Expression of four rat CYP2D isoforms in Saccharomyces cerevisiae and their catalytic specificity

Kinetics of dextromethorphan-O-demethylase activity and distribution of CYP2D in four commonly-used subcellular fractions of rat brain

The purpose of this study was to compare the enzymatic kinetics and distribution of cytochrome P450 2D (CYP2D) among different rat brain subcellular fractions. Rat brains were used to prepare total membrane, crude mitochondrial, purified mitochondrial, and microsomal fractions, in addition to total homogenate. Michaelis-Menten kinetics of the brain CYP2D activity was estimated based on the conversion of dextromethorphan (DXM) to dextrorphan using UPLC-MS/MS. Protein levels of CYP2D and subcellular markers were determined by Western blot. Microsomal CYP2D exhibited high affinity and low capacity, compared with the mitochondrial CYP2D that had a much lower (∼50-fold) affinity but a higher (∼six-fold) capacity. The apparent CYP2D affinity and capacity of the crude mitochondria were in between those of the microsomes and purified mitochondria. Additionally, the CYP2D activity in the whole homogenate was much higher than that in the total membranes at higher DXM concentrations. A CYP2D immune-reactive band in the brain mitochondria appeared at a lower MW but had a much higher intensity than that in the microsomes. Mitochondrial brain CYP2D has a much higher capacity than its microsomal counterpart. Additionally, brain homogenate is more representative of the overall CYP2D activity than the widely-used total membrane fraction.

Interindividual variations in metabolism and pharmacokinetics of 3-(6-methylpyridine-3-yl-sulfanyl)-6-(4H-[1,2,4]triazole-3-yl-sulfanyl)-N-(1,3-thiazole-2-yl)-2-pyridine carboxamide, a glucokinase activator, in rats caused by the genetic polymorphism of CYP2D1

3-(6-Methylpyridine-3-yl-sulfanyl)-6-(4H-[1,2,4]triazole-3-yl-sulfanyl)-N-(1,3-thiazole-2-yl)-2-pyridine carboxamide (Cpd-D) is a novel glucokinase activator that is being developed for the treatment of type 2 diabetes. Large interindividual variations were observed in the pharmacokinetics of Cpd-D in male Sprague-Dawley (SD) rats, which were subsequently divided into two phenotypes; >6-fold longer terminal-phase half-life and ∼10-fold larger AUC0-∞ values were observed in slow metabolizers (SM) than in fast metabolizers (FM) after the oral administration of Cpd-D. The thiohydantoic acid analog (M2) was the predominant metabolite detected in the urine, bile, and plasma after the oral administration of [(14)C]Cpd-D to the FM phenotypes of bile-duct cannulated SD rats. The liver microsomes prepared from FM phenotyped rats extensively formed M2 with the highest affinity (Km = 0.09 μM) and largest Vmax/Km value in primary metabolism, whereas those from SM phenotypes had little capacity to form M2. Of the rat cytochrome P450 isoforms tested, the formation of M2 was only catalyzed by recombinant CYP2D1. Sequence substitutions (418A/421C and 418G/421T) were detected in the CYP2D1 gene and were designated F and S alleles, respectively. The genotype-phenotype correlation analysis indicated that two S alleles were homozygous (S/S) in the SM phenotypes, whereas the FM phenotypes were either homozygous for the F-alleles (F/F) or heterozygous (F/S). These results indicated that the CYP2D1 polymorphism caused by nucleotide substitutions (418A/421C versus 418G/421T) was responsible for interindividual variations leading to the polymorphism in the major metabolism and pharmacokinetics of Cpd-D in male SD rats.

Simultaneous alterations of brain and plasma serotonin concentrations and liver cytochrome P450 in rats fed on a tryptophan-free diet

Our previous study suggested involvement of the brain serotonergic system in the regulation of liver cytochrome P450 (CYP). The aim of the present study was to demonstrate simultaneous responsiveness of liver CYP and the peripheral and brain serotonergic systems to a tryptophan deficient diet during three days and one or three weeks of ingestion. The concentrations of serotonin, noradrenaline, dopamine and their metabolites were measured in blood plasma, the hypothalamus and brain stem of male rats. The enzyme activity and protein levels in the liver were determined for isoforms CYP1A, CYP2A, CYP2B, CYP2C6, CYP2C11, CYP2D and CYP3A. A three-day tryptophan-free diet increased serotonin content in the hypothalamus (but not in the brain stem or plasma). After one week, the level of serotonin was not changed in the brain, but was markedly increased in the plasma. A three week tryptophan restriction significantly reduced the concentration of serotonin in the brain and plasma. Changes in CYP2C6 and CYP2C11 (an increase and a decrease, respectively) were maintained throughout the experiment, while those found in other CYP isoforms varied, which usually resulted in a gradual increase in the enzyme activity within three weeks. The observed alterations in liver CYPs suggest involvement of both central and peripheral serotonin in the regulation of liver CYP expression whose mechanism is discussed. In conclusion, a deficit in tryptophan in the diet may be responsible for very serious food-cytochrome P450 and food-drug metabolism interactions. Interactions of this type may also refer to drugs acting via serotonergic system.

Functional characterization of a first avian cytochrome P450 of the CYP2D subfamily (CYP2D49)

The CYP2D family members are instrumental in the metabolism of 20-25% of commonly prescribed drugs. Although many CYP2D isoforms have been well characterized in other animal models, research concerning the chicken CYP2Ds is limited. In this study, a cDNA encoding a novel CYP2D enzyme (CYP2D49) was cloned from the chicken liver for the first time. The CYP2D49 cDNA contained an open reading frame of 502 amino acids that shared 52%-57% identities with other CYP2Ds. The gene structure and neighboring genes of CYP2D49 are conserved and similar to those of human CYP2D6. Additionally, similar to human CYP2D6, CYP2D49 is un-inducible in the liver and expressed predominantly in the liver, kidney and small intestine, with detectable levels in several other tissues. Metabolic assays of the CYP2D49 protein heterologously expressed in E. coli and Hela cells indicated that CYP2D49 metabolized the human CYP2D6 substrate, bufuralol, but not debrisoquine. Moreover, quinidine, a potent inhibitor of human CYP2D6, only inhibited the bufuralol 1'-hydroxylation activity of CYP2D49 to a negligible degree. All these results indicated that CYP2D49 had functional characteristics similar to those of human CYP2D6 but measurably differed in the debrisoquine 4'-hydroxylation and quinidine inhibitory profile. Further structure-function investigations that employed site-directed mutagenesis and circular dichroism spectroscopy identified the importance of Val-126, Glu-222, Asp-306, Phe-486 and Phe-488 in keeping the enzymatic activity of CYP2D49 toward bufuralol as well as the importance of Asp-306, Phe-486 and Phe-488 in maintaining the conformation of CYP2D49 protein. The current study is only the first step in characterizing the metabolic mechanism of CYP2D49; further studies are still required.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Functional characterization of two novel CYP2C19 variants (CYP2C19*18andCYP2C19*19) found in a Japanese population

Cytochrome P450 2C19 (CYP2C19) plays an important role in the metabolism of a wide range of therapeutic drugs and exhibits genetic polymorphism with interindividual differences in metabolic activity. We have previously described two CYP2C19 allelic variants, namely CYP2C19*18 and CYP2C19*19 with Arg329His/Ile331Val and Ser51Gly/Ile331Val substitutions, respectively. In order to investigate precisely the effect of amino acid substitutions on CYP2C19 function, CYP2C19 proteins of the wild-type (CYP2C19.1B having Ile331Val) and variants (CYP2C19.18 and CYP2C19.19) were heterologously expressed in yeast cells, and their S-mephenytoin 4'-hydroxylation activities were determined. The K(m) value of CYP2C19.19 for S-mephenytoin 4'-hydroxylation was significantly higher (3.0-fold) than that of CYP2C19.1B. Although no significant differences in V(max) values on the basis of microsomal and functional CYP protein levels were observed between CYP2C19.1B and CYP2C19.19, the V(max)/K(m) values of CYP2C19.19 were significantly reduced to 29-47% of CYP2C19.1B. By contrast, the K(m), V(max) or V(max)/K(m) values of CYP2C19.18 were similar to those of CYP2C19.1B. These results suggest that Ser51Gly substitution in CYP2C19.19 decreases the affinity toward S-mephenytoin of CYP2C19 enzyme, and imply that the genetic polymorphism of CYP2C19*19 also causes variations in the clinical response to drugs metabolized by CYP2C19.

