Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:1.14.13.97 (CYP3A4)
 6,365 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Cytochromes P450 comprise a remarkably diverse superfamily of heme-thiolate proteins critical in the metabolism of numerous endogenous ligands and xenobiotics. Among the myriad of P450 substrates are many compounds of toxicological and pharmacological significance. The precise complement of cytochrome P450 isoforms in any given tissue may therefore be an important determinant of susceptibility to chemical-mediated toxicity. We have used a histological approach to study the distribution of individual P450s in human and rabbit gastro-intestinal tissues. We have focused primarily on P450 enzymes of importance in the metabolism of carcinogens, namely CYP1A1, CYP1A2, CYP2E1, CYP3A4/3A5 and CYP4B1. Here we give an overview of the distribution of these enzymes in human and rabbit tissues and discuss the possible toxicological implications of the results. In addition we will discuss the value of archival human tissue specimens for histological analysis of P450 distribution.
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PMID:Localization of cytochromes P450 in human tissues: implications for chemical toxicity. 874 22

The calcium channel blocker verapamil [2,8-bis-(3,4-dimethoxyphenyl)-6-methyl-2-isopropyl-6-azaoctanitrile+ ++] undergoes extensive biotransformation in man. We have previously demonstrated cytochrome P450 (CYP) 3A4 and 1A2 to be the enzymes responsible for verapamil N-dealkylation (formation of D-617 [2-(3,4-dimethoxyphenyl)-5-methylamino-2-isopropylvaleronitrile], and verapamil N-demethylation (formation of norverapamil [2,8-bis-(3,4-dimethoxyphenyl)-2-isopropyl-6-azaoctanitrile]), while there was no involvement of CYP3A4 and CYP1A2 in the third initial metabolic step of verapamil, which is verapamil O-demethylation. This pathway yields formation of D-703 [2-(4-hydroxy-3-methoxyphenyl)-8-(3,4-dimethoxyphenyl)-6-methyl-2-isopro pyl-6-azaoctanitrile] and D-702 [2-(3,4-dimethoxyphenyl)-8-(4-hydroxy-3-methoxyphenyl)-6-methyl-2-isopro pyl-6-azaoctanitrile]. The enzymes catalyzing verapamil O-demethylation have not been characterized so far. We have therefore identified and characterized the enzymes involved in verapamil O-demethylation in humans by using the following in vitro approaches: (I) characterization of O-demethylation kinetics in the presence of the microsomal fraction of human liver, (II) inhibition of verapamil O-demethylation by specific antibodies and selective inhibitors and (III) investigation of metabolite formation in microsomes obtained from yeast strain Saccharomyces cerevisiae W(R), that was genetically engineered for stable expression of human CYP2C8, 2C9 and 2C18. In human liver microsomes (n=4), the intrinsic clearance (CLint), as derived from the ratio of Vmax/Km, was significantly higher for O-demethylation to D-703 compared to formation of D-702 following incubation with racemic verapamil (13.9 +/- 1.0 vs 2.4 +/- 0.6 ml*min-1*g-1, mean+/-SD; p<0.05), S-verapamil (16.8 +/- 3.3 vs 2.2 +/- 1.2 ml* min-1*g-1, p<0.05) and R-verapamil (12.1 +/- 2.9 vs 3.6 +/- 1.3 ml*min-1*g-1; p<0.05), thus indicating regioselectivity of verapamil O-demethylation process. The CLint of D-703 formation in human liver microsomes showed a modest but significant degree of stereoselectivity (p<0.05) with a S/R-ratio of 1.41 +/- 0.17. Anti-LKM2 (anti-liver/kidney microsome) autoantibodies (which inhibit CYP2C9 and 2C19) and sulfaphenazole (a specific CYP2C9 inhibitor) reduced the maximum rate of formation of D-703 by 81.5 +/- 4.5% and 45%, that of D-702 by 52.7 +/- 7.5% and 72.5%, respectively. Both D-703 and D-702 were formed by stably expressed CYP2C9 and CYP2C18, whereas incubation with CYP2C8 selectively yielded D-703. In conclusion, our results show that enzymes of the CYP2C subfamily are mainly involved in verapamil O-demethylation. Verapamil therefore has the potential to interact with other drugs which inhibit or induce these enzymes.
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PMID:Cytochromes of the P450 2C subfamily are the major enzymes involved in the O-demethylation of verapamil in humans. 875 Sep 25

