Interaction of azole antifungal agents with cytochrome P-45014DM purified from Saccharomyces cerevisiae microsomes

Differential inhibition of human CYP3A4 and Candida albicans CYP51 with azole antifungal agents

The inhibition by azole antifungals of human cytochrome CYP3A4, the major form of drug metabolising enzyme within the liver, was compared with their inhibitory activity against their target enzyme, Candida albicans sterol 14alpha-demethylase (CYP51), following heterologous expression in Saccharomyces cerevisiae. IC(50) values for ketoconazole and itraconazole CYP3A4 inhibition were 0.25 and 0. 2 microM. These values compared with much lower doses required for the complete inhibition of C. albicans CYP51, where IC(50) values of 0.008 and 0.0076 microM were observed for ketoconazole and itraconazole, respectively. Additionally, stereoselective inhibition of CYP3A4 and CYP51 was observed with enantiomers of the azole antifungal compounds diclobutrazol and SCH39304. In both instances, the RR(+) configuration at their asymmetric carbon centres was most active. Interestingly, the SS(-) enantiomeric form of SCH39304 was inactive and failed to bind CYP3A4, as demonstrable by Type II binding spectra.

Potent antimalarial activity of clotrimazole in in vitro cultures of Plasmodium falciparum

The increasing resistance of the malaria parasite Plasmodium falciparum to currently available drugs demands a continuous effort to develop new antimalarial agents. In this quest, the identification of antimalarial effects of drugs already in use for other therapies represents an attractive approach with potentially rapid clinical application. We have found that the extensively used antimycotic drug clotrimazole (CLT) effectively and rapidly inhibited parasite growth in five different strains of P. falciparum, in vitro, irrespective of their chloroquine sensitivity. The concentrations for 50% inhibition (IC(50)), assessed by parasite incorporation of [(3)H]hypoxanthine, were between 0.2 and 1.1 microM. CLT concentrations of 2 microM and above caused a sharp decline in parasitemia, complete inhibition of parasite replication, and destruction of parasites and host cells within a single intraerythrocytic asexual cycle (approximately 48 hr). These concentrations are within the plasma levels known to be attained in humans after oral administration of the drug. The effects were associated with distinct morphological changes. Transient exposure of ring-stage parasites to 2.5 microM CLT for a period of 12 hr caused a delay in development in a fraction of parasites that reverted to normal after drug removal; 24-hr exposure to the same concentration caused total destruction of parasites and parasitized cells. Chloroquine antagonized the effects of CLT whereas mefloquine was synergistic. The present study suggests that CLT holds much promise as an antimalarial agent and that it is suitable for a clinical study in P. falciparum malaria.

The R467K amino acid substitution in Candida albicans sterol 14alpha-demethylase causes drug resistance through reduced affinity

The cytochrome P450 sterol 14alpha-demethylase (CYP51) of Candida albicans is involved in an essential step of ergosterol biosynthesis and is the target for azole antifungal compounds. We have undertaken site-directed mutation of C. albicans CYP51 to produce a recombinant mutant protein with the amino acid substitution R467K corresponding to a mutation observed clinically. This alteration perturbed the heme environment causing an altered reduced-carbon monoxide difference spectrum with a maximum at 452 nm and reduced the affinity of the enzyme for fluconazole, as shown by ligand binding studies. The specific activity of CYP51(R467K) for the release of formic acid from 3beta-[32-(3)H]hydroxylanost-7-en-32-ol was 70 pmol/nmol of P450/min for microsomal protein compared to 240 pmol/nmol of P450/min for microsomal fractions expressing wild-type CYP51. Furthermore, inhibition of activity by fluconazole revealed a 7.5-fold-greater azole resistance of the recombinant protein than that of the wild type. This study demonstrates that resistance observed clinically can result from the altered azole affinity of the fungal CYP51 enzyme.

Molecular aspects of azole antifungal action and resistance

During the past three decades azole compounds have been developed as medical and agricultural agents to combat fungal diseases. During the 1980s they were introduced as orally active compounds in medicine and the number of such azole drugs is likely to expand in the near future. They represent a successful strategy for antifungal development, but as the incidence of fungal infection has increased coupled to prolonged use of the drugs, the (almost) inevitable emergence of resistance has occurred. This was after resistance had already been encountered as a serious problem in the field, where a larger number of azole fungicides had been employed commercially. In this review the molecular basis of how azoles work is discussed together with how fungi overcome the inhibitory effect of these compounds: through alterations in the primary target molecule (cytochrome P45051; Erg11p; sterol 14alpha-demethylase); through drug efflux mechanisms and through a suppressor mechanism allowing growth on 14-methylated sterols. Copyright 1999 Harcourt Publishers Ltd.

Y132H substitution in Candida albicans sterol 14alpha-demethylase confers fluconazole resistance by preventing binding to haem

Fungal cytochrome P450 sterol 14alpha-demethylase (CYP51) is required for ergosterol biosynthesis and is the target for azole antifungal compounds. The amino acid substitution Y132H in CYP51 from clinical isolates of Candida albicans can cause fluconazole resistance by a novel change in the protein. Fluconazole binding to the mutant protein did not involve normal interaction with haem as shown by inducing a Type I spectral change. This contrasted to the wild-type protein where fluconazole inhibition was reflected in coordination to haem as a sixth ligand and where the typical Type II spectrum was obtained. The Y132H substitution occurred without drastic perturbation of the haem environment or activity allowing resistant mutants to produce ergosterol and retain fitness, an efficient strategy for resistance in nature.

Cytochrome P450105D1 (CYP105D1) from Streptomyces griseus: heterologous expression, activity, and activation effects of multiple xenobiotics

The open reading frame of CYP105D1, a soluble cytochrome P450 from Streptomyces griseus, was cloned behind the tac promoter of the bacterial expression vector pSPg1910L and expressed in Escherichia coli. The recombinant protein retained normal spectral characteristics having a Soret peak at 448 nm in the reduced carbon monoxide difference spectrum. CYP105D1 was active, obtaining reducing equivalents from endogenous E. coli ferredoxin and ferredoxin reductase redox partners present in E. coli. In vitro activity studies revealed CYP105D1 to catalyse the NADH- and NADPH-dependent oxidation of the xenobiotic substrates benzo[a]pyrene, erythromycin, warfarin, and testosterone. Furthermore, this activity could be stimulated in the presence of either alpha-benzoflavone or beta-benzoflavone in an analogous manner to that reported for mammalian P450 forms including human liver cytochrome P4503A4 (CYP3A4). The system produces an alternative to whole-cell biotransformation of xenobiotic for the production of drug metabolites and an experimental system for probing the structural features of a cytochrome P450 with a broad substrate range.

