Roles of NADPH-P450 reductase and apo- and holo-cytochrome b5 on xenobiotic oxidations catalyzed by 12 recombinant human cytochrome P450s expressed in membranes of Escherichia coli

Oxidation of 1-chloropyrene by human CYP1 family and CYP2A subfamily cytochrome P450 enzymes: catalytic roles of two CYP1B1 and five CYP2A13 allelic variants

1. 1-Chloropyrene, one of the major chlorinated polycyclic aromatic hydrocarbon contaminants, was incubated with human cytochrome P450 (P450 or CYP) enzymes including CYP1A1, 1A2, 1B1, 2A6, 2A13, 2B6, 2C9, 2D6, 2E1, 3A4 and 3A5. Catalytic differences in 1-chloropyrene oxidation by polymorphic two CYP1B1 and five CYP2A13 allelic variants were also examined. 2. CYP1A1 oxidized 1-chloropyrene at the 6- and 8-positions more actively than at the 3-position, while both CYP1B1.1 and 1B1.3 preferentially catalyzed 6-hydroxylation. 3. Five CYP2A13 allelic variants oxidized 8-hydroxylation much more than 6- and 3-hydroxylation, and the variant CYP2A13.3 was found to slowly catalyze these reactions with a lower k_cat value than other CYP2A13.1 variants. 4. CYP2A6 catalyzed 1-chloropyrene 6-hydroxylation at a higher rate than the CYP2A13 enzymes, but the rate was lower than the CYP1A1 and 1B1 variants. Other human P450 enzymes had low activities towards 1-chloropyrene. 5. Molecular docking analysis suggested differences in the interaction of 1-chloropyrene with active sites of CYP1 and 2 A enzymes. In addition, a naturally occurring Thr134 insertion in CYP2A13.3 was found to affect the orientation of Asn297 in the I-helix in interacting with 1-chloropyrene (and also 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, NNK) and caused changes in the active site of CYP2A13.3 as compared with CYP2A13.1.

Kinetic processivity of the two-step oxidations of progesterone and pregnenolone to androgens by human cytochrome P450 17A1

Cytochrome P450 (P450, CYP) 17A1 plays a critical role in steroid metabolism, catalyzing both the 17α-hydroxylation of pregnenolone and progesterone and the subsequent 17α,20-lyase reactions to form dehydroepiandrosterone (DHEA) and androstenedione (Andro), respectively, critical for generating glucocorticoids and androgens. Human P450 17A1 reaction rates examined are enhanced by the accessory protein cytochrome b₅ (b₅), but the exact role of b₅ in P450 17A1-catalyzed reactions is unclear as are several details of these reactions. Here, we examined in detail the processivity of the 17α-hydroxylation and lyase steps. b₅ did not enhance reaction rates by decreasing the k_off rates of any of the steroids. Steroid binding to P450 17A1 was more complex than a simple two-state system. Pre-steady-state experiments indicated lag phases for Andro production from progesterone and for DHEA from pregnenolone, indicating a distributive character of the enzyme. However, we observed processivity in pregnenolone/DHEA pulse-chase experiments. (S)-Orteronel was three times more inhibitory toward the conversion of 17α-hydroxypregnenolone to DHEA than toward the 17α-hydroxylation of pregnenolone. IC₅₀ values for (S)-orteronel were identical for blocking DHEA formation from pregnenolone and for 17α-hydroxylation, suggestive of processivity. Global kinetic modeling helped assign sets of rate constants for individual or groups of reactions, indicating that human P450 17A1 is an inherently distributive enzyme but that some processivity is present, i.e. some of the 17α-OH pregnenolone formed from pregnenolone did not dissociate from P450 17A1 before conversion to DHEA. Our results also suggest multiple conformations of P450 17A1, as previously proposed on the basis of NMR spectroscopy and X-ray crystallography.

Cytochrome b5 plays a dual role in the reaction cycle of cytochrome P450 3A4 during oxidation of the anticancer drug ellipticine

Abstract

Ellipticine is an anticancer agent that forms covalent DNA adducts after enzymatic activation by cytochrome P450 (CYP) enzymes, mainly by CYP3A4. This process is one of the most important ellipticine DNA-damaging mechanisms for its antitumor action. Here, we investigated the efficiencies of human hepatic microsomes and human recombinant CYP3A4 expressed with its reductase, NADPH:CYP oxidoreductase (POR), NADH:cytochrome b₅ reductase and/or cytochrome b₅ in Supersomes™ to oxidize this drug. We also evaluated the effectiveness of coenzymes of two of the microsomal reductases, NADPH as a coenzyme of POR, and NADH as a coenzyme of NADH:cytochrome b₅ reductase, to mediate ellipticine oxidation in these enzyme systems. Using HPLC analysis we detected up to five ellipticine metabolites, which were formed by human hepatic microsomes and human CYP3A4 in the presence of NADPH or NADH. Among ellipticine metabolites, 9-hydroxy-, 12-hydroxy-, and 13-hydroxyellipticine were formed by hepatic microsomes as the major metabolites, while 7-hydroxyellipticine and the ellipticine N²-oxide were the minor ones. Human CYP3A4 in Supersomes™ generated only three metabolic products, 9-hydroxy-, 12-hydroxy-, and 13-hydroxyellipticine. Using the ³²P-postlabeling method two ellipticine-derived DNA adducts were generated by microsomes and the CYP3A4-Supersome system, both in the presence of NADPH and NADH. These adducts were derived from the reaction of 13-hydroxy- and 12-hydroxyellipticine with deoxyguanosine in DNA. In the presence of NADPH or NADH, cytochrome b₅ stimulated the CYP3A4-mediated oxidation of ellipticine, but the stimulation effect differed for individual ellipticine metabolites. This heme protein also stimulated the formation of both ellipticine-DNA adducts. The results demonstrate that cytochrome b₅ plays a dual role in the CYP3A4-catalyzed oxidation of ellipticine: (1) cytochrome b₅ mediates CYP3A4 catalytic activities by donating the first and second electron to this enzyme in its catalytic cycle, indicating that NADH:cytochrome b₅ reductase can substitute NADPH-dependent POR in this enzymatic reaction and (2) cytochrome b₅ can act as an allosteric modifier of the CYP3A4 oxygenase.

Graphical abstract

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

The catalytic function of cytochrome P450 is entwined with its membrane-bound nature

Cytochrome P450, a family of monooxygenase enzymes, is organized as a catalytic metabolon, which requires enzymatic partners as well as environmental factors that tune its complex dynamic. P450 and its reducing counterparts—cytochrome P450-reductase and cytochrome b ₅—are membrane-bound proteins located in the cytosolic side of the endoplasmic reticulum. They are believed to dynamically associate to form functional complexes. Increasing experimental evidence signifies the role(s) played by both protein-protein and protein-lipid interactions in P450 catalytic function and efficiency. However, the biophysical challenges posed by their membrane-bound nature have severely limited high-resolution understanding of the molecular interfaces of these interactions. In this article, we provide an overview of the current knowledge on cytochrome P450, highlighting the environmental factors that are entwined with its metabolic function. Recent advances in structural biophysics are also discussed, setting up the bases for a new paradigm in the study of this important class of membrane-bound enzymes.

