Cytochrome P450 Oxygenase Research Articles

ChemInform Abstract: Reactivity and Mechanisms of Metalloporphyrin-Catalyzed Oxidations

Effective systems for the catalytic oxygenation of hydrocarbons have been developed based on metalloporphyrins containing iron, manganese and ruthenium. These metalloporphyrin catalysts were inspired by studies of the mechanism of the heme oxygenase cytochrome P450 and related enzymes. Oxometalloporphyrins, which have now been spectroscopically characterized, have very high reactivity as oxygen transfer agents. A combination of experimental and computational methods has begun to provide an understanding of the structure and reactivity of these oxometalloporphyrin complexes and the mechanisms of oxygen transfer reactions they catalyze.

Candida albicans NADPH-P450 reductase: Expression, purification, and characterization of recombinant protein

Candida albicans is responsible for serious fungal infections in humans. Analysis of its genome identified NCP1 gene coding for a putative NADPH-P450 reductase (NPR) enzyme. This enzyme appears to supply reducing equivalents to cytochrome P450 or heme oxygenase enzymes for fungal survival and virulence. In this study, we report the characterization of the functional features of NADPH-P450 reductase from C. albicans. The recombinant C. albicans NPR protein harboring a 6×(His)-tag was expressed heterologously in Escherichia coli, and was purified. Purified C. albicans NPR has an absorption maximum at 453 nm, indicating the feature of an oxidized flavin cofactor, which was decreased by the addition of NADPH. It also evidenced NADPH-dependent cytochrome c or nitroblue tetrazolium reducing activity. This purified reductase protein was successfully able to substitute for purified mammalian NPR in the reconstitution of the human P450 1A2-catalyzed O-deethylation of 7-ethoxyresorufin. These results indicate that purified C. albicans NPR is an orthologous reductase protein that supports cytochrome P450 or heme oxygenase enzymes in C. albicans.

Direct Detection of the Oxygen Rebound Intermediates, Ferryl Mb and NO2, in the Reaction of metMyoglobin with Peroxynitrite

Oxygenated hemoproteins are known to react rapidly with nitric oxide (NO) to produce peroxynitrite (PN) at the heme site. This process could lead either to attenuation of the effects of NO or to nitrosative protein damage. Peroxynitrite is a powerful nitrating and oxidizing agent that has been implicated in a variety of cell injuries. Accordingly, it is important to delineate the nature and variety of reaction mechanisms of PN reactions with heme proteins. Here, we present direct evidence that ferrylMb and NO(2) are both produced during the reaction of PN and metmyoglobin (metMb). Kinetic evidence indicates that these products evolve from initial formation of a caged radical intermediate [Fe(IV)=O *NO(2)]. This caged pair reacts mainly via internal return with a rate constant k(r) to form metMb and nitrate in an oxygen rebound scenario. Detectable amounts of ferrylMb are observed by stopped-flow spectrophotometry, appearing at a rate consistent with the rate, k(obs), of heme-mediated PN decomposition. Freely diffusing NO(2), which is liberated concomitantly from the radical pair (k(e)), preferentially nitrates Tyr103 in horse heart myoglobin. The ratio of the rates of in-cage rebound and cage escape, k(r)/k(e), was found to be approximately 10 by examining the nitration yields of fluorescein, an external NO(2) trap. This rebound/escape model for the metMb/PN interaction is analogous to the behavior of alkyl hyponitrites and the well-studied geminate recombination processes of deoxymyoglobin with O(2), CO, and NO. The scenario is also similar to the stepwise events of substrate hydroxylation by cytochrome P450 and other oxygenases. It is likely, therefore, that the reaction of metMb with ONOO(-) and that of oxyMb with NO proceed through the same [Fe(IV)=O *NO(2)] caged radical intermediate and lead to similar outcomes. The results indicate that while oxyMb may reduce the concentration of intracellular NO, it would not eliminate the formation of NO(2) as a decomposition product of peroxynitrite.

