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

Processive kinetics in the three-step lanosterol 14α-demethylation reaction catalyzed by human cytochrome P450 51A1

Cytochrome P450 (P450, CYP) family 51 enzymes catalyze the 14α-demethylation of sterols, leading to critical products used for membranes and the production of steroids, as well as signaling molecules. In mammals, P450 51 catalyzes the 3-step, 6-electron oxidation of lanosterol to form (4β,5α)-4,4-dimethyl-cholestra-8,14,24-trien-3-ol (FF-MAS). P450 51A1 can also use 24,25-dihydrolanosterol (a natural substrate in the Kandutsch-Russell cholesterol pathway). 24,25-Dihydrolanosterol and the corresponding P450 51A1 reaction intermediates, the 14α-alcohol and -aldehyde derivatives of dihydrolanosterol, were synthesized to study the kinetic processivity of the overall 14α-demethylation reaction of human P450 51A1. A combination of steady-state kinetic parameters, steady-state binding constants, dissociation rates of P450-sterol complexes, and kinetic modeling of the time course of oxidation of a P450-dihydrolanosterol complex showed that the overall reaction is highly processive, with koff rates of P450 51A1-dihydrolanosterol and the 14α-alcohol and 14α-aldehyde complexes being 1 to 2 orders of magnitude less than the forward rates of competing oxidations. epi-Dihydrolanosterol (the 3α-hydroxy analog) was as efficient as the common 3β-hydroxy isomer in the binding and formation of dihydro FF-MAS. The common lanosterol contaminant dihydroagnosterol was found to be a substrate of human P450 51A1, with roughly one-half the activity of dihydrolanosterol. Steady-state experiments with 14α-methyl deuterated dihydrolanosterol showed no kinetic isotope effect, indicating that C-14α C-H bond breaking is not rate-limiting in any of the individual steps. The high processivity of this reaction generates higher efficiency and also renders the reaction less sensitive to inhibitors.

Pollen sterols are associated with phylogeny and environment but not with pollinator guilds

Phytosterols are primary plant metabolites that have fundamental structural and regulatory functions. They are also essential nutrients for phytophagous insects, including pollinators, that cannot synthesize sterols. Despite the well-described composition and diversity in vegetative plant tissues, few studies have examined phytosterol diversity in pollen. We quantified 25 pollen phytosterols in 122 plant species (105 genera, 51 families) to determine their composition and diversity across plant taxa. We searched literature and databases for plant phylogeny, environmental conditions, and pollinator guilds of the species to examine the relationships with pollen sterols. 24-methylenecholesterol, sitosterol and isofucosterol were the most common and abundant pollen sterols. We found phylogenetic clustering of twelve individual sterols, total sterol content and sterol diversity, and of sterol groupings that reflect their underlying biosynthesis pathway (C-24 alkylation, ring B desaturation). Plants originating in tropical-like climates (higher mean annual temperature, lower temperature seasonality, higher precipitation in wettest quarter) were more likely to record higher pollen sterol content. However, pollen sterol composition and content showed no clear relationship with pollinator guilds. Our study is the first to show that pollen sterol diversity is phylogenetically clustered and that pollen sterol content may adapt to environmental conditions.

A nematode sterol C4α-methyltransferase catalyzes a new methylation reaction responsible for sterol diversity

Primitive sterol evolution plays an important role in fossil record interpretation and offers potential therapeutic avenues for human disease resulting from nematode infections. Recognizing that C4-methyl stenol products [8(14)-lophenol] can be synthesized in bacteria while C4-methyl stanol products (dinosterol) can be synthesized in dinoflagellates and preserved as sterane biomarkers in ancient sedimentary rock is key to eukaryotic sterol evolution. In this regard, nematodes have been proposed to convert dietary cholesterol to 8(14)-lophenol by a secondary metabolism pathway that could involve sterol C4 methylation analogous to the C2 methylation of hopanoids (radicle-type mechanism) or C24 methylation of sterols (carbocation-type mechanism). Here, we characterized dichotomous cholesterol metabolic pathways in Caenorhabditis elegans that generate 3-oxo sterol intermediates in separate paths to lophanol (4-methyl stanol) and 8(14)-lophenol (4-methyl stenol). We uncovered alternate C3-sterol oxidation and Δ⁷ desaturation steps that regulate sterol flux from which branching metabolite networks arise, while lophanol/8(14)-lophenol formation is shown to be dependent on a sterol C4α-methyltransferse (4-SMT) that requires 3-oxo sterol substrates and catalyzes a newly discovered 3-keto-enol tautomerism mechanism linked to S-adenosyl-l-methionine-dependent methylation. Alignment-specific substrate-binding domains similarly conserved in 4-SMT and 24-SMT enzymes, despite minimal amino acid sequence identity, suggests divergence from a common, primordial ancestor in the evolution of methyl sterols. The combination of these results provides evolutionary leads to sterol diversity and points to cryptic C4-methyl steroidogenic pathways of targeted convergence that mediate lineage-specific adaptations.­­

Steroidal antibiotics are antimetabolites of Acanthamoeba steroidogenesis with phylogenetic implications

Pathogenic organisms may be sensitive to inhibitors of sterol biosynthesis, which carry antimetabolite properties, through manipulation of the key enzyme, sterol methyltransferase (SMT). Here, we isolated natural suicide substrates of the ergosterol biosynthesis pathway, cholesta-5,7,22,24-tetraenol (CHT) and ergosta-5,7,22,24(28)-tetraenol (ERGT), and demonstrated their interference in Acanthamoeba castellanii steroidogenesis: CHT and ERGT inhibit trophozoite growth (EC50 of 51 nM) without affecting cultured human cell growth. Washout experiments confirmed that the target for vulnerability was SMT. Chemical, kinetic, and protein-binding studies of inhibitors assayed with 24-AcSMT [catalyzing C28-sterol via Δ24(28)-olefin production] and 28-AcSMT [catalyzing C29-sterol via Δ25(27)-olefin production] revealed interrupted partitioning and irreversible complex formation from the conjugated double bond system in the side chain of either analog, particularly with 28-AcSMT. Replacement of active site Tyr62 with Phe or Leu residues involved in cation-π interactions that model product specificity prevented protein inactivation. The alkylating properties and high selective index of 103 for CHT and ERGT against 28-AcSMT are indicative of a new class of steroidal antibiotic that, as an antimetabolite, can limit sterol expansion across phylogeny and provide a novel scaffold in the design of amoebicidal drugs. Animal studies of these suicide substrates can further explore the potential of their antibiotic properties.