Single-Protein Tracking Reveals That NADPH Mediates the Insertion of Cytochrome P450 Reductase into a Biomimetic of the Endoplasmic Reticulum

Cytochrome P450 reductase (CPR) is the redox partner for most human cytochrome P450 enzymes. It is also believed that CPR is an integral membrane protein exclusively. Herein, we report that, contrary to this belief, CPR can exist as a peripheral membrane protein in the absence of NADPH and will transition to an integral membrane protein in the presence of stoichiometric amounts of NADPH or greater. All experiments were performed in a solid-supported cushioned lipid bilayer that closely matched the chemical composition of the human endoplasmic reticulum and served as an ER biomimetic. The phase characteristics and fluidity of the ER biomimetic was characterized with fluorescence micrographs and temperature-dependent fluorescence recovery after photobleaching. The interactions of CPR with the ER biomimetic were directly observed by tracking single CPR molecules using time-lapse single-molecule fluorescence imaging and subsequent analysis of tracks. These studies revealed dramatic changes in diffusion coefficient and the degree of partitioning of CPR as a function of NADPH concentration.

Marmoset Cytochrome P450 3A4 Ortholog Expressed in Liver and Small-Intestine Tissues Efficiently Metabolizes Midazolam, Alprazolam, Nifedipine, and Testosterone

Common marmosets (Callithrix jacchus), small New World primates, are increasingly attracting attention as potentially useful animal models for drug development. However, characterization of cytochrome P450 (P450) 3A enzymes involved in the metabolism of a wide variety of drugs has not investigated in marmosets. In this study, sequence homology, tissue distribution, and enzymatic properties of marmoset P450 3A4 ortholog, 3A5 ortholog, and 3A90 were investigated. Marmoset P450 3A forms exhibited high amino acid sequence identities (88-90%) to the human and cynomolgus monkey P450 3A orthologs and evolutionary closeness to human and cynomolgus monkey P450 3A orthologs compared with other P450 3A enzymes. Among the five marmoset tissues examined, P450 3A4 ortholog mRNA was abundant in livers and small intestines where P450 3A4 ortholog proteins were immunologically detected. Three marmoset P450 3A proteins heterologously expressed in Escherichia coli membranes catalyzed midazolam 1'- and 4-hydroxylation, alprazolam 4-hydroxylation, nifedipine oxidation, and testosterone 6β-hydroxylation, similar to cynomolgus monkey and human P450 3A enzymes. Among the marmoset P450 3A enzymes, P450 3A4 ortholog effectively catalyzed midazolam 1'-hydroxylation, comparable to microsomes from marmoset livers and small intestines. Correlation analyses with 23 individual marmoset liver microsomes suggested contributions of P450 3A enzymes to 1'-hydroxylation of both midazolam (human P450 3A probe) and bufuralol (human P450 2D6 probe), similar to cynomolgus monkey P450 3A enzymes. These results indicated that marmoset P450 3A forms had functional characteristics roughly similar to cynomolgus monkeys and humans in terms of tissue expression patterns and catalytic activities, suggesting marmosets as suitable animal models for P450 3A-dependent drug metabolism.

Roles of Human CYP2A6 and Monkey CYP2A24 and 2A26 Cytochrome P450 Enzymes in the Oxidation of 2,5,2',5'-Tetrachlorobiphenyl

2,5,2',5'-Tetrachlorobiphenyl (TCB) induced type I binding spectra with cytochrome P450 (P450) 2A6 and 2A13, with Ks values of 9.4 and 0.51 µM, respectively. However, CYP2A6 oxidized 2,5,2',5'-TCB to form 4-hydroxylated products at a much higher rate (∼1.0 minute-1) than CYP2A13 (∼0.02 minute-1) based on analysis by liquid chromatography-tandem mass spectrometry. Formation of 4-hydroxy-2,5,2',5'-TCB by CYP2A6 was greater than that of 3-hydroxy-2,5,2',5'-TCB and three other hydroxylated products. Several human P450 enzymes, including CYP1A1, 1A2, 1B1, 2B6, 2D6, 2E1, 2C9, and 3A4, did not show any detectable activities in oxidizing 2,5,2',5'-TCB. Cynomolgus monkey CYP2A24, which shows 95% amino acid identity to human CYP2A6, catalyzed 4-hydroxylation of 2,5,2',5'-TCB at a higher rate (∼0.3 minute-1) than CYP2A26 (93% identity to CYP2A6, ∼0.13 minute-1) and CYP2A23 (94% identity to CYP2A13, ∼0.008 minute-1). None of these human and monkey CYP2A enzymes were catalytically active in oxidizing other TCB congeners, such as 2,4,3',4'-, 3,4,3',4'-, and 3,5,3',5'-TCB. Molecular docking analysis suggested that there are different orientations of interaction of 2,5,2',5'-TCB with the active sites (over the heme) of human and monkey CYP2A enzymes, and that ligand interaction energies (U values) of bound protein-ligand complexes show structural relationships of interaction of TCBs and other ligands with active sites of CYP2A enzymes. Catalytic differences in human and monkey CYP2A enzymes in the oxidation of 2,5,2',5'-TCB are suggested to be due to amino acid changes at substrate recognition sites, i.e., V110L, I209S, I300F, V365M, S369G, and R372H, based on the comparison of primary sequences.

Oxidation of Acenaphthene and Acenaphthylene by Human Cytochrome P450 Enzymes

Acenaphthene and acenaphthylene, two known environmental polycyclic aromatic hydrocarbon (PAH)pollutants, were incubated at 50 μM concentrations in a standard reaction mixture with human P450s 2A6, 2A13, 1B1,1A2, 2C9, and 3A4, and the oxidation products were determined using HPLC and LC-MS. HPLC analysis showed that P450 2A6 converted acenaphthene and acenaphthylene to several mono- and dioxygenated products. LC-MS analysis of acenaphthene oxidation by P450s indicated the formation of1-acenaphthenol as a major product, with turnover rates of 6.7,4.5, and 3.6 nmol product formed/min/nmol P450 for P4502A6, 2A13, and 1B1, respectively. Acenaphthylene oxidation by P450 2A6 showed the formation of 1,2-epoxyacenaphthene as a major product (4.4 nmol epoxide formed/min/nmol P450) and also several mono- and dioxygenated products.P450 2A13, 1B1, 1A2, 2C9, and 3A4 formed 1,2-epoxyacenaphthene at rates of 0.18, 5.3 2.4, 0.16, and 3.8 nmol/min/nmol P450, respectively. 1-Acenaphthenol, which induced Type I binding spectra with P450 2A13, was further oxidized by P450 2A13 but not P450 2A6. 1,2-Epoxyacenaphthene induced Type I binding spectra with P450 2A6 and 2A13 (K(s) 1.8 and 0.16 μM,respectively) and was also oxidized to several oxidation products by these P450s. Molecular docking analysis suggested different orientations of acenaphthene, acenaphthylene, 1-acenaphthenol, and 1,2-epoxyacenaphthene in their interactions with P450 2A6a nd 2A13. Neither of these four PAHs induced umu gene expression in a Salmonella typhimurium NM tester strain. These results suggest, for the first time, that acenaphthene and acenaphthylene are oxidized by human P450s 2A6 and 2A13 and other P450s to form several mono- and dioxygenated products. The results are of use in considering the biological and toxicological significance of these environmental PAHs in humans.