NADPH oxidase-derived reactive oxygen species involved in angiotensin II-induced monocyte chemoattractant protein-1 expression in mesangial cells

To investigate the origin of oxidative stress induced by angiotensin II (AngII) in human mesangial cells and the role of reactive oxygen species (ROS) in AngII-induced monocyte chemoattractant protein-1 (MCP-1) expression. MCP-1 expression was determined by real time RT-PCR. ROS production was measured by DCFDA fluorescence. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity was examined by lucigenin chemiluminescence. p47phox and p67phox translocation was assayed by Western blot. Twenty-four male mice were randomly divided into three groups: the control, the AngIIinfusion [AngII 400 ng/(kg.min)], and the apocynin treatment. AngII was infused by subcutaneously osmotic minipump for 14 days. Urinary albumin and 8-isoprostane excretion were measured by ELISA. In cultured human mesangial cells, AngII induced the MCP-1 expression in a dose-dependent manner with 3.56 fold increase as compared with the control. AngII increased intracellular ROS production as early as 3 min with the peak at 60 min and was in a time and dose-dependent. Incubation with different dosages of AngII (1 nmol/L, 10 nmol/L, and 100 nmol/L AngII) for 60 min, ROS production increased at 1.82, 2.92, and 4.08 folds respectively. AngII-induced ROS generation was sensitive to diphenyleneiodonium sulfate (DPI, 10 micromol/L) and apocynin (500 micromol/L), two structurally distinct NADPH oxidase inhibitors. In contrast, inhibitors of other oxidant-producing enzymes, including the mitochondrial complex Iinhibitor rotenone, the xanthine oxidase inhibitor allopurinol, the cyclooxygenase inhibitor indomethacin, the lipoxygenase inhibitor nordihydroguiaretic acid, the cytochrome P450 oxygenase inhibitor ketoconazole and the nitric oxide synthase inhibitor G-nitro-L-arginine methyl ester were without an effect. AngII-induced ROS generation was inhibited by the AT1 antagonist losartan (10 micromol/L) but not the AT2 antagonist PD123319 (10 micromol/L). AngII treatment induced translocation of cytosolic of p47phox and p67phox to the membrane. The antioxidants almost abolished AngII-induced MCP-1 expression. AngII infusion increased urinary and p67 translocation by 2.69-, 2.97-, and 2.67-fold, respectively. NADPH oxidase-derived ROS is involved in AngII-induced MCP-1 expression. Inhibition of NADPH oxidase alleviates AngII-induced renal injury.

CYP719A subfamily of cytochrome P450 oxygenases and isoquinoline alkaloid biosynthesis in Eschscholzia californica

Eschscholzia californica produces various types of isoquinoline alkaloids. The structural diversity of these chemicals is often due to cytochrome P450 (P450) activities. Members of the CYP719A subfamily, which are found only in isoquinoline alkaloid-producing plant species, catalyze methylenedioxy bridge-forming reactions. In this study, we isolated four kinds of CYP719A genes from E. californica to characterize their functions. These four cDNAs encoded amino acid sequences that were highly homologous to Coptis japonica CYP719A1 and E. californica CYP719A2 and CYP719A3, which suggested that these gene products may be involved in isoquinoline alkaloid biosynthesis in E. californica, especially in methylenedioxy bridge-forming reactions. Expression analysis of these genes showed that two genes (CYP719A9 and CYP719A11) were preferentially expressed in plant leaf, where pavine-type alkaloids accumulate, whereas the other two showed higher expression in root than in other tissues. They were suggested to have distinct physiological functions in isoquinoline alkaloid biosynthesis. Enzyme assay analysis using recombinant proteins expressed in yeast showed that CYP719A5 had cheilanthifoline synthase activity, which was expected based on the similarity of its primary structure to that of Argemone mexicana cheilanthifoline synthase (deposited at DDBJ/GenBanktrade mark/EMBL). In addition, enzyme assay analysis of recombinant CYP719A9 suggested that it has methylenedioxy bridge-forming activity toward (R,S)-reticuline. CYP719A9 might be involved in the biosynthesis of pavine- and/or simple benzylisoquinoline-type alkaloids, which have a methylenedioxy bridge in an isoquinoline ring, in E. californica leaf.

