Microsomal enzymes of cholesterol biosynthesis. Purification of lanosterol 14 alpha-methyl demethylase cytochrome P-450 from hepatic microsomes

CYP51 as drug targets for fungi and protozoan parasites: past, present and future

The efficiency of treatment of human infections with the unicellular eukaryotic pathogens such as fungi and protozoa remains deeply unsatisfactory. For example, the mortality rates from nosocomial fungemia in critically ill, immunosuppressed or post-cancer patients often exceed 50%. A set of six systemic clinical azoles [sterol 14α-demethylase (CYP51) inhibitors] represents the first-line antifungal treatment. All these drugs were discovered empirically, by monitoring their effects on fungal cell growth, though it had been proven that they kill fungal cells by blocking the biosynthesis of ergosterol in fungi at the stage of 14α-demethylation of the sterol nucleus. This review briefs the history of antifungal azoles, outlines the situation with the current clinical azole-based drugs, describes the attempts of their repurposing for treatment of human infections with the protozoan parasites that, similar to fungi, also produce endogenous sterols, and discusses the most recently acquired knowledge on the CYP51 structure/function and inhibition. It is our belief that this information should be helpful in shifting from the traditional phenotypic screening to the actual target-driven drug discovery paradigm, which will rationalize and substantially accelerate the development of new, more efficient and pathogen-oriented CYP51 inhibitors.

Identification, modeling and ligand affinity of early deuterostome CYP51s, and functional characterization of recombinant zebrafish sterol 14α-demethylase

BACKGROUND

Sterol 14α-demethylase (cytochrome P450 51, CYP51, P45014DM) is a microsomal enzyme that in eukaryotes catalyzes formation of sterols essential for cell membrane function and as precursors in biosynthesis of steroid hormones. Functional properties of CYP51s are unknown in non-mammalian deuterostomes.

METHODS

PCR-cloning and sequencing and computational analyses (homology modeling and docking) addressed CYP51 in zebrafish Danio rerio, the reef fish sergeant major Abudefduf saxatilis, and the sea urchin Strongylocentrotus purpuratus. Following N-terminal amino acid modification, zebrafish CYP51 was expressed in Escherichia coli, and lanosterol 14α-demethylase activity and azole inhibition of CYP51 activity were characterized using GC-MS.

RESULTS

Molecular phylogeny positioned S. purpuratus CYP51 at the base of the deuterostome clade. In zebrafish, CYP51 is expressed in all organs examined, most strongly in intestine. The recombinant protein bound lanosterol and catalyzed 14α-demethylase activity, at 3.2nmol/min/nmol CYP51. The binding of azoles to zebrafish CYP51 gave KS (dissociation constant) values of 0.26μM for ketoconazole and 0.64μM for propiconazole. Displacement of carbon monoxide also indicated zebrafish CYP51 has greater affinity for ketoconazole. Docking to homology models showed that lanosterol docks in fish and sea urchin CYP51s with an orientation essentially the same as in mammalian CYP51s. Docking of ketoconazole indicates it would inhibit fish and sea urchin CYP51s.

CONCLUSIONS

Biochemical and computational analyses are consistent with lanosterol being a substrate for early deuterostome CYP51s.

GENERAL SIGNIFICANCE

The results expand the phylogenetic view of animal CYP51, with evolutionary, environmental and therapeutic implications.

7-Dehydrocholesterol reductase activity is independent of cytochrome P450 reductase

7-Dehydrocholesterol reductase (DHCR7) catalyzes the final step in cholesterol synthesis. The enzyme utilizes NADPH as a source of electrons and has been reported to require NADPH-cytochrome P450 reductase (POR) as its redox partner. To test this hypothesis, microsomes were prepared from the livers of mice in which hepatic cytochrome P450 reductase expression was extinguished during maturation. These microsomes contained negligible levels of POR but had 2.5-fold greater DHCR7 activity than did microsomes from wild-type mice. Consistent with this greater activity, immunoblot analysis of DHCR7 expression indicated that DHCR7 protein levels were elevated 2-fold in POR-null microsomes. Addition of POR to these microsomes provided no stimulation of DHCR7 activity, confirming the lack of a role for POR in DHCR7 activity. Because the original observation that POR was necessary for DHCR7 activity was based, in part, on antibody inhibition studies with POR antibody, the ability of an antibody to the full-length POR protein to inhibit DHCR7 activity and cytochrome c reductase activity was tested; the antibody had no effect on DHCR7 activity but decreased cytochrome c reductase activity (a POR-catalyzed reaction) by 50%. Immunoblot analysis further demonstrated no cross-reactivity between POR and DHCR7 with antibodies to either protein. We conclude that cytochrome P450 reductase is not involved in 7-dehydrocholesterol reductase activity.

