Biomedical subjects

W D Nes

Publications and source records attributed to W D Nes.

At least 37 records · Page 2Linked to original sources

PubMed1995

Novel sterol transformations promoted by Saccharomyces cerevisiae strain GL7: evidence for 9 beta, 19-cyclopropyl to 9(11)-isomerization and for 14-demethylation to 8(14)-sterols.

Cultures of Saccharomyces cerevisiae strain GL7 (a sterol auxotroph) were incubated with nonradioactive and tritium-labeled cycloartenol, and the sterol composition of the cells was examined by chemical (GLC, TLC, HPLC, MS, 1H-NMR, and 13C-NMR) and radiotracer techniques. Several novel sterols were isolated from the cells including 14 alpha-methyl ergosta-9(11),24(28)-dien-3 beta-ol, 24 beta-methyl-9 beta,19-cyclopropyl ergost-8(14)-en-3 beta-ol, and 9 beta,19-cyclopropyl ergosta-7(8),24(28)-dien-3 beta-ol. GL7 converted [2-3H]cycloartenol to [2-3H]ergosterol in low yield (1% incorporation), whereas [2-3H]lanosterol was converted to [2-3H]ergosterol in high yield (41% incorporation). The degree of sterol absorption and transformation by GL7 was dependent on the type and amount of sterol(s) in the growth medium. The results demonstrate for the first time that yeast may transform 9 beta,19-cyclopropyl sterols to 9(11)-sterols and delta 5-sterols and that 14-demethylation of sterols may proceed in GL7 to double bond formation either in the 8(14)-position or in the 14(15)-position.

Culture Media↗

PubMed1995

Comparative studies of metabolism of 4-desmethyl, 4-monomethyl and 4,4-dimethyl sterols in Manduca sexta.

To investigate the metabolism and possible deleterious effects of 4-methyl and 4,4-dimethyl steroids in Manduca sexta, the 4,4-dimethyl sterols lanosterol and cycloartenol, the 4-methyl sterol obtusifoliol and the 4,4-dimethyl pentacyclic triterpenoid alpha-amyrin were fed in an artificial agar-based diet at various concentrations. Utilization and metabolism of these four compounds were compared with sitosterol, stigmasterol, brassicasterol, ergosterol and 24-methylenecholesterol, 24-alkyl sterols that are readily dealkylated and converted to cholesterol in Manduca and in most phytophagous insects. None of the 4-methylated compounds significantly inhibited development except at very high dietary concentrations. The delta 24-bonds of lanosterol and cycloartenol were effectively reduced by the Manduca delta 24-sterol reductase enzyme, as is the delta 24-bond of desmosterol which, in most phytophagous insects, is an intermediate in the conversion of sitosterol, stigmasterol and other C28 and C29 phytosterols to cholesterol. On the other hand, the 24-methylene substituent of obtusifoliol was not dealkylated. Each of the 4-desmethyl C28 and C29 sterols was readily converted to cholesterol, and a significant amount of 7-dehydrocholesterol was derived from ergosterol metabolism. The reason for the differences in substrate specificity of these sterols is not clear, but the information may be useful in the development of new, specific, mechanism-based inhibitors of sterol metabolism.

Animals↗

PubMed1995

Developmental regulation of sterol biosynthesis in Zea mays.

Sixty-one sterols and pentacyclic triterpenes have been isolated and characterized by chromatographic and spectral methods from Zea mays (corn). Several plant parts were examined; seed, pollen, cultured hypocotyl cells, roots, coleoptiles (sheaths), and blades. By studying reaction pathways and mechanisms on plants fed radiotracers ([2-(14)C]mevalonic acid, [2-(14)C]acetate, and [2-(3)H]acetate), and stable isotopes (D2O), we discovered that hydroxymethylgutaryl CoA reductase is not "the" rate-limiting enzyme of sitosterol production. Additionally, we observed an ontogenetic shift and kinetic isotope effect in sterol biosynthesis that was associated with the C-24 alkylation of the sterol side chain. Blades synthesized mainly 24 alpha-ethyl-sterols, sheaths synthesized mainly 24-methyl-sterols, pollen possessed an interrupted sterol pathway, accumulating 24(28)-methylene-sterols, and germinating seeds were found to lack an active de novo pathway. Shoots, normally synthesizing (Z)-24(28)-ethylidine-cholesterol, after incubation with deuterated water, synthesized the rearranged double-bond isomer, stigmasta-5,23-dien-3 beta-ol. Examination of the mass spectrum and 1H nuclear magnetic resonance spectrum of the deuterated 24-ethyl-sterol indicated the Bloch-Cornforth route originating with acetyl-CoA and passing through mevalonic acid to sterol was not operative at this stage of development. An alternate pathway giving rise to sterols is proposed.