Inhibition of sterol 14 alpha-demethylation of Candida albicans with NND-502, a novel optically active imidazole antimycotic agent

To investigate the mode of action of the newly synthesized optically active imidazole compound, NND-502, (-)-(E)-[4-(2, 4-dichlorophenyl)-1,3-dithiolan-2-ylidene]-1-imidazolylacetonit rile, its effect on ergosterol biosynthesis in cell-free extracts of Candida albicans was examined and compared with that of the (S)-enantiomer of NND-502 in addition to lanoconazole and bifonazole, both of which are clinically used for the treatment of dermatomycoses. NND-502 was found to interfere with ergosterol biosynthesis by inhibition of sterol 14alpha-demethylase, while no interference due to the (S)-enantiomer of NND-502 was found, indicating that the stereochemical orientation of the 2, 4-dichlorophenyl group plays an important role in the interaction with the enzyme. In terms of drug concentration exerting 50% inhibition of ergosterol biosynthesis, NND-502 was 2.5 and 28 times more effective than that of lanoconazole and bifonazole, respectively.

The G464S amino acid substitution in Candida albicans sterol 14alpha-demethylase causes fluconazole resistance in the clinic through reduced affinity

Fluconazole selectively inhibits fungal sterol 14alpha-demethylase, a cytochrome P450 enzyme found in plants, animals, fungi, and Mycobacteria. The mutation G464S, observed in the heme-binding domain of sterol 14alpha-demethylase in clinical strains of fluconazole-resistant Candida albicans, is shown here to cause resistance through substantially reducing the inhibitory effect of fluconazole and is associated with perturbation of the heme environment as indicated by spectral data. The protein exhibits 42% of the maximal enzymatic rate of the wild-type protein allowing continued production of the end product of fungal sterol biosynthesis, ergosterol, in resistant strains. This mutation may cause these phenotypes through altering the heme location, thus changing the ability of residues above the heme to bind the drug effectively. This perturbation would also account for the observation of reduced sterol demethylase catalytic activity by changing the location of the 14alpha-methyl group in relation to oxygen-bound heme during the catalytic cycle.

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.

Purification, reconstitution, and inhibition of cytochrome P-450 sterol delta22-desaturase from the pathogenic fungus Candida glabrata

Sterol delta22-desaturase has been purified from a strain of Candida glabrata with a disruption in the gene encoding sterol 14alpha-demethylase (cytochrome P-45051; CYP51). The purified cytochrome P-450 exhibited sterol delta22-desaturase activity in a reconstituted system with NADPH-cytochrome P-450 reductase in dilaurylphosphatidylcholine, with the enzyme kinetic studies revealing a Km for ergosta-5,7-dienol of 12.5 microM and a Vmax of 0. 59 nmol of this substrate metabolized/min/nmol of P-450. This enzyme is encoded by CYP61 (ERG5) in Saccharomyces cerevisiae, and homologues have been shown in the Candida albicans and Schizosaccharomyces pombe genome projects. Ketoconazole, itraconazole, and fluconazole formed low-spin complexes with the ferric cytochrome and exhibited type II spectra, which are indicative of an interaction between the azole moiety and the cytochrome heme. The azole antifungal compounds inhibited reconstituted sterol delta22-desaturase activity by binding to the cytochrome with a one-to-one stoichiometry, with total inhibition of enzyme activity occurring when equimolar amounts of azole and cytochrome P-450 were added. These results reveal the potential for sterol delta22-desaturase to be an antifungal target and to contribute to the binding of drugs within the fungal cell.

Expression, purification, reconstitution and inhibition of Ustilago maydis sterol 14 alpha-demethylase (CYP51; P450(14DM))

Triadimenol and tebuconazole are potent inhibitors of the sterol 14 alpha-demethylation reaction in fungi which is catalysed by CYP51, a haem-thiolate containing enzyme belonging to the cytochrome P450 monooxygenase superfamily. Using CYP51 from the phytopathogen Ustilago maydis, a comparison of the sensitivity of the fungal enzyme to triadimenol and tebuconazole has been carried out. U. maydis CYP51 was purified to homogeneity as determined by SDS-PAGE and specific haem content. Catalytic activity was investigated following reconstitution with its respective NADPH cytochrome P450 reductase and proposed endogenous substrate, 24-methylenedihydrolanosterol. Addition of the triadimenol and tebuconazole induced type II spectral changes in the enzyme, with saturation occurring at equimolar azole concentrations. Inhibition of reconstituted activities showed a one-to-one sensitivity of the fungal CYP51 as judged by IC50 values. The implications for fungicide mode of action and treatment are discussed.

Organism-dependent fungicidal activities of azoles

We investigated the antifungal activities of itraconazole and voriconazole on Aspergillus species by time kill studies, and the results were compared with those obtained for Candida species. Exposure of Aspergillus fumigatus conidia to varying concentrations (1.25 to 10 microg/ml) of itraconazole and voriconazole resulted in cellular death; the cytocidal effect was time and concentration dependent. In contrast, no killing of Candida albicans occurred in the presence of itraconazole and voriconazole at concentrations as high as 10 microg/ml, although candidal growth was inhibited compared to the drug-free control. Amphotericin B (1.25 to 10 microg/ml), on the other hand, killed both A. fumigatus and C. albicans. Similar results were obtained for non-A. fumigatus aspergilli and non-C. albicans Candida species. These observations indicate that both itraconazole and voriconazole are cytocidal agents for Aspergillus species but not for Candida species, suggesting that azoles possess organism-dependent fungicidal activities.

Clotrimazole, an antimycotic drug, inhibits the sarcoplasmic reticulum calcium pump and contractile function in heart muscle

Clotrimazole (CLT), an antimycotic drug, has been shown to inhibit proliferation of normal and cancer cell lines and its systemic use as a new tool in the treatment of proliferative disorders is presently under scrutiny (Benzaquen, L. R., Brugnara, C., Byers, H. R., Gattoni-Celli, S., and Halperin, J. A. (1995) Nature Med. 1, 534-540). The action of CLT is thought to involve depletion of intracellular Ca2+ stores but the underlying mechanism has not been defined. The present study utilized membrane vesicles of rabbit cardiac sarcoplasmic reticulum (SR) to determine the mechanism by which CLT depletes intracellular Ca2+ stores. The results revealed a strong, concentration-dependent inhibitory action of CLT on the ATP-energized Ca2+ uptake activity of SR (50% inhibition with approximately 35 microM CLT). The inhibition was of rapid onset (manifested in <15 s), and was accompanied by a 7-fold decrease in the apparent affinity of the SR Ca2+-ATPase for Ca2+ and a minor decrement in the enzyme's apparent affinity toward ATP. Exposure of SR to CLT in the absence or presence of Ca2+ resulted in irreversible inhibition of Ca2+ uptake demonstrating that the Ca2+-bound and Ca2+-free conformations of the Ca2+-ATPase are CLT-sensitive. Introduction of CLT to the reaction medium subsequent to induction of enzyme turnover with Ca2+ and ATP resulted in instantaneous cessation of Ca2+ transport indicating that an intermediate enzyme species generated during turnover undergoes rapid inactivation by CLT. The inhibition of Ca2+ uptake by CLT was accompanied by inhibition of Ca2+-stimulated ATP hydrolysis and Ca2+-induced phosphoenzyme intermediate formation from ATP in the ATPase catalytic cycle. Phosphorylation of the Ca2+-deprived enzyme with Pi in the reverse direction of catalytic cycle and Ca2+ release from Ca2+-preloaded SR vesicles were unaffected by CLT. It is concluded that CLT depletes intracellular Ca2+ stores by inhibiting Ca2+ sequestration by the Ca2+-ATPase. The mechanism of ATPase inhibition involves a drug-induced alteration in the Ca2+-binding site(s) resulting in paralysis of the enzyme's catalytic and ion transport cycle. CLT (50 microM) caused marked depression of contractile function in isolated perfused, electrically paced rabbit heart preparations. The contractile function recovered gradually following withdrawal of CLT from the perfusate indicating the existence of mechanisms in the intact cell to inactivate, metabolize, or clear CLT from its target site.

