Characterization of a three-component vanillate O-demethylase from Moorella thermoacetica.
(9/56)
The Moorella thermoacetica aromatic O-demethylase was characterized as an inducible three-component system with similarity to the methanogenic methanol, methylamine, and methanethiol methyltransferases and to the O-demethylase system from Acetobacterium dehalogenans. MtvB catalyzes methyl transfer from a phenylmethylether to the cobalt center of MtvC, a corrinoid protein. MtvA catalyzes transmethylation from MtvC to tetrahydrofolate, forming methyltetrahydrofolate. Cobalamin can substitute for MtvC. (+info)
Characterization of dextromethorphan O- and N-demethylation catalyzed by highly purified recombinant human CYP2D6.
(10/56)
The O-demethylation of dextromethorphan to dextrorphan in humans is catalyzed primarily by cytochrome P450 2D6 (CYP2D6). However, contrary to conventional wisdom, preparations of recombinant cytochrome P450 (P450) expressed from CYP2D6*1 cDNA also appear to produce significant amounts of 3-methoxymorphinan, the N-demethylated metabolite of dextromethorphan, when assayed in vitro. We hypothesized that both pathways were intrinsic to 2D6 and here further examine the kinetics of formation using a highly purified preparation of CYP2D6 in a reconstituted lipid system. Purified CYP2D6 protein with a measured molecular weight of 55772.0 (55769.6 Da predicted) was reconstituted into an active, lipid-vesicle environment with purified rat cytochrome P450 reductase before the addition of substrate and NADPH. Reaction kinetics were followed, and apparent Michaelis-Menten constants were determined for the appearance of each metabolite by high-pressure liquid chromatography, using both UV and fluorescence detection. In a 2-min assay, purified 2D6 catalyzed the formation of dextrorphan with an apparent K(m) value of 1.9 +/- 0.2 microM and a V(max) value of 8.5 +/- 0.2 nmol/nmol of P450/min and measured simultaneously the formation of 3-methoxymorphinan with an apparent K(m) value of 5000 +/- 700 microM and V(max) value of 176 +/- 12 nmol (nmol of P450)(-1) min(-1). These results indicate that at least two distinct binding orientations exist for dextromethorphan within the active site of CYP2D6. (+info)
Pisatin demethylase genes are on dispensable chromosomes while genes for pathogenicity on carrot and ripe tomato are on other chromosomes in Nectria haematococca.
(11/56)
Studies on the wide-host-range fungus Nectria haematococca MP VI have shown a linkage between virulence on pea and five of nine PDA genes that encode the ability to detoxify the pea phytoalexin, pisatin. Most of the PDA genes are on chromosomes of approximately 1.6 megabases (Mb) and two of these genes, PDA1-2 and PDA6-1, have been demonstrated to reside on approximately 1.6-Mb chromosomes that can be lost during meiosis. Prior studies also have shown that the dispensable chromosome carrying PDA6-1 contains a gene (MAK1) necessary for maximum virulence on chickpea. The present study evaluated whether the other approximately 1.6-Mb chromosomes that carry PDA genes also are dispensable, their relationship to each other, and whether they contain genes for pathogenicity on hosts other than pea or chickpea. DNA from the PDA1-1 chromosome (associated with virulence on pea) and the PDA6-1 chromosome (associated with virulence on chickpea) were used to probe blots of contour-clamped homogeneous electric field (CHEF) gels of isolates carrying different PDA genes and genetically related Pda- isolates. All of the approximately 1.6-Mb PDA-bearing chromosomes hybridized with both probes, indicating that they share significant similarity. Genetically related Pda-progeny lacked chromosomes of approximately 1.6 Mb and there was no significant hybridization of any chromosomes to the PDA1-1 and PDA6-1 chromosome probes. When isolates carrying different PDA genes and related Pda- isolates were tested for virulence on carrot and ripe tomato, there was no significant difference in lesion sizes produced by Pda+ and Pda- isolates, indicating that genes for pathogenicity on these hosts are not on the PDA-containing chromosomes. These results support the hypothesis that the chromosomes carrying PDA genes are dispensable and carry host-specific virulence genes while genes for pathogenicity on other hosts are carried on other chromosomes. (+info)
O-demethylation and sulfation of 7-methoxylated flavanones by Cunninghamella elegans.
