A novel olefinic rearrangement. The enzymic conversion of cholesta-7,9-dien-3β-ol into cholesta-8,14-dien-3β-ol

1. [3alpha-(3)H]Cholesta-7,9-dien-3beta-ol is converted in high yield into cholesterol by a 10000g(av.) supernatant fraction of rat liver homogenate. 2. Incubation of cholesta-7,9-dien-3beta-ol with [4-(3)H]NADPH and rat liver microsomal fractions under anaerobic conditions resulted in (3)H being incorporated into the 14alpha-position of cholest-7-en-3beta-ol. 3. Under anaerobic conditions in the absence of NADPH cholesta-7,9-dien-3beta-ol was isomerized into cholesta-8,14-dien-3beta-ol by rat liver microsomal fractions.     

Mode of formation of cholesta-5,7-dien-3β-ol from cholest-5-en-3β-ol by larvae of Calliphora erythrocephala

1. The conversion of cholest-5-en-3beta-ol (cholesterol) into cholesta-5,7-dien-3beta-ol by axenic Calliphora erythrocephala larvae was demonstrated. 2. The transformation is probably direct (Delta(5)-->Delta(5,7)) and does not involve a Delta(0) intermediate (Delta(5)-->Delta(0)-->Delta(7)--> Delta(5,7)). 3. Delta(7)-bond formation involves the stereospecific elimination of the 7beta hydrogen atom. 4. The relative amounts of free and esterified sterols were determined in larvae grown on cholesterol as sole sterol source and on 5alpha-cholestan-3beta-ol supplemented with minimal amounts of cholesterol. 5. The significance of the results is assessed in relation to the probable role of cholesta-5,7-dien-3beta-ol as an intermediate in the biosynthesis of ecdysones.     

Microsomal enzymes of cholesterol biosynthesis from lanosterol. Solubilization and purification of steroid 8-isomerase

Steroid-8-ene isomerase that catalyzes isomerization of delta 8- to delta 7-sterols has been solubilized from rat liver microsomes with a mixture of two detergents, octylglucoside and sodium taurodeoxycholic acid. During a 40-fold enrichment of the solubilized enzyme, other enzymes of cholesterol biosynthesis, endogenous lipids, and electron carriers are removed. A comparison of properties of the solubilized and partially purified isomerase with the membrane-bound enzyme shows they are essentially identical with respect to pH profile, effect of inhibitors and cofactors, substrate specificity, and Km values. Addition of phospholipid to the partially purified enzyme stimulates activity as much as 1.8-fold over control rates. Although the relative rate of isomerization of cholesta-8,24-dien-3 beta-ol is six times that observed with cholest-8-en-3 beta-ol, the delta 8 to delta 7 ratio at equilibrium is approximately equal. The reversibility of the reaction has been demonstrated by the direct conversion of cholest-7-en-3 beta-ol to cholest-8-en-3 beta-ol; at equilibrium the delta 7-isomer is predominant (19/1). The purified enzyme does not catalyze isomerization of cholesta-8,14-dien-3 beta-ol and cholest-8(14)-en-3 beta-ol under conditions that result in equilibrium mixtures of isomers from cholest-8(9)-en-3 beta-ol. These results are consistent with the earlier suggestion that delta 8(14)-sterols are neither formed nor metabolized by the same microsomal enzymes that catalyze transformation of lanosterol to cholesterol.     

Sterol synthesis: studies of the metabolism of 14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol

[3 alpha-3H]14 alpha-Methyl-5 alpha-cholest-7-en-3 beta-ol has been prepared by chemical synthesis. The metabolism of this compound has been studied in the 10,000 g supernatant fraction of liver homogenates of female rats. Efficient conversion to cholesterol was observed. Other labeled compounds recovered after incubation of [3 alpha-3H]14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol with the enzyme preparations include the unreacted substrate, 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholesta-8,14-dien-3 beta-ol, cholesta-5,7-dien-3 beta-ol, 5 alpha-cholest-8(14)-en-3 beta-ol, 5 alpha-cholest-8-en-3 beta-ol, and 5 alpha-cholest-7-en-3 beta-ol. In addition, significant amounts of incubated radioactivity were recovered in steryl esters. The steroidal components of these esters were found to contain labeled 14 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol, 5 alpha-cholesta-8,14-dien-3 beta-ol, 5 alpha-cholesta-7,14-dien-3 beta-ol, 5 alpha-cholest-8-en-3 beta-ol, 5 alpha-cholest-7-en-3 beta-ol, and cholesterol.     

