Microsomal delta 8,14-sterol delta 14-reductase in higher plants. Characterization and inhibition by analogues of a presumptive carbocationic intermediate of the reduction reaction

Role of cholesterol metabolism in the anticancer pharmacology of selective estrogen receptor modulators

Selective estrogen receptor modulators (SERMs) are a class of compounds that bind to estrogen receptors (ERs) and possess estrogen agonist or antagonist actions in different tissues. As such, they are widely used drugs. For instance, tamoxifen, the most prescribed SERM, is used to treat ERα-positive breast cancer. Aside from their therapeutic targets, SERMs have the capacity to broadly affect cellular cholesterol metabolism and handling, mainly through ER-independent mechanisms. Cholesterol metabolism reprogramming is crucial to meet the needs of cancer cells, and different key processes involved in cholesterol homeostasis have been associated with cancer progression. Therefore, the effects of SERMs on cholesterol homeostasis may be relevant to carcinogenesis, either by contributing to the anticancer efficacy of these compounds or, conversely, by promoting resistance to treatment. Understanding these aspects of SERMs actions could help to design more efficacious therapies. Herein we review the effects of SERMs on cellular cholesterol metabolism and handling and discuss their potential in anticancer pharmacology.

Inhibition of Phytosterol Biosynthesis by Azasterols

Inhibitors of enzymes in essential cellular pathways are potent probes to decipher intricate physiological functions of biomolecules. The analysis of Arabidopsis thaliana sterol profiles upon treatment with a series of azasterols reveals a specific in vivo inhibition of SMT2, a plant sterol-C-methyltransferase acting as a branch point between the campesterol and sitosterol biosynthetic segments in the pathway. Side chain azasteroids that modify sitosterol homeostasis help to refine its particular function in plant development.

Carbon-carbon double-bond reductases in nature

Reduction of C = C bonds by reductases, found in a variety of microorganisms (e.g. yeasts, bacteria, and lower fungi), animals, and plants has applications in the production of metabolites that include pharmacologically active drugs and other chemicals. Therefore, the reductase enzymes that mediate this transformation have become important therapeutic targets and biotechnological tools. These reductases are broad-spectrum, in that, they can act on isolation/conjugation C = C-bond compounds, α,β-unsaturated carbonyl compounds, carboxylic acids, acid derivatives, and nitro compounds. In addition, several mutations in the reductase gene have been identified, some associated with diseases. Several of these reductases have been cloned and/or purified, and studies to further characterize them and determine their structure in order to identify potential industrial biocatalysts are still in progress. In this study, crucial reductases for bioreduction of C = C bonds have been reviewed with emphasis on their principal substrates and effective inhibitors, their distribution, genetic polymorphisms, and implications in human disease and treatment.

A sterol C-14 reductase encoded by FgERG24B is responsible for the intrinsic resistance of Fusarium graminearum to amine fungicides

Fusarium graminearum, the causal agent of wheat head blight, shows intrinsic resistance to amine fungicides. It is commonly accepted that the amines target sterol C-14 reductase and sterol Δ8–Δ7 isomerase of ergosterol biosynthesis, encoded by the genes ERG24 and ERG2, respectively. Analysis of the genome sequence of F. graminearum revealed that the fungus contains two paralogous FgERG24 genes (FgERG24A and FgERG24B), which are homologous to the ERG24 of Saccharomyces cerevisiae. In this study, we disrupted FgERG24A and FgERG24B in F. graminearum. Compared to the wild-type strain HN9-1, FgERG24A and FgERG24B deletion mutants did not show recognizable phenotypic changes in mycelial growth on potato dextrose agar or in virulence on wheat heads. HPLC analysis showed that the amount of ergosterol in FgERG24A or FgERG24B deletion mutants was not significantly different from that in the wild-type strain. These results indicate that neither of the two genes is essential for growth, pathogenicity or ergosterol biosynthesis in F. graminearum. FgERG24B deletion mutants exhibited significantly increased sensitivity to amine fungicides, including tridemorph, fenpropidin and spiroxamine, but not to non-amine fungicides. In contrast, FgERG24A deletion mutants did not show changed sensitivity to any amine tested. The resistance of the FgERG24B deletion mutant to amines was restored by genetic complementation of the mutant with wild-type FgERG24B. These results indicate that FgERG24B controls the intrinsic resistance of F. graminearum to amines. The finding of this study provides new insights into amine resistance in filamentous fungi.