Marmoset cytochrome P450 4A11, a novel arachidonic acid and lauric acid ω-hydroxylase expressed in liver and kidney tissues

1. A cDNA encoding novel cytochrome P450 (P450) 4A enzyme was cloned from marmoset livers by reverse transcription (RT)-polymerase chain reaction (PCR) based on the marmoset genome sequences. The amino acid sequence deduced from P450 4A11 cDNA contained consensus sequences of six substrate recognition sites and one heme-binding domain. 2. Marmoset P450 4A11, highly identical (85-88%) to cynomolgus monkey and human P450 4A enzymes, was grouped into the same cluster as cynomolgus monkey and human P450 4A enzymes by phylogenetic analysis. 3. The tissue distribution analyses by real-time RT PCR and immunoblotting demonstrated that marmoset P450 4A11 mRNA and proteins were expressed in kidneys and livers. Marmoset P450 4A11 enzyme heterologously expressed in Escherichia coli preferentially catalyzed the ω-hydroxylation of arachidonic acid and lauric acid, similar to cynomolgus monkey and human P450 4A11 enzymes. However, lauric acid ω-hydroxylation activity of marmoset P450 4A11 was low compared with those of marmoset liver microsomes. 4. These results indicated that novel marmoset P450 4A11 was also a fatty acid ω-hydroxylase expressed in kidneys and livers, with the same regioselectivity (at ω-position of fatty acid) as cynomolgus monkey and human P450 4A enzymes.

Structure-Function Studies of Naphthalene, Phenanthrene, Biphenyl, and Their Derivatives in Interaction with and Oxidation by Cytochromes P450 2A13 and 2A6

Naphthalene, phenanthrene, biphenyl, and their derivatives having different ethynyl, propynyl, butynyl, and propargyl ether substitutions were examined for their interaction with and oxidation by cytochromes P450 (P450) 2A13 and 2A6. Spectral interaction studies suggested that most of these chemicals interacted with P450 2A13 to induce Type I binding spectra more readily than with P450 2A6. Among the various substituted derivatives examined, 2-ethynylnaphthalene, 2-naphthalene propargyl ether, 3-ethynylphenanthrene, and 4-biphenyl propargyl ether had larger ΔAmax/Ks values in inducing Type I binding spectra with P450 2A13 than their parent compounds. P450 2A13 was found to oxidize naphthalene, phenanthrene, and biphenyl to 1-naphthol, 9-hydroxyphenanthrene, and 2- and/or 4-hydroxybiphenyl, respectively, at much higher rates than P450 2A6. Other human P450 enzymes including P450s 1A1, 1A2, 1B1, 2C9, and 3A4 had lower rates of oxidation of naphthalene, phenanthrene, and biphenyl than P450s 2A13 and 2A6. Those alkynylated derivatives that strongly induced Type I binding spectra with P450s 2A13 and 2A6 were extensively oxidized by these enzymes upon analysis with HPLC. Molecular docking studies supported the hypothesis that ligand-interaction energies (U values) obtained with reported crystal structures of P450 2A13 and 2A6 bound to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, indole, pilocarpine, nicotine, and coumarin are of use in understanding the basis of possible molecular interactions of these xenobiotic chemicals with the active sites of P450 2A13 and 2A6 enzymes. In fact, the ligand-interaction energies with P450 2A13 4EJG bound to these chemicals were found to relate to their induction of Type I binding spectra.

Orthogonal Assays Clarify the Oxidative Biochemistry of Taxol P450 CYP725A4

Natural product metabolic engineering potentially offers sustainable and affordable access to numerous valuable molecules. However, challenges in characterizing and assembling complex biosynthetic pathways have prevented more rapid progress in this field. The anticancer agent Taxol represents an excellent case study. Assembly of a biosynthetic pathway for Taxol has long been stalled at its first functionalization, putatively an oxygenation performed by the cytochrome P450 CYP725A4, due to confounding characterizations. Here, through combined in vivo (Escherichia coli), in vitro (lipid nanodisc), and metabolite stability assays, we verify the presence and likely cause of this enzyme's inherent promiscuity. Thereby, we remove the possibility that promiscuity simply existed as an artifact of previous metabolic engineering approaches. Further, spontaneous rearrangement and the stabilizing effect of a hydrophobic overlay suggest a potential role for nonenzymatic chemistry in Taxol's biosynthesis. Taken together, this work confirms taxadiene-5α-ol as a primary enzymatic product of CYP725A4 and provides direction for future Taxol metabolic and protein engineering efforts.

A New Marmoset P450 4F12 Enzyme Expressed in Small Intestines and Livers Efficiently Metabolizes Antihistaminic Drug Ebastine

Common marmosets (Callithrix jacchus) are attracting attention as animal models in preclinical studies for drug development. However, cytochrome P450s (P450s), major drug-metabolizing enzymes, have not been fully identified and characterized in marmosets. In this study, based on the four novel P450 4F genes found on the marmoset genome, we successfully isolated P450 4F2, 4F3B, 4F11, and 4F12 cDNAs in marmoset livers. Deduced amino acid sequences of the four marmoset P450 4F forms exhibited high sequence identities (87%-93%) to the human and cynomolgus monkey P450 4F homologs. Marmoset P450 4F3B and 4F11 mRNAs were predominantly expressed in livers, whereas marmoset P450 4F2 and 4F12 mRNAs were highly expressed in small intestines and livers. Four marmoset P450 4F proteins heterologously expressed in Escherichia coli catalyzed the ω-hydroxylation of leukotriene B4 In addition, marmoset P450 4F12 effectively catalyzed the hydroxylation of antiallergy drug ebastine, a human P450 2J/4F probe substrate. Ebastine hydroxylation activities by small intestine and liver microsomes from marmosets and cynomolgus monkeys showed greatly higher values than those of humans. Ebastine hydroxylation activities by marmoset and cynomolgus monkey small intestine microsomes were inhibited (approximately 60%) by anti-P450 4F antibodies, unlike human small intestine microsomes, suggesting that contribution of P450 4F enzymes for ebastine hydroxylation in the small intestine might be different between marmosets/cynomolgus monkeys and humans. These results indicated that marmoset P450 4F2, 4F3B, 4F11, and 4F12 were expressed in livers and/or small intestines and were functional in the metabolism of endogenous and exogenous compounds, similar to those of cynomolgus monkeys and humans.