Decolorization of Victoria blue by the white rot fungus, Phanerochaete chrysosporium

Synthetic textile dyes are among the most dangerous chemical pollutants released in industrial wastewater streams. Recognizing the importance of reducing the environmental impact of these dyes, the ability of the white rot fungus Phanerochaete chrysosporium to decolorize various textile dyes was investigated. This fungus decolorized 6 of the 14 structurally diverse dyes with varying efficiency (between 14% and 52%). There was no discernable pattern of decolorization even among dyes of the same chemical class, suggesting that attack on the dyes is relatively non-specific. Among the three dyes which showed >40% decolorization, Victoria Blue B (VB) was chosen for further analysis because the ability of the fungus to decolorize VB was nearly independent over a relatively broad concentration range. Blocking lignin peroxidase (LiP) and manganese peroxidase (MnP) production by the fungus did not substantially affect VB decolorization. Inhibition of laccase production by adding various inhibitors to shaken cultures reduced VB decolorization significantly suggesting a role for laccase in VB decolorization. When sodium azide and aminotriazole were used to inhibit endogenous catalase and cytochrome P-450 oxygenase activities, there was 100% and 70% reduction in VB decolorization, respectively. Adding benzoate to trap hydrogen peroxide-derived hydroxyl radicals resulted in 50% decolorization of VB. Boiling the extracellular fluid (ECF) for 30 min resulted in approximately 50% reduction in VB decolorization. Collectively, these data suggest that laccase, and/or oxygenase/oxidase and a heat-stable non-enzymatic factor, but not Lip and MnP, play a role in VB decolorization by P. chrysosporium.

CYP725A4 from Yew Catalyzes Complex Structural Rearrangement of Taxa-4(5),11(12)-diene into the Cyclic Ether 5(12)-Oxa-3(11)-cyclotaxane

Taxa-4(5),11(12)-diene is the first committed precursor of functionalized taxanes such as paclitaxel, a successful anticancer drug. Biosynthesis of taxanes in yew involves several oxidations, a number of which have been shown to be catalyzed by cytochrome P-450 oxygenases. Hydroxylation of the C-5alpha of taxa-4(5),11(12)-diene is believed to be the first of these oxidations, and a gene encoding a taxa-4(5),11(12)-diene 5alpha-hydroxylase (CYP725A4) was recently described (Jennewein, S., Long, R. M., Williams, R. M., and Croteau, R. (2004) Chem. Biol. 11, 379-387). In an attempt to produce the early components of the paclitaxel pathway by a metabolic engineering approach, cDNAs encoding taxa-4(5),11(12)-diene synthase and CYP725A4 were introduced in Nicotiana sylvestris for specific expression in trichome cells. Their co-expression did not lead to the production of the expected 5alpha-hydroxytaxa-4(20),11(12)-diene. Instead, taxa-4(5),11(12)-diene was quantitatively converted to a novel taxane that was purified and characterized. Its structure was determined by NMR analysis and found to be that of 5(12)-oxa-3(11)-cyclotaxane (OCT) in which the eight-carbon B-ring from taxa-4(5),11(12)-diene is divided into two fused five-carbon rings. In addition, OCT contains an ether bridge linking C-5 and C-12 from opposite sides of the molecule. OCT was also the sole major product obtained after incubation of taxa-4(5),11(12)-diene with NADPH and microsomes prepared from recombinant yeast expressing CYP725A4. The rearrangement of the taxa-4(5),11(12)-diene ring system is thus mediated by CYP725A4 only and does not rely on additional enzymes or factors present in the plant. The complex structure of OCT led us to propose a reaction mechanism involving a sequence of events so far unknown in P-450 catalysis.