NADPH cytochrome P-450 oxidoreductase and susceptibility to ketoconazole

The phenotype of a strain of Saccharomyces cerevisiae containing a disruption of the gene encoding NADPH cytochrome P-450 oxidoreductase (CPR) was quantified biochemically and microbiologically, as were those of various transformants of this strain after expression of native CPR, cytochrome P-45051 (CYP51), and a fusion protein of CYP51-CPR (FUS). Only a 4-fold decrease in ergosterol biosynthesis was observed for the cpr strain, but ketoconazole sensitivity increased 200-fold, indicating hypersensitivity to the alternative electron donor system in cpr strains. Both phenotypes could be reversed in transformants expressing the CPR and FUS, indicating the availability of the CPR in FUS as well as the expressed native CPR for monoxygenase-associated reactions. The complementation of function was observed both in vitro and in vivo for the monoxygenases squalene epoxidase, CYP51, and CYP61 in the ergosterol biosynthesis pathway with which CPR is coupled. Overexpression of CYP51 and FUS produced different levels of ketoconazole resistance in wild-type cells, indicating that the availability of CPR may limit the potential of overproduction of CYP51 as a mechanism of resistance to azole antifungal agents.

Induction and inhibition of cytochrome P450 isoforms by imazalil, a food contaminant, in mouse small intestine and liver

1. The effects of imazalil, a food contaminant used as a fungicide, were investigated on the expression and activity of cytochrome P450 in the small intestinal mucosa and liver of mice. Imazalil was orally administered to mice daily at 1 or 10 mg/kg for 3 days. 2. Imazalil enhanced cytochrome P450-catalysed ethoxyresorufin O-deethylase and pentoxyresorufin O-depentylase (PROD) activities in both tissue microsomes at the 10 mg/kg/day dose level, indicating the induction of cytochrome P450 subfamilies CYP1A and CYP2B. In addition, immunochemical analyses also demonstrated an enhanced expression of CYP2B, CYP2C and CYP3A subfamilies in both tissues. 3. Imazalil was a potent inhibitor of cytochrome P450-dependent monooxygenase activities (PROD, aminopyrine N-demethylase and erythromycin demethylase) in in vitro assays using both small intestinal and liver microsomes. 4. From these findings, imazalil has been demonstrated to have not only a potent inhibitory activity but also a significant inducing ability of P450 isoforms in the small intestine. Prolonged ingestion of such a food contaminant may modulate the xenobiotic-metabolizing enzyme system at the site of a primary portal of xenobiotic entry to the systemic circulation.

26-hydroxylation of ecdysteroids is catalyzed by a typical cytochrome P-450-dependent oxidase and related to ecdysteroid resistance in an insect cell line

The epithelial cell line from the dipteran Chironomus tentans responds to the insect steroid hormone 20-hydroxyecdysone and the non-steroidal analogue tebufenozide by undergoing a morphogenetic and biochemical differentiation program. Long-term culture in the presence of 20-hydroxyecdysone has resulted in the selection of subclones that are resistant to the steroid but respond normally to the non-steroidal analogue. In the present study, several subclones that were resistant to the steroid hormone have been compared with steroid-sensitive subclones with respect to their capability to metabolize 20-hydroxyecdysone. Homogenates of both types of cells, when incubated with 3H-labelled steroid in the presence of NADPH, producecd 20,26-dihydroxyecdysone, which was further metabolized to two compounds, which behaved less polar than 20-hydroxyecdysone on reverse-phase HPLC. Ecdysone, a less-active hormone precursor, provided 26-hydroxyecdysone as the only product. The metabolites were identified by mass spectrometry coupled to HPLC, chromatography with authentic samples, and formation of acetonides. The structure of 20,26-dihydroxyecydsone was confirmed by 1H-NMR. The enzyme responsible for the synthesis of 20,26-dihydroxyecdysone in the Chironomus cell preparations has been characterized as a typical cytochrome P-450-dependent monooxygenase. It was a strictly microsomal enzyme, sensitive to inhibition by carbon monoxide and imidazole/triazole-based fungicides, and required NADPH for maximal activity. NADH could partly replace NADPH. The Michaelis constant (Km) for 20-hydroxyecdysone was 0.96 microM, and the maximal enzyme velocity (Vmax) was 50 pmol substrate metabolized x mg protein(-1) x min(-1). 26-Hydroxylation of 20-hydroxyecdysone was inhibited by ecdysone, an alternative substrate, and by inokosterone, a product analogue, to 50% at 1.4 microM and 0.73 microM, respectively. When various subclones were compared with respect to their in vitro rate of 20-hydroxyecdysone metabolization, those clones known to be resistant to the steroid were 'high metabolizers' (> 70% relative rate), whereas the sensitive clones were 'poor metabolizers' (< 30% relative rate). Hence, it is tempting to conclude that ecdysteroid resistance of the Chironomus cell clones is due to metabolic inactivation of the steroid hormone.

Stereoselective interaction of the azole antifungal agent SCH39304 with the cytochrome P-450 monooxygenase system isolated from Cryptococcus neoformans

We investigated the stereoselective inhibition of growth and ergosterol biosynthesis by SCH39304 in the pathogenic fungus Cryptococcus neoformans obtained from four AIDS patients who failed fluconazole therapy and compared the results to those obtained with a wild-type strain. For all strains, the MICs of the RR isomer were approximately half those of the racemate, with the SS enantiomer showing no inhibitory activity. The 50% inhibitory concentrations for in vitro ergosterol biosynthesis correlated with the MIC data, indicating stereoselective inhibition of their target P-450 enzyme, sterol 14alpha-demethylase, as the cause of this difference. The RR enantiomer produced classical type II spectra on addition to microsomal extracts of the strains, whereas the SS enantiomer showed an absence of binding. Stereo- and regio-specific localization of N-1 substituent groups of SCH39304 within the active site of the enzyme determined the unique discrimination between its two enantiomers, and the inability to bind to sterol 14alpha-demethylase is also true of other P-450 enzymes contained in the microsomal fraction. As previously observed for other antifungal azoles, isolates obtained following failure of fluconazole therapy showed resistance to SCH39304 and its RR enantiomer. This resistance could be associated with an alteration in the sensitivity of ergosterol biosynthesis in vitro. These alterations did not cause any changes allowing the SS enantiomer to bind to the P-450 mediating sterol 14alpha-demethylation.