Structural basis of human CYP51 inhibition by antifungal azoles

The obligatory step in sterol biosynthesis in eukaryotes is demethylation of sterol precursors at the C14-position, which is catalyzed by CYP51 (sterol 14-alpha demethylase) in three sequential reactions. In mammals, the final product of the pathway is cholesterol, while important intermediates, meiosis-activating sterols, are produced by CYP51. Three crystal structures of human CYP51, ligand-free and complexed with antifungal drugs ketoconazole and econazole, were determined, allowing analysis of the molecular basis for functional conservation within the CYP51 family. Azole binding occurs mostly through hydrophobic interactions with conservative residues of the active site. The substantial conformational changes in the B' helix and F-G loop regions are induced upon ligand binding, consistent with the membrane nature of the protein and its substrate. The access channel is typical for mammalian sterol-metabolizing P450 enzymes, but is different from that observed in Mycobacterium tuberculosis CYP51. Comparison of the azole-bound structures provides insight into the relative binding affinities of human and bacterial P450 enzymes to ketoconazole and fluconazole, which can be useful for the rational design of antifungal compounds and specific modulators of human CYP51.

Three-dimensional quantitative structure-activity relationship analysis of human CYP51 inhibitors

CYP51 fulfills an essential requirement for all cells, by catalyzing three sequential mono-oxidations within the cholesterol biosynthesis cascade. Inhibition of fungal CYP51 is used as a therapy for treating fungal infections, whereas inhibition of human CYP51 has been considered as a pharmacological approach to treat dyslipidemia and some forms of cancer. To predict the interaction of inhibitors with the active site of human CYP51, a three-dimensional quantitative structure-activity relationship model was constructed. This pharmacophore model of the common structural features of CYP51 inhibitors was built using the program Catalyst from multiple inhibitors (n = 26) of recombinant human CYP51-mediated lanosterol 14alpha-demethylation. The pharmacophore, which consisted of one hydrophobe, one hydrogen bond acceptor, and two ring aromatic features, demonstrated a high correlation between observed and predicted IC(50) values (r = 0.92). Validation of this pharmacophore was performed by predicting the IC(50) of a test set of commercially available (n = 19) and CP-320626-related (n = 48) CYP51 inhibitors. Using predictions below 10 microM as a cutoff indicative of active inhibitors, 16 of 19 commercially available inhibitors (84%) and 38 of 48 CP-320626-related inhibitors (79.2%) were predicted correctly. To better understand how inhibitors fit into the enzyme, potent CYP51 inhibitors were used to build a Cerius(2) receptor surface model representing the volume of the active site. This study has demonstrated the potential for ligand-based computational pharmacophore modeling of human CYP51 and enables a high-throughput screening system for drug discovery and data base mining.

Sterol 14alpha-demethylase cytochrome P450 (CYP51), a P450 in all biological kingdoms

The CYP51 family is an intriguing subject for fundamental P450 structure/function studies and is also an important clinical drug target. This review updates information on the variety of the CYP51 family members, including their physiological roles, natural substrates and substrate preferences, and catalytic properties in vitro. We present experimental support for the notion that specific conserved regions in the P450 sequences represent a CYP51 signature. Two possible roles of CYP51 in P450 evolution are discussed and the major approaches for CYP51 inhibition are summarized.

Molecular analysis of two cytochrome P450 monooxygenase genes required for paxilline biosynthesis in Penicillium paxilli, and effects of paxilline intermediates on mammalian maxi-K ion channels

The gene cluster required for paxilline biosynthesis in Penicillium paxilli contains two cytochrome P450 monooxygenase genes, paxP and paxQ. The primary sequences of both proteins are very similar to those of proposed cytochrome P450 monooxygenases from other filamentous fungi, and contain several conserved motifs, including that for a haem-binding site. Alignment of these sequences with mammalian and bacterial P450 enzymes of known 3-D structure predicts that there is also considerable conservation at the level of secondary structure. Deletion of paxP and paxQ results in mutant strains that accumulate paspaline and 13-desoxypaxilline, respectively. These results confirm that paxP and paxQ are essential for paxilline biosynthesis and that paspaline and 13-desoxypaxilline are the most likely substrates for the corresponding enzymes. Chemical complementation of paxilline biosynthesis in paxG (geranygeranyl diphosphate synthase) and paxP, but not paxQ, mutants by the external addition of 13-desoxypaxilline confirms that PaxG and PaxP precede PaxQ, and are functionally part of the same biosynthetic pathway. A pathway for the biosynthesis of paxilline is proposed on the basis of these and earlier results. Electrophysiological experiments demonstrated that 13-desoxypaxilline is a weak inhibitor of mammalian maxi-K channels (Ki=730 nM) compared to paxilline (Ki=30 nM), indicating that the C-13 OH group of paxilline is crucial for the biological activity of this tremorgenic mycotoxin. Paspaline is essentially inactive as a channel blocker, causing only slight inhibition at concentrations up to 1 microM.

