 5,7-sterols, has been shown to inhibit, at higher concentrations (10-4 M), the enzymatic reduction of the delta 14-double bond of cholesta-8,14-dien-3 beta-ol and cholesta-7,14-dien-3 beta-ol.