Biosynthesis of cholestanol: 5-alpha-cholestan-3-one reductase of rat liver

The 3-beta-hydroxysteroid dehydrogenase of rat liver which catalyzes the conversion of 5alpha-cholestan-3-one to 5alpha-cholestan-3beta-ol is localized mainly in the microsomal fraction. The enzyme required NADPH as hydrogen donor and differed from the known 3-beta-hydroxysteroid dehydrogenases of the C(19) series in being inactive in the presence of NADH. The microsomal preparations did not reduce the 3-keto groups of cholest-4-en-3-one, cholest-5-en-3-one, or 5beta-cholestan-3-one to the corresponding 3beta-hydroxy compounds. The conversion of 5alpha-cholestan-3-one to 5alpha-cholestan-3beta-ol was only slightly inhibited by the reaction product or by other monohydroxy steroids, but a strong inhibitory effect was noted with cholest-5-en-3-one, 5alpha-cholestane-3beta, 7alpha-diol and 5alpha-cholestan-7-on-3beta-ol. The microsomes, but not high speed supernatant solution, catalyzed the reverse of the cholestanone reductase reaction, namely the conversion of 5alpha-cholestan-3beta-ol to 5alpha-cholestan-3-one in the presence of oxygen and an NADP-generating system. The action of the microsomal preparations upon 5alpha-cholestan-3-one produced 5alpha-cholestan-3alpha-ol in addition to the 3beta-epimer. The 3-alpha-hydroxysteroid dehydrogenase involved functioned with either NADH or NADPH as hydrogen donor. The ratio of 5alpha-cholestan-3beta-ol to 5alpha-cholestan-3alpha-ol formed from 5alpha-cholestan-3-one was approximately 10:1 and was independent of the sex of the animal from which the microsomes were prepared.     

Correlation between serum levels of some cholesterol precursors and activity of HMG-CoA reductase in human liver

The possibility that the serum concentrations of various cholesterol precursors may reflect the activity of the hepatic HMG-CoA reductase was investigated in humans under different conditions. The serum levels of squalene, free and esterified lanosterol, (4 alpha, 4 beta, 14 alpha-trimethyl-5 alpha-cholest-8, 24-dien-3 beta-ol), two dimethylsterols (4 alpha, 4 beta-dimethyl-5 beta-cholest-8-en-3 beta-ol and 4 alpha, 4 beta-dimethyl-5 alpha-cholest-8, 24-dien-3 beta-ol), two methostenols (4 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol and 4 alpha-methyl-5 alpha-cholest-8-en-3 beta-ol), two lathosterols (5 alpha-cholest-7-en-3 beta-ol and 5 alpha-cholest-8-en-3 beta-ol) and desmosterol (cholest-5, 24-dien-3 beta-ol) were measured in untreated patients (n = 7) and patients treated with cholestyramine (QuestranR, 8 g twice daily for 2-3 weeks, n = 5) or chenodeoxycholic acid (15 mg/kg body weight daily for 3-4 weeks, n = 8) prior to elective cholecystectomy. The activity of the hepatic microsomal HMG-CoA reductase was measured in liver biopsies taken in connection with the operation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for the presence of 7-hydroperoxycholest-5-en-3 beta-ol in oxidized human LDL.

Low density lipoprotein (LDL) cholesterol is known to be oxidized both in vitro and in vivo giving rise to oxygenated sterols. Conflicting results, however, have been reported concerning both the nature and the relative concentrations of these compounds in oxidized human LDL. We examined the extracts obtained from Cu(2+)-oxidized LDL. Thin layer chromatography analysis showed that the sterol mixture became more complex with reaction time. Analysis of the components by thin layer chromatography and mass spectrometry allowed to establish that 7 alpha- and 7 beta-hydroperoxycholest-5-en-3 beta-ol (7 alpha OOH and beta OOH) are largely prevalent among the oxysterols at early times of oxidation. These hydroperoxy derivatives have not been previously identified in oxidized LDL. The concentration of 7-hydroperoxycholest-5-en-3 beta-ol decreased with oxidation time with a concomitant increase of cholest-5-en-3 beta, 7 alpha-diol (7 alpha OH), cholest-5-en-3 beta, 7 beta-diol (7 beta OH), cholesta-3,5-dien-7-one (CD) and cholest-5-en-3 beta-ol-7-one (7CO). After 24 h of oxidation a minor component of the LDL sterols was cholestan-3 beta-ol-5,6-oxide (EP).     