The mechanism causing the difference in kinetic properties between rat CYP2D4 and human CYP2D6 in the oxidation of dextromethorphan and bufuralol

The capacity to oxidize bufuralol (BF) and dextromethorphan (DEX) was compared kinetically between human CYP2D6 and four rat CYP2D (CYP2D1, -2D2, -2D3 and -2D4) isoenzymes in a yeast cell expression system. In BF 1''-hydroxylation and DEX O-demethylation, only CYP2D4 showed hook-shaped Eadie-Hofstee plots, the other four CYP2D enzymes exhibiting linear plots. In DEX N-demethylation, rat CYP2D2 did not show any detectable activity under the conditions used, whereas the other four enzymes yielded linear Eadie-Hofstee plots. To elucidate the mechanisms causing the nonlinear kinetics, four CYP2D4 mutants, CYP2D4-F109I, -V123F, -L216F and -A486F, were prepared. CYP2D4-V123F, -L216F and -A486F yielded linear or linear-like Eadie-Hofstee plots for BF 1''-hydroxylation, whereas only CYP2D4-A486F exhibited linear plots for DEX O-demethylation. The substitution of Phe-109 by isoleucine did not have any effect on the oxidative capacity of CYP2D4 for either BF or DEX. These results suggest that the introduction of phenylalanine in the active-site cavity of CYP2D4 simplifies complicated interactions between the substrates and the amino acid residues, but the mechanisms causing the simplification differ between BF and DEX.

Stereoselective hexobarbital 3'-hydroxylation by CYP2C19 expressed in yeast cells and the roles of amino acid residues at positions 300 and 476

We examined the enzymatic function of recombinant CYP2C19 in enantiomeric hexobarbital (HB) 3'-hydroxylation, and searched the roles of amino acid residues, such as Phe-100, Phe-114, Asp-293, Glu-300, and Phe-476 of CYP2C19 in the stereoselective HB 3'-hydroxylation, using a yeast cell expression system and site-directed mutagenesis method. CYP2C19 wild-type exerted substrate enantioselectivity of (R)-HB>>(S)-HB and metabolite diastereoselectivity of 3'(R)<3'(S) in 3'-hydroxylation of HB enantiomers. The substitution of Asp-293 by alanine failed to yield an observable peak at 450 nm in its reduced carbon monoxide-difference spectrum. CYP2C19-E300A and CYP2C19-E300V with alanine and valine, respectively, in place of Glu-300 exerted total HB 3'-hydroxylation activities of 45 and 108%, respectively, that of the wild-type. Interestingly, these two mutants showed substrate enantioselectivity of (R)-HB<(S)-HB, which is opposite to that of the wild-type, while metabolite diasteroselectivity remained unchanged. The replacement of Phe-476 by alanine increased total HB 3'-hydroxylation activity to approximately 3-fold that of the wild-type. Particularly, 3'(S)-OH-(S)-HB-forming activity elevated to 7-fold that of the wild-type, resulting in the reversal of the substrate enantioselectivity. In contrast, the substitution of phenylalanine at positions 100 and 114 by alanine did not produce a remarkable change in the total activity or the substrate enantioselectivity. These results indicate that Glu-300 and Phe-476 are important in stereoselective oxidation of HB enantiomers by CYP2C19.

Cloning of a cDNA encoding a novel marmoset CYP2C enzyme, expression in yeast cells and characterization of its enzymatic functions

We cloned a cDNA encoding a novel CYP2C enzyme, called P450 M-2C, from a marmoset liver. The deduced amino acid sequence showed high identities to those of human CYP2C8 (87%), CYP2C9 (78%) and CYP2C19 (77%). The P450 M-2C enzyme expressed in yeast cells catalyzed p-methylhydroxylation of only tolbutamide among four substrates tested, paclitaxel as a CYP2C8 substrate, diclofenac and tolbutamide as CYP2C9 substrates and S-mephenytoin as a CYP2C19 substrate. p-Methylhydroxylation of tolbutamide by marmoset liver microsomes showed monophasic kinetics, and the apparent K(m) value (1.2 mM) for the substrate was similar to that of the recombinant P450 M-2C (1.8 mM). Although all of the recombinant human CYP2C8, CYP2C9 and CYP2C19 expressed in yeast cells catalyzed tolbutamide p-methylhydroxylation, the kinetic profile of CYP2C8 was most similar to that of P450 M-2C. Tolbutamide oxidation by the marmoset liver microsomes and the recombinant P450 M-2C was inhibited most effectively by quercetin, a CYP2C8 inhibitor, followed by omeprazole, a CYP2C19 inhibitor, whereas sulfaphenazole, a CYP2C9 inhibitor, was less potent under the conditions used. These results indicate that P450 M-2C is the major tolbutamide p-methylhydroxylase in the marmoset liver.

Hydrocortisone made in yeast: metabolic engineering turns a unicellular microorganism into a drug-synthesizing factory

Inspired by the successful work of converting Saccharomyces cerevisiae into an microorganism capable of synthesizing hydrocortisone, a 27-carbon molecule, from ethanol, a 2-carbon molecule, this review provides an overview of the potential of yeast as a recombinant organism in the 21st century. Yeast has been used by man for more than 6,000 years, and is still paving the way to new discoveries. It was the first eukaryotic organism to be sequenced, in 1996, and the first to produce hydrocortisone in 2003. In addition, extensive genome-wide analyses have been performed with yeast. In this review, we discuss the pros and cons of using yeast to produce small therapeutic molecules. It is obvious that S. cerevisiae has a cutting edge advantage of being a well-known organism and time will tell if yeast "biohydrocortisone" is a unique example or the beginning of a long list of yeast bioproducts. Other organisms, such as plants and bacteria, are competing with yeast. Bacteria produce a wealth of marketed molecules and plants are capable of producing extremely complex molecules with an unbeatable yield. However, S. cerevisiae offers a unique mix of the simplicity of a recombinant organism combined with the complexity of a eukaryote.

Oxidative metabolism of 5-methoxy-N,N-diisopropyltryptamine (Foxy) by human liver microsomes and recombinant cytochrome P450 enzymes

In vitro quantitative studies of the oxidative metabolism of (5-methoxy-N,N-diisopropyltryptamine, 5-MeO-DIPT, Foxy) were performed using human liver microsomal fractions and recombinant CYP enzymes and synthetic 5-MeO-DIPT metabolites. 5-MeO-DIPT was mainly oxidized to O-demethylated (5-OH-DIPT) and N-deisopropylated (5-MeO-IPT) metabolites in pooled human liver microsomes. In kinetic studies, 5-MeO-DIPT O-demethylation showed monophasic kinetics, whereas its N-deisopropylation showed triphasic kinetics. Among six recombinant CYP enzymes (CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6 and CYP3A4) expressed in yeast or insect cells, only CYP2D6 exhibited 5-MeO-DIPT O-demethylase activity, while CYP1A2, CYP2C8, CYP2C9, CYP2C19 and CYP3A4 showed 5-MeO-DIPT N-deisopropylase activities. The apparent Km value of CYP2D6 was close to that for 5-MeO-DIPT O-demethylation, and the Km values of other CYP enzymes were similar to those of the low-Km (CYP2C19), intermediate-Km (CYP1A2, CYP2C8 and CYP3A4) and high-Km phases (CYP2C9), respectively, for N-deisopropylation in human liver microsomes. In inhibition studies, quinidine (1 microM), an inhibitor of CYP2D6, almost completely inhibited human liver microsomal 5-MeO-DIPT O-demethylation at a substrate concentration of 10 microM. Furafylline, a CYP1A2 inhibitor, quercetin, a CYP2C8 inhibitor, sulfaphenazole, a CYP2C9 inhibitor and ketoconazole, a CYP3A4 inihibitor (5 microM each) suppressed about 60%, 45%, 15% and 40%, respectively, of 5-MeO-DIPT N-deisopropylation at 50 microM substrate. In contrast, omeprazole (10 microM), a CYP2C19 inhibitor, suppressed only 10% of N-deisopropylation by human liver microsomes, whereas at the same concentration the inhibitor suppressed the reaction by recombinant CYP2C19 almost completely. These results indicate that CYP2D6 is the major 5-MeO-DIPT O-demethylase, and CYP1A2, CYP2C8 and CYP3A4 are the major 5-MeO-DIPT N-deisopropylase enzymes in the human liver.