Identification of essential amino acid residues in a sterol 8,7-isomerase from Zea mays reveals functional homology and diversity with the isomerases of animal and fungal origin

A putative 8,7SI (sterol 8,7-isomerase) from Zea mays, termed Zm8,7SI, has been isolated from an EST (expressed sequence tag) library and subcloned into the yeast erg2 mutant lacking 8,7SI activity. Zm8,7SI restored endogenous ergosterol synthesis. An in vitro enzymatic assay in the corresponding yeast microsomal extract indicated that the preferred Delta(8)-sterol substrate possesses a single C4alpha methyl group, in contrast with 8,7SIs from animals and fungi, thus reflecting the diversity in the structure of their active site in relation to the distinct sterol biosynthetic pathways. In accordance with the proposed catalytic mechanism, a series of lipophilic ammonium-ion-containing derivatives possessing a variety of structures and biological properties, potently inhibited the Zm8,7SI in vitro. To evaluate the importance of a series of conserved acidic and tryptophan residues which could be involved in the Zm8,7SI catalytic mechanism, 20 mutants of Zm8,7SI were constructed as well as a number of corresponding mutants of the Saccharomyces cerevisiae 8,7SI. The mutated isomerases were assayed in vivo by sterol analysis and quantification of Delta(5,7)-sterols and directly in vitro by examination of the activities of the recombinant Zm8,7SI mutants. These studies have identified His(74), Glu(78), Asp(107), Glu(121), Trp(66) and Trp(193) that are required for Zm8,7SI activity and show that binding of the enzyme-substrate complex is impaired in the mutant T124I. They underline the functional homology between the plant and animal 8,7SIs on one hand, in contrast with the yeast 8,7SI on the other hand, in accordance with their molecular diversity and distinct mechanisms.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Effects of distal cholesterol biosynthesis inhibitors on cell proliferation and cell cycle progression

Cholesterol is a major lipid component of the plasma membrane in animal cells. In addition to its structural requirement, cholesterol is essential in cell proliferation and other cell processes. The aim of the present study was to elucidate the stringency of the requirement for cholesterol as a regulator of proliferation and cell cycle progression, compared with other sterols of the cholesterol biosynthesis pathway. Human promyelocytic HL-60 cells were cultured in cholesterol-free medium and treated with different distal inhibitors of cholesterol biosynthesis (zaragozic acid, SKF 104976, SR 31747, BM 15766, and AY 9944), which allow the synthesis of isoprenoid derivatives and different sets of sterol intermediates, but not cholesterol. The results showed that only the inhibition of sterol Delta7-reductase was compatible with cell proliferation. Blocking cholesterol biosynthesis upstream of this enzyme resulted in the inhibition of cell proliferation and cell cycle arrest selectively in G2/M phase.

Biosynthesis and accumulation of sterols

In recent years, the impressive development of molecular genetics tools, the sequencing of the Arabidopsis thaliana genome, the availability of DNA or transposon tagged mutants, and the multiple possibilities offered by stable transformation with DNA in sense and antisense orientation have enabled the application of a strategy of gain or loss of function to study the sterol biosynthesis pathway. Here we describe the results obtained with these techniques. The results essentially confirm data obtained previously with sterol biosynthesis inhibitors (SBIs) and enable the precise dissection of biosynthetic pathways. We discuss the advantages and disadvantages of molecular genetics techniques as applied to sterol metabolism. The greater selectivity of these techniques constitutes an invaluable advantage and has led to the discovery of a role for sterols in plant development.

Azasteroids as antifungals

Azasteroids and derivatives thereof with antifungal potential are reviewed. Special emphasis is put on steroids with nitrogen as part of the steroidal framework, natural substances, and lines of development emerging from them.