Metabolism of territrem a by liver microsomes of Wistar rats: identification of the metabolites and their metabolic sequence

The metabolism of territrem A (TRA) was investigated in liver microsomes of male Wistar rats. The results indicated that three metabolites were produced from TRA and these metabolic reactions were inhibited by metyrapone, an inhibitor of cytochrome P-450. Based on analysis by high-performance liquid chromatography (HPLC), mass, and nuclear magnetic resonance (NMR) spectroscopic techniques, the structure of these metabolites were identified as 4beta-hydroxymethyl-4beta-demethylterritrem A (MA1), 4beta-oxo-4beta-demethylterritrem A (MAX), and 2-dihydro-4beta-demethylterritrem A (MA2). It was proposed that reactions proceeded by three sequential oxidative reactions in the pyran moiety of TRA: first, hydroxylation at the 4beta-C methyl group of TRA to form MA1; second, oxidation at the 4beta hydroxyl group of MA, to form MAX; and third, decarbonylation at the 4beta-C oxo group of MAX to form MA2.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Forty years of cytochrome P450

The term "cytochrome P450" first appeared in literature in 1962. It was a microsomal membrane-bound hemoprotein without known physiological functions at that time and was characterized by a unique 450-nm optical absorption peak of its carbon monoxide-bound form, which was originally reported as the spectrum of a novel "microsomal carbon monoxide-binding pigment" in 1958. Elucidation of its function as the oxygenase in 1963 triggered a rapid expansion of research on this hemoprotein. Annual numbers of the published papers dealing with cytochrome P450, which were listed in Biological Abstracts, increased from 60 in 1970 to 500 in 1980, 900 in 1990, and 1500 in 1997. Cytochrome P450 is now regarded as the collective name of a large family of hemoproteins, "cytochrome P450 superfamily, "which seems to have diversified from a single ancestral protein to many forms during the course of biological evolution and is distributed widely among various forms of life from animals and plants to fungi and bacteria. Multicellular eukaryotic organisms including animals and plants have about 100 or more P450 genes in their genomes, and those many P450 genes are expressed tissue specifically and developmental stage specifically, indicating their diverse physiological functions. In mammals, various P450s participate in the biosynthesis and metabolism of sterols and steroid hormones and the metabolism of various lipid biofactors including eicosanoids, vitamin D3, and retinoids. Oxidative metabolism of foreign hydrophobic compounds as the first step of their excretion from the animal body is apparently another major function of cytochrome P450, which protects animals from noxious foreign compounds, man-created and natural.

Molecular diversity of sterol 14alpha-demethylase substrates in plants, fungi and humans

Metabolism of lanosterol (LAN), 24-methylene-24,25-dihydrolanosterol (24-methyleneDHL), dihydrolanosterol (DHL) and obtusifoliol (OBT) by purified human, plant (Sorghum bicolor) and fungal (Candida albicans) sterol 14alpha-demethylase (CYP51; P450(14DM)) reconstituted with NADPH cytochrome P450 reductases was studied in order to elucidate the substrate specificity and sterol stereo- and regio-structural requirements for optimal CYP51 activity. Both human and C. albicans CYP51 could catalyse 14alpha-demethylation of each substrate with varying levels of activity, but having slightly higher activity for their respective endogenous substrates in vivo, dihydrolanosterol for human CYP51 (Vmax = 0.5 nmol/min/nmol CYP51) and 24-methylene-24,25-dihydrolanosterol for C. albicans CYP51 (Vmax = 0.3 nmol/min/nmol CYP51). In contrast, S. bicolor CYP51 showed strict substrate specificity and selectivity towards its own endogenous substrate, obtusifoliol (Vmax = 5.5 nmol/min/nmol CYP51) and was inactive towards 14alpha-demethylation of lanosterol, 24-methylene-24,25-dihydrolanosterol and dihydrolanosterol. These findings confirm that the presence of the 4beta-methyl group in the sterol molecule renders the plant CYP51 incapable of 14alpha-demethylation thus revealing the strict active site conservation of plant CYP51 during evolution.