Molecular mechanisms of azole resistance in fungi

This paper reviews the current status of our understanding of azole antifungal resistance mechanisms at the molecular level and explores their implications. Extensive biochemical studies have highlighted a significant diversity in mechanisms conferring resistance to azoles, which include alterations in sterol biosynthesis, target site, uptake and efflux. In stark contrast, few examples document the molecular basis of azole resistance. Those that do refer almost exclusively to mechanisms in laboratory mutants, with the exception of the role of multi-drug resistance proteins in clinical isolates of Candida albicans. It is clear that the technologies required to examine and define azole resistance mechanisms at the molecular level exist, but research appears distinctly lacking in this most important area.

Characterization of Saccharomyces cerevisiae CYP51 and a CYP51 fusion protein with NADPH cytochrome P-450 oxidoreductase expressed in Escherichia coli

Saccharomyces cerevisiae CYP51, target of azole antifungal agents, and CYP51 fused with S. cerevisiae cytochrome P-450 oxidoreductase (FUS protein) were expressed in active forms in Escherichia coli by cloning into pET15b. The expression was monitored immunologically, catalytically, and by using reduced carbon monoxide difference and type II binding spectra. CYP51 and FUS enzymes were located in membranes and produced a Soret peak at 448 nm in the reduced CO difference spectrum. The cytochrome P-450 contents in the membrane fractions containing CYP51 and FUS proteins were 12.8 +/- 2.6 and 17.4 +/- 3.7 pmol/mg of protein, respectively. The NADPH cytochrome P-450 oxidoreductase (CPR) content was estimated to be 15.7 +/- 1.1 pmol/mg of protein in FUS membrane fractions. FUS protein catalyzed the demethylation of substrate at the 14alpha position, with a turnover number of 1.96 +/- 0.37 min(-1) in the presence of NADPH. No reductase activity was observed in membrane fractions containing CYP51, and therefore, CYP51 did not function catalytically in the presence of NADPH, but in the presence of an artificial electron donor, cumene hydroperoxide, activity was comparable to that of the FUS enzyme. Further support for a normal structure for the hemoproteins was obtained from type II binding spectra, in which the spectral response was saturated with an equimolar concentration of ketoconazole.

Isolation and molecular characterisation of the gene encoding eburicol 14 alpha-demethylase (cYP51) from Penicillium italicum

The CYP51 gene encoding eburicol 14 alpha-demethylase (P450(14DM)) was cloned from a genomic library of the filamentous fungal plant pathogen Penicillium italicum, by heterologous hybridisation with the corresponding gene encoding lanosterol 14 alpha-demethylase from the yeast Candida tropicalis. The nucleotide sequence of a 1739-bp genomic fragment and the corresponding cDNA clone comprises an open reading frame (ORF) of 1545 bp, encoding a protein of 515 amino acids with a predicted molecular mass of 57.3 kDa. The ORF is interrupted by three introns of 60, 72 and 62 bp. The C-terminal part of the protein includes a characteristic haem-binding domain, HR2, common to all P450 genes. The deduced P. italicum P450(14DM) protein and the P450(14DM) proteins from Candida albicans, C. tropicalis and Saccharomyces cerevisiae share 47.2, 47.0 and 45.8% amino acid sequence identity. Therefore, the cloned gene is classified as a member of the CYP51 family. Multiple copies of a genomic DNA fragment of Pl italicum containing the cloned P450 gene were introduced into Aspergillus niger by transformation. Transformants were significantly less sensitive to fungicides which inhibit P450(14DM) activity, indicating that the cloned gene encodes a functional eburicol 14 alpha-demethylase.

Synthesis and antifungal activity of alkylthio and alkylsulfonyl derivatives of SM-8668

Triazole analogues which contained alkylthio or alkylsulfonyl groups where synthesized as derivatives of antifungal SM-8668 and estimated for their in vitro and in vivo activity. Derivatives having pentylthio, heptylthio or nonylthio groups showed excellent efficacy against both candidiasis and aspergillosis. Introduction of a hydrophilic group at the end of their alkyl chain made their activity stronger. Especially, 5-hydroxypentylthio and 7-hydroxyheptylthio derivatives showed the strongest antifungal activity.

Identification of 24-methylene-24,25-dihydrolanosterol as a precursor of ergosterol in the yeasts Schizosaccharomyces pombe and Schizosaccharomyces octosporus

Study of the plasma membrane sterol composition in the yeasts Schizosaccharomyces pombe and Schizosaccharomyces octosporus revealed the presence of ergosterol, lanosterol, dehydroergosterol, fecosterol, episterol and 24-methylene-24,25-dihydrolanosterol (eburicol), a C-31 derivative. The growth of both yeasts in the presence of ketoconazole led to a decrease by 85% of the ergosterol content while the levels of lanosterol and eburicol increased. This suggests that in the biosynthetic pathway of ergosterol in Schizosaccharomyces species, the transmethylation process on the C-24 may occur directly on lanosterol and not only on zymosterol. On the other hand, it cannot be excluded that in the genus Schizosaccharomyces two routes exist from lanosterol to ergosterol: the classical one via a direct C-14, C-4 demethylation of lanosterol and the second one via the formation of a C-31 derivative followed by demethylations.

The effects of phenelzine and other monoamine oxidase inhibitor antidepressants on brain and liver I2 imidazoline-preferring receptors