Various complementary approaches were used to elucidate the major cytochrome P450 (CYP) enzyme responsible for mifepristone (RU 486) demethylation and hydroxylation in human liver microsomes: chemical and immunoinhibition of specific CYPs; correlation analyses between initial rates of mifepristone metabolism and relative immunodetectable CYP levels and rates of CYP marker substrate metabolism; and evaluation of metabolism by cDNA-expressed CYP3A4. Human liver microsomes catalyzed the demethylation of mifepristone with mean (+/-SD) apparent K(m) and Vmax values of 10.6 +/- 3.8 microM and 4920 +/- 1340 pmol/min/mg protein, respectively; the corresponding values for hydroxylation of the compound were 9.9 +/- 3.5 microM and 610 +/- 260 pmol/min/mg protein. Progesterone and midazolam (CYP3A4 substrates) inhibited metabolite formation by up to 77%. The CYP3A inhibitors gestodene, triacetyloleandomycin, and 17 alpha-ethynylestradiol inhibited mifepristone demethylation and hydroxylation by 70-80%; antibodies to CYP3A4 inhibited these reactions by approximately 82 and 65%, respectively. In a bank of human liver microsomes from 14 donors, rates of mifepristone metabolism correlated significantly with relative immunodetectable CYP3A levels, rates of midazolam 1'-and 4-hydroxylation and rates of erythromycin N-demethylation, marker CYP3A catalytic activities (all r2 > or = 0.85 and P < 0.001). No significant correlations were observed for analyses with relative immunoreactive levels or marker catalytic activities of CYP1A2, CYP2C9, CYP2C19, CYP2D6, or CYP2E1. Recombinant CYP3A4 catalyzed mifepristone demethylation and hydroxylation with apparent K(m) values 7.4 and 4.1 microM, respectively. Collectively, these data clearly support CYP3A4 as the enzyme primarily responsible for mifepristone demethylation and hydroxylation in human liver microsomes.
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PMID:Identification of CYP3A4 as the principal enzyme catalyzing mifepristone (RU 486) oxidation in human liver microsomes. 876 73

In this paper we describe the kinetics of formation of 1-methylxanthine (1-MX), 3-methylxanthine (3-MX) and 1,3-dimethyluric acid (1,3-DMU) from theophylline in human liver microsomal incubations and use the selective inhibitor approach to define the role of the individual cytochrome P450s (CYP) in each pathway. A biphasic model fitted the data best for the formation of each metabolite. The high-affinity site Km and Vmax values were: 1-MX, Km = 0.29 +/- 0.21 mM, Vmax = 5.92 +/- 3.74 pmol.mg(-1).min(-1) (mean +/- S.D.; n = 4); 3-MX, Km = 0.28 +/- 0.08 mM, Vmax = 3.32 +/- 2.19 pmol.mg(-1).min(-1); 1,3-DMU,Km = 0.31 +/- 0.14 mM, Vmax = 43.3 +/- 9.3 pmol.mg(-1).min(-1). The relative contribution of the high- and the low-affinity enzymes in 1,3-DMU formation was calculated based on the enzyme kinetic parameters. To characterize the high-affinity site, a range of CYP isozyme substrates and inhibitors were incubated with 100 microM theophylline. The CYP1A2 inhibitors furafylline, ellipticine and alpha-naphthoflavone were potent inhibitors of both 1-MX and 3-MX formation with more that 80% of N-demethylase activities inhibited below a concentration of 5 microM. These compounds also markedly inhibited 1,3-DMU formation. Enzyme kinetic and selective inhibition data indicated that about 80% of 1,3-DMU formation was catalyzed by the high-affinity isoform (CYP1A2) at a theophylline concentration of 100 microM. To investigate the role of other isoforms in 8-hydroxylation, experiments were performed involving incubation with a combination of inhibitors. It is evident that in addition to CYP1A2, CYP2E1 has a minor role om 8-hydroxylation. This based on the fact that 80% inhibition was seen on preincubation with furafylline and about 90% inhibition on preincubation with furafylline plus diethyldithiocarbamate. Low concentrations of ketoconazole (selective for CYP3A4) only produced marginal inhibition of 1,3-DMU and, therefore, CYP3A4 is only of minor significance in this reaction. Human B-lymphoblastoid cell lines expressing CYP1A2 catalyzed theophylline metabolism with formation of 1-MX, 3-MX and 1,3-MDU. CYP2E1 cells also catalyzed formation of 1,3-DMU. The CYP3A4 cell line did not catalyze theophylline metabolism.
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PMID:Theophylline metabolism in human liver microsomes: inhibition studies. 878 69