Functional importance for developmental regulation of sterol biosynthesis in Acanthamoeba castellanii

The sterol metabolome of Acanthamoeba castellanii (Ac) yielded 25 sterols. Substrate screening of cloned AcCYP51 revealed obtusifoliol as the natural substrate which converts to ∆8,14-sterol (<95%). The combination of [2H3-methyl]methionine incubation to intact cultures showing C28-ergosterol incorporates 2-2H atoms and C29-7-dehydroporiferasterol incorporates 5 2H-atoms, the natural distribution of sterols, CYP51 and previously published sterol methyltransferase (SMT) data indicate separate ∆24(28)- and ∆25(27)-olefin pathways to C28- and C29-sterol products from the protosterol cycloartenol. In cell-based culture, we observed a marked change in sterol compositions during the growth and encystment phases monitored microscopically and by trypan blue staining; trophozoites possess C28/C29-∆5,7-sterols, viable encysted cells (mature cyst) possess mostly C29-∆5-sterol and non-viable encysted cells possess C28/C29-∆5,7-sterols that turnover variably from stress to 6-methyl aromatic sterols associated with changed membrane fluidity affording lysis. An incompatible fit of steroidal aromatics in membranes was confirmed using the yeast sterol auxotroph GL7. Only viable cysts, including those treated with inhibitor, can excyst into trophozoites. 25-Azacycloartanol or voriconazole that target SMT and CYP51, respectively, are potent enzyme inhibitors in the nanomolar range against the cloned enzymes and amoeba cells. At minimum amoebicidal concentration of inhibitor amoeboid cells rapidly convert to encysted cells unable to excyst. The correlation between stage-specific sterol compositions and the physiological effects of ergosterol biosynthesis inhibitors suggests that amoeba fitness is controlled mainly by developmentally-regulated changes in the phytosterol B-ring; paired interference in the ∆5,7-sterol biosynthesis (to ∆5,7) - metabolism (to ∆5 or 6-methyl aromatic) congruence during cell proliferation and encystment could be a source of therapeutic intervention for Acanthamoeba infections.

Antifungal drug testing by combining minimal inhibitory concentration testing with target identification by gas chromatography-mass spectrometry

Fungal infections and their increasing resistance to antibiotics are an emerging threat to public health. Novel antifungal drugs, as well technologies that can help us bolster the antimicrobial pipeline and understand resistance mechanisms, are needed. The ergosterol biosynthetic pathway is one potential target for antifungal drugs. Here we describe how antifungal susceptibility testing can be combined with target identification in distal ergosterol biosynthesis by means of gas chromatography-mass spectrometry. The fungi are treated with sublethal doses of active components that block ergosterol biosynthesis, and the ergosterol biosynthesis intermediates are analyzed in a targeted metabolomics manner after derivatization (trimethylsilylation). Drug treatment results in distinct sterol patterns that are characteristic of the affected enzyme. Sterol identification based on relative retention times and electron ionization (EI) mass spectra, as well as semiquantitative assessment of ergosterol intermediates, is described. The protocol is applicable to yeasts and molds. The overall analysis time from incubation to test result is not more than 3 d. The assay can be used to determine whether an antifungal compound of interest targets sterol biosynthesis, and, if so, to determine which enzyme in the pathway it targets.

24-Methylenecyclopropane steroidal inhibitors: A Trojan horse in ergosterol biosynthesis that prevents growth of Trypanosoma brucei

A new class of steroidal therapeutics based on phylogenetic-guided design of covalent inhibitors that target parasite-specific enzymes of ergosterol biosynthesis is shown to prevent growth of the protozoan-Trypanosoma brucei, responsible for sleeping sickness. In the presence of approximately 15±5μM 26,27-dehydrolanosterol, T. brucei procyclic or blood stream form growth is inhibited by 50%. This compound is actively converted by the parasite to an acceptable substrate of sterol C24-methyl transferase (SMT) that upon position-specific side chain methylation at C26 inactivates the enzyme. Treated cells show dose-dependent depletion of ergosterol and other 24β-methyl sterols with no accumulation of intermediates in contradistinction to profiles typical of tight binding inhibitor treatments to azoles showing loss of ergosterol accompanied by accumulation of toxic 14-methyl sterols. HEK cells accumulate 26,27-dehydrolanosterol without effect on cholesterol biosynthesis. During exposure of cloned TbSMT to 26,27-dehydrozymosterol, the enzyme is gradually inactivated (k_cat/k_inact=0.13min^-1/0.08min^-1; partition ratio of 1.6) while 26,27-dehydrolanosterol binds nonproductively. GC-MS analysis of the turnover product and bound intermediate released as a C26-methylated diol (C3-OH and C24-OH) confirmed substrate recognition and covalent binding to TbSMT. This study has potential implications for design of a novel class of chemotherapeutic leads functioning as mechanism-based inhibitors of ergosterol biosynthesis to treat neglected tropical diseases.

Fluorinated Sterols Are Suicide Inhibitors of Ergosterol Biosynthesis and Growth in Trypanosoma brucei

Trypanosoma brucei, the causal agent for sleeping sickness, depends on ergosterol for growth. Here, we describe the effects of a mechanism-based inhibitor, 26-fluorolanosterol (26FL), which converts in vivo to a fluorinated substrate of the sterol C24-methyltransferase essential for sterol methylation and function of ergosterol, and missing from the human host. 26FL showed potent inhibition of ergosterol biosynthesis and growth of procyclic and bloodstream forms while having no effect on cholesterol biosynthesis or growth of human epithelial kidney cells. During exposure of cloned TbSMT to 26-fluorocholesta-5,7,24-trienol, the enzyme is gradually killed as a consequence of the covalent binding of the intermediate C25 cation to the active site (kcat/kinact = 0.26 min(-1)/0.24 min(-1); partition ratio of 1.08), whereas 26FL is non-productively bound. These results demonstrate that poisoning of ergosterol biosynthesis by a 26-fluorinated Δ(24)-sterol is a promising strategy for developing a new treatment for trypanosomiasis.

Characterization, mutagenesis and mechanistic analysis of an ancient algal sterol C24-methyltransferase: Implications for understanding sterol evolution in the green lineage

Sterol C24-methyltransferases (SMTs) constitute a group of sequence-related proteins that catalyze the pattern of sterol diversity across eukaryotic kingdoms. The only gene for sterol alkylation in green algae was identified and the corresponding catalyst from Chlamydomonas reinhardtii (Cr) was characterized kinetically and for product distributions. The properties of CrSMT were similar to those predicted for an ancient SMT expected to possess broad C3-anchoring requirements for substrate binding and formation of 24β-methyl/ethyl Δ(25(27))-olefin products typical of primitive organisms. Unnatural Δ(24(25))-sterol substrates, missing a C4β-angular methyl group involved with binding orientation, convert to product ratios in favor of Δ(24(28))-products. Remodeling the active site to alter the electronics of Try110 (to Leu) results in delayed timing of the hydride migration from methyl attack of the Δ(24)-bond, that thereby produces metabolic switching of product ratios in favor of Δ(25(27))-olefins or impairs the second C1-transfer activity. Incubation of [27-(13)C]lanosterol or [methyl-(2)H3]SAM as co-substrates established the CrSMT catalyzes a sterol methylation pathway by the "algal" Δ(25(27))-olefin route, where methylation proceeds by a conserved SN2 reaction and de-protonation proceeds from the pro-Z methyl group on lanosterol corresponding to C27. This previously unrecognized catalytic competence for an enzyme of sterol biosynthesis, together with phylogenomic analyses, suggest that mutational divergence of a promiscuous SMT produced substrate- and phyla-specific SMT1 (catalyzes first biomethylation) and SMT2 (catalyzes second biomethylation) isoforms in red and green algae, respectively, and in the case of SMT2 selection afforded modification in reaction channeling necessary for the switch in ergosterol (24β-methyl) biosynthesis to stigmasterol (24α-ethyl) biosynthesis during the course of land plant evolution.

