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Concept: P450-containing systems


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


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


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


Paraquat, a herbicide linked to Parkinson’s disease, generates reactive oxygen species (ROS), which causes cell death. Because the source of paraquat-induced ROS production remains unknown, we conducted a CRISPR-based positive-selection screen to identify metabolic genes essential for paraquat-induced cell death. Our screen uncovered three genes, POR (cytochrome P450 oxidoreductase), ATP7A (copper transporter), and SLC45A4 (sucrose transporter), required for paraquat-induced cell death. Furthermore, our results revealed POR as the source of paraquat-induced ROS production. Thus, our study highlights the use of functional genomic screens for uncovering redox biology.

Concepts: Gene, Bacteria, Mitochondrion, Oxidative phosphorylation, Hydrogen peroxide, Cytochrome P450, P450-containing systems, Cytochrome P450 reductase


Benzo[a]pyrene (BaP) is an environmental pollutant that, based on evidence largely from in vitro studies, exerts its genotoxic effects after metabolic activation by cytochrome P450s. In the present study, Hepatic Reductase Null (HRN) and Hepatic Cytochrome b 5 /P450 Reductase Null (HBRN) mice have been used to study the role of P450s in the metabolic activation of BaP in vivo. In HRN mice, cytochrome P450 oxidoreductase (POR), the electron donor to P450, is deleted specifically in hepatocytes. In HBRN mice the microsomal haemoprotein cytochrome b 5 , which can also act as an electron donor from cytochrome b 5 reductase to P450s, is also deleted in the liver. Wild-type (WT), HRN and HBRN mice were treated by i.p. injection with 125 mg/kg body weight BaP for 24 h. Hepatic microsomal fractions were isolated from BaP-treated and untreated mice. In vitro incubations carried out with BaP-pretreated microsomal fractions, BaP and DNA resulted in significantly higher BaP-DNA adduct formation with WT microsomal fractions compared to those from HRN or HBRN mice. Adduct formation (i.e. 10-(deoxyguanosin-N2-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-BaP [dG-N2-BPDE]) correlated with observed CYP1A activity and metabolite formation (i.e. BaP-7,8-dihydrodiol) when NADPH or NADH was used as enzymatic cofactors. BaP-DNA adduct levels (i.e. dG-N2-BPDE) in vivo were significantly higher (~ sevenfold) in liver of HRN mice than WT mice while no significant difference in adduct formation was observed in liver between HBRN and WT mice. Our results demonstrate that POR and cytochrome b 5 both modulate P450-mediated activation of BaP in vitro. However, hepatic P450 enzymes in vivo appear to be more important for BaP detoxification than its activation.

Concepts: Metabolism, Enzyme, Liver, Glycogen, Cytochrome P450, In vitro, P450-containing systems, Cytochrome P450 reductase


The interindividual variability of cyclosporin A (CsA) pharmacokinetics might be explained by heterogeneity in the cytochrome P450 3A (CYP3A) subfamily. Altered CYP3A enzyme activity was associated with variant allele of P450 oxidoreductase gene (POR*28). The aim of this study was to assess the impact of age, CYP3A5*3, CYP3A4*22, and POR*28 alleles on CsA pharmacokinetics in pediatric renal transplant recipients.

Concepts: Gene, Genetics, Genotype, Allele, Cytochrome P450, Organ transplant, Transplant rejection, P450-containing systems


Human hepatic cytochromes P450 (CYP) are integral to xenobiotic metabolism. CYP2B6 is a major catalyst of biotransformation of environmental toxicants including polybrominated diphenyl ethers (PBDEs). CYP2B substrates tend to contain halogen atoms, but the biochemical basis for this selectivity and for species specific determinants of metabolism has not been identified. Spectral binding titrations and inhibition studies were performed to investigate interactions of rat CYP2B1, rabbit CYP2B4, and CYP2B6 with a series of phenoxyaniline (POA) congeners that are analogs of PBDEs. For most congeners, there was less than 3-fold difference between the spectral binding constants (KS) and IC50 values. In contrast, large discrepancies between these values were observed for POA and 3-chloro-4-phenoxyaniline. CYP2B1 was the most sensitive enzyme to POA congeners, so the Val-363 residue from that enzyme was introduced into CYP2B4 or CYP2B6. This substitution partially altered the protein-ligand interaction profiles to make them more similar to that of CYP2B1. Addition of cytochrome P450 oxidoreductase (POR) to titrations of CYP2B6 with POA or 2'4'5'TCPOA decreased the affinity of both ligands for the enzyme. Addition of cytochrome b5 (cyt b5) to a recombinant enzyme system containing POR and CYP2B6 increased the POA IC50 value and decreased the 2'4'5'TCPOA IC50 value. Overall, the inconsistency between KS and IC50 values for POA versus 2'4'5'TCPOA is largely due to the effects of redox partner binding. These results provide insight into the biochemical basis of diphenyl ether binding to human CYP2B6 and changes in CYP2B6 mediated metabolism dependent on POA congener and redox partner identity.

Concepts: Alcohol, Metabolism, Enzyme, Cytochrome P450, Drug metabolism, P450-containing systems, Cytochrome P450 reductase, CYP2B6


Heme’s spin-multiplicity is key in determining the enzymatic function of cytochrome P450 (cytP450). The origin of the low-spin state in ferric P450 is still under debate. Here, we report the first experimental demonstration of P450’s membrane interaction altering its spin equilibrium which is accompanied by a stronger affinity for cytochrome b5. These results highlight the importance of lipid membrane for the function of P450.

Concepts: Archaea, Metabolism, Iron, Cytochrome P450, Steroid, Lipid bilayer, P450-containing systems, 2000 albums


Sulfonated steroids are increasingly recognized as a circulating reservoir of precursors for the local production of active steroids in certain target tissues. As an alternative to sulfonation of unconjugated steroids by cytosolic sulfotransferases, their direct formation from sulfonated precursors has been described. However, productivity and physiological relevance of this sulfate pathway of steroidogenesis are still widely unclear. Applying the porcine testis as a model, conversion of pregnenolone sulfate (P5S, sulfate pathway) by CYP17A1 was assessed in comparison to the parallel conversions of pregnenolone (P5, Δ5-pathway) and progesterone (P4, Δ4-pathway). To characterize conversions in the virtual absence of competing enzyme activities, in a first series of experiments porcine recombinant CYP17A1 was incubated with the respective substrate in the presence of bovine recombinant cytochrome P450 oxidoreductase (CPR) and cytochrome b5 (b5). Moreover, porcine testicular microsomal fractions were used as a source of homologous CYP17A1, CPR and b5. Invariably 17α-hydroxylation of P5S was, if at all, only minimal and no formation of dehydroepiandrosterone sulfate from P5S was detectable. Consistent with earlier studies porcine CYP17A1 efficiently metabolized P4 and P5 in both assay systems. Metabolism of P4 and P5 by testicular microsomal protein varied substantially between the five animals tested. In conclusion, a physiologically relevant sulfate pathway for the production of C19-steroids from P5S via CYP17A1 is very unlikely in the porcine testis.

Concepts: Metabolism, Enzyme, Cytochrome P450, Steroid, CYP17A1, P450-containing systems, Pregnenolone, Cytochrome P450 reductase


An incorrect version of the Supplementary Information was inadvertently published with this Article where the wrong file was included. The HTML has been updated to include the correct version of the Supplementary Information.

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