Acetates↗

PubMed1993

The structural requirements of sterols for membrane function in Saccharomyces cerevisiae.

Cultures of Saccharomyces cerevisiae strain GL7 auxotrophic for sterol were incubated with a series of sterols and sterol-like molecules (tetracyclic and pentacyclic triterpenoids) in order to determine the structural requirements of sterols for bulk membrane function. For growth support, the 3 beta-OH group could not be replaced by H, OMe, OBu, NH2, NHOH, OAc, keto, or 3 alpha-OH. A methyl group at C-14 was neither deleterious nor essential for activity. Removal of the C-4 geminal methyl group was obligatory for activity. Thus, no sterol-like triterpenoid supported growth (e.g., tetrahymanol, lanosterol, and cycloartenol). Growth support required a sterol with the longest methylene segment extending from C-20 not to exceed six contiguous C-atoms and the stereochemistry must be C-20 R. No significance could be attributed to branching at C-20 (i.e., to C-21), C-24 (when alkylated), or C-25 (regarding the isopropyl group). Double bonds in the nucleus were not essential for activity since cholestanol supported growth. In several incubations, the addition of trace levels of dietary ergosterol (0.5 microgram/ml) to the medium was necessary to promote growth and transformation of the bulk sterol to a membrane competent sterol(s).

Cell Membrane↗

PubMed1992

Structural requirements for transformation of substrates by the S-adenosyl-L-methionine:delta 24(25)-sterol methyltransferase. Inhibition by analogs of the transition state coordinate.

Microsomes from sunflower seedlings were used to investigate the transition state coordinate for the C-24 methylation reaction that mediates phytosterol biosynthesis. They were then used to study structurally related cationic and uncharged compounds of the natural sterol substrate, which were designed to interfere with the reaction progress. The hypothetical reaction course is described to proceed through an Sn2 formation of an activated complex involving the initial production of a covalent structure with a dative bond (methyl from AdoMet attacks si-face of the 24,25-double bond of the sterol) and the secondary production of a series of high energy intermediates, the stabilization of which determines the final C-24 methylated product. Derivatives of lanosterol and cholesterol with a methyl, hydrogen, oxygen, or bromine atom introduced into the side chain and/or at C-3 in place of the natural nucleophile were studied as inhibitors that interfere with the formation of the hypothetical tertiary isopropylcarbinyl cation intermediate in the conversion of cycloartenal to 24(28)-methylene cycloartanol. The data indicate the most potent inhibitor is a sterol with an aziridine group attached to C-24(25), which mimics the bridged C-24(25) carbenium ion generated in the transition state, and the methyltransferase possesses two strategic sites: one that recognizes the proximal end of the sterol acting as a proton donor and the other that recognizes the distal end that acts as a proton acceptor. The best fit (binding/catalysis) involves a flat sterol (including substrate and inhibitor) with intact unsubstituted nucleophilic centers at C-3 and C-24 and a freely rotating side chain that can assume the pseudocyclic conformation.

Binding Sites↗

PubMed1991

Structural requirements for transformation of substrates by the (S)-adenosyl-L-methionine:delta 24(25)-sterol methyl transferase.