ANG II induces c-Jun NH2-terminal kinase activation and proliferation of human mesangial cells via redox-sensitive transactivation of the EGFR

We previously showed that ANG II induces mesangial cell (MC) proliferation via the JNK-activator protein-1 pathway. The present study attempted to determine the upstream mediators of JNK activation, with emphasis on reactive oxygen species (ROS) and the epidermal growth factor (EGF) receptor (EGFR). In cultured human MCs (HMCs), as early as 3 min, ANG II time dependently increased intracellular ROS production, which was sensitive to 10 microM diphenyleneiodonium sulfate and 500 microM apocynin, two structurally distinct NADPH oxidase inhibitors. In contrast, inhibitors of other oxidant-producing enzymes, including the mitochondrial complex I inhibitor rotenone, the xanthine oxidase inhibitor allopurinol, the cyclooxygenase inhibitor indomethacin, the lipoxygenase inhibitor nordihydroguiaretic acid, the cytochrome P-450 oxygenase inhibitor ketoconazole, and the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester, were without effect. ANG II-induced ROS generation was inhibited by the angiotensin type 1 receptor antagonist losartan (10 muM) but not the angiotensin type 2 receptor antagonist PD-123319 (10 microM). ANG II induced translocation of p47(phox) and p67(phox) from the cytosol to the membrane. The antioxidants almost abolished the ANG II mitogenic response, as assessed by [(3)H]thymidine incorporation and cell number, associated with a remarkable blockade of the activation of EGFR (90% inhibition) and JNK (83% inhibition). The EGFR inhibitor AG-1478 was able to mimic the effect of antioxidants, in that it inhibited the mitogenic response and the JNK activation following ANG II treatment. Together, these data suggest that the ROS-EGFR-JNK pathway is involved in transducing the proliferative effect of ANG II in cultured HMCs.

A medium‐throughput screening assay to determine catalytic activities of oxygen‐consuming enzymes: a new tool for functional characterization of cytochrome P450 and other oxygenases

A challenge of the post-genomic era is to determine the functions of a plethora of orphan genes. This is a more acute problem when dealing with large gene families, such as the superfamily encoding cytochrome P450 enzymes in higher plants. We propose here a new, simple, medium-throughput methodology to screen for potential substrates of orphan P450 mono-oxygenases. The same technique can also be applied to screening for inhibitors of the oxygenases involved in the biosynthesis of compounds essential for plant development, such as growth regulators. The method is based on a commercially available microplate system, which detects the oxygen consumed by the catalytic reaction via an oxygen-sensing fluorophore. It is optimized using as a model CYP73A1, the cinnamic acid hydroxylase from Helianthus tuberosus, expressed in yeast. We show that the procedure is suitable not only for the detection and real-time monitoring, but also for the quantitative evaluation of enzyme activity. This new method has broad application for the identification of candidate substrates and inhibitors in chemical libraries, to support determination of physiological substrates, development of plant growth regulators, investigations on herbicide and pollutant metabolism, synthesis of valuable compounds and drug design. It also provides a fast-assay platform for determination of catalytic and inhibition parameters. The method applies to plant P450 enzymes, but also to cytochromes P450 from other organisms, and all types of oxygenases. The critical steps, calculation of oxygen consumption from fluorescence signal, and limits of the methods are discussed.

Cytochrome P450 oxygenases of monoterpene metabolism

The cytochrome P450 monoterpene oxygenases are largely responsible for imparting structural and functional diversity to this family of natural products. In most cases, cytochrome P450-mediated allylic hydroxylation of a parental monoterpene olefin leads to a series of redox transformations and conjugation reactions which yield a family of structurally related derivatives and isomers. An overview is provided of the extant monoterpene oxygenases, with examples mainly from the mint (Lamiaceae) family of essential oil plants, and, where possible, information on the structure, mechanism, localization and regulation of these enzymes is described. The review concludes with a brief assessment of biotechnological applications and a view to future research in this area.