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate I2 imidazoline-preferring receptors in the rat brain and liver after chronic treatment with various irreversible and reversible monoamine oxidase (MAO) inhibitors. 2. Chronic treatment (7-14 days) with the irreversible MAO inhibitors, phenelzine (1-20 mg kg-1, i.p.), isocarboxazid (10 mg kg-1, i.p.), clorgyline (3 mg kg-1, i.p.) and tranylcypromine (10 mg kg-1, i.p.) markedly decreased (21-71%) the density of I2 imidazoline-preferring receptors in the rat brain and liver. In contrast, chronic treatment (7 days) with the reversible MAO-A inhibitors, moclobemide (1 and 10 mg kg-1, i.p.) or chlordimeform (10 mg kg-1, i.p.) or with the reversible MAO-B inhibitor Ro 16-6491 (1 and 10 mg kg-1, i.p.) did not alter the density of I2 imidazoline-preferring receptors in the rat brain and liver; except for the higher dose of Ro 16-6491 which only decreased the density of these putative receptors in the liver (38%). 3. In vitro, phenelzine, clorgyline, 3-phenylpropargylamine, tranylcypromine and chlordimeform displaced the binding of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors from two distinct binding sites. Phenelzine, 3-phenylpropargylamine and tranylcypromine displayed moderate affinity (KiH = 0.3-6 microM) for brain and liver I2 imidazoline-preferring receptors; whereas chlordimeform displayed high affinity (KiH = 6 nM) for these receptors in the two tissues studied, Clorgyline displayed very high affinity for rat brain (KiH = 40 pM) but not for rat liver I2 imidazoline-preferring receptors (KiH = 169 nM). 4. Preincubation of cortical or liver membranes with phenelzine (10-4 M for 30 min) did not alter the total density of I2 imidazoline-preferring receptors, indicating that this irreversible MAO inhibitor does not irreversibly bind to I2 imidazoline-preferring receptors. In contrast, preincubation with 10-6 Mclorgyline reduced by 40% the Bmax of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors.5. Chronic treatment (7 days) with the inducers of cytochrome P-450 enzymes phenobarbitone (40 or 80 mg kg-1, i.p.), 3-methylcholanthrene (20 mg kg-1, i.p.) or 2-methylimidazole (40 mg kg-1, i.p.) did not alter the binding parameters of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors.The compound SKF 525A, a potent inhibitor of cytochrome P-450 enzymes which forms a tight but reversible complex with the haemoprotein, completely displaced with moderate affinity (KiH = 2-10 microM)the specific binding of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors. Preincubation of total liver homogenates with 3 x 10-4 M phenelzine in the presence of 10-3 M NADH, a treatment that irreversibly inactivates the haeme group of cytochrome P-450, did not reduce the density of liver I2 imidazoline-preferring receptors. These results discounted a possible interaction of [3H]-idazoxan with the haeme group of cytochrome P-450 enzymes.6. Together the results indicate that the down-regulation of I2 imidazoline-preferring receptors is associated with an irreversible inactivation of MAO (at least in the brain) that is not related either to the affinity of the MAO inhibitors for I2 imidazoline-preferring receptors or to an irreversible binding to these putative receptors. These findings indicate a novel effect of irreversible MAO inhibitors in the brain and suggest a new target for these compounds that could be of relevance in the treatment of depression, a disease in which an increased density of brain I2 imidazoline-preferring receptors has been reported.

Selectivity of isoprenoid-containing imidazole antifungal compounds for sterol 14-demethylase P450 (P450(14)DM) and 7-ethoxycoumarin O-deethylase P450 of rat liver microsomes

The imidazole antifungal compound AFK-108 (1-[2-(2,4-dichlorophenyl)-2-((2E)-3,7-dimethylocta-2,6- dienyloxy)ethyl]-1H-imidazole) has been shown to be a potent inhibitor for yeast lanosterol 14 alpha-demethylase (P450(14)DM), interacting specifically with the sterol side-chain recognition part of the substrate site through its geranyl moiety. AFK-108 acted as a potent inhibitor for rat liver P450(14)DM, while its farnesyl (AFK-110) and prenyl (AFK-122) homologues were weak inhibitors. This indicates that AFK-108 interacts with rat liver P450(14(DM in the same manner as with the yeast enzyme. However, the difference between the potency of AFK-108 and the homologues was greater in rat P450(14)DM than in the yeast enzyme. AFK-108 and its homologues partially inhibited 7-ethoxycoumarin O-deethylase activity of rat liver microsomes. The order of potency was AFK-122 > AFK-108 > AFK-110, indicating that some steric hindrance of the isoprenoid moiety might affect their potency. The inhibitory effect of AFK-108 for P450(14)DM was considerably higher than for 7-ethoxycoumarin O-deethylase P450, while the inhibition of AFK-110 and AFK-122 on these enzymes was of the same order of magnitude. These results suggest that azole compounds interacting with the side-chain recognition site of P450(14)DM may be good candidates as antifungal agents selective for fungal P450(14)DM.

Modeling cytochrome P450 14 alpha demethylase (Candida albicans) from P450cam

The tertiary structure of cytochrome P450 14 alpha demethylase--Candida albicans (P450 CA) is modeled on the basis of sequence alignment with two closely related proteins and the crystallographic structure of Pseudomonas putida P450cam. The secondary structure prediction system used combines the information from several algorithms and trains the data to offer an optimized prediction of the known P450cam. The trained algorithm was then used to predict the secondary structure of the other P450 sequences. The prediction of the surface coil regions was aided by an alignment between P450 CA and the homologous sequences P450 14 alpha demethylase--Saccharomyces cerevisiae (66 SD) and P450 14 alpha demethylase--Candida tropicalis (72 SD). The prediction and alignment information was combined to establish an alignment between P450 CA and P450cam, and to assign full secondary structure to the target protein. This secondary structure was folded from the template of P450cam and the predicted structure was relaxed by molecular dynamics. Model checking highlighted minor adjustments in the alignment, correctly orienting hydrophobic and hydrophilic side chains. The model offers explanations for several known experimental results and suggests further investigations that may prove fruitful in understanding the structure and mechanisms of the P450 family (Porter, T.D. and Coon, M.J. Minireview cytochrome P450. J. Biol. Chem. 1991, 266, 13469-13472. Waterman, M.R. Cytochrome P450 cellular distribution and structural considerations. Current Opinion in Structural Biology 1992, 2, 384-387. Aoyama, Y., Yoshida, Y., Sonohdo, Y. and Sato, Y. Structural analysis of the interaction between the side-chain of substrates and the active site of lanosterol 14 alpha demethylase (P450 14DM) of yeast. Biochim. Biophys. Acta 1992, 1122, 251-255.).

Amino- and aminomethylcholesterol derivatives with fungicidal activity

Among a series of aminocholesterol derivatives synthesized, 7-aminocholesterol is the strongest inhibitor of yeast cell growth. Using sterol auxotrophic mutant strains, we showed that this compound inhibits cell proliferation by interfering with ergosterol biosynthesis. The sterol pattern of treated cells revealed that 7-aminocholesterol inhibits delta 8-->delta 7-sterol isomerase and delta 14-sterol reductase as morpholine inhibitors. However, the novel feature of this compound is a strong cytotoxicity to yeast.