Isolation and reconstitution of the heme-thiolate protein obtusifoliol 14alpha-demethylase from Sorghum bicolor (L.) Moench

The heme-thiolate (cytochrome P450) enzyme which catalyzes the 14alpha-demethylation of obtusifoliol has been isolated from microsomes prepared from etiolated seedlings of Sorghum bicolor (L.) Moench. The obtusifoliol 14alpha-demethylase is a key enzyme in plant sterol biosynthesis and a target for the design of phyla-specific sterol 14alpha-demethylase inhibitors. Microsomal cytochrome P450s were solubilized by using the detergents Renex 690 and reduced Triton X-100, and the obtusifoliol 14alpha-demethylase was isolated by DEAE ion exchange and dye affinity column chromatography. The isolated enzyme has an absorption spectrum characteristic for low spin cytochrome P450s and produces a Type I binding spectrum with obtusifoliol as substrate. Binding spectra were not obtained with lanosterol, campesterol, sitosterol, or stigmasterol. Obtusifoliol 14alpha-demethylase has an apparent molecular mass of 53 kDa and is estimated to constitute approximately 20% of the total cytochrome P450 content of the microsomal membranes and about 0.2% of the total microsomal protein. Gas chromatography-mass spectrometry analysis of reconstitution experiments with dilauroylphosphatidylcholine micelles containing isolated obtusifoliol 14alpha-demethylase and sorghum NADPHcytochrome P450 oxidoreductase demonstrated the conversion of obtusifoliol (4alpha,14alpha-dimethyl-5alpha-ergosta-8, 24(28)-dien-3beta-ol) to 4alpha-methyl-5alpha-ergosta-8,14, 24(28)-trien3beta-ol, the 14alpha-demethylated product of obtusifoliol with a double bond introduced at the Delta14 position. The N-terminal amino acid sequence of the protein is MDLADIPQ/KQQRLMAGXALVV. Five internal sequences were obtained after endoproteinase Lys-C and Glu-C digestion. The fragment AAGAFSYISFGGGRH aligns with the unique heme binding domain of mammalian and yeast sterol 14alpha-demethylases which belong to the CYP51 family. Therefore it is conceivable that the obtusifoliol 14alpha-demethylase from plants also belongs to the CYP51 family, the only P450 family so far known to be conserved across the phyla.

The mechanism of the acyl-carbon bond cleavage reaction catalyzed by recombinant sterol 14 alpha-demethylase of Candida albicans (other names are: lanosterol 14 alpha-demethylase, P-45014DM, and CYP51)

The Candida albicans sterol 14 alpha-demethylase gene (P-45014DM, CYP51) was transferred to the yeast plasmid YEp51 placing it under the control of the GAL10 promoter. The resulting construct (YEp51:CYP51) when transformed into the yeast strain GRF18 gave a clone producing 1.5 mu mol of P-450/liter of culture, the microsomal fraction of which contained up to 2.5 nmol of P-450/mg of protein. Two oxygenated precursors for the 14 alpha-demethylase, 3 beta-hydroxylanost-7-en-32-al and 3 beta-hydroxylanost-7-en-32-ol, variously labeled with 2H and 18O at C-32 were synthesized. In this study the conversion of [32-2H,32-16O]- and [32-2H,32-18O]3 beta-hydroxylanost-7-en-32-al with the recombinant 14 alpha-demethylase was performed under 16O2 or 18O2 and the released formic acid analyzed by mass spectrometry. The results showed that in the acyl-carbon bond cleavage step (i.e. the deformylation process) the original carbonyl oxygen at C-32 of the precursor is retained in formic acid and the second oxygen of formate is derived from molecular oxygen; precisely the same scenario that has previously been observed for the acyl-carbon cleavage steps catalyzed by aromatase (P-450arom) and 17 alpha-hydroxylase-17,20-lyase (P-45017 alpha,CYP17). In the light of these results the mechanism of the acyl-carbon bond cleavage step catalyzed by the 14 alpha-demethylase is considered.

Purification and characterization of a cytochrome P450 isozyme catalyzing lanosterol 14 alpha-demethylation (P45014DM) in hamster liver

To characterize cholesterol synthesis in Syrian golden hamster, an isozyme of cytochrome P450 lanosterol 14 alpha-demethylase (P45014DM), which catalyzes the initial step in the biosynthesis of cholesterol from lanosterol, was purified and its mode of induction by microsomal enzyme inducers was characterized. P450450DM was purified from hamster livers by chromatography using aminooctyl-Sepharose CL-6B columns, to a specific content of 12.8 nmol/mg-protein. The purified protein displayed a single band on SDS-polyacrylamide gel electrophoresis with an apparent molecular weight of 52,000. The absorption spectra of the oxidized form of the purified protein showed a Soret peak at 417 nm in a low-spin state and a Soret peak of reduced CO-binding complex at 448 nm. In a reconstituted system, the purified protein catalyzed 14 alpha-demethylation of 24,25-dihydrolanosterol (1.58 nmol/min/nmol-P450), although it did not show any activities toward testosterone and 7-ethoxyresorufin, marker substrates of other P450 families. Immunoblot analysis using an antibody against porcine P45014DM, which inhibited the activity of lanosterol 14-alpha-demethylation in the hamster liver microsomes, demonstrated that the level of this isozyme protein was markedly decreased in dexamethasone-treated hamster livers. This was accompanied by a decrease in the enzyme activity. In contrast, the levels and the activity in the phenobarbital- and 3-methylcholanthrene-treated hamsters were almost equal to that in the untreated animals.