Angiotensin II-stimulated collagen production in cardiac fibroblasts is mediated by reactive oxygen species

The aim of the present study was to determine whether inhibition of reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] oxidase and of various superoxide generating systems could affect the collagen production, the mRNA and protein expression of collagen types I and III in control and angiotensin II-treated cardiac fibroblasts. Cardiac fibroblasts from passage 2 from normal male adult rats were cultured to confluency and incubated in serum-free Dulbecco's modified Eagle's medium for 24 h. The cells were then preincubated with(out) the tested inhibitors for 1 h and then further incubated with(out) angiotensin II (1 micromol/l) for 24 h. Collagen production was measured spectrophotometrically with picrosirius red as dye and with [3H]proline incorporation; collagen type I and III content by enzyme-linked immunosorbent assay and collagen type I and III mRNA expression by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). NAD(P)H-dependent superoxide anion production was assayed as superoxide dismutase-inhibitable cytochrome c reduction. Intracellular formation of reactive oxygen species was assessed with 2',7'-dichlorofluorescein diacetate as fluorescent probe. Angiotensin II stimulated the collagen production, the collagen I and III content and mRNA expression in cardiac fibroblasts, and apocynin, a membrane NAD(P)H oxidase inhibitor, abolished this induction. Rotenone, allopurinol, indomethacin, nordihydroguiaretic acid, ketoconazole and nitro-L-arginine (inhibitors of mitochondrial NAD(P)H oxidase, xanthine oxidase, cyclooxygenase, lipoxygenase, cytochrome P450 oxygenase and nitric oxide synthase, respectively) did not affect the angiotensin II-induced collagen production. Angiotensin II increased the NAD(P)H-dependent superoxide anion production and the intracellular generation of reactive oxygen species in cardiac fibroblasts, and apocynin abrogated this rise. Our data show that in adult rat cardiac fibroblasts the membrane-associated NAD(P)H oxidase complex is the predominant source of superoxide anion and reactive oxygen species generation in angiotensin II-stimulated adult cardiac fibroblasts. Inhibition of this NAD(P)H oxidase complex with apocynin completely blocked the angiotensin II-stimulated collagen production, and collagen I and III protein and mRNA expression.

Structure and Reactivity of a Thermostable Prokaryotic Nitric-oxide Synthase That Forms a Long-lived Oxy-Heme Complex

In an effort to generate more stable reaction intermediates involved in substrate oxidation by nitric-oxide synthases (NOSs), we have cloned, expressed, and characterized a thermostable NOS homolog from the thermophilic bacterium Geobacillus stearothermophilus (gsNOS). As expected, gsNOS forms nitric oxide (NO) from l-arginine via the stable intermediate N-hydroxy l-arginine (NOHA). The addition of oxygen to ferrous gsNOS results in long-lived heme-oxy complexes in the presence (Soret peak 427 nm) and absence (Soret peak 413 nm) of substrates l-arginine and NOHA. The substrate-induced red shift correlates with hydrogen bonding between substrate and heme-bound oxygen resulting in conversion to a ferric heme-superoxy species. In single turnover experiments with NOHA, NO forms only in the presence of H(4)B. The crystal structure of gsNOS at 3.2 AA of resolution reveals great similarity to other known bacterial NOS structures, with the exception of differences in the distal heme pocket, close to the oxygen binding site. In particular, a Lys-356 (Bacillus subtilis NOS) to Arg-365 (gsNOS) substitution alters the conformation of a conserved Asp carboxylate, resulting in movement of an Ile residue toward the heme. Thus, a more constrained heme pocket may slow ligand dissociation and increase the lifetime of heme-bound oxygen to seconds at 4 degrees C. Similarly, the ferric-heme NO complex is also stabilized in gsNOS. The slow kinetics of gsNOS offer promise for studying downstream intermediates involved in substrate oxidation.