Inhibition of sterol synthesis by delta 5-sterols in a sterol auxotroph of yeast defective in oxidosqualene cyclase and cytochrome P-450

Synthesis of ergosterol is demonstrated in the GL7 mutant of Saccharomyces cerevisiae. This sterol auxotroph has been thought to lack the ability to synthesize sterols due both to the absence of 2,3-oxidosqualene cyclase and to a heme deficiency eliminating cytochrome P-450 which is required in demethylation at C-14. However, when the medium sterol was 5 alpha-cholestan-3 beta-ol, 5 alpha-cholest-8(14)-en-3 beta-ol, or 24 beta-methyl-5 alpha-cholest-8(14)-en-3 beta-ol, sterol synthesis was found to proceed yielding 1-3 fg/cell of ergosterol (24 beta-methylcholesta-5,7,22E-trien-3 beta-ol). Ergosterol was identified by mass spectroscopy, gas and high performance liquid chromatography, ultraviolet spectroscopy, and radioactive labeling from [3H]acetate. Except for some cholest-5-en-3 beta-ol (cholesterol) which was derived from the 5 alpha-cholestan-3 beta-ol, the stanol and the two 8(14)-stenols were not significantly metabolized confirming the absence of an isomerase for migration of the double bond from C-8(14) to C-7. Drastic reduction of ergosterol synthesis to not more than 0.06 fg/cell was observed when the medium sterol either had a double bond at C-5, as in the case of cholesterol, or could be metabolized to a sterol with such a bond. Thus, both 5 alpha-cholest-8(9)-en-3 beta-ol and 5 alpha-cholest-7-en-3 beta-ol (lathosterol) were converted to cholesta-5,7-dien-3 beta-ol (7-dehydrocholesterol), and the presence of the latter dienol depressed the level of ergosterol. The most attractive of the possible explanations for our observations is the assumption of two genetic compartments for synthesis of sterols, one of which has and one of which has not been affected by the two mutations. The ability, despite the mutations, to synthesize small amounts of ergosterol which could act to regulate the cell cycle may also explain why this mutant can grow aerobically with cholesterol (acting in the bulk membrane role) as the sole exogenous sterol.     

Conversion of sterols and triterpenes by mycobacteria. II. Transformation of 7-oxygenated sterols into androstane derivatives via a 7-deoxygenation

The bioconversion of 7-oxygenated sterols by Mycobacterium aurum was studied in a preliminary investigation of the microbial conversion of wool wax. 7-Oxocholesterol was found to be transformed mainly into 3,17-dioxygenated androstane derivatives. 7 xi-Hydroxylated sterols were formed in an initial reduction step, and the C-7 hydroxyl group was then eliminated in a dehydration reaction. This was thought to take place during the isomerisation of cholest-4-en-3-one to cholest-5-en-3-one. Deuterium labelling experiments showed that this elimination proceeded faster for the C-7 alpha isomer, although it was not stereospecific. The C-7 alpha and C-7 beta-hydroxy isomers were weakly interconverted via the 7-oxo derivatives. Cholest-4-en-3-one, cholest-1,4-dien-3-one and cholest-4,6-dien-3-one all lost their side chains following a hydrogenation/dehydrogenation reaction. The resulting 3,17-dioxoandrostene or 3,17-androstadiene derivatives were mainly hydrogenated into 5 alpha-androstane-3,17-dione and 5 alpha-androstane-3 beta-ol-17-one. Elimination of the 3 beta-hydroxyl groups giving cholesta-3,5-dien-7-one, and subsequent microbial degradation of the side chain was not observed to any significant extent. The convergence of the bioconversion pathways of cholesterol and the 7-oxygenated cholesterols enabled crude, partially auto-oxidised cholesterol to be used as a substrate for the production of 3,17-dioxygenated androstane derivatives by M. aurum.     

Sterol metabolism. XXVI. Pyrolysis of some sterol allylic alcohols and hydroperoxides

The thermal decomposition of the allylic alcohols 5α-cholest-6-ene-3β,5-diol, cholest-5-ene-3β,7α-diol, and cholest-5-ene-3β,7β-diol and of the allylic hydroperoxides 3β-hydroxy-5α-cholest-6-ene-5-hydroperoxide, 3β-hydroxycho lest-5-ene-7α-hydroperoxide, and 3β-hydroxycholest-5ene-7β-hydroperoxide to six common major pyrolysis products cholest-5-ene-3β,7α-diol, cholest-5-ene-3β,7β-diol, 3β-hydroxycholest-5-en-7-one, cholesta-3,5-dien-7-one, cholesta-4,6-dien-3-one, and cholesta2,4,6-triene was established.     

