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.     

Substrate specificity of the biotransformation enzymes in a Nocardia erythropolis culture

Substrate specificity of biotransformation enzymes of culture Nocardia erythropolis was studied. Products of transformation of cholesterol and three sterols of microbial origin: ergosterol, ergosta-5,7-dien-3 beta-ol and ergosta-7,22-dien-3 beta-ol was identified with a help of thin-layer chromatography, UV spectrophotometry and mass-spectrometry. It was established, that delta 22-bond in the side chains of sterols and delta 7-bond slows and delta 5-bond makes impossible cleavage of side chains of sterols.     

Efficient preparation of steroidal 5,7-dienes of high purity.

Protected forms of dehydroepiandrosterone, delta 5 cholenic acid, (25R)-26-hydroxycholesterol and diosgenin were converted to the corresponding delta 5,7 dienes by successive treatment with 1,3-dibromo-5,5-dimethylhydantoin (dibromantin), tetrabutylammonium bromide and tetrabutylammonium fluoride. The crude products, which contained the delta 5,7 species contaminated by minor amounts of the delta 5 and delta 4,6 steroids, were purified by silica gel-AgNO3 chromatography to give the following steroids in approximately 99% purity and at least 50% yield: 3 beta-acetoxyandrosta-5,7-dien-17-one, methyl 3 beta-acetoxychola-5,7-dien-24-oate, (25R)-3 beta,26-diacetoxycholesta-5,7-diene and (25R)-3 beta-acetoxyspirosta-5,7-diene. Analogous treatment of acetate derivatives of pregnenolone and stigmasterol gave 3 beta-acetoxypregna-5,7-dien-20-one and 3 beta-acetoxystigmasta-5,7,22-triene in approximately 50% yield but of lower purity. Full 1H and 13C NMR assignments are given for seven delta 5,7 steroid acetates and the corresponding delta 5 starting materials. Coupling constants for rings A, B and C of delta 5,7 steroids are presented and stereochemical assignments have been made for the following 1H NMR signals: the C-11 protons of delta 5,7 steroids, the C-16 protons of sterols and bile acids, the C-22 and C-23 protons of bile acid esters and the C-28 protons of stigmasterol derivatives.     

Effect of 5,7-unsaturated sterols on ethanol tolerance in Saccharomyces cerevisiae.

Structural membrane lipids are known to contribute to the high ethanol resistance of Saccharomyces cerevisiae (2, 4, 17). By manipulating the yeast cellular sterol level by changing the carbon-to-nitrogen source ratio in the chemostat growth medium, high delta 5,7-sterol levels were found to increase the resistance of yeast populations to ethanol-induced death. The resistance of the erg2 (delta 8----delta 7-sterol isomerase) mutant to ethanol-induced death was generally comparable with that of the delta 5,7-sterol-synthesizing strain. In contrast, the sensitivity of anaerobic growth to inhibition by ethanol was higher in the erg2 mutant in comparison with the delta 5,7-sterol-synthesizing strains but a high level of those sterols increased the vulnerability of anaerobic growth to ethanol inhibition.     

Fed-Batch cultivation ofSaccharomyces cerevisiae with increased content of Δ5,7-sterols

Cultivation of Saccharomyces cerevisiae with a linear nutrient feed was used to test the validity of a mathematical model describing changes in the physiological state of the culture. Markers of these changes were the concentrations of proteins and delta 5,7-sterols in yeast dry mass. The model was used to optimize the production of these sterols with regard to the magnitude and composition of nutrient feed.     

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.     

Effects of the hypocholesteremic agent trifluperidol on the sterol, steryl ester, and fatty acid metabolism of Saccharomyces cerevisiae.