Development and validation a liquid chromatography mass spectrometry for determination of solasodine in rat plasma and its application to a pharmacokinetic study

Solasodine is a poisonous alkaloid chemical compound that occurs in plants of the Solanaceae family. A simple and selective liquid chromatography mass spectrometry method for determination of solasodine in rat plasma was developed and validated over the range of 3-1,000 ng/mL. Chromatographic separation was achieved on a C18 (2.1 mm×50 mm, 3.5 μm) column with acetonitrile-0.1% formic acid in water as mobile phase with gradient elution. The flow rate was set at 0.4 mL/min. After addition of midazolam as internal standard (IS), liquid-liquid extraction by ethyl acetate was used as sample preparation. An electrospray ionization source was applied and operated in positive ion mode; selective ion monitoring mode was used for quantification with target ions m/z 414 for solasodine and m/z 326 for IS. Mean recoveries of solasodine in rat plasma were in the range of 87.6-94.1%. Matrix effects for solasodine were between 94.9% and 102.3%. Coefficient of variation of intra-day and inter-day precision were both <13%. The accuracy of the method ranged from 94.4% to 105.3%. The method was successfully applied to a pharmacokinetic study of solasodine after oral administration of 20mg/kg in rats.

C-24-methylation of 26-fluorocycloartenols by recombinant sterol C-24-methyltransferase from soybean: evidence for channel switching and its phylogenetic implications

The tightly coupled nature of the electrophilic alkylation reaction sequence catalysed by 24-SMT (sterol C-24-methyltransferase) of land plants and algae can be distinguished by the formation of cationic intermediates that yield phyla-specific product profiles. C-24-methylation of the cycloartenol substrate by the recombinant Glycine max (soybean) 24-SMT proceeds to a single product 24(28)-methylenecycloartanol, whereas the 24-SMT from green algae converts cycloartenol into two products cyclolaudenol [∆(25(27))-olefin] and 24(28)-methylenecycloartanol [(∆24(28))-olefin]. Substrate analogues that differed in the steric-electronic features at either end of the molecule, 26-homocycloartenol or 3β-fluorolanostadiene, were converted by G. max SMT into a single 24(28)-methylene product. Alternatively, incubation of the allylic 26-fluoro cyclosteroid with G. max SMT afforded a bound intermediate that converted in favour of the ∆(25(27))-olefin product via the cyclolaudenol cation formed initially during the C-24-methylation reaction. A portion of the 26-fluorocycloartenol substrate was also intercepted by the enzyme and the corresponding hydrolysis product identified by GC-MS as 26-fluoro-25-hydroxy-24-methylcycloartanol. Finally, the 26-fluorocycloartenols are competitive inhibitors for the methylation of cycloartenol and 26-monofluorocycloartenol generated timedependent inactivation kinetics exhibiting a kinact value of 0.12 min(-1). The ability of soybean 24-SMT to generate a 25-hydroxy alkylated sterol and fluorinated ∆(25(27))-olefins is consistent with our hypothesis that (i) achieving the cyclolaudenyl cation intermediate by electrophilic alkylation of cycloartenol is significant to the overall reaction rate, and (ii) the evolution of variant sterol C-24-methylation patterns is driven by competing reaction channels that have switched in algae from formation of primarily ∆(25(27)) products that convert into ergosterol to, in land plants, formation of ∆(24(28)) products that convert into sitosterol.

Evolutionarily conserved Delta(25(27))-olefin ergosterol biosynthesis pathway in the alga Chlamydomonas reinhardtii

Ergosterol is the predominant sterol of fungi and green algae. Although the biosynthetic pathway for sterol synthesis in fungi is well established and is known to use C24-methylation-C24 (28)-reduction (Δ(24(28))-olefin pathway) steps, little is known about the sterol pathway in green algae. Previous work has raised the possibility that these algae might use a novel pathway because the green alga Chlamydomonas reinhardtii was shown to possess a mevalonate-independent methylerythritol 4-phosphate not present in fungi. Here, we report that C. reinhardtii synthesizes the protosterol cycloartenol and converts it to ergosterol (C24β-methyl) and 7-dehydroporiferasterol (C24β-ethyl) through a highly conserved sterol C24- methylation-C25-reduction (Δ(25(27))-olefin) pathway that is distinct from the well-described acetate-mevalonate pathway to fungal lanosterol and its conversion to ergosterol by the Δ(24(28))-olefin pathway. We isolated and characterized 23 sterols by a combination of GC-MS and proton nuclear magnetic resonance spectroscopy analysis from a set of mutant, wild-type, and 25-thialanosterol-treated cells. The structure and stereochemistry of the final C24-alkyl sterol side chains possessed different combinations of 24β-methyl/ethyl groups and Δ(22(23))E and Δ(25(27))-double bond constructions. When incubated with [methyl-(2)H(3)]methionine, cells incorporated three (into ergosterol) or five (into 7-dehydroporiferasterol) deuterium atoms into the newly biosynthesized 24β-alkyl sterols, consistent only with a Δ(25(27))-olefin pathway. Thus, our findings demonstrate that two separate isoprenoid-24-alkyl sterol pathways evolved in fungi and green algae, both of which converge to yield a common membrane insert ergosterol.

Enhanced squalene production by wild-type Saccharomyces cerevisiae strains using safe chemical means

Interest is increasing in establishing renewable sources for squalene, a functional lipid, as the conventional ones are limited. In the present study, squalene production was achieved in a wild-type laboratory Saccharomyces cerevisiae strain by two safe chemical means using terbinafine (0.05-0.55 mM) and methyl jasmonate (MJ) (0-1.00 mM). Bioprocess kinetics optimized by response surface methodology and monitored by high-performance liquid chromatography revealed a clear dependence of growth and squalene content (SQC) and yield (SQY) on the above regulators. Maximum SQC (10.02±0.53 mg/g dry biomass) and SQY (20.70±1.00 mg/L) were achieved using 0.442 mM terbinafine plus 0.044 mM MJ after 28 h and 0.300 mM terbinafine after 30 h, respectively. A 10-fold increase in SQY was achieved in comparison to that in the absence of regulator. The ruggedness of optimum conditions for SQY was verified for five industrial strains. The cellular lipid fraction (∼12% of dry biomass) was rich in squalene (12-13%). Results are encouraging toward bioprocess scale up.