Azalanstat (RS-21607), a lanosterol 14 alpha-demethylase inhibitor with cholesterol-lowering activity

Agents that inhibit hepatic cholesterol biosynthesis reduce circulating cholesterol levels in experimental animals and humans, and may be of pharmacological importance in the prevention of atherosclerosis. Azalanstat (RS-21607), a synthetic imidazole, has been shown to inhibit cholesterol synthesis in HepG2 cells, human fibroblasts, hamster hepatocytes and hamster liver, by inhibiting the cytochrome P450 enzyme lanosterol 14 alpha-demethylase. When administered orally to hamsters fed regular chow, RS-21607 (50 mg/kg/day) lowered serum cholesterol in a dose-dependent manner (ED50 = 62 mg/kg) in a period of 1 week. It preferentially lowered low density lipoprotein (LDL) cholesterol and apo B relative to high density lipoprotein (HDL) cholesterol and apo A-1. It also lowered plasma cholesterol levels in hamsters fed a high saturated fat and cholesterol diet. RS-21607 inhibited hepatic microsomal hydroxymethylglutaryl-CoA (HMG-CoA) reductase activity in hamsters in a dose-dependent manner (ED50 = 31 mg/kg), and this was highly correlated with serum cholesterol lowering (r = 0.97). Cholesterol lowering by azalanstat and cholestyramine was additive, and the increase in HMG-CoA reductase brought about by cholestyramine was attenuated significantly by azalanstat. In vitro studies with HepG2 cells indicated that this modulation of reductase activity was indirect, occurring at a post-transcriptional step, and it is proposed that a regulatory oxysterol derived from dihydrolanosterol (or lanosterol) may be responsible for this regulation. Azalanstat does not appear to lower circulating cholesterol in the hamster by up-regulation of the hepatic LDL receptor, suggesting that other mechanisms are involved. Orally administered azalanstat (50-75 mg/kg) stimulated hepatic microsomal cholesterol 7 alpha-hydroxylase activity by 50-400% in hamsters, and it is postulated that this may result from modified cholesterol absorption and bile acid synthesis.

Substrate-based inhibitors of lanosterol 14 alpha-methyl demethylase: I. Assessment of inhibitor structure-activity relationship and cholesterol biosynthesis inhibition properties

A series of 15-, 32-, and 15,32-substituted lanost-8-en-3 beta-ols is described which function as inhibitors of cholesterol biosynthesis. These agents inhibit lanosterol 14 alpha-methyl demethylase activity as well as suppress HMG-CoA reduction activity in cultured cells. Several of these agents are extremely potent as both demethylase inhibitors and reductase suppressors, while others are more selective in their activities. Selected regio double bond isomers show preference for demethylase inhibition with the following order: delta 8 > delta 7 > delta 6 = unsaturated sterols. Comparisons also show that 4,4-dimethyl sterols are always more potent demethylase inhibitors and reductase suppressors than their 4,4-bisnomethyl counterparts. However, evaluation of an extensive oxylanosterol series leads us to conclude that demethylase inhibition and reductase suppression are not parallel in the same molecule. In addition, the oxylanosterols, but not the oxycholesterols, are able to disrupt coordinate regulation of HMG-CoA reductase from the LDL receptor. Thus, oxylanosterol treatment at levels which suppress reductase activity enhances LDL receptor activity. These results demonstrate that compounds can be made which (1) are selective reductase suppressors enabling dissection of the dual inhibitor nature of these compounds and (2) maximize reductase suppression and LDL receptor induction without demethylase inhibition which could lead to novel agents for serum cholesterol lowering.

Oxylanosterols as modifiers of cholesterol biosynthesis

In this review, the thinking and strategy that lead to the design of mechanism-based inhibitors of cholesterol biosynthesis have been recounted. This work began with a purely biochemical perspective on the mechanism of lanosterol demethylation. The final efforts focused on pharmacology and drug design thus bringing the basic science effort to a practical application. Most recently, a series of 15-oxalanosterols, which act as pure suppressors of HMG-CoA reductase lacking lanosterol demethylase inhibition properties has been identified. These molecules also lower serum cholesterol and show promise as potential agents for clinical evaluation. The utility of these compounds and validation of our hypothesis will have to await further testing.