The membrane-bound enzyme of microsomes obtained from sunflower embryos that catalyzes the bi-substrate transfer reaction whereby the methyl group of (S)-adenosyl-L-methionine is transferred to C-24 of the sterol side chain has been investigated. Optimal incubation conditions for assay of the microsomal (S)-adenosyl-L-methionine:sterol delta 24-methyl transferase (SMT) have been established for the first time. The microsomal preparation was found to catalyze the formation of a delta 24(28)-sterol and to be free of contaminating methyl transferase enzymes, e.g. those which form delta 23-24 methyl sterols (cyclosadol) and delta 25-24 beta-methyl sterols (cyclolaudenol) and other sterolic enzymes which might transform the acceptor molecule to metabolites which could compete in the assay with the test substrate. From a series of incubations with 27 sterol and sterol-like (triterpenoids) substrates of which 23 compounds possessed a 24,25-double bond, we observed a marked dependence on precise structural features and three-dimensional shape of the acceptor molecule in its ability to be transformed by the SMT. In contrast to the yeast SMT where cycloartenol fails to bind to the SMT and zymosterol is the best substrate for methylation, the sunflower SMT studied here utilizes cycloartenol preferentially to zymosterol and the other substrates. Of the chemical groups which distinguishes cycloartenol, a free 3 beta-OH,9 beta,19-cyclopropyl group, trimethylated saturated nucleus, and delta 24-double bond, only the nucleophilic centers at C-3 and C-24 were obligatory for substrate binding and methylation. Of the bent or flat conformations which cycloartenol may orient in the enzyme-substrate complex, our results indicate a selection for acceptor molecules which possess the shape that closely resembles the crystal state and solution orientation of cycloartenol which is now known to be flat rather than bent (Nes, W. D., Benson, M., Lundin, R. E., and Le, P. H. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 5759-5763).

Alkylation↗

PubMed1991

Regulation of sterol biosynthesis in sunflower by 24(R,S),25-epiminolanosterol, a novel C-24 methyl transferase inhibitor.

Whereas sitosterol and 24(28)-methylene cycloartanol were competitive inhibitors (with Ki = 26 microM and 14 microM, respectively), 24(R,S)-25-epiminolanosterol was found to be a potent non-competitive inhibitor (Ki = 3.0 nM) of the S-adenosyl-L-methionine-C-24 methyl transferase from sunflower embryos. Because the ground state analog, 24(R,S)-oxidolanosterol, failed to inhibit the catalysis and 25-azalanosterol inhibited the catalysis with a Ki of 30 nM we conclude that the aziridine functions in a manner similar to the azasteriod (Rahier, A., et al., J. Biol. Chem. (1984) 259, 15215) as a transition state analog mimicking the carbonium intermediate found in the normal transmethylation reaction. Additionally, we observed that the aziridine inhibited cycloartenol metabolism (the preferred substrate for transmethylation) in cultured sunflower cells and cell growth.

Cell Division↗

PubMed1990

Zymosterol is located in the plasma membrane of cultured human fibroblasts.

Zymosterol (5 alpha-cholesta-8(9),24-dien-3 beta-ol) comprised a negligible fraction of the mass of sterol in cultured human fibroblasts but was well labeled biosynthetically with radioactive acetate. Treatment of cells with triparanol, a potent inhibitor of sterol delta 24-reductase, led to a marked increase in labeled zymosterol while its mass rose to 1 mol% of total sterol. All of this sterol could be chased into cholesterol. Furthermore, cell homogenates converted exogenous radiolabeled zymosterol to cholesterol. Three lines of evidence suggested that biosynthetically labeled zymosterol was associated with the plasma membrane. 1) About 80% of radiolabeled zymosterol was oxidized by the impermeant enzyme, cholesterol oxidase, in glutaraldehyde-fixed intact cells. 2) Sucrose density gradient analysis of homogenates showed that the equilibrium buoyant density profile of newly synthesized zymosterol was identical with that of the plasma membrane. 3) Newly synthesized zymosterol was transferred as readily from fixed intact fibroblasts to exogenous acceptors as was cholesterol. Given that cholesterol is synthesized within the cell, it is unclear why most of the zymosterol is in the plasma membrane. The pathway of cholesterol biosynthesis may compel zymosterol to flux through the plasma membrane. Alternatively, plasma membrane zymosterol may represent a separate pool, in equilibrium with the zymosterol in the intracellular biosynthetic pool.