Sterols regulate development and gene expression in Arabidopsis

Sterols are important not only for structural components of eukaryotic cell membranes but also for biosynthetic precursors of steroid hormones. In plants, the diverse functions of sterol-derived brassinosteroids (BRs) in growth and development have been investigated rigorously, yet little is known about the regulatory roles of other phytosterols. Recent analysis of Arabidopsis fackel (fk) mutants and cloning of the FK gene that encodes a sterol C-14 reductase have indicated that sterols play a crucial role in plant cell division, embryogenesis, and development. Nevertheless, the molecular mechanism underlying the regulatory role of sterols in plant development has not been revealed. In this report, we demonstrate that both sterols and BR are active regulators of plant development and gene expression. Similar to BR, both typical (sitosterol and stigmasterol) and atypical (8, 14-diene sterols accumulated in fk mutants) sterols affect the expression of genes involved in cell expansion and cell division. The regulatory function of sterols in plant development is further supported by a phenocopy of the fk mutant using a sterol C-14 reductase inhibitor, fenpropimorph. Although fenpropimorph impairs cell expansion and affects gene expression in a dose-dependent manner, neither effect can be corrected by applying exogenous BR. These results provide strong evidence that sterols are essential for normal plant growth and development and that there is likely a BR-independent sterol response pathway in plants. On the basis of the expression of endogenous FK and a reporter gene FK::beta-glucuronidase, we have found that FK is up-regulated by several growth-promoting hormones including brassinolide and auxin, implicating a possible hormone crosstalk between sterol and other hormone-signaling pathways.

FACKEL is a sterol C-14 reductase required for organized cell division and expansion in Arabidopsis embryogenesis

In flowering plants, the developing embryo consists of growing populations of cells whose fates are determined in a position-dependent manner to form the adult organism. Mutations in the FACKEL(FK) gene affect body organization of theArabidopsis seedling. We report that FK is required for cell division and expansion and is involved in proper organization of the embryo. We isolated FK by positional cloning. Expression analysis in embryos revealed that FK mRNA becomes localized to meristematic zones. FK encodes a predicted integral membrane protein related to the vertebrate lamin B receptor and sterol reductases across species, including yeast sterol C-14 reductase ERG24. We provide functional evidence that FK encodes a sterol C-14 reductase by complementation of erg24. GC/MS analysis confirmed that fk mutations lead to accumulation of intermediates in the biosynthetic pathway preceding the C-14 reductase step. Although fk represents a sterol biosynthetic mutant, the phenotype was not rescued by feeding with brassinosteroids (BRs), the only plant sterol signaling molecules known so far. We propose that synthesis of sterol signals in addition to BRs is important in mediating regulated cell growth and organization during embryonic development. Our results indicate a novel role for sterols in the embryogenesis of plants.

Identification, characterization, and partial purification of 4 alpha-carboxysterol-C3-dehydrogenase/ C4-decarboxylase from Zea mays

A microsomal preparation from seedlings of Zea mays catalyzed the NAD+-dependent oxidative decarboxylation of several substrates, including 4alpha-carboxy-cholest-7-en-3beta-ol, synthesized according to a new procedure, giving the first in vitro evidence for this enzymatic activity in a higher plant. A GC assay has been developed to detect the Delta7-cholestenone produced and the kinetic parameters of the microsomal system have been established. 4alpha-Carboxysterol decarboxylation shows an exclusive requirement for an oxidized pyridine nucleotide, with NAD+ being more efficient than NADP+. The decarboxylation reaction is independent of molecular oxygen. 4alpha-Carboxysterol-C3-dehydrogenase/C4-decarboxylase (4alpha-CD) is a microsome-bound protein which can be efficiently solubilized by detergents, including Brij W-1 and sodium cholate. The Brij W-1-solubilized enzyme was partially purified 290-fold by a combination of DEAE anion-exchange chromatography, Cibacron blue 3GA-agarose dye chromatography, and gel permeation. The apparent molecular mass of 4alpha-CD in sodium cholate was estimated to be 45 kDa. These results support the contention that demethylation at C4 of plant sterols is composed of two separate processes: an oxygen- and NAD(P)H-dependent oxidation of the 4alpha-methyl group to produce the 4alpha-carboxysterol metabolite (S. Pascal et al., J. Biol. Chem. 268, 11639, 1993) followed by oxygen-independent dehydrogenation/decarboxylation to produce an obligatory 3-ketosteroid.

