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Concept: Cytochrome P450 reductase

171

Cytochrome P450 oxidoreductase (POR) is known as the sole electron donor in the metabolism of drugs by cytochrome P450 (CYP) enzymes in human. However, little is known about the effect of polymorphic variants of POR on drug metabolic activities of CYP3A4 and CYP2B6. In order to better understand the mechanism of the activity of CYPs affected by polymorphic variants of POR, six full-length mutants of POR (e.g., Y181D, A287P, K49N, A115V, S244C and G413S) were designed and then co-expressed with CYP3A4 and CYP2B6 in the baculovirus-Sf9 insect cells to determine their kinetic parameters. Surprisingly, both mutants, Y181D and A287P in POR completely inhibited the CYP3A4 activity with testosterone, while the catalytic activity of CYP2B6 with bupropion was reduced to approximately ~70% of wild-type activity by Y181D and A287P mutations. In addition, the mutant K49N of POR increased the CLint (Vmax/Km) of CYP3A4 up to more than 31% of wild-type, while it reduced the catalytic efficiency of CYP2B6 to 74% of wild-type. Moreover, CLint values of CYP3A4-POR (A115V, G413S) were increased up to 36% and 65% of wild-type respectively. However, there were no appreciable effects observed by the remaining two mutants of POR (i.e., A115V and G413S) on activities of CYP2B6. In conclusion, the extent to which the catalytic activities of CYP were altered did not only depend on the specific POR mutations but also on the isoforms of different CYP redox partners. Thereby, we proposed that the POR-mutant patients should be carefully monitored for the activity of CYP3A4 and CYP2B6 on the prescribed medication.

Concepts: Metabolism, Enzyme, Electrochemistry, Catalysis, Nitrogen, Cytochrome P450, P450-containing systems, Cytochrome P450 reductase

166

We have previously described the development of genetic models to study the in vivo functions of the hepatic cytochrome P450 system, through the hepatic deletion of either cytochrome P450 oxidoreductase (POR; HRN line) or cytochrome b5 (Cyb5; HBN line). However, HRN mice still exhibit low levels of mono-oxygenase activity, in spite of the absence of detectable reductase protein. To investigate whether this is because cytochrome b5 and cytochrome b5 reductase can act as sole electron donors to the P450 system, we have crossed HRN with HBN mice to generate a line lacking hepatic expression of both electron donors (HBRN). HBRN mice exhibited exacerbation of the phenotypic characteristics of the HRN line - liver enlargement, hepatosteatosis and increased expression of certain cytochrome P450s. Also, drug metabolising activities in vitro were further reduced relative to the HRN model, in some cases to undetectable levels. Pharmacokinetic studies in vivo demonstrated that midazolam half-life, Cmax and area under the concentration-time curve (AUC) were increased, and clearance was decreased, to a greater extent in the HBRN line than in either the HBN or HRN model. Microsomal incubations using NADPH concentrations below the apparent Km of cytochrome b5 reductase, but well above that for POR, led to the virtual elimination of 7-benzyloxyquinoline turnover in HRN samples. These data provide strong evidence that cytochrome b5/cytochrome b5 reductase can act as a sole electron donors to the cytochrome P450 system in vitro and in vivo.

Concepts: Cytochrome P450, Paracetamol, CYP3A4, P450-containing systems, Cytochrome P450 reductase

28

Our aim was to characterize Adverse Drug Reactions (ADRs) related to drug-drug interactions (DDIs) related to involvement of cytochrome P450 (CYP450) isoenzymes in a pharmacovigilance database.

Concepts: Pharmacology, Cytochrome P450, Adverse drug reaction, Cytochrome P450 reductase

28

NADPH-cytochrome P450 oxidoreductase (CYPOR) and nitric oxide synthase (NOS), two members of the diflavin oxidoreductase family, are multi-domain enzymes containing distinct FAD and FMN domains connected by a flexible hinge. FAD accepts a hydride ion from NADPH, and reduced FAD donates electrons to FMN, which in turn transfers electrons to the heme center of cytochrome P450 or NOS oxygenase domain. Structural analysis of CYPOR, the prototype of this enzyme family, has revealed the exact nature of the domain arrangement and the role of residues involved in cofactor binding. Recent structural and biophysical studies of CYPOR have shown that the two flavin domains undergo large domain movements during catalysis. NOS isoforms contain additional regulatory elements within the reductase domain that control electron transfer through Ca(2+)-dependent calmodulin (CaM) binding. The recent crystal structure of an iNOS Ca(2+)/CaM-FMN construct, containing the FMN domain in complex with Ca(2+)/CaM, provided structural information on the linkage between the reductase and oxgenase domains of NOS, making it possible to model the holo iNOS structure. This review summarizes recent advances in our understanding of the dynamics of domain movements during CYPOR catalysis and the role of the NOS diflavin reductase domain in the regulation of NOS isozyme activities.

Concepts: Electron, Metabolism, Enzyme, Cytochrome P450, Nitric oxide, Nitric oxide synthase, Enzymes, Cytochrome P450 reductase

27

Interindividual variability in cytochrome P450 (CYP)-mediated xenobiotic metabolism is extensive. CYP metabolism requires two electrons, which can be donated by NADPH cytochrome P450 oxidoreductase (CYPOR) and/or cytochrome b5 (b5). Although substantial number of studies have reported on the function and effect of b5 in CYP-mediated catalysis, its mode of action is still not fully understood.