Gingival hyperplasia is a well-known complication of therapy with cyclosporine, calcium channel blockers, and phenytoin. It is characterized by the presence of inflammation and a marked fibrotic response. The mechanism of this adverse reaction is unknown. We propose that it may be initiated by the metabolic activation of these drugs to form reactive metabolites. These then cause cellular injury and lead to the gingival hyperplasia. To evaluate this hypothesis we examined phenytoin metabolism and the cytochrome P450 contents of gingival tissues from 10 patients undergoing surgery for various periodontal conditions. We found that microsomes obtained from the gingiva show significant phenytoin hydroxylase activity as determined by the production of 5-(4'-hydroxyphenyl)-5-phenylhydantoin (HPPH) (range, 12.8 pmol HPPH/min.mg microsomal protein to 276.9 pmol HPPH/min.mg microsomal protein; rat control, 133.7 +/- 11.5 pmol HPPH/min.mg microsomal protein). We also found that CYP1A1, CYP1A2, CYP2C9, CYP2E1, and CYP3A4 were present in these microsomes. We detected no CYP2B6 or CYP2D6. We believe that these data support our hypothesis that the proliferative inflammation observed with drugs such as phenytoin, nifedipine, and cyclosporine may be initiated by the formation of reactive metabolites and that the formation of these metabolites may be catalyzed by one or more CYPs found in the gingiva. These metabolites may then cause cellular injury and induce a reactive inflammatory response, followed by fibroblastic proliferation. This proliferation leads to the excess collagen deposition observed with gingival hyperplasia.
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PMID:Metabolism of phenytoin by the gingiva of normal humans: the possible role of reactive metabolites of phenytoin in the initiation of gingival hyperplasia. 882 37

Cytochrome P450 (CYP) activity in human liver microsomes was measured after the O-demethylation of [O-methyl 14C]naproxen (NAPase). The formation of [14C]formaldehyde in the presence of microsomes was described by an apparent KM(1) and Vmax(1) of 0.16 +/- 0.09 mM and 4.1 +/- 2.8 nmol HCHO/min/mg protein (mean +/- SD; N = 5 different livers), respectively, over a relatively wide naproxen concentration (5-1600 microM) range. With two sets of microsomes, a high KM NAPase component was also detected (mean KM2 = 2.7 mM; mean Vmax2 = 23 nmol HCHO/min/mg). As expected, the O-demethylation of naproxen (0.4 mM) was found to be highly correlated with tolbutamide hydroxylase (TOLase) activity in a panel of human liver microsomes (r = 0.82, p < 0.01, N = 10) and was inhibited (32-54%) by a number of purported CYP2C (CYP2C9/10) inhibitors/substrates (e.g. phenytoin, sulfaphenazole, tienilic acid, tolbutamide, and ibuprofen). Only marginal decreases in activity (< or = 14%) were observed with inhibitors of other CYP proteins. However, NAPase activity was also found to correlate significantly with CYP1A2 [ethoxyresorufin O-deethylase (ERODase)] activity (r = 0.68, p < 0.05, n = 11). In addition, the reaction was inhibited (36-75%, N = 11 different livers) by furafylline (FURA), a CYP1A2-selective mechanism-based inhibitor. The effect of FURA and tienilic acid was additive, leading to 90 +/- 4.2% inhibition of NAPase activity. FURA-inhibited activity also significantly correlated with ERODase activity (r = 0.78, p < 0.01, N = 11), whereas tienilic acid-inhibited activity correlated with TOLase activity (r = 0.63, p < 0.05, N = 10). In human B-lymphoblast microsomes, cDNA-expressed CYP1A2 exhibited relatively high activity (KM = 0.25 mM; Vmax = 24 nmol/min/nmol CYP), when compared with CYP2A6, CYP2D6, CYP2E1, CYP2B6, and CYP3A4. The kinetic parameters for reconstituted purified human liver microsomal CYP2C9 (KM = 0.43 mM; Vmax = 11 nmol/min/nmol CYP) were comparable with those of CYP1A2. It is concluded that the O-demethylation of naproxen (< or = 0.4 mM) is catalyzed by CYP2C subfamily members (CYP2C9/10) and CYP1A2 in human liver microsomes.
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PMID:[O-methyl 14C]naproxen O-demethylase activity in human liver microsomes: evidence for the involvement of cytochrome P4501A2 and P4502C9/10. 882