Human lamin B receptor exhibits sterol C14-reductase activity in Saccharomyces cerevisiae

Lamin B receptor (LBR), a nuclear protein of avian and mammalian cells, contains an hydrophobic domain that shares extensive structural similarities with the members of the sterol reductase family. To test if the sterol-reductase-like domain of LBR could be enzymatically competent, several sterol reductase-defective strains of Saccharomyces cerevisiae were transformed with a human-LBR expressing vector. LBR production did not change the ergosterol biosynthesis defect in an erg4 mutant impaired in sterol C24(28) reductase. In contrast, the sterol C14 reduction step and ergosterol prototrophy were restored in LBR-producing erg24 transformants which lack endogenous sterol C14 reductase. To test the effects of C14 reductase inhibitors on LBR activity, we constructed EMY54, an ergosterol-requiring strain that is devoid of both sterol C8-C7 isomerase and sterol C14 reductase activities. EMY54 cells recovered the capability of synthesizing ergost-8-en-3beta-ol upon transformation with a vector that expressed either yeast sterol C14 reductase or hLBR. In addition, growth in sterol-free medium was restored in these transformants. Sterol biosynthesis and proliferation of LBR-producing cells were found to be highly susceptible to fenpropimorph and tridemorph, but only moderately susceptible to SR 31747. Our results strongly suggest that hLBR is a sterol C14 reductase.

Sterol biosynthesis: strong inhibition of maize delta 5,7-sterol delta 7-reductase by novel 6-aza-B-homosteroids and other analogs of a presumptive carbocationic intermediate of the reduction reaction

A series of mono- and diazasteroids have been synthesized as analogs of a predicted carbocationic intermediate of delta 5,7-sterol delta 7-reductase (delta 7-SR). 6-Aza-B-homo-5 alpha-cholest-7-en-3 beta-ol (4), a novel compound whose synthesis is described for the first time, and 6,7-diaza-5 alpha-cholest-8(14)-en-3 beta-ol (6) were shown to be very powerful inhibitors of delta 7-SR in a preparation isolated from maize (Zea mays) (K(i),app = 50-70 nM, Ki,app/Km,app = 1.0 x 10(-4) to 1.3 x 10(-4). The data are consistent with a carbonium ion mechanism for the reduction; compounds 4 and 6 probably act as reaction intermediate analogs. Compound 4, in contrast to compound 6, displayed in the same microsomal preparation more than 50-fold selectivity for inhibition of the delta 7-SR versus delta 8-delta 7-sterol isomerase, cycloeucalenol isomerase, and delta 8,14-sterol delta 14-reductase, the mechanism of these four enzymes involving presumptive cationic intermediates centered respectively at C7, C8, C9, and C14. These observations highlight the paramount importance of the location of the positively charged nitrogen atom(s) in the B-ring structure for selectivity among these enzymes involving structurally close cationic reaction intermediates. Efficient in vivo inhibition of sterol biosynthesis in bramble cell suspension cultures by a low concentration of compound 4 was demonstrated and confirmed the in vitro properties of this derivative.)

The immunosuppressant SR 31747 blocks cell proliferation by inhibiting a steroid isomerase in Saccharomyces cerevisiae

SR 31747 is a novel immunosuppressant agent that arrests cell proliferation in the yeast Saccharomyces cerevisiae, SR 31747-treated cells accumulate the same aberrant sterols as those found in a mutant impaired in delta 8- delta 7-sterol isomerase. Sterol isomerase activity is also inhibited by SR 31747 in in vitro assays. Overexpression of the sterol isomerase-encoding gene, ERG2, confers enhanced SR resistance. Cells growing anaerobically on ergosterol-containing medium are not sensitive to SR. Disruption of the sterol isomerase-encoding gene is lethal in cells growing in the absence of exogenous ergosterol, except in SR-resistant mutants lacking either the SUR4 or the FEN1 gene product. The results suggest that sterol isomerase is the target of SR 31747 and that both the SUR4 and FEN1 gene products are required to mediate the proliferation arrest induced by ergosterol depletion.