Catalytic roles of CYP2D6.10 and CYP2D6.36 enzymes in mexiletine metabolism: In vitro functional analysis of recombinant proteins expressed in Saccharomyces cerevisiae

Cytochrome P450 2D6 (CYP2D6) metabolizes approximately one-third of the medicines in current clinical use and exhibits genetic polymorphism with interindividual differences in metabolic activity. To precisely investigate the effect of CYP2D6*10B and CYP2D6*36 frequently found in Oriental populations on mexiletine metabolism in vitro, CYP2D6 proteins of wild-type (CYP2D6.1) and variants (CYP2D6.10 and CYP2D6.36) were heterologously expressed in yeast cells and their mexiletine p- and 2-methyl hydroxylation activities were determined. Both variant CYP2D6 enzymes showed a drastic reduction of CYP2D6 holo- and apoproteins compared with those of CYP2D6.1. Mexiletine p- and 2-methyl hydroxylation activities on the basis of the microsomal protein level at the single substrate concentration (100 microM) of variant CYP2D6s were less than 6% for CYP2D6.10 and 1% for CYP2D6.36 of those of CYP2D6.1. Kinetic analysis for mexiletine hydroxylation revealed that the affinity toward mexiletine of CYP2D6.10 and CYP2D6.36 was reduced by amino acid substitutions. The Vmax and Vmax/Km values of CYP2D6.10 on the basis of the microsomal protein level were reduced to less than 10% of those of CYP2D6.1, whereas the values on the basis of functional CYP2D6 level were comparable to those of CYP2D6.1. Although it was impossible to estimate the kinetic parameters for the mexiletine hydroxylation of CYP2D6.36, the metabolic ability toward mexiletine was considered to be poorer not only than that of CYP2D6.1 but also than that of CYP2D6.10. The same tendency was also observed in kinetic analysis for bufuralol 1''-hydroxylation as a representative CYP2D6 probe. These findings suggest that CYP2D6*36 has a more drastic impact on mexiletine metabolism than CYP2D6*10.

COMPARISON OF MOUSE AND RAT CYTOCHROME P450-MEDIATED METABOLISM IN LIVER AND INTESTINE

The liver is considered to be the major site of first-pass metabolism, but the small intestine is also able to contribute significantly. The improvement of existing in vitro techniques and the development of new ones, such as intestinal slices, allow a better understanding of the intestine as a metabolic organ. In this paper, the formation of metabolites of several human CYP3A substrates by liver and intestinal slices from rat and mouse was compared. The results show that liver slices exhibited a higher metabolic rate for the majority of the studied substrates, but some metabolites were produced at a higher rate by intestinal slices, compared with liver slices. Coincubation with ketoconazole inhibited the metabolic conversion in intestinal slices almost completely, but inhibition was variable in liver slices. To better understand the role of CYP3A in mice, we studied the relative mRNA expression of different CYP3A isoforms in intestine and liver from mice because, in this species, CYP3A expression has not been well described in these organs. It was found that in mice, CYP3A13 is more expressed in the intestine, whereas CYP3A11, CYP3A25, and CYP3A41 are more expressed in the liver, comparable to similar findings in the rat. Altogether, these data demonstrate that, in addition to liver, the intestine from mouse and rat may have an important role in the process of first-pass metabolism, depending on the substrate. Moreover, we show that intestinal slices are a useful in vitro technique to study gut metabolism.

Different effects of desipramine on bufuralol 1''-hydroxylation by rat and human CYP2D enzymes

Inhibitory effects of desipramine (DMI) on rat and human CYP2D enzymes were studied using bufuralol (BF) 1''-hydroxylation as an index. Inhibition was examined under the following two conditions: 1) DMI was co-incubated with BF and NADPH in the reaction mixture containing rat or human liver microsomes or yeast cell microsomes expressing rat CYP2D1, CYP2D2 or human CYP2D6 (co-incubation); 2) DMI was preincubated with NADPH and the same enzyme sources prior to adding the substrate (preincubation). When either rat liver microsomes or recombinant CYP2D2 was employed, the preincubation with DMI (0.3 microM) caused a greater inhibition of BF 1''-hydroxylation than the co-incubation did, whereas BF 1''-hydroxylation by rat CYP2D1 was not markedly affected under the same conditions. The inhibitory effect of DMI on BF 1''-hydroxylation by human liver microsomal fractions or recombinant CYP2D6 was much lower than that on the hydroxylation by rat liver microsomes or CYP2D2. Kinetic studies demonstrated that the inhibition-type changed from competitive for the co-incubation to noncompetitive for the preincubation in the case of CYP2D2, whereas the inhibition-type was competitive for both the co-incubation and the preincubation in the case of CYP2D6. Furthermore, the loss of activity of rat CYP2D2 under the preincubation conditions followed pseudo-first-order kinetics. Binding experiments employing the recombinant enzymes and [(3)H]-DMI revealed that CYP2D2 and CYP2D6 were the only prominent proteins to which considerable radioactive DMI metabolite(s) bound. These results indicate that rat CYP2D2 biotransforms DMI into reactive metabolite(s), which covalently bind to CYP2D2, resulting in inactivation of the enzyme. In contrast, human CYP2D6 may also biotransform DMI into some metabolite(s) that covalently bind to CYP2D6, but that do not inactivate the enzyme.

IMPORTANCE OF CYP2D3 IN POLYMORPHISM OF DIAZEPAMP-HYDROXYLATION IN RATS

Diazepam was metabolized to three primary metabolites, 3-hydroxy-diazepam, N-desmethyl-diazepam, and p-hydroxy-diazepam. Our previous studies reported metabolic position-specific inter- or intrastrain differences in diazepam metabolism among Sprague-Dawley, Brown Norway, Dark Agouti, and Wistar rats. Especially, there were marked ( approximately 300 fold) inter- or intrastrain differences in diazepam p-hydroxylation activity at low concentration of substrate. In this study, we investigated the enzyme that catalyzes diazepam p-hydroxylation. The activity toward diazepam p-hydroxylation was inhibited by anti-cytochrome P450 2D (CYP2D) antibody, suggesting that this activity was catalyzed by CYP2D isoforms. Comparing the expression levels of the CYP2D subfamily in liver microsomes from various strains of rats using anti-CYP2D2 antibody, we found that there was a band of protein that was consistent with the phenotype of diazepam p-hydroxylation. N-terminal amino acid sequences of the specific protein exactly corresponded to those of CYP2D3, indicating that CYP2D3 might be involved in diazepam p-hydroxylation. Moreover, using rat CYP2D isoforms expressed in yeast, we tested CYP2Ds to catalyze diazepam p-hydroxylation. CYP2D1 and CYP2D2 practically did not participate in diazepam metabolism. On the other hand, diazepam p-hydroxylation was catalyzed by CYP2D3. CYP2D4 had high activity toward diazepam N-desmethylation, but not p-hydroxylation. In conclusion, the polymorphic expression of CYP2D3 caused the inter- or intrastrain differences in diazepam p-hydroxylation among rat strains or individuals.

Alteration in catalytic properties of human CYP2D6 caused by substitution of glycine-42 with arginine, lysine and glutamic acid

The effects of the substitution of glycine at position 42 with various other amino acid residues on the functions of CYP2D6 were studied using debrisoquine (DB) and bunitrolol (BTL) 4-hydroxylations as indices of drug-metabolizing enzymes. The substitution with hydrophobic amino acid residues such as valine and phenylalanine did not affect the enzymatic properties such as reduced CO-difference spectra, microsomal CYP contents and oxidation activities towards DB and BTL. The substitution of glycine-42 with a polar but noncharged amino acid residue (serine) exhibited a similar reduced CO-different spectrum, but the substitution with a charged basic (lysine and arginine) or acidic (glutamic acid) amino acid residue commonly produced a peak at 420 nm in addition to a Soret peak at 450 nm. Cytochrome P450 contents and microsomal contents of G42S, G42K, G42R and G42E estimated spectrophotometrically and estimated by Western blot analysis, respectively, were lower than those of the wild-type. Kinetic analysis revealed that the substitution of glycine-42 with charged amino acid residues such as lysine, arginine and glutamic acid markedly increased the apparent K(m) values for DB and BTL oxidations without remarkable changes in the V(max) values. The subsitution with noncharged amino acid residues such as serine, valine and phenylalanine did not cause such a marked change in the K(m) values. Efficiencies (V(max)/K(m)) as DB and BTL 4-hydroxylases of CYP2D6 mutant proteins having charged amino acid residues were found to be decreased mainly by increasing their K(m) values. These results indicate that the properties of amino acid residues at position 42 affect the behavior of CYP2D6 proteins such as anchoring into ER membranes, conversion of P450 to P420 and incorporation of heme into apoproteins.