Synthesis of the marine epoxy sterol 9 alpha,11 alpha-epoxy-5 alpha-cholest-7-ene-3 beta,5,6 beta-triol

The synthesis of 9 alpha,11 alpha-epoxy-5 alpha-cholest-7-ene-3 beta,5,6 beta-triol (1), a highly oxygenated marine sterol containing a 9,11-epoxide moiety in the nucleus, is described. Epoxy sterol 1 was synthesized from cholesta-5,7-dien-3 beta-ol. Oxidation of this sterol with m-chloroperbenzoic acid followed by hydrolysis and acetylation furnished 5 alpha-cholest-7-ene-3 beta,5,6 alpha-triol 3,6-diacetate (2). Mercuric acetate dehydrogenation of diacetate 2, followed by oxidation with manganese dioxide and epoxidation with m-chloroper-benzoic acid, afforded 9 alpha,11 alpha-epoxy-3 beta,5-dihydroxy-5 alpha-cholest-7-en-6-one (5). Reduction of 5 with lithium aluminum hydride gave the desired compound 1. The structures of all synthetic intermediates were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. A reassignment of resonances for carbons 1, 8, and 15 in the 13C NMR spectrum of 1, based on 2D-NMR correlation spectroscopy, has been accomplished.     

In vitro inhibition of rat liver cholest-5en-3 beta-ol (cholesterol) biosynthesis by non-mercurial sulfhydryl reagents.

In vitro conversion of 2-14C-mevalonate to cholest-5en-3 beta-ol (cholesterol) in rat liver homogenates is inhibited by arsenite, beta-mercaptoethanol, dithiothreitol and ethanethiol. Two sterols containing 20 carbon atoms accumulate under these conditions. One of these is identified as 4,4 dimethyl-5alpha-cholest-8en-3beta-ol and the other tentatively identified as 4,4 dimethyl-5alpha-cholest-8,24-dien-3beta-ol. Based on these observations, these non-mercurial sulfhydryl reagents do not inhibit 5alpha-lanosta-8,24-dien-3beta-ol 14alpha demethylase.     

Oxysterols from human bile induce apoptosis of canine gallbladder epithelial cells in monolayer culture

Oxysterols have been detected in various mammalian organs and blood. Biliary epithelium is exposed to high concentrations of cholesterol, and we have identified three keto-oxysterols (cholest-4-en-3-one, cholesta-4,6-dien-3-one, cholesta-3,5-dien-7-one) in human bile and gallstones. Because the effects of oxysterols on biliary physiology are not well defined, we investigated their biological effects on dog gallbladder epithelial cells. Enriched medium (culture medium containing taurocholate and lecithin and cholesterol +/- various oxysterols) was applied to confluent monolayers of dog gallbladder epithelial cells in culture. Cytotoxicity and apoptosis were studied by morphological analysis and flow cytometry. Oxysterols in the mitochondrial fraction were identified by gas chromatography/mass spectrometry, whereas release of cytochrome c from mitochondria was assayed by spectrophotometry and Western blot analysis. Compared with cells treated with culture medium or with enriched medium containing cholesterol, oxysterol-treated cells showed significantly increased apoptosis (P < 0.05). Exogenously applied oxysterols were recovered from the mitochondrial fraction. Cytochrome c release from mitochondria was increased significantly by cholest-4-en-3-one, cholesta-4,6-dien-3-one, and 5beta-cholestan-3-one (all P < 0.05). Thus oxysterols recovered from human bile and gallstones induce apoptosis of biliary epithelium via a mitochondrial-dependent pathway and may play a role in the pathogenesis of chronic inflammation and carcinogenesis in the gallbladder.     

Studies on delta8-delta7 isomerization and methyl transfer of sterols in ergosterol biosynthesis of yeast.