Disposition of azole antifungal agents. II. Hepatic binding and clearance of dichlorophenyl-bis-triazolylpropanol (DTP) in the rat

DTP (dichlorophenyl-bis-triazolylpropanol) was evaluated as a probe of drug-cytochromes P450 interactions in vitro and in vivo. Studies with rat liver microsomes demonstrate that DTP shows similar P450 binding affinity to its analog, ketoconazole, as determined by P450 difference spectra and inhibition of the metabolism of methoxycoumarin. As a more polar azole, DTP shows less affinity for rat plasma albumin (fraction unbound 0.56) than ketoconazole (fraction unbound 0.037). DTP metabolism is simpler than that of ketoconazole, with only one pathway, N-dealkylation which removes a triazole ring to yield DTP glycol. This primary metabolite is further metabolised to a carboxylic acid, a glycol glucuronide and a third unknown secondary metabolite (probably an acid glucuronide). Over a dose range of 0.1-24mg/kg there is complete mass balance recovery in urine via the five metabolites and unchanged drug. However DTP metabolism is dose dependent and while the affinity of DTP for the cytochromes P450 carrying out the initial dealkylation is high (1.5 microM based on unbound blood concentration), the capacity of the reaction is low (1 nmole/min). Under linear conditions, metabolic clearance is low (19ml/h), but ten-fold higher than renal clearance. The liver is the major distribution site for both DTP and ketoconazole. At low DTP concentrations, a specific high affinity process dominates the hepatic binding of DTP resulting in a liver:blood partition coefficient of approximately 30. Hepatic binding is concentration dependent and the progressive decrease in partition coefficient observed as the dose of DTP is escalated is coincident with a decrease in volume of distribution. The two saturable processes involved in the disposition of DTP result in an unusual concentration dependency in the blood concentration-time profile of this azole. Following administration of a high dose (10mg/kg) of DTP the log concentration-time profile is sigmoidal. At high concentrations (above 1mg/L) both the N-dealkylation and the hepatic binding of DTP are saturated, but as concentrations fall to approximately 0.05mg/L the former process becomes linear and the time profile is convex over this concentration range. At later times as DTP concentrations decline further, the tissue binding also reaches the linear region and the time profile becomes concave. Only at low concentrations (below 0.05mg/L) do both processes become first order and the true half life is evident.

Cloning and characterization of a yeast cytochrome b5-encoding gene which suppresses ketoconazole hypersensitivity in a NADPH-P-450 reductase-deficient strain

Cytochrome P-450 (Cyp) 51 or lanosterol-C14-demethylase is the main target for antifungal compounds of the triazole family like ketoconazole (Kz). Disruption of the associated NADPH-P-450 reductase-encoding gene (YRED) is not lethal, but decreases by about 20-fold the Kz resistance (KzR) of wild-type (wt) Saccharomyces cerevisiae. Transformation of a YRED-disrupted strain by a yeast genomic library based on a multicopy vector allowed us to identify a suppressor of Kz hypersensitivity. Deletion analysis of the 5-kb cloned fragment indicated that yeast cytochrome b5-encoding gene (CYB5), which encodes a 120-amino-acid (aa) protein, is required and sufficient for the suppressor effect. The encoded polypeptide shares about 30% aa identity with mammalian cytochromes b5 (Cyb5). CYB5 disruption and tetrad analysis demonstrate that yeast Cyb5 is not required for growth in a Yred+ strain. Determination of the microsomal content of b-type cytochromes by differential spectra indicated the presence of a strongly decreased or null Cyb5 level in the disrupted strain. This confirms that we have cloned the gene encoding the major microsomal form of Cyb5 which appears not to be essential. Minor Cyb5 isoforms could also be present in yeast or other redox proteins could substitute for the pleiotropic roles of Cyb5 in the sterol and lipid biosynthesis pathways.

Characterization of receptor-mediated and store-regulated Ca2+ influx in human neutrophils

1. It is not known to what extent the emptying of intracellular Ca2+ stores participates in the mediation of chemoattractant-induced Ca2+ influx in human neutrophils. To study this question, we compared the properties of bivalent-cation influx in response to the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenyl-alanine (f-MLP) and to the microsomal Ca(2+)-ATPase inhibitor thapsigargin. 2. The influx pathway activated by f-MLP and thapsigargin had identical properties of permeation. Mn2+ influx became saturated at around 1 mM extracellular Mn2+, whereas Ca2+ influx did not become saturated up to concentrations of 10 mM. 3. The influx of the two bivalent cations, Mn2+ and Ca2+, was activated to a similar extent and with identical kinetics of activation. 4. The Mn2+ influx activated by f-MLP and thapsigargin was blocked, with identical dose-inhibition curves, by four imidazole analogues. 5. The same relationship between the emptying of Ca2+ stores and bivalent-cation influx was observed for f-MLP and thapsigargin, with a half-maximal activation of the influx at 40% emptying of intracellular stores. 6. In conclusion, neutrophils possess a single type of Ca(2+)-influx pathway that is activated by receptor agonists and by store depletion. Receptor agonists activate this influx pathway to a large extent, if not completely, through the depletion of intracellular Ca2+ stores.

Ro 09-1470 is a selective inhibitor of P-450 lanosterol C-14 demethylase of fungi

Ro 09-1470 is a new antifungal agent that belongs to a series of compounds characterized by a tetrahydropyran skeleton with glycine and alkenyl side chains and that inhibits P-450 lanosterol C-14 demethylase (P-450(14DM)) of fungi (Y. Aoki, T. Yamazaki, M. Kondoh, Y. Sudoh, N. Nakayama, Y. Sekine, H. Shimada, and M. Arisawa, J. Antibiot. 45:160-170, 1992; S. Matsukuma, T. Ohtsuka, H. Kotaki, H. Sawairi, T. Sano, K. Watanabe, N. Nakayama, Y. Itezono, M. Fujiu, N. Shimma, K. Yokose, and T. Okuda, J. Antibiot. 45:151-159, 1992). We have studied the compound's mode of interaction with fungal P-450(14DM) and its selectivity for the fungal versus mammalian P-450 enzymes. Ro 09-1470 bound to the Saccharomyces cerevisiae P-450(14DM) by coordinating to the heme with one-to-one stoichiometry. Unlike the azole compounds, it interacted with both ferric and ferrous heme. It was active also against the P-450(14DM) of Candida albicans. Ro 09-1470 preferentially inhibited the yeast P-450(14DM), showing a 50% inhibitory concentration (IC50) of 0.47 to approximately 1.1 microM, which is much lower than the IC50s for rat hepatic P-450s catalyzing cholesterol biosynthesis (IC50 = 341 microM), p-nitroanisol O-demethylation (> 1,000 microM), aniline hydroxylation (> 1,000 microM), and aminopyrine N-demethylation (920 microM). The degree of selectivity for yeast P-450 was higher than that of ketoconazole.