Inhibition of rat liver CYP2D in vitro and after 1-day and long-term exposure to neuroleptics in vivo-possible involvement of different mechanisms

The aim of the present study was to investigate the influence of classic and atypical neuroleptics on the activity of rat CYP2D measured as a rate of ethylmorphine O-deethylation. The reaction was studied in control liver microsomes in the presence of neuroleptics, as well as in microsomes of rats treated intraperitoneally (i.p.) for 1-day or 2-weeks (twice a day) with pharmacological doses of the drugs (promazine, levomepromazine, thioridazine, perazine 10 mg kg(-1); chlorpromazine 3 mg kg(-1); haloperidol 0.3 mg kg(-1); risperidone 0.1 mg kg(-1); sertindole 0.05 mg kg(-1)), in the absence of the neuroleptics in vitro. Neuroleptics added in vitro to control liver microsomes decreased the activity of the rat CYP2D by competitive or mixed inhibition of the enzyme. Thioridazine (Ki=15 microM) was the most potent inhibitor of the rat CYP2D among the drugs studied, whose effect was more pronounced than that of the other neuroleptics tested: phenothiazines (Ki=18-23 microM), haloperidol (Ki=32 microM), sertindole (Ki=51 microM) or risperidone (Ki=165 microM). The investigated neuroleptics-when given to rats in vivo-also seemed to exert an inhibitory effect on CYP2D via other mechanisms. One-day exposure of rats to the classic neuroleptics decreased the activity of CYP2D in rat liver microsomes. After chronic treatment with the investigated neuroleptics, the decreased CYP2D activity produced by the phenothiazines was still maintained, while that caused by haloperidol diminished. Moreover, risperidone decreased the activity of that enzyme. The obtained results indicate drug- and time-dependent interactions between the investigated neuroleptics and the CYP2D subfamily of rat cytochrome P-450, which may proceed via different mechanisms: (1) competitive or mixed inhibition of CYP2D shown in vitro, the inhibitory effects of phenothiazines being stronger than those of haloperidol or atypical neuroleptics, but weaker than the effects of the respective drugs on human CYP2D6; (2) in vivo inhibition of CYP2D, produced by both 1-day and chronic treatment with phenothiazines, which suggests inactivation of enzyme by intermediate metabolites; (3) in vivo inhibition of CYP2D by risperidone, produced only by chronic treatment with the drug, which suggests its influence on the enzyme regulation.

cDNA-directed expression of a functional zebrafish CYP1A in yeast

A cytochrome P450 1A (CYP1A) cDNA was isolated from an adult zebrafish (Danio rerio) library. The 2580-bp clone (GenBank Accession No. AF210727) contained a 62-bp 5'-unstranslated region (UTR), 1557-bp coding region and 962-bp 3'-UTR. The deduced 519-residue protein (calculated molecular weight 58,556, pI = 7.58) shared 74% identity with rainbow trout CYP1A and 57 and 54% identities with mouse and human CYP1A1s, respectively. The zebrafish CYP1A protein coding region was cloned into the pDONR201 entry vector and then transferred to a yeast expression vector pYES-DEST52. Expression of zebrafish CYP1A in Saccharomyces cerevisiae transformants was induced by galactose to a maximum level of 493 pmol CYP1A per mg microsomal protein or about 8 nmol/l of culture. Recombinant CYP1A protein expressed in yeast was mainly in the denatured P420 form under normal microsomal preparation conditions but when the oxygen concentration was reduced in the buffer by degassing and the yeast cells were maintained at less than 10 degrees C, the integrity of the CYP1A was preserved and it exhibited a characteristic reduced CO-difference spectrum maximum at 448 nm. The recombinant zebrafish CYP1A demonstrated 7-ethoxyresorufin O-deethylase (EROD) activity with an apparent Km (Km(app)) and Vmax values at 30 degrees C of 0.31 +/- 0.04 microM and 0.70 +/- 0.10 nmol/min/nmol CYP, respectively. The recombinant protein also metabolized benzo(a)pyrene with a Km(app) and Vmax values of 5.34 +/- 0.58 microM and 1.16 +/- 0.13 nmol/min/nmol CYP, respectively. These results show the recombinant expression of a functional zebrafish CYP in yeast and validated yeast as a host for heterologous expression of zebrafish CYP1A and potentially for other zebrafish CYPs.

The roles of amino acid residues at positions 43 and 45 in microsomal contents and enzymatic functions of rat CYP2D1 and CYP2D2

The effects of the substitution of amino acid residues at positions 43 and 45 of rat CYP2D1 and CYP2D2 on their microsomal contents and enzymatic functions were examined. The substitution of Val-45 of CYP2D1 by glycine decreased the microsomal content, whereas the substitution of Gly-45 of CYP2D2 by valine increased. The substitution of Leu-43 of CYP2D2 by tryptophan also increased the microsomal protein content. In reduced CO-difference spectra, CYP2D2 showed a P420 peak as well as a P450 peak, whereas CYP2D1 gave only a P450 peak. The substitution of Leu-43 and Gly-45 of CYP2D2 by valine and tryptophan, respectively, markedly decreased the P420 peak in parallel with an increase in P450 content. These substitutions did not cause remarkable changes in drug oxidation capacities (bufuralol 1''-hydroxylation and debrisoquine 4-hydroxylation) of the recombinant enzymes in terms of nmol/min/nmol CYP. The results indicate that amino acid residues at positions 43 and 45 are important for anchoring of the rat CYP2D proteins and their stabilities in the endoplasmic reticulum membrane.

Down-regulation of hepatic CYP3A in chronic renal insufficiency

PURPOSE

The objective of this study was to investigate the mechanisms underlying the decrease in hepatic clearance of some drugs metabolized by CYP450 enzymes in chronic renal insufficiency (CRI).

METHODS

CRI was induced in male Sprague-Dawley rats (n = 7) by the remnant kidney model (RKM); control animals (C) (n = 12) underwent sham surgery, of which n = 6 rats were pair-fed (CPF) with CRI rats and others (n = 6) had free access to food. Serum creatinine (Scr) and urea nitrogen (SUN) were monitored every 2 weeks. On day 36, livers were isolated, and microsomes were prepared. Catalytic activities were measured through O-demethylation (CYP2D) and N-demethylation of dextromethorphan (CYP3A) and O-deethylation of 7-ethoxyresorufin (CYP1A2). CYP450 protein and mRNA levels were also measured.

RESULTS

Compared with CPF, Scr and SUN levels in CRI rats were increased twofold (p < 0.01) and 2.5-fold (p < 0.01), respectively. No effect on CYP1A2 and CYP2D activities, mRNA, or protein levels was observed between the groups. There was a reduction (41.8 +/- 20%, p < 0.01) in CYP3A activity, mRNA (p < 0.05), and protein levels (p < 0.05) in CRI rats compared to CPF.

CONCLUSIONS

CRI induced by RKM does not have an effect on hepatic CYP1A2 and CYP2D enzymes but does reduce CYP3A activity, probably through down-regulation of CYP3A2.

Cloning and functional expression of a novel marmoset cytochrome P450 2D enzyme, CYP2D30: comparison with the known marmoset CYP2D19

Using a primer set designed on the cDNA encoding the known marmoset cytochrome P450 2D19 (CYP2D19), a cDNA encoding a novel CYP2D enzyme (CYP2D30) was cloned from the liver of a female marmoset bred at Kyoto University (KYU). In addition, a cDNA encoding CYP2D19 was cloned from the liver of a female marmoset bred at Kagoshima University (KAU). CYP2D30 and CYP2D19 showed homologies of 93.6 and 93.4% in their nucleotide and amino acid sequences, respectively. Reverse transcription polymerase chain reaction (RT-PCR) and digestion with NdeI demonstrated that the KYU-marmoset liver contained mainly mRNA for CYP2D30, while the KAU-marmoset liver contained mainly mRNA for CYP2D19. Marmoset CYP2D30, like human CYP2D6, exhibited high debrisoquine (DB) 4-hydroxylase activity and relatively low DB 5-, 6-, 7- and 8-hydroxylase activities, whereas CYP2D19 lacked DB 4-hydroxylase but exhibited marked 5-, 6-, 7- and 8-hydroxylase activities. The two marmoset recombinant enzymes showed enantioselective bufuralol (BF) 1"-hydroxylase activities, similar to CYP2D6. BF 1"-hydroxylation by CYP2D30 exhibited product-enantioselectivity of (1"R-OH-BF << 1"S-OH-BF), similar to that observed with human CYP2D6, whereas CYP2D19 showed a reversed selectivity of (1"R-OH-BF > or = 1"S-OH-BF). BF 1"-hydroxylation in marmoset liver microsomes from both sources was inhibited by antibodies raised against rat CYP2D1 in a concentration-dependent manner. A known inhibitor of CYP2D6, quinidine, effectively inhibited the BF 1"-hydroxylation activities in liver microsomal fractions prepared from KYU- and KAU-marmosets. These results suggest that CYP2D19 and CYP2D30 proteins can be expressed as functional enzymes in marmoset livers, although it is unresolved whether both enzymes coexist in the same marmoset liver.