The formation of cholesta-7,24-dien-3 beta-ol and its activity as a substrate for the sterol side-chain methyltransferase in yeast have not previously been studied. Experiments with acetone-powder extracts of yeast showed that the sterol is formed from zymosterol by delta8-delta7 isomerization. However, direct conversion of cholesta-7,24-dien-3 beta-ol into zymosterol could not be demonstrated. The reversibility of the reaction was proved by the detection of 3H-incorporation into cholesta-8-en-3 beta-ol (with lathosterol as a carrier) from [3H]H2O in the medium. Incubation of cholesta-7,24-dien-3 beta-ol and S-adenosyl-L-[methyl-14C]methionine with the acetone-powder extract resulted in methylation of the sterol to form episterol. Similar incubation of zymosterol gave fecosterol and episterol, suggesting that fecosterol initially formed by the methylation was isomerized to episterol. In intact cells, however, an alternative pathway (zymosterol yields cholesta-7,24-dien-3 beta-ol yields episterol) may also operate. The relative importance of the two pathways is not known.     

The oxidation of Delta2, Delta2,4 and Delta4,6 steroids with RuO4

In order to find new ways for the functionalization of the A and B rings of the steroid nucleus, the reaction of 5alpha-androst-2-en-17beta-ol 17-acetate (1), cholesta-2,4-diene (4) and cholesta-4,6-dien-3beta-ol 3-acetate (7) was examined using stoichiometric amounts of ruthenium tetraoxide to yield 1,2-cis diols and/or alpha-hydroxy ketones. The reaction of 5alpha-cholest-2-en-3-ol 3-acetate (9) with ruthenium tetraoxide was also carried out and afforded, apart from an alpha-hydroxy ketone, also a diketone and a seco-dicarboxylic acid. The structures of all new steroids, including stereochemical details, were deduced by analysis of spectral data.     

Occurrence of isomeric dehydrocholesterols in human plasma.

Three isomeric dehydrocholesterols were found in plasma from healthy subjects and patients with abnormal production or metabolism of cholesterol. These chemically labile steroids were isolated by a mild liquid-solid extraction procedure using octadecylsilane-bonded silica as sorbent. Sterol-protein interactions were minimized by diluting plasma with aqueous isopropanol. The dehydrocholesterols were identified by high-performance liquid chromatography-ultraviolet spectroscopy and gas chromatography-mass spectrometry as cholesta-5,7-dien-3 beta-ol (7-dehydrocholesterol), 5 alpha-cholesta-6,8(9)-dien-3 beta-ol (isodehydrocholesterol), and tentatively as cholesta-5,8(9)-dien-3 beta-ol. There was a strong positive correlation between plasma levels of the two former compounds, isodehydrocholesterol levels usually being about 1.4 times higher than those of 7-dehydrocholesterol. The median concentration of 7-dehydrocholesterol in plasma from healthy subjects was 52 ng/ml. Similar concentrations were found in colectomized patients (median concentration 47 ng/ml) and patients with extrahepatic cholestasis and alcoholic liver cirrhosis (median concentrations 79 and 67 ng/ml, respectively). Patients with ileal resection or under treatment with cholestyramine had elevated levels (median concentrations 142 and 160 ng/ml, respectively) whereas patients with primary biliary cirrhosis had subnormal levels (median concentration 26 ng/ml). The results are consistent with a positive correlation between levels of the dehydrocholesterols in plasma and the rate of cholesterol synthesis. The sterols were also analyzed in human skin and bile and the results indicate that the liver may be an important source of isodehydrocholesterol.     

The conversion of cholest-5-en-3beta-ol into cholest-7-en-3beta-ol by the echinoderms Asterias rubens and Solaster papposus.

1. The echinoderms Asterias rubens and Solaster papposus (Class Asteroidea) metabolize injected [4(-14)C]cholest-5-en-3beta-ol to produce labelled 5alpha-cholestan-3beta-ol and 5alpha-cholest-7-en-3beta-ol. 2. Conversion of 5alpha-[4(-14)C]cholestan-3beta-ol into 5alpha-cholest-7-en-3beta-ol was demonstrated in A. Rubens. 3. Incubations of A. rubens with [4(-14)C]cholest-4-en-3-one resulted in the production of labelled 5alpha-cholestan-3-one, 5alpha-cholestan-3beta-ol and 5alpha-cholest-7-en-3beta-ol. 4. [4(-14)C]Sitosterol was metabolized by A. rubens to give 5alpha-stigmastan-3beta-ol and 5alpha-stigmast-7-en-3beta-ol. 5. The significance of these results in relation to the presence of alpha7 sterols in starfish is discussed.     