Stimulation by paraquat of microsomal and cytochrome P-450-dependent oxidation of glycerol to formaldehyde

Glycerol can be oxidized to formaldehyde by microsomes in a reaction that is dependent on cytochrome P-450. An oxidant derived from the interaction of H2O2 with iron was responsible for oxidizing the glycerol, with P-450 suggested to be necessary to produce H2O2 and reduce non-haem iron. The effect of paraquat on formaldehyde production from glycerol and whether paraquat could replace P-450 in supporting this reaction were studied. Paraquat increased NADPH-dependent microsomal oxidation of glycerol; the stimulation was inhibited by glutathione, catalase, EDTA and desferrioxamine, but not by superoxide dismutase or hydroxyl-radical scavengers. The paraquat stimulation was also inhibited by inhibitors, substrate and ligand for P-4502E1 (pyrazole-induced P-450 isozyme), as well as by anti-(P-4502E1) IgG. These results suggest that P-450 still played an important role in glycerol oxidation, even in the presence of paraquat. Purified NADPH-cytochrome P-450 reductase did not oxidize glycerol to formaldehyde; some oxidation, however, did occur in the presence of paraquat. Reductase plus P-4502E1 oxidized glycerol, and a large stimulation was observed in the presence of paraquat. Rates in the presence of P-450, reductase and paraquat were more than additive than the sums from the reductase plus P-450 and reductase plus paraquat rates, suggesting synergistic interactions between paraquat and P-450. These results indicate that paraquat increases oxidation of glycerol to formaldehyde by microsomes and reconstituted systems, that H2O2 and iron play a role in the overall reaction, and that paraquat can substitute, in part, for P-450 in supporting oxidation of glycerol. However, cytochrome P-450 is required for elevated rates of formaldehyde production even in the presence of paraquat.

General resistance to sterol biosynthesis inhibitors in Saccharomyces cerevisiae

Screening for resistance to fenpropimorph was undertaken in order to isolate yeast mutants affected in the regulation of the ergosterol pathway. Among the mutants isolated, one bearing the recessive fen1-1 mutation was characterized by a 1.5-fold increase in the ergosterol level and a general resistance to sterol biosynthesis inhibitors. The fen1-1 mutation was linked to MAT locus on chromosome III. The measurement of enzyme activities involved in the ergosterol pathway revealed that isopentenyl diphosphate (IPP) isomerase activity was specifically increased 1.5-fold as compared to the wild type strain. However, overexpression of IPP isomerase in the wild type strain was not by itself sufficient to lead to sterol increase or resistance to sterol biosynthesis inhibitors, showing that IPP isomerase is not a limiting step in the pathway. The fen1-1 mutation permits viability in aerobiosis of yeast disrupted for sterol-14 reductase in absence of exogenous ergosterol supplementation, whereas the corresponding strain bearing the wild type FEN1 allele grows only in anaerobiosis. This result shows that ignosterol is able to efficiently replace ergosterol as bulk membrane component and that the fen1-1 mutation eliminates the specific ergosterol requirement in yeast.

Cytochrome P450 in trypanosomatids

Post-mitochondrial supernatant extracts prepared from bloodstream forms of Trypanosoma brucei brucei, T. cruzi epimastigotes, Leishmania donovani promastigotes and Crithidia fasciculata have been found to catalyse cytochrome P450-dependent reactions. Appreciable ethoxycoumarin deethylase and ethoxyresorufin deethylase activities were found in all of the above trypanosomatids, with T. cruzi epimastigotes having the highest activity (57.1 and 10.7 pmol/min/mg protein, respectively). In all four species these reactions were inhibited by the cytochrome P450 inhibitors carbon monoxide, proadifen and metyrapone. In contrast to rat liver microsomes, the trypanosomatid extracts showed no detectable pentoxyresorufin depentylase or pentamidine hydroxylase activity. Both C. fasciculata and T. b. brucei post-mitochondrial supernatants showed carbon monoxide difference spectra consistent with the presence of cytochrome P450 (9.6 and 6.3 pmol/mg protein, respectively). An additional hemoprotein which gave a carbon monoxide difference peak at 420 nm was also detected in C. fasciculata and T. b. brucei microsomes and C. fasciculata mitochondria. Subcellular fractionation of both early and late log C. fasciculata showed that the ethoxycoumarin deethylase activity was enriched in the microsomal fraction.

Disposition of azole antifungal agents. I. Nonlinearities in ketoconazole clearance and binding in rat liver

The disposition of ketoconazole was characterized in the rat over a wide dose/concentration range. Bolus dose (0.03-10 mg/kg) studies indicate that plasma concentration-time profiles for ketoconazole are not superimposable when dose normalized because of nonlinearities occurring in both volume of distribution and clearance. The volume of distribution decreases from 3 to less than 1 L/kg, while the plasma clearance decreases 10-fold from 25 mL/min/kg as the dose is escalated. From these results, infusion rates were calculated to maintain the plasma concentrations achieved with particular bolus doses. The curvilinear relationship between steady-state plasma concentration (0.015-8.3 mg/L) and ketoconazole infusion rate (0.021-2.45 mg/hr/kg) was analyzed in terms of Michaelis-Menten kinetics. A Vmax of 3.2 mg/hr/kg and Km of 2.1 mg/L were obtained by nonlinear regression analysis. At the end of the ketoconazole infusion, liver, adrenals and kidneys were removed and assayed for ketoconazole. Tissue-to-plasma partition coefficients for the liver and adrenals showed a marked dependence upon steady-state plasma concentration. Both parameters (liver, 22; and adrenals, 53) showed a decrease of approximately 10-fold as the plasma concentrations were increased. In contrast, the kidney:plasma partition coefficient (1.8), blood:plasma concentration ratio (0.6), and plasma binding (96%) of ketoconazole did not show a concentration dependence over the range studied. It is concluded that the liver is an important determinant of ketoconazole's volume of distribution and that saturation of this process accounts largely for the reduction in volume of distribution with increasing dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by a novel azole antifungal agent with a geranyl group on lanosterol 14 alpha-demethylase of yeast

AFK-108 (1-[2-(2,4-dichlorophenyl)-2-((2E)-3,7-dimethylocta-2,6- dienyloxy)ethyl]-1H-imidazole) is a new imidazole derivative characterized by a geranyl substituent showing strong antifungal activity. Azole antifungal agents are known to be potent inhibitors of lanosterol 14 alpha-demethylase (P450(14)DM) of fungi. The role of the geranyl group of AFK-108 on interaction of AFK-108 with the target was studied by using Saccharomyces cerevisiae P450(14)DM as the model enzyme. AFK-108 and some of its derivatives bound to oxidized P450(14)DM with one-to-one stoichiometry and inhibited the demethylase activity. AFK-108 derivatives having the longer farnesyl or the shorter prenyl group showed lower affinity than AFK-108 for the enzyme. AFK-108 caused 100% inhibition at the equivalent concentration to P450(14)DM in the reaction mixture (0.07 microM), while the farnesyl derivative inhibited the activity by 60% at the same concentration. AFK-108 interfered with the binding of CO to the ferrous P450(14)DM. However, the interfering effect of the prenyl derivative was lower than that of AFK-108. Another AFK-108 derivative having the saturated 3,7-dimethyloctyl group was also a weaker inhibitor than AFK-108. These experiments suggest that the geranyl group of AFK-108 interacts with the substrate binding site of P450(14)DM that recognises the side chain of the substrate. AFK-108 is the first example of an azole derivative interacting with the side chain recognising region of the substrate binding site of P450(14)DM.