Impaired 17,20-Lyase Activity in Male Mice Lacking Cytochrome b5 in Leydig Cells

Androgen and estrogen biosynthesis in mammals requires the 17,20-lyase activity of cytochrome P450 17A1 (steroid 17-hydroxylase/17,20-lyase). Maximal 17,20-lyase activity in vitro requires the presence of cytochrome b₅ (b5), and rare cases of b5 deficiency in human beings causes isolated 17,20-lyase deficiency. To study the consequences of conditional b5 removal from testicular Leydig cells in an animal model, we generated Cyb5^flox/flox:Sf1-Cre (LeyKO) mice. The LeyKO male mice had normal body weights, testis and sex organ weights, and fertility compared with littermates. Basal serum and urine steroid profiles of LeyKO males were not significantly different than littermates. In contrast, marked 17-hydroxyprogesterone accumulation (100-fold basal) and reduced testosterone synthesis (27% of littermates) were observed after human chorionic gonadotropin stimulation in LeyKO animals. Testis homogenates from LeyKO mice showed reduced 17,20-lyase activity and a 3-fold increased 17-hydroxylase to 17,20-lyase activity ratio, which were restored to normal upon addition of recombinant b5. We conclude that Leydig cell b5 is required for maximal androgen synthesis and to prevent 17-hydroxyprogesterone accumulation in the mouse testis; however, the b5-independent 17,20-lyase activity of mouse steroid 17-hydroxylase/17,20-lyase is sufficient for normal male genital development and fertility. LeyKO male mice are a good model for the biochemistry but not the physiology of isolated 17,20-lyase deficiency in human beings.

Marmoset cytochrome P450 2J2 mainly expressed in small intestines and livers effectively metabolizes human P450 2J2 probe substrates, astemizole and terfenadine

1. Common marmoset (Callithrix jacchus), a New World Monkey, has potential to be a useful animal model in preclinical studies. However, drug metabolizing properties have not been fully understood due to insufficient information on cytochrome P450 (P450), major drug metabolizing enzymes. 2. Marmoset P450 2J2 cDNA was isolated from marmoset livers. The deduced amino acid sequence showed a high-sequence identity (91%) with cynomolgus monkey and human P450 2J2 enzymes. A phylogenetic tree revealed that marmoset P450 2J2 was evolutionarily closer to cynomolgus monkey and human P450 2J2 enzymes, than P450 2J forms in pigs, rabbits, rats or mice. 3. Marmoset P450 2J2 mRNA was abundantly expressed in the small intestine and liver, and to a lesser extent in the brain, lung and kidney. Immunoblot analysis also showed expression of marmoset P450 2J2 protein in the small intestine and liver. 4. Enzyme assays using marmoset P450 2J2 protein heterologously expressed in Escherichia coli indicated that marmoset P450 2J2 effectively catalyzed astemizole O-demethylation and terfenadine t-butyl hydroxylation, similar to human and cynomolgus monkey P450 2J2 enzymes. 5. These results suggest the functional characteristics of P450 2J2 enzymes are similar among marmosets, cynomolgus monkeys and humans.

Binding of bile acids by pastry products containing bioactive substances during in vitro digestion

The modern day consumer tends to choose products with health enhancing properties, enriched in bioactive substances. One such bioactive food component is dietary fibre, which shows a number of physiological properties including the binding of bile acids. Dietary fibre should be contained in everyday, easily accessible food products. Therefore, the aim of this study was to determine sorption capacities of primary bile acid (cholic acid - CA) and secondary bile acids (deoxycholic - DCA and lithocholic acids - LCA) by muffins (BM) and cookies (BC) with bioactive substances and control muffins (CM) and cookies (CC) in two sections of the in vitro gastrointestinal tract. Variations in gut flora were also analysed in the process of in vitro digestion of pastry products in a bioreactor. Enzymes: pepsin, pancreatin and bile salts: cholic acid, deoxycholic acid and lithocholic acid were added to the culture. Faecal bacteria, isolated from human large intestine, were added in the section of large intestine. The influence of dietary fibre content in cookies and concentration of bile acids in two stages of digestion were analysed. Generally, pastry goods with bioactive substances were characterized by a higher content of total fibre compared with the control samples. These products also differ in the profile of dietary fibre fractions. Principal Component Analysis (PCA) showed that the bile acid profile after two stages of digestion depends on the quality and quantity of fibre. The bile acid profile after digestion of BM and BC forms one cluster, and with the CM and CC forms a separate cluster. High concentration of H (hemicellulose) is positively correlated with LCA (low binding effect) and negatively correlated with CA and DCA contents. The relative content of bile acids in the second stage of digestion was in some cases above the content in the control sample, particularly LCA. This means that the bacteria introduced in the 2nd stage of digestion synthesize the LCA.

Novel Marmoset Cytochrome P450 2C19 in Livers Efficiently Metabolizes Human P450 2C9 and 2C19 Substrates, S-Warfarin, Tolbutamide, Flurbiprofen, and Omeprazole

The common marmoset (Callithrix jacchus), a small New World monkey, has the potential for use in human drug development due to its evolutionary closeness to humans. Four novel cDNAs, encoding cytochrome P450 (P450) 2C18, 2C19, 2C58, and 2C76, were cloned from marmoset livers to characterize P450 2C molecular properties, including previously reported P450 2C8. The deduced amino acid sequence showed high sequence identities (>86%) with those of human P450 2Cs, except for marmoset P450 2C76, which has a low sequence identity (∼70%) with any human P450 2Cs. Phylogenetic analysis showed that marmoset P450 2Cs were more closely clustered with those of humans and macaques than other species investigated. Quantitative polymerase chain reaction analysis showed that all of the marmoset P450 2C mRNAs were predominantly expressed in liver as opposed to the other tissues tested. Marmoset P450 2C proteins were detected in liver by immunoblotting using antibodies against human P450 2Cs. Among marmoset P450 2Cs heterologously expressed in Escherichia coli, marmoset P450 2C19 efficiently catalyzed human P450 2C substrates, S-warfarin, diclofenac, tolbutamide, flurbiprofen, and omeprazole. Marmoset P450 2C19 had high Vmax and low Km values for S-warfarin 7-hydroxylation that were comparable to those in human liver microsomes, indicating warfarin stereoselectivity similar to findings in humans. Faster in vivo S-warfarin clearance than R-warfarin after intravenous administration of racemic warfarin (0.2 mg/kg) to marmosets was consistent with the in vitro kinetic parameters. These results indicated that marmoset P450 2C enzymes had functional characteristics similar to those of humans, and that P450 2C-dependent metabolic properties are likewise similar between marmosets and humans.