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

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

Biosynthesis of monoterpenes: inhibition of (+)-pinene and (-)-pinene cyclases by thia and aza analogs of the 4R- and 4S-alpha-terpinyl carbocation

(+)-Pinene cyclase (synthase) from Salvia officinalis leaf catalyzes the cyclization of geranyl pyrophosphate, via (3R)-linalyl pyrophosphate and the (4R)-alpha-terpinyl cation, to (+)-alpha-pinene and to lesser quantities of stereochemically related monoterpene olefins, whereas (-)-pinene cyclase converts the same achiral precursor, via (3S)-linalyl pyrophosphate and the (4S)-alpha-terpinyl cation, to (-)-alpha-pinene and (-)-beta-pinene and to lesser amounts of related olefins. Racemic thia analogs of the linalyl and alpha-terpinyl carbocation intermediates of the reaction sequence were previously shown to be good uncompetitive inhibitors of monoterpene cyclases, and inhibition was synergized by the presence of inorganic pyrophosphate. These results suggested that the normal reaction proceeds through a series of carbocation:pyrophosphate anion paired intermediates. Both the (4R)- and the (4S)-thia and -aza analogs of the alpha-terpinyl cation were prepared and tested as inhibitors with the antipodal pinene cyclases, both in the absence and in the presence of inorganic pyrophosphate. Although the inhibition kinetics were complex, cooperative binding of the analogs and inorganic pyrophosphate was demonstrated, consistent with ion pairing of intermediates in the course of the normal reaction. Based on the antipodal reactions catalyzed by the pinene cyclases, stereochemical differentiation between the (4R)- and the (4S)-analogs was anticipated; however, neither enzyme effectively distinguished between enantiomers of the thia and aza analogs of the alpha-terpinyl carbocation. Enantioselectivity in the enzymatic conversion of (RS)-alpha-terpinyl pyrophosphate to limonene by the pinene cyclases was also examined. Consistent with the results obtained with the thia and aza analogs, the pinene cyclases were unable to discriminate between enantiomers of alpha-terpinyl pyrophosphate in this unusual reaction. Either the alpha-terpinyl antipodes are too similar to allow differentiation by the pinene cyclases, or these enzymes lack an inherent requirement to distinguish the (4R)- and (4S)-forms because they encounter only one enantiomer in the course of the normal reaction from geranyl pyrophosphate.

Construction and growth properties of a yeast strain defective in sterol 14-reductase

We have transformed Saccharomyces cerevisiae with a genomic library contained in the replicative vector pFL44. The resulting transformants were screened for resistance to fenpropidin, a specific inhibitor of sterol 14-reductase. A plasmid was isolated that transformed yeast both to resistance to fenpropidin and to an increased specific activity of sterol 14-reductase. Sterol analysis of transformed cells grown in the presence of increasing concentrations of the inhibitor confirmed that resistance was a consequence of over-production of sterol 14-reductase. By chromosomal gene disruption, we have, for the first time, constructed yeast strains defective in sterol 14-reductase. As expected, since yeast in unable to take up sterols in aerobiosis, the disrupted strains do not grow in the presence of oxygen, even if exogenous sterols are supplied. However, disrupted cells grow in anaerobiosis with exogenous oleic acid and ergosterol supplements. They also grow in aerobiosis if they bear an additional mutation allowing sterol uptake. In this last growth condition the cells require a "sparking" ergosterol supplementation (25 nM) and accumulate ignosterol (ergosta-8,14-dienol) as the end-product of the sterol pathway. These results reveal that ignosterol is not obviously toxic to yeast membranes and strongly suggest that the molecular basis of the antifungal-activity morpholine and piperidine is directly related to the specific inhibition of ergosterol formation.