Acetates↗

PubMed1990

Sterol phylogenesis and algal evolution.

The stereochemistry of several sterol precursors and end products synthesized by two fungal-like micro-organisms Prototheca wickerhamii (I) and Dictyostelium discoideum (II) have been determined by chromatographic (TLC, GLC, and HPLC) and spectral (UV, MS, and 1H NMR) methods. From I and II the following sterols were isolated from the cells: cycloartenol, cyclolaudenol, 24(28)-methylenecycloartanol, ergosterol, protothecasterol, 4alpha-methylergostanol, 4alpha-methylclionastanol, clionastanol, 24beta-ethylcholesta-8,22-enol, and dictyosterol. In addition, the mechanism of C-24 methylation was investigated in both organisms by feeding to I [2-3H]lanosterol, [2-3H]cycloartenol, [24-3H]lanosterol, and [methyl-2H3]methionine and by feeding to II [methyl-2H3]methionine. The results demonstrate that the 24beta configuration is formed by different alkylation routes in I and II. The Delta25(27) route operates in I while the Delta24(28) route operates in II. Based on what is known in the literature regarding sterol distribution and phylogenesis together with our findings that the stereochemical outcome of squalene oxide cyclization leads to the production of cycloartenol rather than lanosterol (characteristic of the fungal genealogy) and the chirality of the C-24 alkyl group is similar in the two nonphotosynthetic microbes (beta oriented), we conclude that Prototheca is an apoplastic Chlorella (i.e., an alga) and that Dictyostelium as well as the other soil amoebae that synthesize cycloartenol evolved from algal rather than fungal ancestors.

Journal Article↗

PubMed1989

Metabolism of 24(R,S),25-epiminolanosterol to 25-aminolanosterol and lanosterol by Gibberella fujikuroi.

Mycelia of Gibberella fujikuroi metabolized 2-tritio-24(R,S), 25-epiminolanosterol, a novel fungal sterol biosynthesis inhibitor that regulates delta 24-sterol methyltransferase, to 25-aminolanosterol (a new sterol) and lanosterol. The identities of the two sterols were established by cochromatography with authentic samples, mass spectroscopy, and isotopic dilution and recrystallization to constant specific activity. The newly biosynthesized tritiolanosterol was demonstrably introduced into the sterol pathway as evidenced by significant tritium label in the radiochemically purified sterols; 24(28)-methylene-24,25-dihydrolanosterol and 14-norlanosterol. The results demonstrate for the first time the direct reduction in situ of an aziridine ring to a tertiary amine and the stereocontrolled deamination of a C-25 aminosteroid to produce a delta 24, rather than a delta 25(27), steroid.

Ergosterol↗

PubMed1989

Inhibition of cholesterol synthesis and cell growth by 24(R,S),25-iminolanosterol and triparanol in cultured rat hepatoma cells.

24(R,S),25-Iminolanosterol (IL) and triparanol added to cultures of rat hepatoma cells, H4-II-C3 (H4), interrupt the conversion of lanosterol to cholesterol and, depending on their concentrations, cause the accumulation in the cells of intermediates in the lanosterol to cholesterol conversion. At 45 microM, both substances cause the accumulation of 5 alpha-cholesta-8(9),24-dien-3 beta-ol (zymosterol), and at the low concentration of 4.5 microM, they cause the accumulation of cholesta-5.24-dien-3 beta-ol (desmosterol). The effect of intermediate concentrations of 9 or 22.5 microM of either substance is to cause the accumulation in the cells of three sterols: cholesta-5,7,24-trien-3 beta-ol, zymosterol, and desmosterol. The synthesis of these intermediary sterols, not found normally in H4 cells, is particularly pronounced in cultures kept in lipid-depleted media that contain the inhibitors and proceeds by the use of endogenous substrates at the expense of cholesterol. The synthesis of cholesterol from [14C]acetate or [2-14C]mevalonate is completely blocked by either inhibitor even at 4.5 microM. IL or triparanol inhibits the growth of H4 cells. Cells seeded into either full growth or lipid-depleted medium containing 22.5 microM IL will not grow unless the media are supplemented with low density lipoproteins (60 micrograms/ml). Supplementation of the media with 4.6 mM mevalonate does not counteract the inhibitory effect of IL on cell growth.