The pharmacokinetics of fluvoxamine, a selective serotonin reuptake inhibitor (SSRI) with antidepressant properties, are well established. After oral administration, the drug is almost completely absorbed from the gastrointestinal tract, and the extent of absorption is unaffected by the presence of food. Despite complete absorption, oral bioavailability in man is approximately 50% on account of first-pass hepatic metabolism. Peak plasma fluvoxamine concentrations are reached 4 to 12 hours (enteric-coated tablets) or 2 to 8 hours (capsules, film-coated tablets) after administration. Steady-state plasma concentrations are achieved within 5 to 10 days after initiation of therapy and are 30 to 50% higher than those predicted from single dose data. Fluvoxamine displays nonlinear steady-state pharmacokinetics over the therapeutic dose range, with disproportionally higher plasma concentrations with higher dosages. Plasma fluvoxamine concentrations show no clear relationship with antidepressant response or severity of adverse effects. Fluvoxamine undergoes extensive oxidative metabolism, most probably in the liver. Nine metabolites have been identified, none of which are known to be pharmacologically active. The specific cytochrome P450 (CYP) isoenzymes involved in the metabolism of fluvoxamine are unknown. CYP2D6, which is crucially involved in the metabolism of paroxetine and fluoxetine, appears to play a clinically insignificant role in the metabolism of fluvoxamine. The drug is excreted in the urine, predominantly as metabolites, with only negligible amounts ( < 4%) of the parent compound. Fluvoxamine shows a biphasic pattern of elimination with a mean terminal elimination half-life of 12 to 15 hours after a single oral dose; this is prolonged by 30 to 50% at steady-state. Plasma protein binding of fluvoxamine (77%) is low compared with that of other SSRIs. Fluvoxamine pharmacokinetics are substantially unaltered by increased age or renal impairment. However, its elimination is prolonged in patients with hepatic cirrhosis. Fluvoxamine inhibits oxidative drug metabolising enzymes (particularly CYP1A2, and less potently and much less potently CYP3A4 and CYP2D6, respectively) and has the potential for clinically significant drug interactions. Drugs whose metabolic elimination is impaired by fluvoxamine include tricyclic antidepressants (tertiary, but not secondary, amines), alprazolam, bromazepam, diazepam, theophylline, propranolol, warfarin and, possibly, carbamazepine. Fluvoxamine is a second generation antidepressant that selectively inhibits neuronal reuptake of serotonin (5-hydroxytryptamine; 5-HT). Fluvoxamine exhibits antidepressant activity similar to that of the tricyclic antidepressants, but has a somewhat improved tolerability profile, particularly with respect to a lower incidence of anticholinergic effects and reduced cardiotoxic potential. However, gastrointestinal adverse effects, especially nausea, are seen more frequently with fluvoxamine than with the tricyclic antidepressants. Fluvoxamine does not have an asymmetric carbon in its structure (fig. 1) and therefore does not exist as optical isomers. For this reason, the potentially confounding problem of stereoisomerism does not arise with fluvoxamine.
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PMID:Overview of the pharmacokinetics of fluvoxamine. 884 17