Plant sterol biosynthesis: novel potent and selective inhibitors of cytochrome P450-dependent obtusifoliol 14 alpha-methyl demethylase

The R-(-) isomer of methyl 1-(2,2-dimethylindan-1-yl)imidazole-5-carboxylate (CGA 214372; 2) strongly inhibited P450-dependent obtusifoliol 14 alpha-demethylase (P450OBT.14DM) (I50 = 8 x 10(-9) M, I50/Km = 5 x 10(-5) in a maize (Zea mays) microsomal preparation. Kinetic studies indicated uncompetitive inhibition with respect to obtusifoliol. The corresponding S-(+) isomer was a 20-fold weaker inhibitor for P450OBT.14DM. The molecular features of a variety of analogues of 2 were related to their potency as inhibitors of P450OBT.14DM in vitro, allowing delineation of the key structural requirements governing inhibition of the demethylase. CGA 214372 proved to have a high degree of selectivity for P450OBT.14DM. This allowed easy distinction of this activity from other P450-dependent activities present in the maize microsomal preparation and gave strong evidence that P450OBT.14DM is a herbicidal target. Microsomal maize P450OBT.14DM and yeast P450LAN.14DM, the only known examples of P450-dependent enzymes carrying out an identical metabolic function in different eukaryotes, showed distinct inhibition patterns with CGA 214372 and ketoconazole, a substituted imidazole anti-mycotic.

Hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol by rat prostate microsomes: potent inhibition by imidazole-type antimycotic drugs and lack of inhibition by steroid 5 alpha-reductase inhibitors

5 alpha-Dihydrotestosterone, the principal androgen mediating prostate growth and function in the rat, is formed from testosterone by steroid 5 alpha-reductase. The inactivation of 5 alpha-dihydrotestosterone involves reversible reduction to 5 alpha-androstane-3 beta,17 beta-diol by 3 beta-hydroxysteroid oxidoreductase followed by 6 alpha-, 7 alpha-, or 7 beta-hydroxylation. 5 alpha-Androstane-3 beta,17 beta-diol hydroxylation represents the ultimate inactivation step of dihydrotestosterone in rat prostate and is apparently catalyzed by a single, high-affinity (Km approximately 0.5 microM) microsomal cytochrome P450 enzyme. The present studies were designed to determine if 5 alpha-androstane-3 beta,17 beta-diol hydroxylation by rat prostate microsomes is inhibited by agents that are known inhibitors of androgen-metabolizing enzymes. Inhibitors of steroid 5 alpha-reductase (4-azasteroid analogs; 10 microM) or inhibitors of 3 beta-hydroxysteroid oxidoreductase (trilostane, azastene, and cyanoketone; 10 microM) had no appreciable effect on the 6 alpha-, 7 alpha-, or 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol (10 microM) by rat prostate microsomes. Imidazole-type antimycotic drugs (ketoconazole, clotrimazole, and miconazole; 0.1-10 microM) all markedly inhibited 5 alpha-androstane-3 beta,17 beta-diol hydroxylation in a concentration-dependent manner, whereas triazole-type antimycotic drugs (fluconazole and itraconazole; 0.1-10 microM) had no inhibitory effect. The rank order of inhibitory potency of the imidazole-type antimycotic drugs was miconazole greater than clotrimazole greater than ketoconazole. In the case of clotrimazole, the inhibition was shown to be competitive in nature, with a Ki of 0.03 microM. The imidazole-type antimycotic drugs inhibited all three pathways of 5 alpha-androstane-3 beta,17 beta-diol hydroxylation to the same extent, which provides further evidence that, in rat prostate microsomes, a single cytochrome P450 enzyme catalyzes the 6 alpha-, 7 alpha-, and 7 beta-hydroxylation of 5 alpha-androstane-3 beta,17 beta-diol. These studies demonstrate that certain imidazole-type compounds are potent, competitive inhibitors of 5 alpha-androstane-3 beta,17 beta-diol hydroxylation by rat prostate microsomes, which is consistent with the effect of these antimycotic drugs on cytochrome P450 enzymes involved in the metabolism of other androgens and steroids.

Comparison of SCH 39304, fluconazole, and ketoconazole for treatment of systemic infections in mice

SCH 39304 was compared with fluconazole and ketoconazole in a systemic Candida albicans infection in mice (10(6) CFU per mouse). Results were based on survival rates and CFU in kidneys following once-daily oral treatment of 2, 5, or 10 days duration. In normal mice, SCH 39304 (dose to reduce kidney counts by 4 log units, 0.5 mg/kg of body weight) was 3 and 200 times more active than fluconazole and ketoconazole, respectively. In immunocompromised mice (gamma irradiation, 600 rads), SCH 39304 (dose to reduce kidney counts by 4 log units, 1.3 mg/kg) was 35 and greater than 100 times more active than fluconazole and ketoconazole, respectively. In normal mice, when the infecting inoculum varied from 10(5) to 10(7) CFU, only a fivefold increase in the dose to reduce kidney counts by 4 log units was observed with SCH 39304. Excellent protection was also seen when mice were treated with a single oral dose of SCH 39304 up to 24 h prior to infection with C. albicans. Studies in a systemic C. albicans infection model indicated that SCH 39304 is equally efficacious following either oral or intravenous administration. In a systemic Aspergillus flavus infection, mice treated with SCH 39304 (5 mg/kg) survived twice as long (16 days) as those treated with fluconazole (50 mg/kg) or controls did.

Agonist-induced Ca2+ influx into human platelets is secondary to the emptying of intracellular Ca2+ stores

We have studied the relation between the filling state of the intracellular Ca2+ stores and the plasma-membrane permeability to Mn2+, used here as a Ca2+ surrogate for Ca2+ channels. Emptying of the intracellular Ca2+ stores either by incubation in Ca(2+)-free medium or by treatment with low concentrations of the Ca2+ ionophore ionomycin accelerated the influx of Mn2+. Refilling of the Ca2+ stores by incubation in Ca(2+)-containing medium restores low Mn2+ permeability. This Ca(2+)-store-regulated permeability was inhibited by Ni2+ and by cytochrome P-450 inhibitors. Stimulation of platelets with thrombin produced Ca2+ release from the intracellular stores, which was followed, after a temperature-dependent lag (2 s at 37 degrees C; 5 s at 18 degrees C), by an acceleration of Mn2+ influx. Cytochrome P-450 inhibitors prevented the thrombin-induced Mn2+ influx, with little effect on the Ca2+ mobilization from the intracellular stores. Ki values were similar to those estimated for inhibition of the store-regulated permeability in non-stimulated platelets. Similar results were found in platelets stimulated by platelet-activating factor or by ADP. We propose that agonist-induced Ca2+ (Mn2+) influx in platelets is secondary to the emptying of the intracellular Ca2+ stores. The activation of the plasma-membrane Ca2+ (Mn2+) pathway may take place by a mechanism involving microsomal cytochrome P-450, similar to that described previously in thymocytes [Alvarez, Montero & García-Sancho (1991) Biochem. J. 274, 193-197] and neutrophils [Montero, Alvarez & García-Sancho (1991) Biochem. J. 277, 73-79].