Quantitative Deuterium Isotopic Profiling at Natural Abundance Indicates Mechanistic Differences for Δ12-Epoxidase and Δ12-Desaturase in Vernonia galamensis

Quantitative (2)H NMR spectroscopy can determine the natural abundance ((2)H/(1)H) ratio at each site of a molecule. In natural products, variation in these values is related to the reaction mechanisms in the pertinent biosynthetic pathway. For the first time, this novel approach has been exploited to probe for mechanistic differences in the introduction of different functionalities into a long-chain fatty acid. Vernolic acid, a major component of the seed oil of Vernonia galamensis, contains both an epoxide and a desaturation. The site-specific isotopic distribution ((2)H/(1)H)(i) has been determined for both vernolic acid and linoleic acid isolated from the same V. galamensis oil. It is found that the ((2)H/(1)H) ratio of vernolic acid shows a pattern along the entire length of the chain, consistent with linoleic acid being its immediate precursor. Notably, the C13 relates to the C13 of linoleic acid but not to the C13 of oleic acid. Furthermore, the C12 and C13 positions in vernolic acid are less depleted, consistent with a change in hybridization state from sp(2) to sp(3). However, the C11 position shows a marked relative enrichment in the vernolic acid, implying that it plays a role in the epoxidase but not the desaturase mechanism. Thus, although it can be concluded that the catalytic mechanisms for the epoxidase and desaturase activities are similar, marked differences in the residual ((2)H/(1)H) patterns indicate that the reaction mechanisms are not identical.

Coexpression in yeast ofTaxuscytochrome P450 reductase with cytochrome P450 oxygenases involved in Taxol biosynthesis

To maximize redox coupling efficiency with recombinant cytochrome P450 hydroxylases from yew (Taxus) species installed in yeast for the production of the anticancer drug Taxol, a cDNA encoding NADPH:cytochrome P450 reductase from T. cuspidata was isolated. This single-copy gene (2,154 bp encoding a protein of 717 amino acids) resembles more closely other reductases from gymnosperms (approximately 90% similarity) than those from angiosperms (<80% similarity). The recombinant reductase was characterized and compared to other reductases by heterologous expression in insect cells and was shown to support reconstituted taxoid 10beta-hydroxylase activity with an efficiency comparable to that of other plant-derived reductases. Coexpression in yeast of the reductase along with T. cuspidata taxoid 10beta-hydroxylase, which catalyzes an early step of taxoid biosynthesis, demonstrated significant enhancement of hydroxylase activity compared to that supported by the endogenous yeast reductase alone. Functional transgenic coupling of the Taxus reductase with a homologous cytochrome P450 taxoid hydroxylase represents an important initial step in reconstructing Taxol biosynthesis in a microbial host.

Structural and functional diversity in heme monooxygenases.

Recent advances in understanding structure-function relationships in cytochrome P450 (P450), nitric-oxide synthase (NOS), and heme oxygenase are summarized. Of particular importance is the role that dynamics plays in P450 function, where the active site undergoes large open/close motions to enable substrates to bind and products to leave. In sharp contrast, the heme-containing active site of NOS is rigid and remains relatively exposed compared with P450s. This difference in dynamics and active site exposure requires that the O(2) activation machinery operate somewhat differently in P450 and NOS. Owing to the open NOS active site, the NOS-oxy complex could be subject to nonspecific protonation that short-circuits the normal reaction path. One working hypothesis holds that NOS recruited the cofactor, tetrahydrobiopterin, to bind near the heme for very rapid coupled electron/proton transfer to the oxy complex, which avoids indiscriminate reaction with bulk solvent. Despite these differences, P450, NOS, and also heme oxygenase use a very similar network of H-bonded water molecules in the active site that are required for oxygen activation. Both P450 and NOS are important drug targets. With NOS, the structural basis for isoform-selective inhibition by a class of dipeptide inhibitors has been worked out, thus providing the basis for structure-based drug design.