Sterol composition in larvae of the silkworm, Bombyx mori

Sterols in silkworm larvae were analyzed. Cholesterol was predominantly detected in all tissues examined. Dietary phytosterols and desmosterol, a putative biosynthetic intermediate from phytosterols to cholesterol, were also detected, indicating that imperfect intestinal conversion from phytosterols to cholesterol influences the sterol composition in larval tissues.

A role for MGA2, but not SPT23, in activation of transcription of ERG1 in Saccharomyces cerevisiae

The SaccharomycescerevisiaeMGA2 gene encodes an important regulator of unsaturated fatty acid production, by controlling transcription and mRNA stability of OLE1, the gene encoding the Δ9 fatty acid desaturase. Lipid composition studies indicated that the mga2Δ strain contains elevated relative amounts of squalene when compared to wild-type cells. The deletion of the MGA2 homologue SPT23 did not impact squalene levels. To explore the role of MGA2 in the regulation of sterol synthesis, the transcription of the ERG1 gene, which encodes squalene epoxidase, was studied using an ERG1 promoter-lacZ reporter gene construct. We report here that in addition to MGA2's role in regulation of unsaturated fatty acids, MGA2 is required for full basal expression of ERG1. Mga2p was found to be controlled by a novel regulator in its activation of ERG1, as neither unsaturated fatty acids nor cobalt affected ERG1 expression, as had previously been shown for Mga2p's regulation of OLE1. Further, response to miconazole treatment, which inhibits production of ergosterol at a later step in the sterol biosynthetic pathway and results in up-regulation of several genes in ergosterol synthesis, was not affected in the mga2Δ mutant. In each case, the spt23Δ mutant strain shows similar ERG1 expression to wild-type cells, while the mga2Δ/spt23Δ strain shows reduced ERG1 expression, comparable to the mga2Δ, suggesting that the role of regulation of ERG1 transcription is unique to Mga2p.

Cloning, functional expression and phylogenetic analysis of plant sterol 24C-methyltransferases involved in sitosterol biosynthesis

Sterol 24C-methyltransferases (SMTs) constitute a group of sequence-related proteins that catalyze the distinct patterns of 24-alkyl sterols that occur throughout nature. Two SMT cDNAs (SMT2-1 and SMT2-2) were cloned by homology based PCR methods from young leaves of Glycine max (soybean) and the corresponding enzymes were expressed functionally in Escherichia coli. The full-length cDNA for SMT2-1 and SMT2-2 have open reading frames of 1086 bp and 1092 bp, respectively, and encode proteins of 361 and 363 residues with a calculated molecular mass of 40.3 and 40.4 kDa, respectively. The substrate preference of the two isoforms was similar yet they differed from SMT1; kinetically SMT2-1 and SMT2-2 generated k(cat) values for the optimal substrate 24(28)methylene lophenol of 0.8 min(-1) and 1.34 min(-1), respectively, compared to the activity of SMT1 that generated a k(cat) for the optimal substrate cycloartenol of 0.6 min(-1). SMT2-2 was purified to homogeneity and the subunit organization shown to be tetrameric in similar fashion to other cloned SMTs. Analysis of the accumulated products catalyzed by the recombinant enzymes demonstrated that soybean SMT2-1 and SMT2-2 operate transalkylation activities analogous to the soybean plant SMT1. Metabolite analyses correlated with transcript profiling of the three SMT isoforms during soybean maturation clearly demonstrated that SMT isoform expression determines specific C24-methyl to C24-ethyl ratios to flowering whereas with seed development there is a disconnection such that the SMT transcript levels decrease against an increase in sterol content; generally SMT2-2 is expressed more than SMT2-1 or SMT1. These observations suggest that the genes that encode SMT1 and SMT2 in sitosterol biosynthesis may have undergone divergent evolution. In support of this proposition, the genomic organization for SMT1 of fungi and protozoa align very closely with one another and to those of the plant SMT2; both sets of SMTs lack introns. Unexpectedly, the SMT1 from Glycine max and other embryophytes of diverse origin possess disparate intron-exon characteristics that can be shown relates back to the algae. Our results suggest that the order of SMT1 appearing before SMT2 in phytosterol synthesis arose recently in plant evolution in response to duplication of a more primitive SMT gene likely to have been bifunctional and catalytically promiscuous.

Purification of dinosterol for hydrogen isotopic analysis using high-performance liquid chromatography-mass spectrometry

A semi-preparative normal-phase high-performance liquid chromatography-mass spectrometry (HPLC-MS) method is presented for the purification of various alcohol fractions from total lipid extracts derived from sediments, for the purpose of hydrogen isotopic measurement by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). 4-methylsterols, including the dinoflagellate-specific marker dinosterol (4,23,24-trimethylcholestan-22-en-3beta-ol), were successfully separated from notoriously co-eluting plant-derived pentacyclic triterpenoid alcohols and alkyl alcohols. We find that substantial hydrogen isotope fractionation occurs during chromatographic separation, demonstrating the importance of recovering the entire peak when subsequent hydrogen isotope analyses are to be performed. This is the first report of such hydrogen isotopic fractionation for a natural unlabelled compound.

Separation of free sterols by high temperature liquid chromatography

Increasing the column temperature accelerates markedly elution in HPLC. The separation of five free sterols was studied on three packing materials that can withstand high temperatures. These stationary phases included graphitic carbon, a polymeric C18 silica, and a zirconia-based adsorbent. Measurements of retention data were made at up to 150 degrees C with mobile phases of different compositions. Since the columns tested afford different retention mechanisms, a variety of elution patterns were observed, with some being more advantageous than others for certain sterol separations. Effects observed include some selectivity improvements and some elution order reversals. The separation of free sterols in selected fruit juices is also presented. Albeit at the expense of a longer analysis time, the graphitic carbon column produced the best separation of the sterols in this study.