Marmoset cytochrome P450 2D8 in livers and small intestines metabolizes typical human P450 2D6 substrates, metoprolol, bufuralol and dextromethorphan

1. Although the New World non-human primate, the common marmoset (Callithrix jacchus), is a potentially useful animal model, comprehensive understanding of drug metabolizing enzymes is insufficient. 2. A cDNA encoding a novel cytochrome P450 (P450) 2D8 was identified in marmosets. The amino acid sequence deduced from P450 2D8 cDNA showed a high sequence identity (83-86%) with other primate P450 2Ds. Phylogenetic analysis showed that marmoset P450 2D8 was closely clustered with human P450 2D6, unlike P450 2Ds of miniature pig, dog, rabbit, guinea pig, mouse or rat. 3. Marmoset P450 2D8 mRNA was predominantly expressed in the liver and small intestine among the tissues types analyzed, whereas marmoset P450 2D6 mRNA was expressed predominantly in the liver where P450 2D protein was detected by immunoblotting. 4. By metabolic assays using marmoset P450 2D8 protein heterologously expressed in Escherichia coli, although P450 2D8 exhibits lower catalytic efficiency compared to marmoset and human P450 2D6 enzymes, P450 2D8 mediated O-demethylations of metoprolol and dextromethorphan and bufuralol 1'-hydroxylation. 5. These results suggest that marmoset P450 2D8 (also expressed in the extrahepatic tissues) has potential roles in drug metabolism in a similar manner to those of human and marmoset P450 2D6.

Cytochrome b5 modulates multiple reactions in steroidogenesis by diverse mechanisms

Cytochrome b5 (cyt-b5) is a relatively small haemoprotein which plays an important role in the regulation of mammalian steroidogenesis. This unique protein has the ability to modulate the activity of key steroidogenic enzymes via a number of diverse reaction mechanisms. Cyt-b5 can augment the 17,20-lyase activity of CYP17A1 by promoting the interaction of CYP17A1 and POR; enhance the 16-ene-synthase activity of CYP17A1 by acting as an electron donor; and enhance the activity of 3βHSD by increasing the affinity of 3βHSD for its cofactor NAD(+). We review the modulation of CYP17A1 and 3βHSD activity by cyt-b5 and discuss the reaction mechanisms associated with each activity. The physiological importance of cyt-b5 in regulating mammalian steroidogenesis is presented and the impact of inactivating cyt-b5 mutations are reviewed. This article is part of a Special Issue entitled 'Steroid/Sterol signaling'.

Effect of Cytochrome b5 Content on the Activity of Polymorphic CYP1A2, 2B6, and 2E1 in Human Liver Microsomes

Human cytochrome b5 (Cyt b5) plays important roles in cytochrome P450 (CYP)-mediated drug metabolism. However, the expression level of Cyt b5 in normal human liver remains largely unknown. The effect of Cyt b5 on overall CYP activity in human liver microsomes (HLM) has rarely been reported and the relationship between Cyt b5 and the activity of polymorphic CYP has not been systematically investigated. In this study, we found that the median value of Cyt b5 protein was 270.01 pmol/mg from 123 HLM samples, and 12- and 19-fold individual variation was observed in Cyt b5 mRNA and protein levels, respectively. Gender and smoking clearly influenced Cyt b5 content. In addition, we found that Cyt b5 protein levels significantly correlated with the overall activity of CYP1A2, 2B6, and 2E1 in HLM. However, when the CYP activities were sorted by single nucleotide polymorphisms (SNP), the effect of Cyt b5 protein on the kinetic parameters varied greatly. There were significant correlations between Cyt b5 content and V_max and CL_int of CYP1A2 wild-types (3860GG, 2159GG, and 5347CC) as well as homozygous mutants (163AA and 3113GG). In contrast to V_max and CL_int, the K_m of CYP2B6 516GG and 785AA genotypes was inversely associated with Cyt b5 content. Correlations between Cyt b5 content and V_max and CL_int of CYP2E1 -1293GG, -1293GC, 7632TT, 7632TA, -333TT, and -352AA genotypes were also observed. In conclusion, Cyt b5 expression levels varied considerably in the Chinese cohort from this study. Cyt b5 had significant impact on the overall activity of CYP1A2, 2B6, and 2E1 in HLM and the effects of Cyt b5 protein on polymorphic CYP1A2, 2B6, and 2E1 activity were SNP-dependent. These findings suggest that Cyt b5 plays an important role in CYP-mediated activities in HLM and may possibly be a contributing factor for the individual variation observed in CYP enzyme activities.

Activation and deactivation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine by cytochrome P450 enzymes and flavin-containing monooxygenases in common marmosets (Callithrix jacchus)

The potential proneurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces Parkinson-like syndromes in common marmosets, other primates, and humans. MPTP is metabolically activated to 1-methyl-4-phenyl-2,3-dihydropyridinium and 1-methyl-4-phenylpyridinium ions (MPDP(+) and MPP(+), respectively) by desaturation reactions. MPTP is deactivated to 4-phenyl-1,2,3,6-tetrahydropyridine (PTP) by N-demethylation and is also deactivated to MPTP N-oxide. The roles of cytochrome P450 (P450) enzymes and flavin-containing monooxygenases (FMOs) in the oxidative metabolism of MPTP-treated marmosets are not yet fully clarified. This study aimed to elucidate P450- and FMO-dependent MPTP metabolism in marmoset liver and brain. Rates of MPTP N-oxygenation in liver microsomes were similar to those in brain microsomes from 11 individual marmosets (substrate concentration, 50 μM) and were correlated with rates of benzydamine N-oxygenation (r = 0.75, P < 0.05); the reactions were inhibited by methimazole (10 μM). MPTP N-oxygenation was efficiently mediated by recombinantly expressed marmoset FMO3. Rates of PTP formation by MPTP N-demethylation in marmoset liver microsomes were correlated with bufuralol 1'-hydroxylation rates (r = 0.77, P < 0.01) and were suppressed by quinidine (1 μM), thereby indicating the importance of marmoset CYP2D6 in PTP formation. MPTP transformations to MPDP(+) and MPP(+) were efficiently catalyzed by recombinant marmoset CYP2D6 and human CYP1A2. These results indicated the contributions of multiple drug-metabolizing enzymes to MPTP oxidation, especially marmoset FMO3 in deactivation (N-oxygenation) and marmoset CYP2D6 for both MPTP deactivation and MPTP activation to MPDP(+) and MPP(+). These findings provide a foundation for understanding MPTP metabolism and for the successful production of preclinical marmoset models.

Regioselective hydroxylation of steroid hormones by human cytochromes P450

This article reviews in vitro metabolic activities [including Michaelis constants (Km), maximal velocities (Vmax) and Vmax/Km] and drug-steroid interactions [such as induction and cooperativity (activation)] of cytochromes P450 (P450 or CYP) in human tissues, including liver and adrenal gland, for 14 kinds of endogenous steroid compounds, including allopregnanolone, cholesterol, cortisol, cortisone, dehydroepiandrosterone, estradiol, estrone, pregnenolone, progesterone, testosterone and bile acids (cholic acid). First, we considered the drug-metabolizing P450s. 6β-Hydroxylation of many steroids, including cortisol, cortisone, progesterone and testosterone, was catalyzed primarily by CYP3A4. CYP1A2 and CYP3A4, respectively, are likely the major hepatic enzymes responsible for 2-/4-hydroxylation and 16α-hydroxylation of estradiol and estrone, steroids that can contribute to breast cancer risk. In contrast, CYP1A1 and CYP1B1 predominantly metabolized estrone and estradiol to 2- and 4-catechol estrogens, which are endogenous ultimate carcinogens if formed in the breast. Some metabolic activities of CYP3A4, including dehydroepiandrosterone 7β-/16α-hydroxylation, estrone 2-hydroxylation and testosterone 6β-hydroxylation, were higher than those for polymorphically expressed CYP3A5. Next, we considered typical steroidogenic P450s. CYP17A1, CYP19A1 and CYP27A1 catalyzed steroid synthesis, including hydroxylation at 17α, 19 and 27 positions, respectively. However, it was difficult to predict which hepatic drug-metabolizing P450 or steroidogenic P450 will be mainly responsible for metabolizing each steroid hormone in vivo based on these results. Further research is required on the metabolism of steroid hormones by various P450s and on prediction of their relative contributions to in vivo metabolism. The findings collected here provide fundamental and useful information on the metabolism of steroid compounds.