Animals↗

PubMed1988

Comparison of the chromatographic properties of sterols, select additional steroids and triterpenoids: gravity-flow column liquid chromatography, thin-layer chromatography, gas-liquid chromatography and high-performance liquid chromatography.

The chromatographic properties of approximately 100 sterols, select steroids of plant origin (sapogenins and steroidal alkaloids) and triterpenoids has been evaluated in this laboratory by monitoring their elution characteristics in adsorption (gravity column and thin-layer methods with and without the addition of silver nitrates), gas and reversed-phase high-performance liquid chromatography. The utility of each methodology to act in one or another chromatographic mode-separation, radio-chemical purification, quantitation and structural elucidation, is discussed. The importance of the tilt of the -OH group at C-3 as well as the polarity, size, an shape of the rest of the molecule as it effects the hydrogen-bonding ability of the -OH group is demonstrated through changes in chromatographic behavior that result from the step-wise introduction of double bonds, methyl, bromo, oxygen, nitrogen and cyclopropyl groups into 5 alpha-cholestanol. An independent aid in the structure identification and quantitation of the compounds was use of a multiple-wavelength diode array detector in which different wavelengths of the UV spectrum (200-400 nm) were simultaneously monitored following passage of the sample through a reversed-phase C18 column.

Chromatography, Gas↗

PubMed1988

Biosynthesis of cholesterol in the yeast mutant erg6.

A mutant (erg6) of Saccharomyces cerevisiae defective in S-adenosylmethionine: delta 24-sterol-C-methyl transferase (EC2.1.1.41) which normally produces cholesta-5,7,24-trienol and cholesta-5,7,22,24-tetraenol as the major sterols (total 4,4-desmethyl sterol content-8.3 fg/cell) was shown to synthesize trace levels of cholesterol (0.08 fg/cell). The identity of cholesterol was established by co-chromatography in TLC, GLC and HPLC with an authentic sample, mass spectroscopy and after an incubation with [1-14C]acetate by isotopic dilution and recrystallization of the radiochemically purified material to constant specific activity.

Cholestanols↗

PubMed1988

Conformational analysis of 9beta,19-cyclopropyl sterols: Detection of the pseudoplanar conformer by nuclear Overhauser effects and its functional implications.

Nuclear Overhauser difference spectroscopy and variable temperature studies of the 9beta,19-cyclopropyl sterols 24,25-dehydropollinastanol (4,4-desmethyl-5alpha-cycloart-24-en-3beta-ol) and cyclolaudenol [(24S)-24-methyl-5alpha-cycloart-25(27)-en-3beta-ol] have shown the solution conformation of the B/C rings to be twist-chair/twist-boat rather than boat/chair as suggested in the literature. This is very similar to the known crystal structure conformation of 9beta,19-cyclopropyl sterols. The effect of these conformations on the molecular shape is highly significant: the first conformation orients into a pseudoplanar or flat shape analogous to lanosterol, whereas the latter conformation exhibits a bent shape. The results are interpreted to imply that, for conformational reasons, cyclopropyl sterols can be expected to maintain the pseudoplanar shape in membrane bilayers.

Journal Article↗

PubMed1987

Effects of 2,3-iminosqualene on cultured cells.