Cytochrome P450 2D catalyze steroid 21-hydroxylation in the brain

mRNA of cytochrome P450 21-hydroxylase (P450c21) is expressed in the brain, but little is known about the enzymatic properties of P450c21 in the brain. In the present study, we showed, by using various recombinant cytochrome P450 (CYP)2D enzymes and anti-CYP2D4- or P450c21-specific antibodies, that rat brain microsomal steroid 21-hydroxylation is catalyzed not by P450c21, but by CYP2D isoforms. Rat CYP2D4 and human CYP2D6, which are the predominant CYP2D isoforms in the brain, possess 21-hydroxylation activity for both progesterone and 17alpha-hydroxyprogesterone. In rat brain microsomes, these activities were not inhibited by anti-P450c21 antibodies, but they were effectively inhibited by the CYP2D-specific chemical inhibitor quinidine and by anti-CYP2D4 antibodies. mRNA and protein of CYP2D4 were expressed throughout the brain, especially in cerebellum, striatum, pons, and medulla oblongata, whereas the mRNA and protein levels of P450c21 were extremely low or undetectable. These results support the idea that CYP2D4, not P450c21, works as steroid 21-hydroxylase in the brain. Allopregnanolone, a representative gamma-aminobutyric acid receptor modulator, was also hydroxylated at the C-21 position by recombinant CYP2D4 and CYP2D6. Rat brain microsomal allopregnanolone 21-hydroxylation was inhibited by fluoxetine with an IC(50) value of 2 microm, suggesting the possibility that the brain CYP2D isoforms regulate levels of neurosteroids such as allopregnanolone, and that this regulation is modified by central nervous system-active drugs such as fluoxetine.

Poor correlation between intestinal and hepatic metabolic rates of CYP3A4 substrates in rats

PURPOSE

To clarify the contribution of the intestinal first-pass metabolism to the drug bioavailability, the correlation between the intestinal and hepatic metabolism of human CYP3A4 substrates was investigated in rats.

METHODS

The metabolic rates of four compounds (lidocaine, quinidine, nifedidpine, and rifabutin) were examined with excised intestinal tissues and liver microsomes. The intestinal and hepatic expression of CYP3A1/23 and CYP3A2 was evaluated by Western blot analysis.

RESULTS

Rifabutin was metabolized fastest, and lidocaine was metabolized slowest in excised intestinal tissues. By contrast, lidocaine was metabolized fastest and rifabutin was the slowest in liver microsomes. The hepatic metabolism of lidocaine was inhibited by a CYP2D6 substrate desipramine, not by a CYP3A4 inhibitor ketoconazole. In addition, members of the CYP3A subfamily expressed in the intestine were different from those expressed in the liver.

CONCLUSIONS

Poor correlation between the intestinal and hepatic metabolism of human CYP3A4 substrates in rats may be caused by the contribution of the CYP2D subfamily to the drug metabolisms in the liver and also by the unique expression of the CYP3A subfamily in the intestine.

CYP2D in the Brain

CYP2D1, 2D2, 2D3, and 2D4 are major CYP2D isoforms expressed in the rat. In humans, only CYP2D6 is expressed. In rat brain, the mRNA for CYP2D4 is most abundant in cerebellum, striatum, pons and medulla oblongata. In human brain, CYP2D6 mRNA expression was detected in all regions with highest levels observed in cerebellum. CYP2D isoforms are involved in the metabolism of not only xenobiotics such as antidepressants, beta-adrenergic blockers, antiarrhysthmics, and antihypertensives, but also endogenous compounds such as trace amine and neurosteroids. Among 11 isoforms of human recombinant P450s, only CYP2D6 exhibited an ability to efficiently convert tyramine which exists in the brain, to dopamine. CYP2D4 and CYP2D6 which are the predominant CYP2D isoforms in the rat and human brain, respectively, possess 21-hydroxylation activity for both progesterone and allopregnanolone. CYP2D4, not P450c21, works as a steroid 21-hydroxylase in the brain. These results suggested that CYP2D in the brain may be involved in the metabolism of neuronal amines and steroids and in the regulation of the central nervous system.

Inhibition and possible induction of rat CYP2D after short- and long-term treatment with antidepressants

The aim of this study was to investigate the influence of tricyclic antidepressants (imipramine, amitriptyline, clomipramine, desipramine), selective serotonin reuptake inhibitors (SSRIs: fluoxetine, sertraline) and novel antidepressant drugs (mirtazapine, nefazodone) on the activity of CYP2D, measured as a rate of ethylmorphine O-deethylation. The reaction was studied in control liver microsomes in the presence of the antidepressants, as well as in microsomes of rats treated intraperitoneally for one day or two weeks (twice a day) with pharmacological doses of the drugs (imipramine, amitriptyline, clomipramine, nefazodone 10 mg kg(-1) i.p.; desipramine, fluoxetine, sertraline 5 mg kg(-1) i.p.; mirtazapine 3 mg kg(-1) i.p.), in the absence of the antidepressants in-vitro. Antidepressants decreased the activity of the rat CYP2D by competitive inhibition of the enzyme, the potency of their inhibitory effect being as follows: clomipramine (K(i) = 14 microM) > sertraline approximate, equals fluoxetine (K(i) = 17 and 16 microM, respectively) > imipramine approximate, equals amitriptyline (K(i) = 26 and 25 microM, respectively) > desipramine (K(i) = 44 microM) > nefazodone (K(i) = 55 microM) > mirtazapine (K(i) = 107 microM). A one-day treatment with antidepressants caused a significant decrease in the CYP2D activity after imipramine, fluoxetine and sertraline. After prolonged administration of antidepressants, the decreased CYP2D activity produced by imipramine, fluoxetine and sertraline was still maintained. Moreover, amitriptyline and nefazodone significantly decreased, while mirtazapine increased the activity of the enzyme. Desipramine and clomipramine did not produce any effect when administered in-vivo. The obtained results indicate three different mechanisms of the antidepressants-CYP2D interaction: firstly, competitive inhibition of CYP2D shown in-vitro, the inhibitory effects of tricyclic antidepressants and SSRIs being stronger than those of novel drugs; secondly, in-vivo inhibition of CYP2D produced by both one-day and chronic treatment with tricyclic antidepressants (except for desipramine and clomipramine) and SSRIs, which suggests inactivation of the enzyme apoprotein by reactive metabolites; and thirdly, in-vivo inhibition by nefazodone and induction by mirtazapine of CYP2D produced only by chronic treatment with the drugs, which suggests their influence on the enzyme regulation.

Complementary DNA cloning and characterization of cytochrome P450 2D29 from Japanese monkey liver

A cDNA was cloned from Japanese monkey liver mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR) using oligonucleotide primers based on the marmoset cytochrome P450 2D19 (CYP2D19) nucleotide sequence. The full-length cDNA encoded a 497 amino acid protein (designated CYP2D29) that is 96, 91, and 88% homologous to human CYP2D6, cynomolgus monkey CYP2D17, and marmoset monkey CYP2D19, respectively. Yeast cells (Saccharomyces cerevisiae AH-22 strain) transfected with pGYR1 vectors containing the CYP2D29 cDNA were cultured, and microsomal fractions were obtained. Reduced carbon monoxide-difference spectra and western blot analysis using polyclonal antibodies raised against rat CYP2D2 demonstrated that in yeast cell microsomal fractions, the level of CYP2D29 holoenzyme was similar to that of CYP2D6 holoenzyme. However, western blot analysis indicated that the level of CYP2D29 in Japanese monkey liver microsomes might be much higher than that of CYP2D6 in human liver microsomes. Japanese monkey liver microsomes exhibited much higher activities than did human liver microsomes, expressed as nmol/min/mg protein, for debrisoquine (DB) 4-hydroxylation and bufuralol (BF) 1"-hydroxylation (typical reactions catalyzed by CYP2D6), whereas recombinant CYP2D29 activity, expressed as nmol/min/nmol CYP, was similar to that of CYP2D6 for DB and BF hydroxylation. In kinetic analyses, the K(m) value of CYP2D29 for DB 4-hydroxylation was much lower than that of Japanese monkey liver microsomes, whereas the K(m) value of CYP2D6 for DB 4-hydroxylation was similar to that of human liver microsomes. In contrast, K(m) values for BF 1"-hydroxylation were similar for Japanese monkey and human liver microsomes and yeast cell microsomal fractions expressing recombinant CYP2D29 or CYP2D6. These results suggest that the properties of Japanese monkey CYP2D29 are similar to those of human CYP2D6, but their populations and/or some other factors in liver microsomes may cause the difference in microsomal DB 4-hydroxylase activities between Japanese monkeys and humans.