Concepts: Metabolism, Enzyme, Cytochrome P450, CYP1A2, Xenobiotic metabolism, P450-containing systems, Cytochrome P450 reductase, Xenobiotic

23

Cytochrome P450s are associated with metabolizing of a wide range of compounds including insecticides. CYP353D1v2 has been found over-expressed in the imidacloprid-resistant strain of Laodelphax striatellus. Thus, this study was conducted to express CYP353D1v2 in Sf9 cell as a recombinant protein, to assess its ability in metabolizing imidacloprid.

Concepts: DNA, Protein, Metabolism, Endoplasmic reticulum, Escherichia coli, Cytochrome P450, Steroid, Cytochrome P450 reductase

0

CYP725A4 is a P450 enzyme from Taxus cuspidata that catalyzes the formation of taxadiene-5α-ol (T5α-ol) from taxadiene in paclitaxel biosynthesis. Past attempts expressing CYP725A4 in heterologous hosts reported the formation of 5(12)-oxa-3(11)-cyclotaxane (OCT) and/or 5(11)-oxa-3(11)-cyclotaxane (iso-OCT) instead of, or in addition to, T5α-ol. Here we report that T5α-ol is produced as a minor product by Escherichia coli expressing both taxadiene synthase (TS) and CYP725A4. The major products were OCT and iso-OCT, while trace amounts of unidentified monooxygenated taxanes were also detected by gas chromatography-mass spectrometry. Since OCT and iso-OCT had not been found in nature, we tested the hypothesis that protein-protein interaction of CYP725A4 with redox partners, such as cytochrome P450 reductase (CPR) and cytochrome b5, may affect the products formed by CYP725A4, possibly favoring the formation of T5α-ol over OCT and iso-OCT. Our results show that coexpression of CYP725A4 with CPR from different organisms did not change the relative ratios of OCT, iso-OCT, and T5α-ol, while cytochrome b5 decreased overall levels of the products formed. Although unsuccessful in finding conditions that promote T5α-ol formation over other products, we used our results to clarify conflicting claims in the literature and discuss other possible approaches to produce paclitaxel via metabolic and enzyme engineering. This article is protected by copyright. All rights reserved.

Concepts: Protein, Metabolism, Enzyme, Escherichia coli, Cytochrome P450, Copyright, P450-containing systems, Cytochrome P450 reductase

0

To summarize the available body of evidence guiding the management of supratherapeutic concentrations of calcineurin inhibitors (CNI) using cytochrome P450 (CYP450) enzyme inducers.

Concepts: Metabolism, Cytochrome P450, Cytochrome P450 reductase

0

NADPH-cytochrome P450 reductase is a multi-domain redox enzyme which is a key component of the P450 mono-oxygenase drug-metabolizing system. We report studies of the conformational equilibrium of this enzyme using small-angle neutron scattering, under conditions where we are able to control the redox state of the enzyme precisely. Different redox states have a profound effect on domain orientation in the enzyme and we analyse the data in terms of a two-state equilibrium between compact and extended conformations. The effects of ionic strength show that the presence of a greater proportion of the extended form leads to an enhanced ability to transfer electrons to cytochrome c. Domain motion is intrinsically linked to the functionality of the enzyme, and we can define the position of the conformational equilibrium for individual steps in the catalytic cycle.

Concepts: Metabolism, Enzyme, Hydrogen, Electrochemistry, Catalysis, Scattering, Cytochrome P450, Cytochrome P450 reductase

0

We examined the effects of thyrotoxic rubber antioxidants, 2-mercaptobenzimidazole (MBI, 0.3 mmol/kg/day) and its methyl derivatives, methyl-MBIs [4-methyl-MBI (4-MeMBI, 0.6 mmol/kg/day), 5-methyl-MBI (5-MeMBI, 0.6 mmol/kg/day), and 4(or 5)-methyl-MBI (4(5)-MeMBI, 0.6 or 1.2 mmol/kg/day)], on the drug-metabolizing activity in male rat liver microsomes by 8-day repeated oral administration. The weight of liver and thyroid were increased by all the test chemicals; MBI was most potent, and there was no additive or synergistic effect between 4-MeMBI and 5-MeMBI. MBI decreased the cytochrome P450 (CYP) content, NADPH-cytochrome P450 reductase (POR) activity, 7-ethoxycoumarin O-deethylation (ECOD) activity, and flavin-containing monooxygenase (FMO) activity, but increased the 7-pentoxyresorufin O-depentylation (PROD) activity, suggesting inhibition of the drug-metabolizing activity on the whole but induce some activities such as the CYP2B activity. On the contrary, all the methyl-MBIs increased the CYP content, CYB5 content, ECOD activity, 7-ethoxyresorufin O-deethylation (EROD) activity, and PROD activity, indicating that they are mostly inducible of the CYP activity. However, the methyl-MBIs decreased the FMO activity, and 5-MeMBI and 4(5)-MeMBI appeared inhibitory for CYPs 2C11 and 2C13. Between 4-MeMBI and 5-MeMBI, there was no additive or synergistic effect on the drug-metabolizing activity, but was counteraction. It was concluded that MBI and methyl-MBIs had both inhibitory and inducible effects on the drug-metabolizing activity in rat liver microsomes at thyrotoxic doses. The effects of 4(5)-MeMBI indicated that the increased liver weight alone can be a hepatotoxic sign but not an adaptive no-adverse response in toxicity studies. The present results were related to the toxicokinetic profiles of MBI and 4(5)-MeMBI in the repeated toxicity studies.

Concepts: Metabolism, Liver, Cytochrome P450, Paracetamol, Microsome, P450-containing systems, Cytochrome P450 reductase, CYP2B6