P450 inhibitors of use in medical treatment: focus on mechanisms of action

A number of cytochrome P450s are targets for compounds that are clinically used or under clinical evaluation for treatment of patients with mycotic infections, such as dermatophytosis, superficial and systemic candidiasis, cryptococcosis and aspergillosis, with skin diseases, such as psoriasis or ichthyosis, and other retinoid-sensitive malignancies, e.g., neuro-ectodermal glioma. Some of the P450 inhibitors are candidates for the treatment of hirsutism or prostate cancer, others are potent inhibitors of the P450 isomerase involved in the synthesis of thromboxane A2, a potent platelet aggregation inducer and vasoconstrictor.

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.

Selective inhibition of mammalian lanosterol 14 alpha-demethylase by RS-21607 in vitro and in vivo

The discovery of selective lanosterol 14 alpha-demethylase inhibitors may lead to novel hypolipidemic drugs. RS-21607, (2S,4S)-cis-2[1H-imidazol-1-yl)methyl]-2-[2-(4-chlorophenyl)ethyl]-4- [[(4-aminophenyl)thio]methyl]-1,3-dioxolane, was characterized as a tight-binding, competitive inhibitor of lanosterol 14 alpha-demethylase purified from rat liver. The apparent Ki was determined to be 840 pM and found to be similar in hepatic microsomes from human, rat, and hamster. RS-21607, which contains two chiral centers, was a more effective lanosterol 14 alpha-demethylase inhibitor than its three stereoisomers. In vitro, RS-21607 had a greater affinity for lanosterol 14 alpha-demethylase than the other cytochromes P450 evaluated: CYP7, CYP27, CYP11A1, CYP19, CYP17, CYP11B1, CYP21, CYP3A4, CYP4A, CYP2D6, CYP1A2, CYP2C9, and 27-hydroxycholesterol 7 alpha-hydroxylase. The other stereoisomers were not as selective as RS-21607. Doses of 3-30 mg/kg RS-21607 given orally to hamsters caused a dose-dependent decrease in cholesterol biosynthesis with a corresponding accumulation of 24,25-dihydrolanosterol. RS-21607 inhibited the enzyme and cholesterol biosynthesis in hamster liver by 50% at 18 h following a 30 mg/kg oral dose. This was interpreted to indicate that RS-21607 is able to distribute to the site of action in hamsters and inhibit the target enzyme. In the same dose range, the plasma concentrations of testosterone, corticosterone, and progesterone, the endpoints for the cytochromes P450 involved in steroid biosynthesis, were relatively unaffected. These data show RS-21607 to be an effective and selective inhibitor of lanosterol 14 alpha-demethylase, both in vivo and in vitro. RS-21607 interacted with the purified enzyme to produce a type II binding spectrum, consistent with an interaction between the imidazole moiety and the heme. The electrostatic contribution of the imidazole binding was investigated using the desimidazole analog of RS-21607. The apparent Ki for the desimidazole compound (65 microM) was similar to the apparent Km for the substrate DHL (79 microM). Together, these data confirm that the ligand attached to the imidazole in RS-21607 is a good non-sterol substitute for DHL, i.e., binding to the enzyme with similar affinity, and that the coordination of the imidazole to the heme provides a major electrostatic contribution for the inhibition of lanosterol 14 alpha-demethylase by RS-21607. RS-21607 was also observed to increase the accumulation of 3 beta-hydroxy-24,25-dihydrolanost-8-en-32-al, the second intermediate in the multistep oxidation, but not the first intermediate. 24,25-dihydrolanost-8-ene-3 beta,32-diol.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural analysis of the interaction between the side-chain of substrates and the active site of lanosterol 14 alpha-demethylase (P-450(14)DM) of yeast

The role of the side-chain of lanosterol in the enzyme-substrate interaction of yeast P-450(14)DM (lanosterol 14 alpha-demethylase) was analyzed with lanosterol derivatives having functional groups on the side-chain. Purified P-450(14)DM from Saccharomyces cerevisiae catalyzed 14 alpha-demethylation of 26-hydroxylanosterol and 25-hydroxy-24,25-dihydrolanosterol with a lower activity than lanosterol and 24,25-dihydrolanosterol. This enzyme demethylated the (Z)-24-ethylidene-24,25-dihydrolanosterol with a low rate, but did not metabolize the E-isomer. The apparent Km of 26-hydroxylanosterol was 10.8 microM, which was higher than that of lanosterol, but lower than that of 24,25-dihydrolanosterol. On the other hand, competition experiments suggested that the affinity of 25-hydroxy-24,25-dihydrolanosterol and (Z)-24-ethylidene-24,25-dihydrolanosterol for P-450(14)DM was significantly lower than that of 24,25-dihydrolanosterol. Integration of the present results with the preceding ones (Aoyama, Y., Yoshida, Y., Sonoda, Y. and Sato, Y. (1991) Biochim. Biophys. Acta, 1081, 262-266 and Aoyama, Y. and Yoshida, Y. (1991) Biochem. Biophys. Res. Commun., 178, 1064-1071) suggests that yeast P-450(14)DM recognizes two parts of the side-chain, the structure around C-24 and the terminal fork consisting of C-25, C-26 and C-27.