Cytochrome b5 is a major determinant of human cytochrome P450 CYP2D6 and CYP3A4 activity in vivo

The cytochrome P450-dependent mono-oxygenase system is responsible for the metabolism and disposition of chemopreventive agents, chemical toxins and carcinogens, and >80% of therapeutic drugs. Cytochrome P450 (P450) activity is regulated transcriptionally and by the rate of electron transfer from P450 reductase. In vitro studies have demonstrated that cytochrome b5 (Cyb5) also modulates P450 function. We recently showed that hepatic deletion of Cyb5 in the mouse (HBN) markedly alters in vivo drug pharmacokinetics; a key outstanding question is whether Cyb5 modulates the activity of the major human P450s in drug disposition in vivo. To address this, we crossed mice humanized for CYP2D6 or CYP3A4 with mice carrying a hepatic Cyb5 deletion. In vitro triazolam 4-hydroxylation (probe reaction for CYP3A4) was reduced by >50% in hepatic microsomes from CYP3A4-HBN mice compared with controls. Similar reductions in debrisoquine 4-hydroxylation and metoprolol α-hydroxylation were observed using CYP2D6-HBN microsomes, indicating a significant role for Cyb5 in the activity of both enzymes. This effect was confirmed by the concentration-dependent restoration of CYP3A4-mediated triazolam turnover and CYP2D6-mediated bufuralol and debrisoquine turnover on addition of Escherichia coli membranes containing recombinant Cyb5. In vivo, the peak plasma concentration and area under the concentration time curve from 0 to 8 hours (AUC0-8 h) of triazolam were increased 4- and 5.7-fold, respectively, in CYP3A4-HBN mice. Similarly, the pharmacokinetics of bufuralol and debrisoquine were significantly altered in CYP2D6-HBN mice, the AUC0-8 h being increased ∼1.5-fold and clearance decreased by 40-60%. These data demonstrate that Cyb5 can be a major determinant of CYP3A4 and CYP2D6 activity in vivo, with a potential impact on the metabolism, efficacy, and side effects of numerous therapeutic drugs.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

Regulation of cytochrome b5 expression by miR-223 in human liver: effects on cytochrome P450 activities

PURPOSE

Cytochrome b5 (b5) is a hemoprotein that transfers electrons to several enzymes to fulfill functions in fatty acid desaturation, methemoglobin reduction, steroidogenesis, and drug metabolism. Despite the importance of b5, the regulation of b5 expression in human liver remains largely unknown. We investigated whether microRNA (miRNA) might be involved in the regulation of human b5.

METHODS

Twenty-four human liver specimens were used for correlation analysis. In silico analysis and luciferase assay were performed to determine whether the predicted miRNAs functionally target to b5. The miR-223 was overexpressed into HepG2 cells infected with adenovirus expressing human cytochrome P450.

RESULTS

In human livers, the b5 protein levels were not positively correlated with the b5 mRNA levels, and miR-223 levels were inversely correlated with the b5 mRNA levels or the translational efficiencies. The luciferase assay showed that miR-223 functionally binds to the element in the 3′-untranslated region of b5 mRNA. The overexpression of miR-223 significantly reduced the endogenous b5 protein level and the mRNA stability in HepG2 cells. Moreover, the overexpression of miR-223 significantly reduced CYP3A4-catalyzed testosterone 6β-hydroxylation activity and CYP2E1-catalyzed chlorzoxazone 6-hydroxylase activity but not CYP1A2-catalyzed 7-ethoxyresorufin O-deethylase activity.

CONCLUSIONS

miR-223 down-regulates b5 expression in the human liver, modulating P450 activities.

Flexible docking-based molecular dynamics/steered molecular dynamics calculations of protein-protein contacts in a complex of cytochrome P450 1A2 with cytochrome b5

Formation of transient complexes of cytochrome P450 (P450) with another protein of the endoplasmic reticulum membrane, cytochrome b5 (cyt b5), dictates the catalytic activities of several P450s. Therefore, we examined formation and binding modes of the complex of human P450 1A2 with cyt b5. Docking of soluble domains of these proteins was performed using an information-driven flexible docking approach implemented in HADDOCK. Stabilities of the five unique binding modes of the P450 1A2-cyt b5 complex yielded by HADDOCK were evaluated using explicit 10 ns molecular dynamics (MD) simulations in aqueous solution. Further, steered MD was used to compare the stability of the individual P450 1A2-cyt b5 binding modes. The best binding mode was characterized by a T-shaped mutual orientation of the porphyrin rings and a 10.7 Å distance between the two redox centers, thus satisfying the condition for a fast electron transfer. Mutagenesis studies and chemical cross-linking, which, in the absence of crystal structures, were previously used to deduce specific P450-cyt b5 interactions, indicated that the negatively charged convex surface of cyt b5 binds to the positively charged concave surface of P450. Our simulations further elaborate structural details of this interface, including nine ion pairs between R95, R100, R138, R362, K442, K455, and K465 side chains of P450 1A2 and E42, E43, E49, D65, D71, and heme propionates of cyt b5. The universal heme-centric system of internal coordinates was proposed to facilitate consistent classification of the orientation of the two porphyrins in any protein complex.

Kinetic analysis of lauric acid hydroxylation by human cytochrome P450 4A11

Cytochrome P450 (P450) 4A11 is the only functionally active subfamily 4A P450 in humans. P450 4A11 catalyzes mainly ω-hydroxylation of fatty acids in liver and kidney; this process is not a major degradative pathway, but at least one product, 20-hydroxyeicosatetraenoic acid, has important signaling properties. We studied catalysis by P450 4A11 and the issue of rate-limiting steps using lauric acid ω-hydroxylation, a prototypic substrate for this enzyme. Some individual reaction steps were studied using pre-steady-state kinetic approaches. Substrate and product binding and release were much faster than overall rates of catalysis. Reduction of ferric P450 4A11 (to ferrous) was rapid and not rate-limiting. Deuterium kinetic isotope effect (KIE) experiments yielded low but reproducible values (1.2–2) for 12-hydroxylation with 12-²H-substituted lauric acid. However, considerable “metabolic switching” to 11-hydroxylation was observed with [12-²H₃]lauric acid. Analysis of switching results [Jones, J. P., et al. (1986) J. Am. Chem. Soc.108, 7074–7078] and the use of tritium KIE analysis with [12-³H]lauric acid [Northrop, D. B. (1987) Methods Enzymol.87, 607–625] both indicated a high intrinsic KIE (>10). Cytochrome b₅ (b₅) stimulated steady-state lauric acid ω-hydroxylation ∼2-fold; the apoprotein was ineffective, indicating that electron transfer is involved in the b₅ enhancement. The rate of b₅ reoxidation was increased in the presence of ferrous P450 mixed with O₂. Collectively, the results indicate that both the transfer of an electron to the ferrous·O₂ complex and C–H bond-breaking limit the rate of P450 4A11 ω-oxidation.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Variability in human in vitro enzyme kinetics