Inhibition of 2,3-oxidosqualene cyclases

Monocyclic and tricyclic compounds possessing a nitrogen atom situated at a position corresponding to the carbenium ion of high energy intermediates or transition states involved during cyclization of 2,3-oxidosqualene to tetra- and pentacyclic triterpenes have been synthesized. These compounds were tested as inhibitors of 2,3-oxidosqualene cycloartenol, lanosterol-, and beta(alpha)-amyrin-cyclases in vitro and in vivo, and their affinity was compared to that of formerly synthesized 8-aza-bicyclic compounds [Taton et al. (1986) Biochem. Biophys. Res. Commun. 138, 764-770]. A monocyclic N-alkyl-hydroxypiperidine was shown to be the strongest inhibitor of the series upon cycloartenol-cyclase (I50 = 1 microM) from maize embryos but was much less effective on the beta(alpha)-amyrin-cyclases from Rubus fruticosus suspension cultures or pea cotyledons. In contrast, 13-aza-tricyclic derivatives displayed little inhibition on 2,3-oxidosqualene cycloartenol-, lanosterol-, and beta(alpha)-amyrin-cyclases. The obtained data exemplify the differences existing in the cyclization process between cycloartenol- (lanosterol-) cyclases on one hand and beta(alpha)-amyrin-cyclases on the other. The results are discussed with respect to current mechanisms postulated for 2,3-oxidosqualene cyclization. Because of its activity in vivo and in vitro the monocyclic N-alkyl-hydroxypiperidine appears to be a potent and promising tool to study sterol biosynthesis regulation.

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.

Sterol biosynthesis inhibitors: their current status and modes of action

The mechanism of each of the reactions in the post-squalene segment of the fungal and higher plant sterol biosynthetic pathway is outlined. The inhibitors of the enzymes catalyzing the reactions are described and how inhibition is brought about is explained in the areas where it is known.

Properties and structural requirements for substrate specificity of cytochrome P-450-dependent obtusifoliol 14 alpha-demethylase from maize (Zea mays) seedlings

The biochemical properties of cytochrome P-450-dependent obtusifoliol 14 alpha-demthylase (P-450OBT.14DM) from maize (Zea mays) seedlings were defined. In particular, the enzyme was shown by differential centrifugation to be localized in the endoplasmic reticulum. P-450OBT.14DM had an apparent Km of 160 +/- 5 microM and an apparent Vmax of 65 +/- 5 pmol/min per mg of protein for its best substrate, obtusifoliol. The substrate specificity of P-450OBT.14DM was thoroughly investigated by comparing the demethylation of obtusifoliol with that of a series of 15 natural or novel synthetic analogues of obtusifoliol. The results obtained clearly indicate that three distinct domains of the sterol substrate are governing obtusifoliol demethylation by P-450OBT.14DM. They revealed that (i) P-450OBT.14DM has probably a specific apolar binding site for the side chain, (ii) the delta 8-double bond is an absolute requirement for substrate demethylation and (iii) the 3-hydroxy group plays a critical role in the enzyme-substrate interaction. Interestingly the binding site, beyond the C-3 position, contains a cleft which cannot accommodate a 4 beta-methyl substituent present in lanosterol or eburicol, the precursors of 14-desmethylsterols respectively in mammals and yeast. This result indicates that P-450OBT.14DM is a novel constitutive cytochrome P-450 with a high degree of substrate and product specificity.

Radio-detection high-performance liquid chromatographic enzyme assay for inhibitors of fungal sterol delta 14-reductase

An enzyme assay for inhibitors of fungal sterol delta 14-reductase employing isocratic reversed-phase high-performance liquid chromatography is described. A Hypersil 5-microns octadecylsilyl (ODS) column (250 mm x 4.6 mm I.D.) was used and a mobile phase consisting of methanol-water-ethanol (86:4:10, v/v) was pumped at a flow-rate of 1.5 ml/min. Typical analysis times were 15 min. Using [4-14C]ignosterol as a substrate and an enzyme preparation from Saccharomyces cerevisiae, this method was used to compare the inhibition of sterol delta 14-reductase by the fungicides fenpropidin and fenpropimorph with three N-substituted 8-azadecaline compounds.

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

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