Catalytic specificity of CYP2D isoforms in rat and human

In rats, six cytochrome P450 (P450) 2D isoforms have been genetically identified. Nonetheless, there is little evidence of catalytic properties of each CYP2D isoform. In this study, using recombinant CYP2D isoforms (rat CYP2D1, CYP2D2, CYP2D3, and CYP2D4 and human CYP2D6) or hepatic microsomes, we investigated the catalytic specificity toward bufuralol, debrisoquine, and propranolol, which are frequently used as CYP2D substrates. Bufuralol was oxidized to three metabolites by rat and human hepatic microsomes. 1'-Hydroxybufuralol was the major metabolite. 1'2'-Ethenylbufuralol, one of the others, was identified as a novel metabolite. The formation of 1'-hydroxybufuralol and 1'2'-ethenylbufuralol in hepatic microsomes was inhibited by anti-CYP2D antibody, suggesting that these metabolites were formed by CYP2D isoforms. All rat and human recombinant CYP2D isoforms possessed activity for the 1'-hydroxylation of bufuralol, indicating that this catalytic property was common to all CYP2D isoforms. However, the 1'2'-ethenylation of bufuralol was catalyzed only by rat CYP2D4 and human CYP2D6. Debrisoquine was oxidized to two metabolites, 3-hydroxydebrisoquine, and 4-hydroxydebrisoquine, by hepatic microsomes. Recombinant CYP2D2 and CYP2D6 had very high levels of activity for the 4-hydroxylation of debrisoquine with low K(m) values. Only CYP2D1 had a higher level of 3-hydroxylation than 4-hydroxylation activity. Propranolol 4-hydroxylation was catalyzed by CYP2D2, CYP2D4, and CYP2D6. The 7-hydroxylation of propranolol was catalyzed only by CYP2D2. In conclusion, in rats, bufuralol 1'2'-ethenylation activity was specific to CYP2D4 and debrisoquine 4-hydroxylation and propranolol 7-hydroxylation activities were specific to CYP2D2. These catalytic activities are useful as a probe for rat CYP2D isoforms.

4-Hydroxylation of debrisoquine by human CYP1A1 and its inhibition by quinidine and quinine

A panel of 15 recombinant cytochromes P450 expressed in human B-lymphoblastoid cells was used to study debrisoquine 4-hydroxylation. Both CYP2D6 and CYP1A1 carried out the reaction. The apparent K(m) (micromolar) and V(max) (picomoles per minute per picomole of P450) for CYP2D6 were 12.1 and 18.2 and for CYP1A1 were 23.1 and 15.2, respectively. CYP1A1 debrisoquine 4-hydroxylase was inhibited by the CYP1A1 inhibitor alpha-naphthoflavone and the CYP1A1 substrate 7-ethoxyresorufin. Additionally and surprisingly, this reaction was also inhibited by quinidine and quinine, with respective IC(50) values of 1.38 +/- 0.10 and 3.31 +/- 0.14 microM, compared with those for CYP2D6 debrisoquine 4-hydroxylase of 0.018 +/- 0.05 and 3.75 +/- 2.07 microM, respectively. Anti-CYP1A1 monoclonal antibody (mAb) 1-7-1 abolished CYP1A1 debrisoquine hydroxylase and anti-CYP2D6 mAb 50-1-3 eradicated CYP2D6 debrisoquine 4-hydroxylase. Three further CYP2D6-specific reactions were tested: dextromethorphan O-demethylation, bufuralol 1'-hydroxylation, and sparteine dehydrogenation. The CYP2D6 specificity, judged by the CYP2D6/CYP1A1 activity ratios was 18.5, 7.0, 6.0, and 1.6 for dextromethorphan, bufuralol, sparteine, and debrisoquine, respectively. Thus, debrisoquine is not a specific CYP2D6 substrate and quinidine is not a specific CYP2D6 inhibitor. These findings have significant implications for the conduct of in vitro drug metabolism inhibition studies and underscore the fallacy of "specific chemical inhibitors" of a supergene family of enzymes that have overlapping substrate specificities. The use of highly specific mAbs in such studies is mandated. It is unclear as yet whether these findings have implications for the relationship between CYP2D6 genotype and in vivo debrisoquine 4-hydroxylase activity.

CYP2C9*3 influences the metabolism and the drug-interaction of candesartan in vitro

Candesartan cilexetil is an angiotensin II receptor antagonist, and candesartan, its active metabolite, is metabolized by CYP2C9. However, the effect of CYP2C9*3 on candesartan metabolism is not established. We characterized the kinetics of candesartan by CYP2C9*1/*1 and CYP2C9*1/*3 in human liver microsomes. The difference between the two was not significant. Subsequently, CYP2C9*1 and CYP2C9*3 (Leu359) were expressed in yeast, and the kinetics of candesartan were determined. The wild-type showed the lower Km (345 vs 439 microM; 3/4) and higher Vmax/Km (1/3) than the Leu359 variant. Also, we investigated potential interaction between candesartan and warfarin with both the wild-type and the Leu359 variant. Candesartan had no effect on S-warfarin 7-hydroxylation. In contrast, S-warfarin inhibited candesartan metabolism by the wild-type (K = 17microM) greater than by the Leu359 variant (Ki = 36 microM). These findings suggest that CYP2C9*3 may change not only the metabolic activity but also the inhibitory susceptibility compared with CYP2C9*1.

Change in the gene expression of hepatic tamoxifen-metabolizing enzymes during the process of tamoxifen-induced hepatocarcinogenesis in female rats

Altered gene expression of the enzymes responsible for tamoxifen metabolism during the process of tamoxifen-induced hepatocarcinogenesis in female Sprague-Dawley rats was examined by the RT-PCR method. Treatment of rats with tamoxifen (20 mg/kg body/day) for 52 weeks, but not the 1 day, 2 or 12 week treatments, resulted in the formation of the liver hyperplastic nodules. The gene expression of CYP3A subfamily enzymes, especially CYP3A1, responsible for not only detoxification (N-demethylation) but also activation (alpha-hydroxylation) of tamoxifen, was increased by the tamoxifen treatments for 2 and 12 weeks, whereas after the 52 week treatment, the expression in the induced nodules returned to the control level. The gene expression of SULT2A subfamily sulfotransferases, especially HSTa, responsible for metabolic activation of alpha-hydroxytamoxifen was decreased to a level <20% of the control in the nodules, although no significant change in the expression was observed in the liver of rats treated with tamoxifen for 1 day, 2 or 12 weeks. On the other hand, the gene expression of CYP3A2 and flavin-containing monooxygenase 1 (FMO1), responsible for the N-demethylation and N-oxidation, respectively, of tamoxifen was increased in a time-dependent fashion up to the 52 week treatment. Although the gene expression of UDP-glucuronosyltransferase(s), which might be responsible for detoxification of tamoxifen, was also increased by the tamoxifen treatment for 2 or 12 weeks, it decreased to the control level in the nodules after the 52 week treatment. The present findings demonstrate that in the early stage of the formation of the liver hyperplastic nodules by tamoxifen, the genes of the enzymes responsible for not only detoxification but also activation of tamoxifen were activated, whereas in the later stage (in the nodules), the genes of the detoxification enzymes, CYP3A2 and FMO1, remained active, but those of the activation enzymes such as CYP3A1 and HSTa were suppressed.