Cloning and characterization of CYP51 from Mycobacterium avium

Mycobacterium avium complex (MAC) causes chronic lung disease in immunocompetent people and disseminated infection in patients with AIDS. MAC is intrinsically resistant to many conventional antimycobacterial agents, it develops drug resistance rapidly to macrolide antibiotics, and patients with MAC infection experience frequent relapses or the inability to completely eradicate the infection with current treatment. Treatment regimens are prolonged and complicated by drug toxicity or intolerances. We sought to identify biochemical pathways in MAC that can serve as targets for novel antimycobacterial treatment. The cytochrome P450 enzyme, CYP51, catalyzes an essential early step in sterol metabolism, removing a methyl group from lanosterol in animals and fungi, or from obtusifoliol in plants. Azoles inhibit CYP51 function, leading to an accumulation of methylated sterol precursors. This perturbation of normal sterol metabolism compromises cell membrane integrity, resulting in growth inhibition or cell death. We have cloned and characterized a CYP51 from MAC that functions as a lanosterol 14alpha-demethylase. We show the direct interactions of azoles with purified MAC-CYP51 by absorbance and electron paramagnetic resonance spectroscopy, and determine the minimum inhibitory concentrations (MICs) of econazole, ketoconazole, itraconazole, fluconazole, and voriconazole against MAC. Furthermore, we demonstrate that econazole has a MIC of 4 mug/ml and a minimum bacteriocidal concentration of 4 mug/ml, whereas ketoconazole has a MIC of 8 mug/ml and a minimum bacteriocidal concentration of 16 mug/ml. Itraconazole, voriconazole, and fluconazole did not inhibit MAC growth to any significant extent.

Characterizing Sterol Defect Suppressors Uncovers a Novel Transcriptional Signaling Pathway Regulating Zymosterol Biosynthesis

erg26-1ts cells harbor defects in the 4alpha-carboxysterol-C3 dehydrogenase activity necessary for conversion of 4,4-dimethylzymosterol to zymosterol. Mutant cells accumulate toxic 4-carboxysterols and are inviable at high temperature. A genetic screen aimed at cloning recessive mutations remediating the temperature sensitive growth defect has resulted in the isolation of four complementation groups, ets1-4 (erg26-1ts temperature sensitive suppressor). We describe the characterization of ets1-1 and ets2-1. Gas chromatography/mass spectrometry analyses demonstrate that erg26-1ts ets1-1 and erg26-1ts ets2-1 cells do not accumulate 4-carboxysterols, rather these cells have increased levels of squalene and squalene epoxide, respectively. ets1-1 and ets2-1 cells accumulate these same sterol intermediates. Chromosomal integration of ERG1 ERG7 at their loci in erg26-1ts ets1-1 and erg26-1ts and ets2-1 mutants, respectively, results in the loss of accumulation of squalene and squalene epoxide, re-accumulation of 4-carboxysterols and cell inviability at high temperature. Enzymatic assays demonstrate that mutants harboring the ets1-1 allele have decreased squalene epoxidase activity, while those containing the ets2-1 allele show weakened oxidosqualene cyclase activity. Thus, ETS1 and ETS2 are allelic to ERG1 and ERG7, respectively. We have mapped mutations within the erg1-1/ets1-1 (G247D) and erg7-1/ets2-1 (D530N, V615E) alleles that suppress the inviability of erg26-1ts at high temperature, and cause accumulation of sterol intermediates and decreased enzymatic activities. Finally using erg1-1 and erg7-1 mutant strains, we demonstrate that the expression of the ERG25/26/27 genes required for zymosterol biosynthesis are coordinately transcriptionally regulated, along with ERG1 and ERG7, in response to blocks in sterol biosynthesis. Transcriptional regulation requires the transcription factors, Upc2p and Ecm22p.

ATP-binding cassette (ABC) transporters mediate nonvesicular, raft-modulated sterol movement from the plasma membrane to the endoplasmic reticulum

Little is known about the mechanisms of intracellular sterol transport or how cells maintain the high sterol concentration of the plasma membrane (PM). Here we demonstrate that two inducible ATP-binding cassette (ABC) transporters (Aus1p and Pdr11p) mediate nonvesicular movement of PM sterol to the endoplasmic reticulum (ER) in Saccharomyces cerevisiae. This transport facilitates exogenous sterol uptake, which we find requires steryl ester synthesis in the ER. Surprisingly, while expression of Aus1p and Pdr11p significantly increases sterol movement from PM to ER, it does not alter intracellular sterol distribution. Thus, ER sterol is likely rapidly returned to the PM when it is not esterified in the ER. We show that the propensity of PM sterols to be moved to the ER is largely determined by their affinity for sterol sphingolipid-enriched microdomains (rafts). Our findings suggest that raft association is a primary determinant of sterol accumulation in the PM and that Aus1p and Pdr11p facilitate sterol uptake by increasing the cycling of sterol between the PM and ER.

Disruption of ergosterol biosynthesis, growth, and the morphological transition inCandida albicansby sterol methyltransferase inhibitors containing sulfur at C-25 in the sterol side chain

The sterol substrate analog 25-thialanosterol and its corresponding sulfonium salt were evaluated for their ability to serve as antifungal agents and to inhibit sterol methyltransferase (SMT) activity in Candida albicans. Both compounds inhibited cell proliferation, were fungistatic, interrupted the yeast-like-form to germ-tube-form transition, and resulted in the accumulation of zymosterol and related delta24-sterols concurrent with a decrease in ergosterol, as was expected for the specific inhibition of SMT activity. Feedback on sterol synthesis was evidenced by elevated levels of cellular sterols in treated vs. control cultures. However, neither farnesol nor squalene accumulated in significant amounts in treated cultures, suggesting that carbon flux is channeled from the isoprenoid pathway to the sterol pathway with minor interruption or redirection until blockage at the C-methylation step. Activity assays using solubilized C. albicans SMT confirmed the inhibitors impair SMT action. Kinetic analysis indicated that 25-thialanosterol inhibited SMT with the properties of a time-dependent mechanism-based inactivator Ki of 5 microM and apparent kinact of 0.013 min(-1), whereas the corresponding sulfonium salt was a reversible-type transition state analog exhibiting a Ki of 20 nM. The results are interpreted to imply changes in ergosterol homeostasis as influenced by SMT activity can control growth and the morphological transition in C. albicans, possibly affecting disease development.

Subcellular localization of yeast Sec14 homologues and their involvement in regulation of phospholipid turnover

Sec14p of the yeast Saccharomyces cerevisiae is involved in protein secretion and regulation of lipid synthesis and turnover in vivo, but acts as a phosphatidylinositol-phosphatidylcholine transfer protein in vitro. In this work, the five homologues of Sec14p, Sfh1p-Sfh5p, were subjected to biochemical and cell biological analysis to get a better view of their physiological role. We show that overexpression of SFH2 and SFH4 suppressed the sec14 growth defect in a more and SFH1 in a less efficient way, whereas overexpression of SFH3 and SFH5 did not complement sec14. Using C-terminal yEGFP fusions, Sfh2p, Sfh4p and Sfh5p are mainly localized to the cytosol and microsomes similar to Sec14p. Sfh1p was detected in the nucleus and Sfh3p in lipid particles and in microsomes. In contrast to Sec14p, which inhibits phospholipase D1 (Pld1p), overproduction of Sfh2p and Sfh4p resulted in the activation of Pld1p-mediated phosphatidylcholine turnover. Interestingly, Sec14p and the two homologues Sfh2p and Sfh4p downregulate phospholipase B1 (Plb1p)-mediated turnover of phosphatidylcholine in vivo. In summary, Sfh2p and Sfh4p are the Sec14p homologues with the most pronounced functional similarity to Sec14p, whereas the other Sfh proteins appear to be functionally less related to Sec14p.