Trifluperidol (TFP), at a concentration of 100 muM, inhibited the 24-h growth of Saccharomyces cerevisiae by about 30%. Effects on lipid metabolism were investigated by monitoring the incorporation of [1-14C]sodium acetate into various lipid fractions after 4 and 24 h of growth in the presence of several concentrations of TFP. Although little effect was noted on the amount of free sterols, 24-h incorporation of label into steryl esters was increased two- to fourfold by 100 muM TFP. Major sterol components of the steryl ester fraction isolated from an untreated culture were zymosterol (48%) and ergosterol (24%), whereas from the TFP-treated culture delta8,24(28)-ergostadienol (66.6%) and delta8-ergostenol (14.7%) were most abundant. Free sterols present in the highest concentration in the untreated culture were ergosterol (78.2%) and lanosterol (13%); whereas delta8,22-ergostadienol (38.5%), delta8-ergostenol (35.4%), and delta8,24(28)-ergostadienol (25.4%) were the most abundant free sterols obtained from the TFP-treated culture. Thus, the major block in the sterol biosynthetic pathway in yeast appears to be delta8 leads to delta7 isomerization. In these same cultures the relative amounts of C12 and C14 acids isolated from both steryl ester and miscellaneous lipid fractions were increased more than threefold over controls.     

Sterol-phospholipid interaction in model membranes: role of C5-C6 double bond in cholesterol

Several double-bond isomers of cholesterol where the normal C5-C6 double bond (delta 5) has been moved to different positions in the ring skeleton, i.e., delta 1, delta 4, delta 7, delta 8(9), delta 8(14), and delta 14, have been synthesized and incorporated in phosphotidylcholine vesicles. In addition, dienes like delta 5,7, delta 7,14, and delta 8,14 have also been studied. Many of these cholesterol analogues are intermediates in the sterol biosynthesis in different organisms. The incorporation studied indicated that more than 90% of the sterol was present in the vesicles. The effect of these cholesterol analogues was studied by glucose permeability, electron spin resonance, and fluorescence polarization spectroscopy. These studies indicated that delta 14-cholesten-3 beta-ol was most effective in restricting glucose permeability or in increasing the order parameter but was still not as effective as cholesterol. This was followed by delta 8(14)- and delta 8(9)-cholesten-3 beta-ol. The delta 1, delta 4, and delta 7 analogues and the dienols were relatively less effective in condensing the membrane. These studies indicate that the double bond at C5-C6 in cholesterol is most effective for optimal sterol-phospholipid interaction and may have formed the basis of the migration of the double bond from rings C and D in sterols to C5-C6 during the evolution of cholesterol.     

Dehydrodinosterol,dinosterone and related sterols of a non-photosynthetic dinoflagellate, Crypthecodinium cohnii

The heterotrophic dinoflagellate Crypthecodinium cohnii contained the 4α-methyl sterols, dinosterol, dehydrodinosterol (4α,23,24-trimethylcholesta-5,22-dien-3β-ol) and the tentatively identified 4α,24-dimethyl-cholestan-3β-ol and 4α,24-dimethylcholest-5-en-3β-ol. The major 4-demethyl sterol was cholesta-5,7-dien-3β-ol which was accompanied by a smaller amount of cholesterol and traces of several other C₂₇,C₂₈ and C₂₉ sterols. In addition, a 3-oxo-steroid fraction was isolated and the major component identified as dinosterone (4α,23,24-trimethylcholest-22-en-3-one). The possible biosynthetic relationships of these compounds are discussed.     

Occurrence and seasonal variation of 19-norcholest-4-en-3-one and 3 beta-monohydroxy sterols in the Californian gorgonian, Muricea californica

The first natural occurrence of 19-norcholestenone is reported, together with 17 sterols and one other delta 4-3-ketone in the extracts of the Californian gorgonian, Muricea californica (Aurivillius). Six additional demethyl sterols and five additional 4-monomethyl sterols which remain unidentified were also detected. Lipid extracts of M. californica from a winter and summer collection were split by various chromatographic methods into free sterol, steryl ester, and steryl conjugate fractions. Sterol compositions (determined by CG and CG-MS) of each fraction, subsequent to hydrolysis, are tabulated and discussed with respect to plausible origins of observed variations. The possible relationship of the Muricea 19-nor-steroidal ketone to other naturally occurring 19-nor-steroids is discussed.     

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.     