Molecular cloning and characterization of a cytochrome P450 taxoid 2α-hydroxylase involved in Taxol biosynthesis

The Taxol biosynthetic pathway, arising from the primary isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate in yew ( Taxus), consists of approximately twenty steps, at least nine of which are thought to be cytochrome P450-mediated oxygenations. Several oxygenases involved in the early hydroxylation steps of the pathway have been identified and the corresponding genes have been cloned; however, defining the enzymes and their genes responsible for oxygenations in the central portion of the pathway is more difficult because neither the exact sequence of reactions nor the relevant intermediates are known. A surrogate substrate, (+)-taxusin (taxa-4(20),11(12)-dien-5α,9α,10β,13α-tetraol tetraacetate), that was previously employed in the isolation of a taxoid 7β-hydroxylase, was used here to functionally screen a family of cytochrome P450 oxygenases originating from a Taxus cell EST library. This in vivo screen in yeast led to the identification of a 1488 bp cDNA clone (encoding a 495 residue protein) that was capable of producing 2α-hydroxytaxusin from taxusin with a K M value of 10.5 ± 2.7 μM and k cat of about 0.05 s −1 for the surrogate substrate. This structurally typical cytochrome P450 resembles most closely the previously isolated taxoid 7β-hydroxylase, which also uses taxusin as a substrate, and both 2α- and 7β-hydroxylases are capable of the reciprocal conversion of their respective pentaol tetraacetate products to the common hexaol tetraacetate. This C2-hydroxylase would appear to mediate the mid-pathway functionalization of the C2-position of the taxane core that ultimately bears a benzoyl group as an important Taxol pharmacophore. Overexpression of this cytochrome P450 taxoid 2α-hydroxylase in Taxus cells may improve Taxol yields and could prove useful in the production of other 2α-hydroxy taxoids as starting materials for subsequent acylation at this position.

Cytochrome p450 taxadiene 5alpha-hydroxylase, a mechanistically unusual monooxygenase catalyzing the first oxygenation step of taxol biosynthesis.

The first oxygenation step in the biosynthesis of the anticancer drug taxol in Taxus species is the cytochrome p450-mediated hydroxylation (with double bond migration) of the diterpene olefin precursor taxa-4(5),11(12)-diene to taxa-4(20),11(12)-dien-5alpha-ol. A homology-based cloning strategy, employing an induced Taxus cell library, yielded a cDNA encoding taxadiene 5alpha-hydroxylase, which was functionally expressed in yeast and insect cells. The recombinant enzyme was characterized and shown to efficiently utilize both taxa-4(5),11(12)-diene and taxa-4(20),11(12)-diene (as an adventitious substrate) to synthesize taxa-4(20),11(12)-dien-5alpha-ol. This hydroxylase resembles, in sequence and properties, other cytochrome p450 oxygenases of taxol biosynthesis. The utilization of both taxadiene isomers in the formation of taxa-4(20),11(12)-dien-5alpha-ol is novel, suggesting a reaction mechanism involving promiscuous radical abstraction with selective oxygen insertion rather than epoxidation of the C4,C5-alkene of the natural substrate and allylic rearrangement of the resulting taxa-11(12)-en-4,5epoxide.

A novel IS element, IS Mpa1, in Mycobacterium avium subsp. paratuberculosis

A novel insertion element belonging to the IS 110 family was identified in Mycobacterium avium subsp. paratuberculosis. The IS element, IS Mpa1, is 1500 bp and has one ORF encoding a putative transposase. Three copies of IS Mpa1 were identified in the M. avium subsp. paratuberculosis genome. The element had inserted into the 3′ end of the highly conserved mycobacterial genes prrB and a homologue of M. tuberculosis Rv 1593c, and between a putative cytochrome p450 oxygenase and a putative hydrolase. The IS element was present in all ( n=11) M. avium subsp. paratuberculosis strains but not detected in most other mycobacterial species examined, including 10 M. avium subsp. avium isolates of human, avian and porcine origin. However two porcine isolates of M. avium subsp. avium and the reference strain IWGMT49 did harbour IS Mpa1. These three strains belong to a previously described subgroup of M. avium subsp. avium based on IS 1245 restriction fragment length polymorphism (RFLP) pattern and serovars. All of the M. avium subsp. paratuberculosis strains examined had an identical RFLP pattern when probed with sequences corresponding to the 5′ end of IS Mpa1, whereas a different pattern was seen in the positive M. avium subsp. avium strains. This novel IS element might be a useful tool in strain classification of M. avium subsp. avium and also for the identification of M. avium subsp. paratuberculosis when used in combination with IS 900.