The 4 beta-methyl group of substrate does not affect the activity of lanosterol 14 alpha-demethylase (P-450(14)DM) of yeast: difference between the substrate recognition by yeast and plant sterol 14 alpha-demethylases

Interaction of obtusifoliol and 24,28-dihydroobtusifoliol with yeast lanosterol 14 alpha-demethylase (P-450(14)DM) was studied to elucidate the role of the 4 beta-methyl group of substrate. P-450(14)DM of Saccharomyces cerevisiae catalyzed 14 alpha-demethylation of obtusifoliol. Apparent Vmax of obtusifoliol demethylation (15.4 nmol/min/nmol P-450) was similar to that of 24-methylene-24,25-dihydrolanosterol demethylation and was a little higher than those of lanosterol and 24,25-dihydrolanosterol demethylations. Apparent Km for obtusifoliol (12.0 microM) was higher than those for lanosterol and 24-methylene-24,25-dihydrolanosterol but was lower than that for 24,25-dihydrolanosterol. 24,28-Dihydroobtusifoliol was a very poor substrate for yeast P-450(14)DM. These facts suggest that the 4 beta-methyl group of sterol slightly affects the activity of yeast P-450(14)DM, while hydrogenation of a double bond in the sterol side-chain considerably impairs the activity. This finding is a contrast to the fact that the plant P-450(14)DM could not catalyze demethylation of sterols having 4 beta-methyl group, but favorably interacts with sterols having saturated side chain.

Different substrate specificities of lanosterol 14a-demethylase (P-45014DM) of Saccharomyces cerevisiae and rat liver for 24-methylene-24,25-dihydrolanosterol and 24,25-dihydrolanosterol

The purified lanosterol 14a-demethylase (P-45014DM) of S. cerevisiae catalyzed the 14a-demethylation of 24-methylene-24,25-dihydrolanosterol (24-methylenelanost-8-en-3 beta-ol, 24-methylene-DHL), the natural substrate of the demethylase of filamentous fungi, as well as its natural substrate, lanosterol. Lanosterol 14a-demethylase of rat liver microsomes also catalyzed the 14a-demethylation of 24-methylene-DHL, but the activity was considerably lower than that for lanosterol. The activity of the rat liver enzyme for 24-methylene-DHL was also lower than that for 24,25-dihydrolanosterol (DHL), while the activity of yeast P-45014DM for 24-methylene-DHL was considerably higher than that for DHL. Since 24-substituted sterols are not found in mammals and DHL is not an intermediate of ergosterol biosynthesis by yeast, above-mentioned different substrate specificities between the yeast and the mammalian 14a-demethylases may reflect certain evolutional alteration in their active sites in relation to the difference in their sterol biosynthetic pathways.

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.

Purification and characterization of cytochrome P-45014DM (lanosterol 14 alpha-demethylase) from pig liver microsomes

Cytochrome P-45014DM, which catalyzes lanosterol 14 alpha-demethylation, from pig liver microsomes was purified to a state of virtually homogeneous by gel electrophoresis. Its apparent monomeric molecular weight was estimated to be 53,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the amino-terminal amino acid sequence was Gly-Leu-Leu-Thr-Gly(Leu)-Asp-Leu-Leu-Gly-Ile. When reconstituted with NADPH-cytochrome P-450-reductase, the enzyme showed a high activity for lanosterol and 24,25-dihydrolanosterol 14 alpha-demethylation. Furthermore, the oxygenated intermediates of 24,25-dihydrolanosterol 14 alpha-demethylation, 32-hydroxy-24,25-dihydrolanosterol and 32-oxo-24,25-dihydrolanosterol, were converted to the 32-nor compound, 4,4-dimethylcholesta-8,14-dien-3 beta-ol, by the reconstituted enzyme system.

Identification of lanosterol 14 alpha-methyl demethylase in human tissues

Lanosterol 14 alpha-methyl demethylase was investigated in human tissues using a radio-HPLC assay to detect the 4,4-dimethyl-5 alpha-cholesta-8, 14-dien-3 beta-ol (diene) metabolite. The sequence of events leading to the demethylated product in human liver microsomes involves the conversion of the diol to the aldehyde followed by diene formation. Enzyme activity displayed a greater than 10 fold variation among the 9 liver samples studied. Kinetic parameters were determined and shown to differ between two separate liver samples. Addition of inhibitors of yeast lanosterol 14 alpha demethylase, ketoconazole and miconazole, resulted in extensive inhibition of formation of the demethylated metabolite. The enzyme, detected in microsomes isolated from human kidney and lymphocytes, also catalyzed the conversion of dihydrolanosterol to oxylanosterol intermediates and the diene. The presence of this enzyme in microsomes from various human tissues suggests that it may play a role in cellular regulation of cholesterol synthesis.