(12/56)
Metabolism of 7-O-methylnaringenin (sakuranetin) by Cunninghamella elegans NRRL 1392 yielded naringenin and naringenin-4'-sulfate. C. elegans also converted 5, 3', 4'-trihydroxy-7-methoxyflavanone into eriodictyol-4'-sulfate. Furthermore, incubation of 5, 4'-dihydroxy-7, 3'-dimethoxyflavanone with the same fungus gave homoeriodictyol (5, 7, 4'-trihydroxy-3'-methoxyflavanone) and homoeriodicytol-7-sulfate. The structures of the new metabolites were established by spectral analysis including 2D-NMR, HR-ESI-FT-MS beside hydrolysis by acid. (+info)
Identification of CYP1A2 as the main isoform for the phase I hydroxylated metabolism of genistein and a prodrug converting enzyme of methylated isoflavones.
(13/56)
This study determined the cytochrome P450 (P450) isoforms responsible for metabolism of isoflavones using human liver microsomes (HLM) and expressed P450s. The primary metabolite of genistein is 3'-OH-genistein, as identified with an authentic chemically synthesized standard. CYP1A2 was predominantly responsible for 3'-OH-genistein formation since its formation was inhibited (>50%, p < 0.05) by a monoclonal antibody specific for CYP1A2, was correlated with CYP1A2 activities of HLM, and was catalyzed by expressed CYP1A2. In addition to CYP1A2, CYP2E1 also catalyzed, although to a lesser extent, its formation. The contribution of these P450s to the formation of 3'-OH-genistein was also confirmed with a panel of expressed enzymes. Methylated isoflavones biochanin A, prunetin, and formononetin (10-100 microM) were rapidly converted by HLM and expressed CYP1A2 to more active genistein and daidzein. The conversion of biochanin A to genistein appears to be mainly mediated by CYP1A2 because of the strong correlation between the conversion rates and CYP1A2 activities in HLM. Thus, CYP1A2 is an effective prodrug-converting enzyme for less active methylated isoflavones. CYP1A2-catalyzed conversion of biochanin A to genistein (Km, 7.80 microM; Vmax, 903 pmol/min/mg of protein; Vmax/Km, 116 microl/min/mg of protein) was much faster than 3'-hydroxylation of genistein (Km, 12.7 microM and Vmax, 109 pmol/min/mg of protein; Vmax/Km, 8.6 microl/min/mg of protein). The interaction studies showed that genistein inhibited formation of acetaminophen from phenacetin with an IC50 value of 16 microM. Additional studies showed that phenacetin and genistein were mutually inhibitory. In conclusion, CYP1A2 and CYP2E1 metabolized genistein and CYP1A2 acted as prodrug-converting enzymes for other less active methylated isoflavones. (+info)
Demethylation of radiolabelled dextromethorphan in rat microsomes and intact hepatocytes.
(14/56)
Liver microsomal preparations are routinely used to predict drug interactions that can occur in vivo as a result of inhibition of cytochrome P450 (CYP)-mediated metabolism. However, the concentration of free drug (substrate and inhibitor) at its intrahepatic site of action, a variable that cannot be directly measured, may be significantly different from that in microsomal incubation systems. Intact cells more closely reflect the environment to which CYP substrates and inhibitors are exposed in the liver, and it may therefore be desirable to assess the potential of a drug to cause CYP inhibition in isolated hepatocytes. The objective of this study was to compare the inhibitory potencies of a series of CYP2D inhibitors in rat liver microsomes and hepatocytes. For this, we developed an assay suitable for rapid analysis of CYP-mediated drug interactions in both systems, using radiolabelled dextromethorphan, a well-characterized probe substrate for enzymes of the CYP2D family. Dextromethorphan demethylation exhibited saturable kinetics in rat microsomes and hepatocytes, with apparent Km and Vmax values of 2.1 vs. 2.8 microM and 0.74 nM x min(-1) per mg microsomal protein vs. 0.11 nM x min(-1) per mg cellular protein, respectively. Quinine, quinidine, pyrilamine, propafenone, verapamil, ketoconazole and terfenadine inhibited dextromethorphan O-demethylation in rat liver microsomes and hepatocytes with IC50 values in the low micromolar range. Some of these compounds exhibited biphasic inhibition kinetics, indicative of interaction with more than one CYP2D isoform. Even though no important differences in inhibitory potencies were observed between the two systems, most inhibitors, including quinine and quinidine, displayed 2-3-fold lower IC50 in hepatocytes than in microsomes. The cell-associated concentrations of quinine and quinidine were found to be significantly higher than those in the extracellular medium, suggesting that intracellular accumulation may potentiate the effect of these compounds. Studies of CYP inhibition in intact hepatocytes may be warranted for compounds that concentrate in the liver as the result of cellular transport. (+info)
Stable expression of human cytochrome P450 2D6*10 in HepG2 cells.