Functional evaluation of cytochrome P450 2D6 with Gly42Arg substitution expressed in Saccharomyces cerevisiae

A single amino acid-substituted mutant protein, CYP2D6 (G42R) was expressed in Saccharomyces cerevisiae and its enzymatic properties were compared with those of other single (P34S, R296C and S486T) and double amino acid-substituted mutant proteins (P34S/S486T and R296C/S486T) expressed in yeast cells, all of which were known to occur in the CYP2D6 gene as single nucleotide polymorphisms. The protein levels of G42R, P34S and P34S/S486T in microsomal fractions and their oxidation capacities towards debrisoquine as a prototypic substrate and bunitrolol as a chiral substrate were different from those of wild-type CYP2D6, while the R296C, S486T and R296C/S486T behaved similarly to the wild-type in these indices. The CYP contents both in yeast microsomal and in whole cell fractions indicated that some part of G42R protein was localized in the endoplasmic reticulum membrane fraction, whereas most of G42R protein was in some subcellular fractions other than endoplasmic reticulum. In kinetic analysis, the G42R substitution increased apparent Km and decreased Vmax for debrisoquine 4-hydroxylation, while it increased both Km and Vmax for bunitrolol 4-hydroxylation. The P34S substitution did not drastically change Km but decreased Vmax for debrisoquine 4-hydroxylation, whereas Km was increased and Vmax unchanged or decreased for bunitrolol 4-hydroxylation by P34S substitution. These results suggest that the G42R substitution causes a change in the CYP2D6 conformation, which may be different from the change produced by the P34S substitution.

Progesterone oxidation by cytochrome P450 2D isoforms in the brain

The existence of cytochrome P450 2D isoforms in the brain has been demonstrated, although their physiological functions remain to be elucidated. In this study we demonstrated that recombinant rat cytochrome P450 2D1 and 2D4 and human cytochrome P450 2D6 possess progesterone 6 beta- and 16 alpha- hydroxylation activities; 2 beta- and 21-hydroxylation activities; and 2 beta-, 6 beta-, 16 alpha- and 21-hydroxylation activities, respectively. Cytochrome P450 2D4 had the lowest K(m) value and the highest maximum velocity value toward these activities. Progesterone 2 beta- and 21-hydroxylation activities were also detected in rat brain microsomes, and these activities were completely inhibited by anticytochrome P450 2D antibodies. The presence of endogenous 2 beta- and 21-hydroxyprogesterones in rat brain tissues was also demonstrated. The mRNAs of cytochrome P450 2D4, CYP11A, and 3 beta-hydroxysteroid dehydrogenase were detected in the rat brain, suggesting that progesterone was generated from cholesterol by CYP11A and 3 beta-hydroxysteroid dehydrogenase and then underwent hydroxylation to hydroxyprogesterones by cytochrome P450 2D4 in rat brain. Collectively, our findings support the idea that cytochrome P450 2D may be involved in the regulation (metabolism and/or synthesis) of endogenous neuroactive steroids, such as progesterone and its derivatives, in brain tissues.

A transgenic mouse expressing human CYP4B1 in the liver

The human CYP4B1 protein was expressed in the liver of a transgenic mouse line under the control of the promoter of the human apolipoprotein E (apo E) gene. Hepatic microsomes of transgenic mice catalyzed omega-hydroxylation of lauric acid and also activated 2-aminofluorene (2-AF), which is a typical substrate for CYP4B1, to mutagenic compounds detected by an umu gene expression assay. These activities observed in transgenic mouse were efficiently inhibited by CYP4B1 antibody. However, such inhibition was not observed in control mice. This is the first report to indicate catalytic activities of human CYP4B1. For further characterization of human CYP4B1, a fusion protein of CYP4B1 and NADPH-P450 reductase was expressed in yeast cells. It was able to activate 2-AF and was also able to catalyze omega-hydroxylation of lauric acid. This transgenic mouse line and the recombinant fusion protein provide a useful tool to study human CYP4B1 and its relation to chemical toxicity and carcinogenesis.

Molecular cloning and characterization of three novel cytochrome P450 2D isoforms, CYP2D20, CYP2D27, and CYP2D28 in the Syrian hamster (Mesocricetus auratus)

We cloned three novel cytochrome P450 (CYP) 2D cDNAs in the Syrian hamster (Mesocricetus auratus). Each clone contained an open reading frame of 1500 nucleotides encoding a protein of 500 amino acids. The deduced amino acid sequences of these had high identities with those of the other CYP2D members, therefore, the clones were assigned as CYP2D20, CYP2D27, and CYP2D28. Northern blot analysis showed that the CYP2D27 mRNA was expressed in liver, but not in kidney, small intestine, and brain, while the CYP2D20 and CYP2D28 mRNAs were not detected in these tissues examined. The expression of CYP2D27 mRNA in liver did not show sex difference and was not induced by either 3-methylcholanthrene or phenobarbital treatment. We characterized the enzyme activities of recombinant CYP2D27 expressed in COS-7 cells. The CYP2D27 protein had the bufuralol 1'-hydroxylase and debrisoquine 4-hydroxylase activities that are specific to the CYP2D subfamily.

The decreased in vivo clearance of CYP2D6 substrates by CYP2D6*10 might be caused not only by the low-expression but also by low affinity of CYP2D6

CYP2D6 exhibits genetic polymorphism with interindividual differences in metabolic activity. We have found a significant influence on the pharmacokinetics of venlafaxine by the CYP2D6*10 allele in a Japanese population. CYP2D6.10, which is translated from CYP2D6*10, has two amino acid substitutions: Pro34 --> Ser and Ser486 --> Thr. In this study, CYP2D6.10 was expressed in Saccharomyces cerevisiae and its catalytic activity for CYP2D6 substrates was investigated. The CYP2D6*10B- and *10C-associated cDNA were isolated from human lymphocyte genotyped as CYP2D6*10. In addition, three forms of CYP2D6, Pro34/Thr486 (PT), Ser34/Ser486 (SS), and Pro34/Ser486 (wild type, CYP2D6.1), were constructed by PCR-site mutagenesis to clarify the effects of the two amino-acid substitutions. The expression of CYP2D6 protein was confirmed by immunoblotting using CYP2D antibody. The absorbance at 450 nm was measured by CO-reduced difference spectra from five all microsome preparations. The CYP2D6 forms with Pro34 --> Ser amino acid substitution were at a lower expression than CYP2D6.1 from the findings of immunoblotting and spectral analysis. The apparent K(m) values of CYP2D6.1, CYP2D6.10A, and CYP2D6.10C were 1.7, 8.5, and 49.7 microM, respectively, for bufuralol 1'-hydroxylation, and 9.0, 51.9, and 117.4 microM, respectively, for venlafaxine O-demethylation, respectively. The V(max) values were not significantly different among the three variants. These findings suggest that the decreased in vivo clearance by CYP2D6*10 was caused not only by low expression of but also the increased K(m) value of CYP2D6.

Regional and cellular distribution of CYP2D subfamily members in rat brain

1. Human CYP2D6 is present in brain, metabolizes many drugs and has been implicated in Parkinson's and Alzheimer's diseases and some cancers. It is still unclear which of the six known rat CYP2D subfamily members is/are homologous to human CYP2D6. 2. In this study, RT-PCR, Southern and Western blotting and immunohistochemical techniques were used to study the distribution of CYP2D subfamily member mRNA and proteins across 10 rat brain regions. CYP2D subfamily mRNA and protein levels were correlated with brain dextromethorphan O-demethylation (DOD), a measure of human CYP2D6 and rat CYP2D1 activities. 3. The data showed a strong relationship between CYP2D1 and CYP2D1-18 with brain DOD activity. In addition, it was shown that CYP2D proteins are present in brain mitochondrial as well as microsomal membranes. CYP2D subfamily member mRNA and proteins varied across brain regions and were highly concentrated in specific cell types. 4. These data strongly suggest that CYP2D1 and not CYP2D5 mediates DOD activity in rat brain, and may be the rat homologue of human CYP2D6. The highly localized nature of CYP2D indicates that in specific neurones enzyme levels may approach hepatic levels and, hence, contribute to local alterations in brain drug metabolism.

Comparative in vitro metabolism of indinavir in primates--a unique stereoselective hydroxylation in monkey

1. The in vitro metabolism of indinavir (CRIXIVAN, MK-0639, L-735,524), an HIV protease inhibitor, was evaluated using liver microsomes from cynomolgus monkey, rhesus monkey, chimpanzee and human. Indinavir exhibited marked species differences in metabolism. The overall rate of indinavir metabolism varied > 4-fold among primates (84 pmol/min/mg protein in cynomolgus monkey versus 20.4 pmol/min/mg protein in human) and followed the rank order: cynomolgus monkey > rhesus monkey > chimpanzee > human. 2. The cis-(indan)hydroxylated metabolite of indinavir was formed only in cynomolgus and rhesus monkey livers, whereas trans-(indan)hydroxylation and N-dealkylation were observed as the major metabolites in all primates tested. Inhibition studies with P450-selective inhibitors (ketoconazole, quinine, quinidine) and monoclonal antibodies (against CYP2D6 or CYP3A4) indicated that a cytochrome P450 isoform of the CYP2D subfamily is involved in the formation of the unique cis-(indan) hydroxylated metabolite in monkey, whereas all other oxidative metabolites, including the trans-(indan)hydroxylated metabolite, are formed by CYP3A isoform(s). 3. The present study has demonstrated that monkeys were unique in their abilities to form the stereoselective metabolite and were not appropriate surrogates for the qualitative prediction of indinavir metabolism in human.