Minor and trace sterols in marine invertebrates. 61. Isolation and structure elucidation of new A-nor sterols from the marine sponge Phakellia aruensis

An examination of the Australian sponge Phakellia aruensis led to the isolation and identification of sterols with six different nuclei. Eight new sterols were isolated which included three delta 15-A-nor sterols, three delta 7-A-nor sterols, and two saturated A-nor sterols. Their structures were established through mass spectrometry and 1H-NMR spectral studies.     

Steinernema feltiae (= Neoaplectana carpocapsae): effect of sterols and hypolipidemic agents on development

The structure and concentration of sterol in a lipid-defined artificial medium affected the development of the entomogenous nematode, Steinernema feltiae (= Neoaplectana carpocapsae). The nematode grew normally in vitro when the medium was supplemented with delta 5-desalkylsterol (cholesterol) or delta 5-desalkylsteryl ester (cholesterol oleate). The minimum amount of cholesterol in the medium that was necessary to support the development of S. feltiae to the climax population (i.e., dauer stage) was 0.0025%. The nematode also completed its life cycle normally when delta 0- or delta 7-desalkylsterols (cholestanol and lathosterol) were substituted for cholesterol. In contrast, development was inhibited when the medium contained delta 5,7-desalkylsterol (7-dehydrocholesterol); however, the nematode population reached the climax stage, in medium containing this sterol, when cholesterol was also present. S. feltiae was able to utilize delta 5- and delta 0-24 alpha-ethylsterols (sitosterol and sitostanol) as dietary sterols; however, when a delta 22-bond was introduced into the side chain (stigmasterol) the rate of development of the nematode slowed significantly. The growth of the nematode was also retarded when the medium contained delta 5,7,22-24 beta-methylsterol (ergosterol). The nematode population reached the climax stage in medium containing delta 8,24-4,14 alpha-trimethylsterol (lanosterol) only when cholesterol was also present. When S. feltiae was exposed to certain hypolipidemic agents, which are known to lower the level of lipids in human plasma (clofibrate, cholestyramine resin, niacin, and D-thyroxine), all but D-thyroxine affected the growth and development of the nematode in vivo (in Heliothis zea) and/or in vitro. Therefore further studies are warranted to determine how these drugs affect the lipid biochemistry of this nematode.     

Identification of delta 5,7-sterol-delta 7-reductase in higher plant microsomes

A microsomal preparation from seedlings of Zea mays catalyzed the NADPH dependent reduction of the delta 7-bond of delta 5,7-cholestadienol (1) giving the first in vitro evidence for the intermediacy of delta 5,7-sterols in plant sterol biosynthesis. Using a GC assay developed to detect the cholesterol (2) produced, the properties of the microsomal enzyme have been established with respect to cofactor requirements and kinetics. The potent in vitro inhibition of the plant delta 5,7-sterol-delta 7-reductase by the ammonium-ion containing fungicides, tridemorph2 (3), fenpropimorph (4) and AY 9944 (5) was demonstrated. The high affinities observed for these derivatives, especially for (4) (I50 = 8 x 10(-8) M, I50/Km = 2 x 10(-4)), are in full accordance with the previously proposed cationic mechanism involved in this reduction reaction.     

Chemical composition and biological properties of Erythraea centaurium Rafn

The steroidic fraction from the ethereal extract of the aereal parts of Erythraea centaurium Rafn. has showed the presence of the following sterols: beta-sitosterol, stigmasterol, campesterol, brassicasterol and delta 7 stigmastenol, isolated by H.P.L.C. and identified by G.L.C., 1H-N.M.R. and MS. Then the free amino-acids from the aqueous extract of E. centaurium were determined.     

The functional importance of structural features of ergosterol in yeast.