Role of the side chain of lanosterol in substrate recognition and catalytic activity of lanosterol 14 alpha-demethylase (cytochrome P-450 (14DM)) of yeast

The 14 alpha-demethylation of 24,25-dihydrolanosterol (DHL) derivatives having trimmed side chains, 27-nor-DHL, 26,27-dinor-DHL, 25,26,27-trinor-DHL, 24,25,26,27-tetranor-DHL, 23,24,25,26,27-pentanor-DHL and 22,23,24,25,26,27-hexanor-DHL, was studied with the reconstituted lanosterol 14 alpha-demethylase system consisting of cytochrome P-450(14DM) and NADPH-cytochrome P-450 reductase both purified from yeast microsomes. The demethylase catalyzed the 14 alpha-demethylation of the derivatives having the side chains longer than tetranor but the activities for the trinor- and tetranor-derivatives were lower. Kinetic analysis indicated that affinity of the trinor-derivative for the demethylase was considerably higher than that of DHL. The affinities of the 27-nor- and dinor-derivatives were increased by this order and were the intermediates of DHL and the trinor derivative. On the other hand, Vmax values of the demethylase for the DHL derivatives were decreased depending on their side-chain lengths, and the substrate-dependent reduction rate of cytochrome P-450(14DM) was also decreased in the same manner. Based on these observations, it was concluded that interaction of the side chain of lanosterol especially C-25, 26 and 27 with the substrate site of lanosterol 14 alpha-demethylase was necessary for enhancing the catalytic activity of the enzyme. However, this interaction was considered not to be essential for substrate binding.

Cytochrome P450 of fungi: primary target for azole antifungal agents

Cytochromes of fungi are essentially similar to those of animals. Cytochromes of fungi constitute two electron transport systems occurring in mitochondria and the endoplasmic reticulum. The former system, called the respiratory chain, contributes to cellular respiration and ATP generation, whereas the later system, named the microsomal electron transport system, is responsible for biosynthesis of several cellular components. The oxidative metabolism of lanosterol, that is included in the biosynthetic pathway of ergosterol, is one of the important functions of the microsomal electron transport system, which is catalyzed by P450(14DM). Many azole antifungal agents avidly combine with P450(14DM) and inhibit the oxidative removal of C-32 (the 14 alpha-demethylation) of lanosterol. This inhibition causes depletion of ergosterol and accumulation of 14-methylsterols in the membrane of fungal cells. Such change in sterol composition disturbs membrane function and results in growth inhibition and death of the fungal cells. Accordingly, P450(14DM) is considered as the primary target for azole antifungal agents. Cytochrome P450, which mediates the 14 alpha-demethylation of lanosterol, is also present in mammalian cells. Mammalian cells contain various species of cytochrome P450 which are responsible for many important cellular metabolic functions. If azole antifungal agents inhibit mammalian cytochrome P450 too, their systemic use may result in potentially significant adverse reactions. The high selectivity of azole antifungal agents for fungal P450(14DM) will be necessary for their systemic application. Binding ability of an azole antifungal agent to P450(14DM) is predominantly determined by the substituent at N-1 of the azole group, and the substituent must interact with the substrate site of the cytochrome. Extensive modification of the N-1 substituents and the screening of newly developed compounds with respect to the selectivity to fungal P450(14DM) with some conventional methods will be necessary. For this project, a biochemical understanding of cytochrome P450 and other cytochromes is important.

Primary structure of the P450 lanosterol demethylase gene from Saccharomyces cerevisiae

We have sequenced the structural gene and flanking regions for lanosterol 14 alpha-demethylase (14DM) from Saccharomyces cerevisiae. An open reading frame of 530 codons encodes a 60.7-kDa protein. When this gene is disrupted by integrative transformation, the resulting strain requires ergosterol and, as expected, grows only in the absence of oxygen. The deduced amino acid sequence of 14DM includes a hydrophobic segment near the amino terminus which may be a transmembrane domain. The deduced sequence has been compared with those of eight other eukaryotic P450s, each from a different family within the P450 superfamily. These comparisons indicate that this yeast gene is the first member of a new P450 family, P450LI. The P450, designated P450LIA1, is more closely related to mammalian P450s than to the bacterial P450cam. In fact, both the yeast P450 and several mammalian P450s have equivalent alignment scores when each is compared with the bovine P450scc. Matrix comparisons of the amino acid sequence of this P450 with those of mammalian P450s reveal three conserved regions. The DNA region 5' to the structural 14DM gene includes poly(dA:dT) sequences and a repeating hexamer sequence.