Cytochromes P450 Involved in Cyclophosphamide, Paclitaxel and Docetaxel Metabolism in Rats

We investigated involvement of cytochromes P450 (CYPs) of rat liver microsomes in metabolism of two anticancer drugs, paclitaxel (PCT) and docetaxel (DTX), by an indirect method. This method is based on the presumption that the compound competitively inhibiting oxidation of the CYP-selective substrate should also be a substrate for the CYP enzyme. The validity of this approach was confirmed using the model drug, cyclophosphamide (CPA). Indeed, CPA competitively inhibited oxidation of substrates specific for CYP2B1 and CYP3A1/2, enzymes previously reported to be capable of metabolizing CPA. Using this method, we identified CYP enzymes participating in PCT and DTX metabolism. The CYP2D1/2/3 and CYP3A1/2 are enzymes oxidizing PCT while CYP3A1/2 and CYP2E1 are responsible for metabolism of DTX. Here, we report a suitable method serving for easy and fast estimation of CYP enzymes involved in drug metabolism.

Expression and localisation of CYP2D enzymes in rat basal ganglia

P450 enzymes in the CYP2D subfamily have been suggested to contribute to the susceptibility of individuals in developing Parkinson's disease. We have used specific anti-peptide antisera and peroxidase immunohistochemistry to investigate the expression of CYP2D enzymes in the rat brain and some possible factors that may affect their regulation. In male Wistar rats, CYP2D1 was not detected in the basal ganglia or in any other brain region. CYP2D2 was weakly expressed within neurones of the subthalamic nucleus, substantia nigra and interpeduncular nucleus as well as in the hippocampus, dentate gyrus, red nucleus and pontine nucleus. CYP2D3 and CYP2D4 were absent from the basal ganglia, although moderate amounts of CYP2D3 were detected within fibres of the oculomotor root, and very low levels of CYP2D4 were present in white matter tracts. In contrast, CYP2D5 was extensively expressed in the basal ganglia, including neurones in the subthalamic nucleus, substantia nigra and interpeduncular nucleus, as well as other areas of the brain, including the ventral tegmental area, piriform cortex, hippocampus, dentate gyrus, medial habenular nucleus, thalamic nucleus and pontine nucleus. Lesioning of the nigro-striatal tract to cause almost a complete loss of tyrosine hydroxylase containing neurones in the substantia nigra, also reduced the number of neurones expressing CYP2D5 by 50%, indicating that CYP2D5 is expressed in dopaminergic neurones. Castration of pre-pubertal or adult Wistar rats had no effect on the number of CYP2D5-positive neurones in the substantia nigra. Although Dark Agouti rats lack hepatic CYP2D2, expression in the midbrain was similar to that of Wistar rats; furthermore, there was no difference in expression or distribution between male and female rats. In contrast to naive rats, extensive expression of CYP2D4 was found throughout the basal ganglia and in other brain nuclei in Wistar rats treated with not only clozapine, but also saline, suggesting that CYP2D4 may be induced as a result of mild stress. The function of CYP2D enzymes in the brain remains unknown, but their selective localisation suggests a physiological role in neuronal activity and in adaptation to abnormal situations.

In vitro biotransformation of atrazine by rat liver microsomal cytochrome P450 enzymes

We studied atrazine (ATZ) metabolism in male and female rat liver microsomes in vitro, and the major metabolite was deisopropylatrazine (DeiPr-ATZ) with deethylatrazine (DeEt-ATZ) and 1-hydroxyisopropylatrazine (iPrOH-ATZ) as minor metabolites in both sexes. The enzyme kinetics of ATZ biotransformation were examined by means of Eadie-Hofstee analyses. Although no remarkable sex difference of Michaelis Menten values for each pathway was observed, Cl(int)S (Vmax/Km) for DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ were slightly higher in female than in male rats. The formation of DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ from ATZ was substantially inhibited by SKF-525A, metyrapone, diallyl sulfide, 7-ethoxycoumarin, benzphetamine, nicotine, testosterone and lauric acid in both sexes. Cimetidine effectively inhibited the formation of all metabolites in male rats. On the other hand, the inhibition rates of the formation of DeiPr-ATZ and iPrOH-ATZ by cimetidine in female rats were lower than those in male rats, and DeEt-ATZ was hardly affected by the chemicals. In contrast with the results for cimetidine, the inhibition of ATZ biotransformation by bufuralol was more effective in female than in male rats. Anti-rat CYP2B1 and CYP2E1 antibodies effectively inhibited DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ formations in both sexes. Anti-rat CYP2C11 antibody also inhibited the three metabolites in both sexes, with the inhibition rates higher in male than in female rats, similar to cimetidine. In the case of anti-rat CYP2D1 antibody, the inhibitory effect on ATZ biotransformation in male rats was less than that in female rats. On the other hand, anti-rat CYP1A2, CYP3A2 and CYP4A1 antibodies did not affect the ATZ biotransformation in either sex. There was no significant correlation between the formation rate of ATZ metabolites and P450 isoform levels in either sex. These results may mean that CYP2B2, CYP2C11, CYP2D1 (only iPrOH-ATZ formation) and CYP2E1 in male rats, and CYP2B2, CYP2D1 and CYP2E1 in female rats are involved ATZ metabolism in liver, and that the substrate specificity of P450 isoforms for ATZ is broad.

Isolation of partially purified P450 2D18 and characterization of activity toward the tricyclic antidepressants imipramine and desipramine

Previous reports have shown that rat brain microsomes are capable of metabolizing tricyclic antidepressants such as imipramine. Subsequent studies have shown that the protein products of several clones isolated from rat brain cDNA libraries are capable of metabolizing imipramine to both its active metabolite, desipramine, and its inactive hydroxylated metabolites. We report here the overexpression and partial purification of P450 2D18 using the baculovirus expression system and the incorporation of a C-terminal [His]4 tag. P450 2D18 was partially purified to a specific content of 4.8 nmol/mg protein and shown to be electrophoretically pure. The apparent KM values for P450 2D18 toward imipramine and desipramine were 374 and 314 microM, respectively. While apparent KM values were similar, P450 2D18 was shown to have a fivefold increased Vmax (2.2 nmol/min/nmol P450) for imipramine compared to desipramine (0.44 nmol/min/nmol P450), suggesting a primary involvement in the activation of imipramine to desipramine. We also examined the effect of the CYP2D6 inhibitor quinidine, the CYP3A inhibitor ketoconazole, and the dopamine reuptake inhibitor GBR-12935 for their ability to inhibit P450 2D18-mediated metabolism of imipramine. These results, when compared with previous studies using rat brain microsomes, suggest that P450 2D18 may play an important role in the conversion of imipramine to its active metabolite desipramine in the rat brain.

Dopamine formation from tyramine by CYP2D6

Dopamine is formed form L-tyrosine by tyrosine hydroxylase and aromatic L-amino acid decarboxylase. In addition to this pathway, however, the formation of catecholamines, including dopamine, from trace amines such as tyramine by hepatic microsomes has been demonstrated. In this study, we investigated the formation of dopamine from trace amines, using human hepatic microsomes and human cytochrome P450 (CYP) isoforms expressed in yeast. Among the 11 isoforms of human CYP expressed in yeast, CYP2D6 was the only isoform exhibiting strong ability to convert p-tyramine and m-tyramine to dopamine. In studies with human hepatic microsomes, the hydroxylation of tyramine to dopamine was inhibited by bufuralol, a typical substrate for CYP2D isoforms, and anti-CYP2D1 antiserum. This is the first report showing that CYP2D is capable of converting tyramine to dopamine. The Km values of CYP2D6, expressed in yeast, for p-tyramine and m-tyramine were 190.1 +/- 19.5 microM and 58.2 +/- 13.8 microM, respectively. Tyramine is an endogenous compound which exists in the brain as a trace amine but is also an exogenous compound which is found in foods such as cheese and wine. Our results suggest that dopamine is formed from endogenous and/or exogenous tyramine by this CYP2D isoform.