Liver-specific Deletion of the NADPH-Cytochrome P450 Reductase Gene: IMPACT ON PLASMA CHOLESTEROL HOMEOSTASIS AND THE FUNCTION AND REGULATION OF MICROSOMAL CYTOCHROME P450 AND HEME OXYGENASE

A mouse model with liver-specific deletion of the NADPH-cytochrome P450 reductase (Cpr) gene (designated Alb-Cre/Cprlox mice) was generated and characterized in this study. Hepatic microsomal CPR expression was significantly reduced at 3 weeks and was barely detectable at 2 months of age in the Alb-Cre+/-/Cprlox+/+ (homozygous) mice, with corresponding decreases in liver microsomal cytochrome P450 (CYP) and heme oxygenase (HO) activities, in pentobarbital clearance, and in total plasma cholesterol level. Nevertheless, the homozygous mice are fertile and are normal in gross appearance and growth rate. However, at 2 months, although not at 3 weeks, the homozygotes had significant increases in liver weight, accompanied by hepatic lipidosis and other pathologic changes. Intriguingly, total microsomal CYP content was increased in the homozygotes about 2-fold at 3 weeks and about 3-fold at 2 months of age; at 2 months, there were varying degrees of induction in protein (1-5-fold) and mRNA expression (0-67-fold) for all CYPs examined. There was also an induction of HO-1 protein (nearly 9-fold) but no induction of HO-2. These data indicate the absence of significant alternative redox partners for liver microsomal CYP and HO, provide in vivo evidence for the significance of hepatic CPR-dependent enzymes in cholesterol homeostasis and systemic drug clearance, and reveal novel regulatory pathways of CYP expression associated with altered cellular homeostasis. The Alb-Cre/Cprlox mouse represents a unique model for studying the in vivo function of hepatic HO and microsomal CYP-dependent pathways in the biotransformation of endogenous and xenobiotic compounds.

Calcium dobesilate potentiates endothelium-derived hyperpolarizing factor-mediated relaxation of human penile resistance arteries.

1 We have evaluated the participation of endothelium-derived hyperpolarizing factor (EDHF) in the endothelium-dependent relaxation of isolated human penile resistance arteries (HPRA) and human corpus cavernosum (HCC) strips. In addition, the effect of the angioprotective agent, calcium dobesilate (DOBE), on the endothelium-dependent relaxation of these tissues was investigated. 2 Combined inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX) nearly abolished the endothelium-dependent relaxation to acetylcholine (ACh) in HCC, while 60% relaxation of HPRA was observed under these conditions. Endothelium-dependent relaxation of HPRA resistant to NOS and COX inhibition was prevented by raising the extracellular concentration of K(+) (35 mM) or by blocking Ca(2)(+)-activated K(+) channels, with apamin (APA; 100 nM) and charybdotoxin (CTX; 100 nM), suggesting the involvement of EDHF in these responses. 3 Endothelium-dependent relaxation to ACh was markedly enhanced by DOBE (10 micro M) in HPRA but not in HCC. The potentiating effects of DOBE on ACh-induced responses in HPRA, remained after NOS and COX inhibition, were reduced by inhibition of cytochrome P450 oxygenase with miconazole (0.3 mM) and were abolished by high K(+) or a combination of APA and CTX. 4 In vivo, DOBE (10 mg kg(-1) i.v.) significantly potentiated the erectile responses to cavernosal nerve stimulation in male rats. 5 EDHF plays an important role in the endothelium-dependent relaxation of HPRA but not in HCC. DOBE significantly improves endothelium-dependent relaxation of HPRA mediated by EDHF and potentiates erectile responses in vivo. Thus, EDHF becomes a new therapeutic target for the treatment of erectile dysfunction (ED) and DOBE could be considered a candidate for oral therapy for ED.