There are many factors which are known to cause variability in human in vitro enzyme kinetic data. Factors such as the source of enzyme and how it was prepared, the genetics and background of the donor, how the in vitro studies are designed, and how the data are analyzed contribute to variability in the resulting kinetic parameters. It is important to consider not only the factors which cause variability within an experiment, such as selection of a probe substrate, but also those that cause variability when comparing kinetic data across studies and laboratories. For example, the artificial nature of the microsomal lipid membrane and microenvironment in some recombinantly expressed enzymes, relative to those found in native tissue microsomes, has been shown to influence enzyme activity and thus can be a source of variability when comparing across the two different systems. All of these factors, and several others, are discussed in detail in the chapter below.

Two surfaces of cytochrome b5 with major and minor contributions to CYP3A4-catalyzed steroid and nifedipine oxygenation chemistries

Conserved human cytochrome b5 (b5) residues D58 and D65 are critical for interactions with CYP2E1 and CYP2C19, whereas E48 and E49 are essential for stimulating the 17,20-lyase activity of CYP17A1. Here, we show that b5 mutations E48G, E49G, D58G, and D65G have reduced capacity to stimulate CYP3A4-catalyzed progesterone and testosterone 6β-hydroxylation or nifedipine oxidation. The b5 double mutation D58G/D65G fails to stimulate these reactions, similar to CYP2E1 and CYP2C19, whereas mutation E48G/E49G retains 23-42% of wild-type stimulation. Neither mutation impairs the activity stimulation of wild-type b5, nor does mutation D58G/D65G impair the partial stimulation of mutations E48G or E48G/E49G. For assays reconstituted with a single phospholipid, phosphatidyl serine afforded the highest testosterone 6β-hydroxylase activity with wild-type b5 but the poorest activity with b5 mutation E48G/E49G, and the activity stimulation of mutation E48G/E49G was lost at [NaCl]>50mM. Cross-linking of CYP3A4 and b5 decreased in the order wild-type>E48G/E49G>D58G/D65G and varied with phospholipid. We conclude that two b5 acidic surfaces, primarily the domain including residues D58-D65, participate in the stimulation of CYP3A4 activities. Our data suggest that a minor population of CYP3A4 molecules remains sensitive to b5 mutation E48G/E49G, consistent with phospholipid-dependent conformational heterogeneity of CYP3A4.

CYP3A4 intron 6 C>T polymorphism (CYP3A4*22) is associated with reduced CYP3A4 protein level and function in human liver microsomes

Effects of the CYP3A4 intron 6 C>T (CYP3A4*22) polymorphism, which has recently been reported to have a critical role in vivo, were investigated by measuring CYP3A4 protein expression levels and CYP3A4-dependent drug oxidation activities in individual human liver microsomes in vitro. Prior to protein analysis, analysis of DNA samples indicated that 36 Caucasian subjects were genotyped as CYP3A4*1/*1 and five subjects were CYP3A4*1/*22, with a CYP3A4*22 allelic frequency of 6.1%. No CYP3A4*22 alleles were found in the Japanese samples (106 alleles). Individual differences in CYP2D6-dependent dextromethorphan O-demethylation activities in liver microsomes from Caucasians were not affected by either the CYP3A4*1/*22 or CYP3A5*1/*3 genotype. Liver microsomes genotyped as CYP3A4*1/*22 (n = 4) showed significantly lower CYP3A-dependent dextromethorphan N-demethylation, midazolam 1'-hydroxylation, and testosterone 6β-hydroxylation activities, as well as lower expression levels of CYP3A protein (28% of control), compared with those of the CYP3A4*1/*1 group (n = 19). The other polymorphism, CYP3A5*1/*3, did not show these differences (n = 4). The CYP3A4*22 polymorphism was associated with reduced CYP3A4 protein expression levels and resulted in decreased CYP3A4-dependent activities in human livers. The present results suggest an important role of low expression of CYP3A4 protein associated with the CYP3A4*22 allele in the individual differences in drug clearance.

A role for cytochrome b5 in the In vivo disposition of anticancer and cytochrome P450 probe drugs in mice

The role of microsomal cytochrome b5 (Cyb5) in defining the rate of drug metabolism and disposition has been intensely debated for several decades. Recently we described mouse models involving the hepatic or global deletion of Cyb5, demonstrating its central role in in vivo drug disposition. We have now used the cytochrome b5 complete null (BCN) model to determine the role of Cyb5 in the metabolism of ten pharmaceuticals metabolized by a range of cytochrome P450s, including five anticancer drugs, in vivo and in vitro. The extent to which metabolism was significantly affected by the absence of Cyb5 was substrate-dependent; AUC increased (75-245%) and clearance decreased (35-72%) for phenacetin, metoprolol, and chlorzoxazone. Tolbutamide disposition was not significantly altered by Cyb5 deletion, while for midazolam clearance was decreased by 66%. The absence of Cyb5 had no effect on gefitinib and paclitaxel disposition, while significant changes in the in vivo pharmacokinetics were measured for: cyclophosphamide [maximum plasma concentration (Cmax) and terminal half-life increased 55% and 40%, respectively], tamoxifen (AUClast and Cmax increased 370% and 233%, respectively), and anastrozole (AUC and terminal half-life increased 125% and 62%, respectively; clearance down 80%). These data provide strong evidence that both hepatic and extrahepatic Cyb5 levels are an important determinant of in vivo drug disposition catalyzed by a range of cytochrome P450s, including currently prescribed anticancer agents, and that individuality in Cyb5 expression could be a significant determinant in rates of drug disposition in man.

Specificity determinants of CYP1B1 estradiol hydroxylation

Cytochrome P450 (P450)-catalyzed oxidation of the aromatic ring of estradiol can result in 2- or 4-hydroxylation. Which of these products is formed is biologically important, as the 4-hydroxylated metabolite is carcinogenic, whereas the 2-hydroxylated metabolite is not. Most human P450 enzymes, including CYP1A1 and CYP1A2, exhibit a high preference for estradiol 2-hydroxylation, but human CYP1B1 greatly favors 4-hydroxylation. Here we show that heterologous expression of the human, monkey, dog, rat, and mouse CYP1B1 enzymes yields active proteins that differ in their estradiol hydroxylation specificity. The monkey and dog orthologs, like the human enzyme, preferentially catalyze 4-hydroxylation, but the rat and mouse enzymes favor 2-hydroxylation. Analysis of the CYP1B1 sequences in light of these findings suggested that one residue, Val395 in human CYP1B1, could account for the differential hydroxylation specificities. In fact, mutation of this valine in human CYP1B1 to the leucine present in the rat enzyme produces a human enzyme that has the 2-hydroxylation specificity of the rat enzyme. The converse is true when the leucine in the rat enzyme is mutated to the human valine. The role of CYP1B1 in estradiol carcinogenicity thus depends on the identity of this single amino acid residue.