(15/56)
AIM: Over 90% of drugs are metabolized by the cytochrome P-450 (CYP) family of liver isoenzymes. The most important enzymes are CYP1A2, 3A4, 2C9/19, 2D6 and 2E1. Although CYP2D6 accounts for <2% of the total CYP liver enzyme content, it mediates metabolism in almost 25% of drugs. In order to study its enzymatic activity for drug metabolism, its cDNA was cloned and a HepG2 cell line stably expressing CYP2D6 was established. METHODS: Human CYP2D6 cDNA was amplified with reverse transcription-polymerase chain reaction (RT-PCR) from total RNA extracted from human liver tissue and cloned into pGEM-T vector. cDNA segment was identified by DNA sequencing and subcloned into a mammalian expression vector pREP9. A cell line was established by transfecting the recombinant plasmid of pREP9-CYP2D6 to hepatoma HepG2 cells. Expression of mRNA was validated by RT-PCR. Enzyme activity of catalyzing dextromethorphan O-demethylation in postmitochondrial supernatant (S9) fraction of the cells was determined by high performance liquid chromatography (HPLC). RESULTS: The cloned cDNA had 4 base differences, e.g. 100 C-T, 336 T-C, 408 C-G and 1 457 G-C, which resulted in P34S, and S486T amino acid substitutions, and two samesense mutations were 112 F and 136 V compared with that reported by Kimura et al (GenBank accession number: M33388). P34S and S486T amino acid substitutions were the characteristics of CYP2D6*10 allele. The relative activity of S9 fraction of HepG2-CYP2D6*10 metabolized detromethorphan O-demethylation was found to be 2.31 +/- 0.19 nmol/min(-1)/mg(-1) S9 protein (n=3), but was undetectable in parental HepG2 cells. CONCLUSION: cDNA of human CYP2D6*10 can be successfully cloned. A cell line, HepG2-CYP2D6*10, expressing CYP2D6*10 mRNA and having metabolic activity, has been established. (+info)
A novel tetrahydrofolate-dependent O-demethylase gene is essential for growth of Sphingomonas paucimobilis SYK-6 with syringate.
(16/56)
Sphingomonas paucimobilis SYK-6 degrades syringate to 3-O-methylgallate (3MGA), which is finally converted to pyruvate and oxaloacetate via multiple pathways in which protocatechuate 4,5-dioxygenase, 3MGA dioxygenase, and gallate dioxygenase are involved. Here we isolated the syringate O-demethylase gene (desA), which complemented the growth deficiency on syringate of a Tn5 mutant of the SYK-6 derivative strain. The desA gene is located 929 bp downstream of ferA, encoding feruloyl-coenzyme A synthetase, and consists of a 1,386-bp open reading frame encoding a polypeptide with a molecular mass of 50,721 Da. The deduced amino acid sequence of desA showed 26% identity in a 325-amino-acid overlap with that of gcvT of Escherichia coli, which encodes the tetrahydrofolate (H(4)folate)-dependent aminomethyltransferase involved in glycine cleavage. The cell extract of E. coli carrying desA converted syringate to 3MGA only when H(4)folate was added to the reaction mixture. DesA catalyzes the transfer of the methyl moiety of syringate to H(4)folate, forming 5-methyl-H(4)folate. Vanillate and 3MGA were also used as substrates for DesA; however, the relative activities toward them were 3 and 0.4% of that toward syringate, respectively. Disruption of desA in SYK-6 resulted in a growth defect on syringate but did not affect growth on vanillate, indicating that desA is essential to syringate degradation. In a previous study the ligH gene, which complements the growth deficiency on vanillate and syringate of a chemical-induced mutant of SYK-6, DC-49, was isolated (S. Nishikawa, T. Sonoki, T. Kasahara, T. Obi, S. Kubota, S. Kawai, N. Morohoshi, and Y. Katayama, Appl. Environ. Microbiol. 64:836-842, 1998). Disruption of ligH resulted in the same phenotype as DC-49; its cell extract, however, was found to be able to convert vanillate and syringate in the presence of H(4)folate. The possible role of ligH is discussed. (+info)