Deletion of the Hsp70 chaperone gene SSB causes hypersensitivity to guanidine toxicity and curing of the [PSI+] prion by increasing guanidine uptake in yeast

Yeast Ssb proteins (Ssbp) are ribosome-associated Hsp70 chaperones that function in translation. Elevated levels of Ssbp enhance the ability of over-expressed Hsp104 chaperone to eliminate the yeast [PSI+] prion, while depletion of Ssbp reduces this effect. Millimolar concentrations of guanidine in the growth medium cure yeast cells of prions by inactivating Hsp104. Guanidine is also toxic to yeast, irrespective of the status of Hsp104 and [PSI+]. Strains that lack Ssbp are hypersensitive to guanidine toxicity. Here we show that ssb- cells have normal numbers of [PSI+] "seeds", but can be cured of [PSI+] using one-sixth of the guanidine concentration required to eliminate [PSI+] from SSB cells. Correspondingly, the level of intracellular guanidine was eight-fold higher in ssb- cells than in wild-type cells, which explains all effects of Ssbp depletion on susceptibility to guanidine. The sensitivity of wild-type cells to the effects of guanidine also correlated with guanidine uptake, which was enhanced at low temperature. Guanidine sensitivity of strains mutated in any of 16 ABC membrane transporters, which are implicated in multidrug resistance, was normal. We found that an erg6 mutant that has an altered membrane lipid composition was hypersensitive to guanidine toxicity, but the lipid composition of ssb- cells was identical to that of wild-type cells. Our results suggest that Ssbp depletion does not affect prion seed regeneration, and that elevated guanidine uptake by ssb- cells may be due to increased retention rather than to an alteration in active or passive transport of the compound.

Yeast cells lacking the ARV1 gene harbor defects in sphingolipid metabolism. Complementation by human ARV1

arv1Delta mutant cells have an altered sterol distribution within cell membranes (Tinkelenberg, A.H., Liu, Y., Alcantara, F., Khan, S., Guo, Z., Bard, M., and Sturley, S. L. (2000) J. Biol. Chem. 275, 40667-40670), and thus it has been suggested that Arv1p may be involved in the trafficking of sterol in the yeast Saccharomyces cerevisiae and also in humans. Here we present data showing that arv1Delta mutants also harbor defects in sphingolipid metabolism. [(3)H]inositol and [(3)H]dihydrosphingosine radiolabeling studies demonstrated that mutant cells had reduced rates of biosynthesis and lower steady-state levels of complex sphingolipids while accumulating certain hydroxylated ceramide species. Phospholipid radiolabeling studies showed that arv1Delta cells harbored defects in the rates of biosynthesis and steady-state levels of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylglycerol. Neutral lipid radiolabeling studies indicated that the rate of biosynthesis and steady-state levels of sterol ester were increased in arv1Delta cells. Moreover, these same studies demonstrated that arv1Delta cells had decreased rates of biosynthesis and steady-state levels of total fatty acid and fatty acid alcohols. Gas chromatography/mass spectrometry analyses examining different fatty acid species showed that arv1Delta cells had decreased levels of C18:1 fatty acid. Additional gas chromatography/mass spectrometry analyses determining the levels of various molecular sterol species in arv1Delta cells showed that mutant cells accumulated early sterol intermediates. Using fluorescence microscopy we found that GFP-Arv1p localizes to the endoplasmic reticulum and Golgi. Interestingly, the heterologous expression of the human ARV1 cDNA suppressed the sphingolipid metabolic defects of arv1Delta cells. We hypothesize that in eukaryotic cells, Arv1p functions in the sphingolipid metabolic pathway perhaps as a transporter of ceramides between the endoplasmic reticulum and Golgi.

Mutations in erg4 affect the sensitivity of Saccharomyces cerevisiae to medium-chain fatty acids

We have found that the medium-chain fatty acids (MCFAs) undecanoic acid (11:0), 10-undecenoic acid (11:1 Delta 10), and lauric acid (12:0) can affect the growth of Saccharomyces cerevisiae in a dose-dependent manner. The principal effect was a longer lag phase in MCFA-containing medium, although higher concentrations of 11:1 Delta 10 inhibited growth. Their relative order of inhibitory action was 11:1 Delta 10>11:0>12:0. Cellular content with MCFA supplementation was dependent on the concentration and the particular species of fatty acid, with 12:0 showing the highest relative accumulation and 11:1 Delta 10 the lowest at all concentrations. We have isolated and characterized a mutant that is hypersensitive to MCFA supplementation and is unable to grow at the normally permissive condition of 1 mM 11:1 Delta 10. However, it does not appear to accumulate higher relative levels of the fed MCFA compared to wild-type cells. Complementation of the mutant revealed that the ERG4 gene, encoding the enzyme that catalyzes the last step in ergosterol biosynthesis, had been mutated. The fatty acid composition of the erg4 Delta mutant differs only slightly from wild-type cells, mainly involving an increase in the relative amount of 12:0. These results indicate that yeast require ergosterol for optimal growth on certain MCFAs. We discuss the role ergosterol may have in cells responding to exogenous MCFAs and in supporting optimal cell growth.

Chromatographic analysis of plant sterols in foods and vegetable oils

This paper reviews recently published chromatographic methods for the analysis of plant sterols in various sample matrices with emphasis on vegetable oils. An overview of structural complexities and biological/nutritional aspects including hypocholesterolemic activities of phytosterols is provided in the Section 1. The principal themes of the review highlight the development and application of chromatographic techniques for the isolation, purification, separation and detection of the title compounds. Pertinent gas chromatographic and high-performance liquid chromatographic methods from the literature are tabulated to illustrate common trends and methodological variability. The review also covers specific analyses of natural/synthetic standard mixtures to shed light on potential applicability in plant sample assays. Examples of combined chromatographic techniques linked in tandem for the analysis of complex samples are included. Elution characteristics of sterol components are discussed in the context of analyte substituent effects, structural factors and stationary/mobile phase considerations.