As an approach to the study of the relationship between the structure of sterols and their capacity to function in the lipid leaflet of membranes, various sterols were examined for their ability to support the growth of anaerobic Saccharomyces cerevisiae. A marked dependence on precise structural features was observed in growth-response and morphology. Of the chemical groups which distinguish ergosterol, the main sterol of S. cerevisiae, the hydroxyl group at C-3 was obligatory, and the other groups were found to be of the following relative importance: 24beta-methyl-delta22-grouping greater than 24beta-methyl group greater than delta5,7-diene system = delta5-bond approximately or equal to no double bond. Methyl groups at C-4 and C-14 were inconsistent with activity. Consequently, the data strongly suggest that the normal biosynthetic processes removal of methyl groups from the nucleus and introduction of one in the side chain are of functional significance. A double bond between C-17 and C-20 joining the steroidal side chain to the nucleus had no deleterious effect on the growth process but only if C-22 was trans-oriented to C-13. In the cis-case no growth at all proceeded. This means the natural sterol probably acts functionally in the form of its preferred conformer in which C-22 is to the right ("right-handed") in the usual view. Since the placing of a substituent (OH or CH3) in the molecule at C-20 in such a way that it appears on the front side in the right-handed conformer completely destroyed activity, the sterol apparently presents its front face to protein or phospholipid when complexing occurs.     

Chemical synthesis of 7- and 8-dehydro derivatives of pregnane-3,17alpha,20-triols,potential steroid metabolites in Smith-Lemli-Opitz syndrome

Pregnane-3,17 alpha,20-triols bearing unsaturation at delta(7), delta(8), delta(5,7), or delta(5,8) have been tentatively identified as steroid metabolites in Smith-Lemli-Opitz syndrome (SLOS). Starting with 17 alpha-hydroxypregnenolone diacetate, we have synthesized 13 unsaturated C(21) triols by four different routes in one to four steps. These multifunctional steroids were prepared by a series of regio- and stereoselective transformations chosen to minimize facile olefin isomerization and 17-deoxygenation. The results include a study of stereoselectivity in the reduction of 17 alpha-hydroxy-20-ketosteroids, an alternative method for reducing diethyl azodicarboxylate adducts of delta(5,7) steroids, and an efficient oxidation-isomerization of a delta(5,7) steroid using cholesterol oxidase. The 13 triols and their synthetic precursors were fully characterized by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. The NMR data, together with molecular modeling, indicated unanticipated conformational heterogeneity for two synthetic intermediates, 17 alpha-hydroxypregna-4,7-diene-3,20-dione and 17 alpha-hydroxy-5 beta-pregn-7-ene-3,20-dione. The unsaturated C(21) triols are useful as reference standards to study adrenal steroid production in SLOS and to develop methods for pre- and postnatal diagnosis of this congenital disorder.     

Composition sterolique et morphogeneses reproductrices chez Gnomonia leptostyla

Sterol components of Gnomonia leptostyla mycelia have been investigated from in vitro cultures in which sexual and asexual morphogenesis are induced by temperature and light conditions. The nature and content of free sterols and sterol esters were determined by MIKE spectrometry. Relations between sterol composition (total sterols; Δ^5,7 and Δ⁵ sterols) and reproductive morphogenesis are discussed, particularly with respect to the degree of sexuality induced.     

On the origin of terpenes in symbiotic associations between marine invertebrates and algae (zooxanthellae). Culture studies and an application of 13C/12C isotope ratio mass spectrometry

13C/12C ratios of sets of compounds, algal sterols and terpenes, isolated from dinoflagellate symbiont (zooxanthellae)-bearing soft corals and gorgonians were determined. In most cases, a significant difference was found between the delta 13C values of the terpenes and of the algal sterols from the same set, the algal sterols containing less 13C than the terpenes. These results can only be explained if terpenes are synthesized by the host. Cultured zooxanthellae, isolated from symbiotic associations, do not make terpenes. Algal sterols of the various sets do not all have the same delta 13C value: average values range from -18.2 to -24.3%. A consistent difference of about 7% in the delta 13C values of sterols of cultured symbionts isolated from two of the gorgonians was found. This has potential applications for the taxonomy of zooxanthellae, most of which are believed by some specialists to be one discrete species.     

Sterols of the cultured euglenid Eutreptia viridis: A novel Δ23-unsaturated sterol.

The sterol mixture isolated from the marine alga $\text{Eutreptia viridis}$ consists mainly of Δ^5,7-dienes which account for 80% of the free sterols. Eighteen different sterols were detected, including a novel sterol with the rare Δ²³-unsaturation, viz. 24-ethylcholesta-5,7,23Z-trien-3β-ol.