2,3-Iminosqualene (ISq) is a powerful inhibitor of squalene oxide:lanosterol cyclase (EC 5.4.99.7). When added to lipid-depleted culture media (LDM) of rat hepatoma (H-4-II-E-C3) or Chinese hamster ovary (CHO) cells at a concentration of 10 micrograms ml-1, it causes the cells to float off the substratum in a few days. Lipoproteins in the culture medium completely counteract this effect. Cells in lipoprotein-containing media (FGM) grow normally in the presence of ISq. Irrespective of the culture medium, ISq at 10 micrograms ml-1 causes an almost complete and apparently irreversible inactivation of the squalene oxide cyclase in CHO and H4 cells and the accumulation in the cells of squalene, of squalene 2,3-oxide (mostly), and of squalene 2,3-22,23-dioxide when [14C]acetate or [14C]mevalonate is fed to the cells. Chronic treatment of H4 cells with ISq failed to elicit induction of the cyclase, but increased the conversion of mevalonate into squalene and squalene dioxide, and depressed the conversion of squalene oxide to the dioxide. Cells loaded with squalene and the squalene oxides from mevalonate in the presence of ISq get rid of these substances by rapidly secreting them into the media and by some unidentified metabolic processes.

Animals↗

PubMed1986

Control of fungal sterol C-24 transalkylation: importance to developmental regulation.

When cultures of Gibberella fujikuroi are incubated with 24-epiiminolanosterol the introduction of a methyl group into sterol side chains at C-24 is blocked inducing a mycelial accumulation of lanosterol and 24-desalkylsterols, i.e., having the cholestane side chain. The altered sterol composition lead to aberrant mycelial membranes resulting in growth inhibition. A compensatory physiological response to the ensuing hyphal death was induction of asexual sporulation. The results are interpreted to imply that regulation of sterol C-24 transalkylation may be a mechanism to mediate life cycle events of fungi.

Alkylation↗

PubMed1986

Physiological requirement for biosynthesis of multiple 24 beta-methyl sterols in Gibberella fujikuroi.

Studies with Gibberella fujikuroi have been designed to examine the relationship between the biosynthesis and function of fungal sterols. Evidence was obtained through appropriate feeding and trapping experiments for the existence of multiple end products which are produced by separate routes in the later stages of sterol biosynthesis. The three end products, ergosterol (24 beta-methylcholesta-5,7,22E-trien-3 beta-ol), brassicasterol (24 beta-methylcholesta-5,22E-dien-3 beta-ol), and 22(23)-dihydrobrassicasterol (24 beta-methyl-cholesterol), were found to be non-interconvertible during logarithmic phase growth; thus the metabolic route delta 5,7,22-24 beta-CH3----delta 5,22-24 beta-CH3----delta 5-24 beta-CH3 was ruled out. Ergosterol can be further metabolized, viz., to 24 beta-methylcholesta-5,7,9(11),22-tetraen-3 beta-ol, but only as the culture enters into the stationary phase. In the presence of growth inhibitory concentrations of 2,3-iminosqualene, a partial reversal of growth cessation was obtained when all three sterols were concurrently supplied to the medium. Since neither ergosterol nor the other two sterols added individually to the medium was able to overcome the inhibitor's deleterious effect, ergosterol cannot play a dual architectural role (bulk and regulatory) in this fungus as it apparently can do in other fungal systems, i.e., yeast. For G. fujikuroi each sterol end product appears to possess a unique physiological role. Mycelial growth requires more than simply ergosterol.

Chromatography, High Pressure Liquid↗

PubMed1984

Rotational isomerism about the 17(20)-bond of steroids and euphoids as shown by the crystal structures of euphol and tirucallol.

The influence of configuration at C-20 on rotation about the 17(20)-bond in steroids and euphoids was examined by x-ray crystallographic studies of the C-20 epimers euphol and tirucallol. The H atom on C-20 was in back next to C-18 in the crystal structures of both of the compounds, and C-22 was found to be cis-oriented ("left-handed") to C-13 in euphol and trans-oriented to it ("right-handed") in tirucallol. The results, which are consistent with the known left-handed crystal structure of 24(25)-dihydroeuphol and right-handed crystal structure of cholesterol and other natural sterols, lend further credence to the earlier suggestion that rotational isomerism at the 17(20)-bond can arise in C-20 epimers and that there is preference for an arrangement with the 20-H atom adjacent to C-18.

Isomerism↗