Soybean sterol composition and utilization by Phytophthora sojae

The sterol fraction of Glycine max (soybean) was found to contain a mixture of 13 major sterols which differed dramatically in composition between seeds and shoots. Typical C4-desmethyl Delta(5)-sterols, including sitosterol, predominate the sterol mixture of shoots, whereas C4-methyl sterol intermediates, cycloartenol and 24(28)-methylene cycloartanol, accumulate in seeds. The significance of modified sterol profile of shoot compared to seed was relevant to the physiology of Phytophthora sojae, a phytopathogen of soybean shown to be auxotrophic for sterol. Sterols native to the host plant containing a C4-methyl group, such as cycloartenol, were not utilized by the fungus. Alternatively, all Delta(5)-sterols added to the culture media of P. sojae supported normal growth and promoted viable oospore production. The results demonstrate the importance of sterols in plant-fungal interactions and offer the possibility of bioengineering the phytosterol pathway for resistance to phytopathogens which scavenge specific sterols of the host plant to complete the life cycle.

Biochemical characterization and subcellular localization of the sterol C-24(28) reductase, erg4p, from the yeast saccharomyces cerevisiae

The yeast ERG4 gene encodes sterol C-24(28) reductase which catalyzes the final step in the biosynthesis of ergosterol. Deletion of ERG4 resulted in a complete lack of ergosterol and accumulation of the precursor ergosta-5,7,22,24(28)-tetraen-3beta-ol. An erg4 mutant strain exhibited pleiotropic defects such as hypersensitivity to divalent cations and a number of drugs such as cycloheximide, miconazole, 4-nitroquinoline, fluconazole, and sodium dodecyl sulfate. Similar to erg6 mutants, erg4 mutants are sensitive to the Golgi-destabilizing drug brefeldin A. Enzyme activity measurements with isolated subcellular fractions revealed that Erg4p is localized to the endoplasmic reticulum. This view was confirmed in vivo by fluorescence microscopy of a strain expressing a functional fusion of Erg4p to enhanced green fluorescent protein. We conclude that ergosterol biosynthesis is completed in the endoplasmic reticulum, and the final product is supplied from there to its membranous destinations.

Contribution of Are1p and Are2p to steryl ester synthesis in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, two acyl-CoA:sterol acyltransferases (ASATs) that catalyze the synthesis of steryl esters have been identified, namely Are2p (Sat1p) and Are1p (Sat2p). Deletion of either ARE1 or ARE2 has no effect on cell viability, and are1are2 double mutants grow in a similar manner to wild-type despite the complete lack of cellular ASAT activity and steryl ester formation [Yang, H., Bard, M., Bruner, D. A., Gleeson, A., Deckelbaum, R. J., Aljinovic, G., Pohl, T. M., Rothstein, R. & Sturley, S. L. (1996) Science 272, 1353-1356; Yu, C., Kennedy, J., Chang, C. C. Y. & Rothblatt, J. A. (1996) J. Biol. Chem. 271, 24157-24163]. Here we show that both Are2p and Are1p reside in the endoplasmic reticulum as demonstrated by measuring ASAT activity in subcellular fractions of are1 and are2 deletion strains. This localization was confirmed by fluorescence microscopy using hybrid proteins of Are2p and Are1p fused to green fluorescent protein (GFP). Lipid analysis of are1 and are2 deletion strains revealed that Are2p and Are1p utilize sterol substrates in vivo with different efficiency; Are2p has a significant preference for ergosterol as a substrate, whereas Are1p esterifies sterol precursors, mainly lanosterol, as well as ergosterol. The specificity towards fatty acids is similar for both isoenzymes. The lack of steryl esters in are1are2 mutant cells is largely compensated by an increased level of free sterols. Nevertheless, terbinafine, an inhibitor of ergosterol biosynthesis, inhibits growth of are1are2 cells more efficiently than growth of wild-type. In a growth competition experiment are1are2 cells grow more slowly than wild-type after several rounds of cultivation, suggesting that Are1p and Are2p or steryl esters, the product formed by these two enzymes, are more important in the natural environment than under laboratory conditions.

1-Acyldihydroxyacetone-phosphate reductase (Ayr1p) of the yeast Saccharomyces cerevisiae encoded by the open reading frame YIL124w is a major component of lipid particles

Biosynthesis of phosphatidic acid through the dihydroxyacetone phosphate pathway requires NADPH-dependent reduction of the intermediate 1-acyldihydroxyacetone phosphate before the second step of acylation. Studies with isolated subcellular fractions of the yeast Saccharomyces cerevisiae revealed that lipid particles and the endoplasmic reticulum harbor 1-acyldihydroxyacetone-phosphate reductase (ADR) activity. Deletion of the open reading frame YIL124w (in the following named AYR1) abolished reduction of 1-acyldihydroxyacetone phosphate in lipid particles, whereas ADR activity in microsomes of the deletion strain was decreased approximately 3-fold as compared with the wild-type level. This result indicates that (i) both lipid particles and microsomes harbor Ayr1p, which was confirmed by immunological detection of the protein in these two cellular compartments, and (ii) microsomes contain at least one additional ADR activity. As a consequence of this redundancy, deletion of AYR1 neither results in an obvious growth phenotype nor affects the lipid composition of a haploid deletion strain. When a heterozygous AYR1(+)/ayr1(-) diploid strain was subjected to sporulation; however, spores bearing the ayr1 defect failed to germinate, suggesting that Ayr1p plays an essential role at this stage. Overexpression of Ayr1p at a 5- to 10-fold level of wild type caused growth arrest. Heterologous expression of Ayr1p in Escherichia coli resulted in gain of ADR activity in the prokaryote, confirming that YIL124w is the structural gene of the enzyme and does not encode a regulatory or auxiliary component required for reduction of 1-acyldihydroxyacetone phosphate. Taken together, these results identified Ayr1p of the yeast as the first ADR from any organism at the molecular level.

Specific sterols required for the internalization step of endocytosis in yeast

Sterols are major components of the plasma membrane, but their functions in this membrane are not well understood. We isolated a mutant defective in the internalization step of endocytosis in a gene (ERG2) encoding a C-8 sterol isomerase that acts in the late part of the ergosterol biosynthetic pathway. In the absence of Erg2p, yeast cells accumulate sterols structurally different from ergosterol, which is the major sterol in wild-type yeast. To investigate the structural requirements of ergosterol for endocytosis in more detail, several erg mutants (erg2Delta, erg6Delta, and erg2Deltaerg6Delta) were made. Analysis of fluid phase and receptor-mediated endocytosis indicates that changes in the sterol composition lead to a defect in the internalization step. Vesicle formation and fusion along the secretory pathway were not strongly affected in the ergDelta mutants. The severity of the endocytic defect correlates with changes in sterol structure and with the abundance of specific sterols in the ergDelta mutants. Desaturation of the B ring of the sterol molecules is important for the internalization step. A single desaturation at C-8,9 was not sufficient to support internalization at 37 degrees C whereas two double bonds, either at C-5,6 and C-7,8 or at C-5,6 and C-8,9, allowed internalization.