Studies were carried out to test the hypothesis that inflammatory liver disease increases the expression of specific cytochrome P-450 isoenzymes involved in aflatoxin B1 (AFB) activation. The immunohistochemical expression and localization of various human cytochrome P-450 isoforms, including CYP2A6, CYP1A2, CYP3A4, and CYP2B1, were examined in normal human liver and liver with hepatitis and cirrhosis. The constitutive expression of CYP3A4 in normal liver showed a characteristic pattern of distribution in centrilobular hepatocytes, whereas CYP1A2, CYP2A6, and CYP2B1 were expressed uniformly throughout the liver acinus. In sections of liver infected with hepatitis B virus (HBV) or hepatitis C virus (HCV), the expression of CYP2A6 was markedly increased in hepatocytes immediately adjacent to areas of fibrosis and inflammation. CYP3A4 and CYP2B1 were induced to a lesser degree, and expression of CYP1A2 was unaffected. In HBV-infected liver, double immunostaining revealed that overexpression of CYP2A6 occurred in hepatocytes expressing the HBV core antigen. In HCV-infected liver, CYP2A6, CYP3A4, and CYP2B1 were overexpressed in hepatocytes with hemosiderin pigmentation. These results suggest that alterations in phenotypic expression of specific P-450 isoenzymes in hepatocytes associated with hepatic inflammation and cirrhosis might increase susceptibility to AFB genotoxicity.
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PMID:Overexpression of cytochrome P-450 isoforms involved in aflatoxin B1 bioactivation in human liver with cirrhosis and hepatitis. 886 87

We systematically characterized the levels and substrate specificity of P450s from humans and rats to extrapolate drug metabolism data from experimental animals to humans. Human P450s (CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C18, 2D6, 2E1, and 3A4) were expressed in Saccharomyces cerevisiae and purified. Rat P450s were purified from hepatic microsomes of rats. We investigated the catalytic activities of purified P450s in a reconstituted system. Human CYP2B6 and rat CYP2B1 had high lidocaine N-deethylation activity. Human and rat CYP2D forms had high debrisoquine 4-hydroxylation activity. Human CYP3A4 and rat CYP3A2 had high testosterone 2 beta- and 6 beta-hydroxylation activities in a modified reconstituted system with a lipid mixture. The hydroxylation site of testosterone by CYP2B6 (16 alpha- and 16 beta-positions) agreed with that by rat CYP2B1. Human CYP2E1 had the highest lauric acid (omega-1)-hydroxylation activity and also had catalytic properties similar to those of rat CYP2E1. Human CYP2A and 2C forms had catalytic properties in testosterone metabolism different from those of rats. Antibodies raised against purified P450s were used to measure the levels of hepatic P450s. The level of CYP3A4 was the highest in human hepatic microsomes, comprising 30-40% of the total P450. CYP2C9 comprised 10-20% of the total. The levels of CYP1A2, 2A6, 2C8, 2D6, and 2E1 were moderate (5-15% of total P450). CYP2B6 content was very low. The information of this study is useful for drug metabolism and toxicological studies.
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PMID:Multiple forms of human P450 expressed in Saccharomyces cerevisiae. Systematic characterization and comparison with those of the rat. 886 26

Cytochrome P-450 (CYP) catalyzes phase I metabolic reactions of psychotropic drugs. The main isoenzymes responsible for their biotransformation are CYP1A2, CYP2D6, CYP3A4 and these of the subfamily CYP2C. The majority of metabolites of psychotropic drugs are biologically active. Some of them retain pharmacological properties of parent compounds (eg. selective serotonin reuptake inhibitors, risperidone, carbamazepine, benzodiazepines), but others display quite different (eg. amitriptyline, buspirone) or even opposite (trazodone) profiles. They are present in vivo in concentrations high enough to contribute to pharmacological and clinical effects of the administrated drugs. Active metabolites of psychotropics are also characterized by pharmacokinetic properties different from their parent compounds, e.g. half-life time, plasma protein binding, blood-brain-barrier penetration, the cerebrospinal fluid (CSF) protein binding and tissue binding. These properties lead, in turn, to differences in the brain/plasma and the CSF/plasma concentration ratios between a drug and its metabolites. Therefore studies relating a pharmacological or therapeutic response of psychotropic drug to its plasma concentrations should not disregard the presence of its active metabolites, considering their distinct pharmacological and pharmacokinetic properties. With regard to a low therapeutic index of psychotropics, interindividual differences in the rate of their metabolism, genetic polymorphism of their main metabolic pathways and metabolic interactions in clinical drug combinations, the phenotyping of patients at the beginning of therapy and a control of drug concentrations (and its active metabolites) at a steady state and during coadministration of another drug, may increase the efficiency and safety of the pharmacotherapy of psychiatric disorders.
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PMID:Metabolism of psychotropic drugs: pharmacological and clinical relevance. 886 27


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