Drug interactions of diclofenac and its oxidative metabolite with human liver microsomal cytochrome P450 1A2-dependent drug oxidation

1.  The purpose of this study was to investigate the inhibitory effects of diclofenac on human cytochrome P450 1A2-, 2C19- and 3A4-mediated drug oxidations and to evaluate the drug interaction potential of diclofenac and 4'-hydroxydiclofenac. 2.  Diclofenac was converted to 4'-hydroxydiclofenac by recombinantly expressed human P450 1A2 with Km and Vmax values of 33 µM and 0.20 min(-1), respectively. Diclofenac and 4'-hydroxydiclofenac suppressed flurbiprofen 4'-hydroxylation by P450 2C9 strongly and moderately, respectively; however, they did not affect P450 2C19-dependent S-mephenytoin hydroxylation or P450 3A4-dependent midazolam hydroxylation. 3.  Although the caffeine 3-N-demethylation activity of liver microsomal P450 1A2 was inhibited by simultaneous incubation with diclofenac, the riluzole N-hydroxylation activities of recombinant P450 1A2 and human liver microsomes were inhibited after preincubation with diclofenac or 4'-hydroxydiclofenac for 20 min in the presence of NADPH. Using the inhibition constant (37 µM) of diclofenac on caffeine 3-N-demethylation and the reported 95th percentiles of maximum plasma concentration (10.5 µM) after an oral dose of diclofenac, the in vivo estimated increase in area under the plasma concentration-time curve was 29%. 4.  These results suggest that diclofenac could inhibit drug clearance to a clinically important degree that depends on P450 1A2. Clinically relevant drug interactions in vivo with diclofenac are likely to be invoked via human P450 1A2 function in addition to those caused by the effect of diclofenac on P450 2C9.

Functional characterization of eight human CYP1A2 variants: the role of cytochrome b5

BACKGROUND

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.

OBJECTIVE

The aim of this work was to examine the effect of b5 on the activities of eight natural-occurring variants of human CYP1A2, namely, T83M, S212C, S298R, G299S, I314V, I386F, C406Y, and R456H.

MATERIALS AND METHODS

An approach, as used in our former study was applied, coexpressing these polymorphic CYP1A2 variants separately with CYPOR and b5 in the bacterial cell model BTC-CYP. For each variant, 16 different activity parameters were measured, using eight different substrates. This heterogeneous data set was merged with the one of our former study (i.e. without b5) and a multivariate analysis was carried out.

RESULTS

This analysis indicated that b5 seems to have the ability to affect CYP1A2 variants to behave more like the wild-type variant. This was especially the case for variant I386F, for which the presence of b5 was crucial to show activity. Variants T83M and C406Y showed considerably different activity-profiles when in the presence of b5. Furthermore, our data seem to implicate CYP1A2 residue G299 in its interaction with CYPOR and/or b5.

CONCLUSION

Results indicate the ability of b5 to affect CYP1A2 variants to behave more like the wild-type variant, attenuating detrimental effects of structural mutations of these variants, seemingly through extensive allosteric effects.

Protein expression profiling of nuclear membrane protein reveals potential biomarker of human hepatocellular carcinoma

Background

Complex molecular events lead to development and progression of liver cirrhosis to HCC. Differentially expressed nuclear membrane associated proteins are responsible for the functional and structural alteration during the progression from cirrhosis to carcinoma. Although alterations/ post translational modifications in protein expression have been extensively quantified, complementary analysis of nuclear membrane proteome changes have been limited. Deciphering the molecular mechanism that differentiate between normal and disease state may lead to identification of biomarkers for carcinoma.

Results

Many proteins displayed differential expression when nuclear membrane proteome of hepatocellular carcinoma (HCC), fibrotic liver, and HepG2 cell line were assessed using 2-DE and ESI-Q-TOF MS/MS. From the down regulated set in HCC, we have identified for the first time a 15 KDa cytochrome b5A (CYB5A), ATP synthase subunit delta (ATPD) and Hemoglobin subunit beta (HBB) with 11, 5 and 22 peptide matches respectively. Furthermore, nitrosylation studies with S-nitrosocysteine followed by immunoblotting with anti SNO-cysteine demonstrated a novel and biologically relevant post translational modification of thiols of CYB5A in HCC specimens only. Immunofluorescence images demonstrated increased protein S-nitrosylation signals in the tumor cells and fibrotic region of HCC tissues. The two other nuclear membrane proteins which were only found to be nitrosylated in case of HCC were up regulated ATP synthase subunit beta (ATPB) and down regulated HBB. The decrease in expression of CYB5A in HCC suggests their possible role in disease progression. Further insight of the functional association of the identified proteins was obtained through KEGG/ REACTOME pathway analysis databases. String 8.3 interaction network shows strong interactions with proteins at high confidence score, which is helpful in characterization of functional abnormalities that may be a causative factor of liver pathology.

Conclusion

These findings may have broader implications for understanding the mechanism of development of carcinoma. However, large scale studies will be required for further verification of their critical role in development and progression of HCC.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Monkey liver cytochrome P450 2C9 is involved in caffeine 7-N-demethylation to form theophylline

Caffeine (1,3,7-trimethylxanthine) is a phenotyping substrate for human cytochrome P450 1A2. 3-N-Demethylation of caffeine is the main human metabolic pathway, whereas monkeys extensively mediate the 7-N-demethylation of caffeine to form pharmacological active theophylline. Roles of monkey P450 enzymes in theophylline formation from caffeine were investigated using individual monkey liver microsomes and 14 recombinantly expressed monkey P450 enzymes, and the results were compared with those for human P450 enzymes. Caffeine 7-N-demethylation activity in microsomes from 20 monkey livers was not strongly inhibited by α-naphthoflavone, quinidine or ketoconazole, and was roughly correlated with diclofenac 4'-hydroxylation activities. Monkey P450 2C9 had the highest activity for caffeine 7-N-demethylation. Kinetic analysis revealed that monkey P450 2C9 had a high Vmax/Km value for caffeine 7-N-demethylation, comparable to low Km value for monkey liver microsomes. Caffeine could dock favorably with monkey P450 2C9 modeled for 7-N-demethylation and with human P450 1A2 for 3-N-demethylation. The primary metabolite theophylline was oxidized to 8-hydroxytheophylline in similar ways by liver microsomes and by recombinant P450s in both humans and monkeys. These results collectively suggest a high activity for monkey liver P450 2C9 toward caffeine 7-N-demethylation, whereas, in humans, P450 1A2-mediated caffeine 3-N-demethylation is dominant.