Lipid composition of subcellular membranes of an FY1679-derived haploid yeast wild-type strain grown on different carbon sources

The aim of the project EUROFAN (European Functional Analysis Network) is to elucidate the function of unknown genes of the yeast Saccharomyces cerevisiae at a large scale. Functional analysis is based on general and specific tests with yeast deletion strains. A prerequisite for these studies is a profound knowledge of the biochemistry and cell biology of the corresponding wild-type strain FY1679. As a contribution from our laboratory we present here a systematic lipid analysis of the major organelles isolated from FY1679 grown in the presence of different carbon sources. Phospholipid, sterol and fatty acid composition are characteristic for each organelle. Moreover, growth of the yeast on glucose, ethanol or lactate causes in some cases marked changes of the organelle lipid pattern. As the most prominent example, cultivation of the yeast on non-fermentable carbon sources results in an increase of mitochondrial cardiolipin. As another example, the ratio of unsaturated to saturated fatty acids is enhanced in cells grown on ethanol or lactate as compared to glucose. Thus, the lipid composition of yeast subcellular membranes reflects in a significant way the nutrient conditions caused by variation of the carbon source.

Identification and characterization of major lipid particle proteins of the yeast Saccharomyces cerevisiae

Lipid particles of the yeast Saccharomyces cerevisiae were isolated at high purity, and their proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Major lipid particle proteins were identified by mass spectrometric analysis, and the corresponding open reading frames (ORFs) were deduced. In silicio analysis revealed that all lipid particle proteins contain several hydrophobic domains but none or only few (hypothetical) transmembrane spanning regions. All lipid particle proteins identified by function so far, such as Erg1p, Erg6p, and Erg7p (ergosterol biosynthesis) and Faa1p, Faa4p, and Fat1p (fatty acid metabolism), are involved in lipid metabolism. Based on sequence homology, another group of three lipid particle proteins may be involved in lipid degradation. To examine whether lipid particle proteins of unknown function are also involved in lipid synthesis, mutants with deletions of the respective ORFs were constructed and subjected to systematic lipid analysis. Deletion of YDL193w resulted in a lethal phenotype which could not be suppressed by supplementation with ergosterol or fatty acids. Other deletion mutants were viable under standard conditions. Strains with YBR177c, YMR313c, and YKL140w deleted exhibited phospholipid and/or neutral lipid patterns that were different from the wild-type strain and thus may be further candidate ORFs involved in yeast lipid metabolism.

Characterization and catalytic properties of the sterol 14alpha-demethylase from Mycobacterium tuberculosis

Sterol 14alpha-demethylase encoded by CYP51 is a mixed-function oxidase involved in sterol synthesis in eukaryotic organisms. Completion of the Mycobacterium tuberculosis genome project revealed that a protein having homology to mammalian 14alpha-demethylases might be present in this bacterium. Using genomic DNA from mycobacterial strain H(37)Rv, we have established unambiguously that the CYP51-like gene encodes a bacterial sterol 14alpha-demethylase. Expression of the M. tuberculosis CYP51 gene in Escherichia coli yields a P450, which, when purified to homogeneity, has the predicted molecular mass, ca. 50 kDa on SDS/PAGE, and binds both sterol substrates and azole inhibitors of P450 14alpha-demethylases. It catalyzes 14alpha-demethylation of lanosterol, 24, 25-dihydrolanosterol, and obtusifoliol to produce the 8,14-dienes stereoselectively as shown by GC/MS and (1)H NMR analysis. Both flavodoxin and ferredoxin redox systems are able to support this enzymatic activity. Structural requirements of a 14alpha-methyl group and Delta(8(9))-bond were established by comparing binding of pairs of sterol substrate that differed in a single molecular feature, e.g., cycloartenol paired with lanosterol. These substrate requirements are similar to those established for plant and animal P450 14alpha-demethylases. From the combination of results, the interrelationships of substrate functional groups within the active site show that oxidative portions of the sterol biosynthetic pathway are present in prokaryotes.

PDR16 and PDR17, two homologous genes of Saccharomyces cerevisiae, affect lipid biosynthesis and resistance to multiple drugs

The Saccharomyces cerevisiae open reading frame YNL231C was recently found to be controlled by the multiple drug resistance regulator Pdr1p. Here we characterize YNL231C (PDR16) and its homologue YNL264C (PDR17). Deletion of PDR16 resulted in hypersensitivity of yeast to azole inhibitors of ergosterol biosynthesis. While no increase in drug sensitivity was found upon deletion of PDR17 alone, a Deltapdr16,Deltapdr17 double mutant was hypersensitive to a broad range of drugs. Both mutations caused significant changes of the lipid composition of plasma membrane and total cell extracts. Deletion of PDR16 had pronounced effects on the sterol composition, whereas PDR17 deletion mainly affected the phospholipid composition. Thus, Pdr16p and Pdr17p may regulate yeast lipid synthesis like their distant homologue, Sec14p. The azole sensitivity of the PDR16-deleted strain may be the result of imbalanced ergosterol synthesis. Impaired plasma membrane barrier function resulting from a change in the lipid composition appears to cause the increased drug sensitivity of the double mutant strain Deltapdr16,Deltapdr17. The uptake rate of rhodamine-6-G into de-energized cells was shown to be almost 2-fold increased in a Deltapdr16,Deltapdr17 strain as compared with wild-type and Deltapdr5 strains. Collectively, our results indicate that PDR16 and PDR17 control levels of various lipids in various compartments of the cell and thereby provide a mechanism for multidrug resistance unrecognized so far.

Sterol uptake in Saccharomyces cerevisiae heme auxotrophic mutants is affected by ergosterol and oleate but not by palmitoleate or by sterol esterification

The relationship between sterol uptake and heme competence in two yeast strains impaired in heme synthesis, namely, G204 and H12-6A, was analyzed. To evaluate heme availability, a heterologous 17alpha-hydroxylase cytochrome P-450 cDNA (P-450c17) was expressed in these strains, and its activity was measured in vivo. Heme deficiency in G204 led to accumulation of squalene and lethality. The heterologous cytochrome P-450 was inactive in this strain. The leaky H12-6A strain presented a slightly modified sterol content compared to that for the wild type, and the P-450c17 recovered partial activity. By analyzing sterol transfer on nongrowing cells, it was shown that the cells were permeable toward exogenous cholesterol when they were depleted of endogenous sterols, which was the case for G204 but not for H12-6A. It was concluded that the fully blocked heme mutant (G204) replenishes its diminishing endogenous sterol levels during growth by replacement with sterol from the outside medium. Endogenous sterol biosynthesis appears to be the primary factor capable of excluding exogenous sterol. Oleate but not palmitoleate was identified as a component that reduced but did not prevent sterol transfer. Sterol transfer was only slightly affected by a lack of esterification. It is described herein how avoidance of the potential cytotoxicity of the early intermediates of the mevalonate pathway could be achieved by a secondary heme mutation in erg auxotrophs.