Evaluation of the vitamin D activity of yeasts with altered sterol metabolism

The sterol content of Saccharomyces strains with altered ergosterol metabolism was studied by UV-spectrophotometry, thin-layer chromatography and chromatographic mass-spectroscopy. A technique for estimation of D-vitamin activity of the yeast strains is proposed. The irradiated biomass of the strains accumulated ergosta-5,7-dien-3 beta-ol and also cholesta-5,7,24-trien-3 beta-ol and cholesta-5,7,22,24-tetraen-3 beta-ol is characterized by high antirachitic activity.     

The formation and reduction of the 14,15-double bond in cholesterol biosynthesis

It was shown that 100mug quantities of 4,4'-dimethyl[2-(3)H(2)]cholesta-8,14-dien-3beta-ol (IIIa), tritiated cholesta-8,14-dien-3beta-ol, 4,4'-dimethyl[2-(3)H(2)]cholesta-7,14-dien-3beta-ol, dihydro[2-(3)H(2)]lanosterol and [24-(3)H]lanosterol were converted by a 10000g supernatant of rat liver homogenate into cholesterol in 17%, 54%, 6%, 9.5% and 24% yields respectively. From an incubation of dihydro[3alpha-(3)H]lanosterol with a rat liver homogenate in the presence of a trap up to 38% of the radioactivity was found to be associated with a fraction that was unambiguously shown to be 4,4'-dimethylcholesta-8,14-dien-3beta-ol. Another related compound, 4,4'-dimethylcholesta-7,14-dien-3beta-ol was also shown to be equally effective in its ability to trap compound (IIIa) from an incubation of dihydro[3alpha-(3)H]lanosterol. The mechanism of the further conversion of the compound (IIIa) into cholesterol occurred by the reduction of the 14,15-double bond and involved the addition of a hydrogen atom from the medium to C-15 and another from the 4-position of NADPH to C-14. Two possible mechanisms for the removal of the 14alpha-methyl group in sterol biosynthesis are discussed.     

Inhibitors of ergosterol biosynthesis and growth of the trypanosomatid protozoan Crithidia fasciculata

Six nitrogen-, sulfur- and cyclopropane-containing derivatives of cholestanol were examined as inhibitors of growth and sterol biosynthesis in the trypanosomatid protozoan Crithidia fasciculata. The concentrations of inhibitors in the culture medium required for 50% inhibition of growth were 0.32 microM for 24-thia-5 alpha,20 xi-cholestan-3 beta-ol (2), 0.009 microM for 24-methyl-24-aza-5 alpha,20 xi-cholestan-3 beta-ol (3), 0.95 microM for (20,21),(24,-25)-bis-(methylene)-5 alpha,20 xi-cholestan-3 beta-ol (4), 0.13 microM for 22-aza-5 alpha,20 xi-cholestan-3 beta-ol (5), and 0.3 microM for 23-azacholestan-3-ol (7). 23-Thia-5 alpha-cholestan-3 beta-ol (6) had no effect on protozoan growth at concentrations as high as 20 microM. Ergosterol was the major sterol observed in untreated C. fasciculata, but significant amounts of ergost-7-en-3 beta-ol, ergosta-7,24(28)-dien-3 beta-ol, ergosta-5,7,22,24(28)-tetraen-e beta-ol, cholesta-8,24-dien-3 beta-ol, and, in an unusual finding, 14 alpha-methyl-cholesta-8,24-dien-3 beta-ol were also present. When C. fasciculata was cultured in the presence of compounds 2 and 3, ergosterol synthesis was suppressed, and the principal sterol observed was cholesta-5,7,24-trien-3 beta-ol, a sterol which is not observed in untreated cultures. The presence of this trienol strongly suggests that 2 and 3 specifically inhibit the S-adenosylmethionine:sterol C-24 methyltransferase but do not interfere with the normal enzymatic processing of the sterol nucleus. When C. fasciculata was cultured in the presence of compounds 5 and 7, the levels of ergosterol and ergost-7-en-3 beta-ol were suppressed, but the amounts of the presumed immediate precursors of these sterols, ergosta-5,7,22,24(28)-tetraen-3 beta-ol and ergosta-7,24-(28)-dien-3 beta-ol, respectively, were correspondingly increased. These findings suggest that 5 and 7 specifically inhibit the reduction of the delta 24(28) side chain double bond. When C. fasciculata was cultured in the presence of compound 4, ergosterol synthesis was suppressed, but the sterol distribution in these cells was complex and not easily interpreted. Compound 6 had no significant effect on sterol synthesis in C. fasciculata.