Characterization of two P-450 isozymes placed in the rat CYP2D subfamily

Comparison of CYP2D metabolism and hepatotoxicity of the myocardial metabolic agent perhexiline in Sprague-Dawley and Dark Agouti rats

1. Perhexiline, a chiral anti-anginal agent, may be useful to develop new cardiovascular therapies, despite its potential hepatotoxicity. 2. This study compared Dark Agouti (DA) and Sprague-Dawley (SD) rats, as models of perhexiline's metabolism and hepatotoxicity in humans. Rats (n = 4/group) received vehicle or 200 mg/kg/d of racemic perhexiline maleate for 8 weeks. Plasma and liver samples were collected to determine concentrations of perhexiline and its metabolites, hepatic function and histology. 3. Median (range) plasma and liver perhexiline concentrations in SD rats were 0.09 (0.04-0.13) mg/L and 5.42 (0.92-8.22) ng/mg, respectively. In comparison, DA rats showed higher (p < 0.05) plasma 0.50 (0.16-1.13) mg/L and liver 24.5 (9.40-54.7) ng/mg perhexiline concentrations, respectively, 2.5- and 3.7-fold higher cis-OH-perhexiline concentrations, respectively (p < 0.05), and lower plasma metabolic ratio (0.89 versus 1.55, p < 0.05). In both strains, the (+):(-) enantiomer ratio was 2:1. Perhexiline increased plasma LDH concentrations in DA rats (p < 0.05), but had no effect on plasma biochemistry in SD rats. Liver histology revealed lower glycogen content in perhexiline-treated SD rats (p < 0.05), but no effects on lipid content in either strain. 4. DA rats appeared more similar to humans with respect to plasma perhexiline concentrations, metabolic ratio, enantioselective disposition and biochemical changes suggestive of perhexiline-induced toxicity.

The importance of heterogeneous nuclear ribonucleoprotein K on cytochrome P450 2D2 gene regulation: its binding is reduced in Dark Agouti rats

Cytochrome P450 (P450) 2D2 (CYP2D2) enzyme is known to metabolize the majority of typical substrates of the human CYP2D6 enzyme, which is the most extensively characterized polymorphic drug-metabolizing enzyme. Despite its impact on drug metabolism in rats, the transcriptional regulation of CYP2D2 remains to be elucidated. We clarified the molecular mechanism of CYP2D2 gene expression. The CYP2D2 gene was positively regulated by the poly(C)-binding protein heterogeneous nuclear ribonucleoprotein K (hnRNP K) through a transcriptional regulatory element located in the 5'-flanking region from -94 to -113. To date, nothing is known about the potential role of hnRNP K in P450 gene regulation. Thus, this is the first report that hnRNP K protein is involved in CYP2D2 gene regulation. Furthermore, we elucidated the genetic basis of the extremely low expression of CYP2D2 mRNA in Dark Agouti (DA) rats. Because of its relatively low abundance, DA rats have been frequently used for the study of CYP2D substrate metabolism as the animal model of the poor metabolizer phenotype for CYP2D6 compared with Sprague-Dawley rats as an extensive metabolizer phenotype. We found a single substitution within the transcriptional regulatory element of the CYP2D2 gene in DA rats. The mutation was detected in the polypyrimidine sequence that is the preferred binding site for hnRNP K protein. The mutation within the transcriptional regulatory element attenuated the binding of hnRNP K protein. In conclusion, decreased recruitment of hnRNP K protein to the mutated sequence causes the low expression of CYP2D2 mRNA in DA rats.

Monoethylglycinexylidide and lidocaine determination in porcine microsomal preparations

A simple, accurate, and sensitive HPLC analysis of monoethylglycinexylidide (MEGX) and lidocaine in porcine microsome samples is described. Lidocaine and MEGX were measured by direct injection after the addition of the internal standard. Chromatography was performed on a muBondapak C(18) column using an isocratic mobile phase of 0.03 M potassium dihydrogen phosphate:acetonitrile (87:13), pH 5.9. UV absorbance was measured at 205 nm. The procedure produced linear curves for the concentration range 50-1000 ng/mL with a limit of detection of 10 ng/mL. Recoveries for both compounds were greater than 90%. This assay produced accurate and repeatable results.

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.

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.

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.

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.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.

Effects of ACTH administration on zonation of the guinea pig adrenal cortex

Experiments were done to determine the actions of ACTH on the morphologic and functional characteristics of the zona fasciculata (ZF) and zona reticularis (ZR) in the guinea pig adrenal cortex. In control guinea pigs, a number of morphologic differences distinguished the ZF from the ZR, including the presence of far more lipid in the ZF than in the ZR. Treatment with ACTH decreased the lipid droplet content of the ZF cells, equalizing the amount of lipid in the two zones. Other morphologic differences between the ZF and ZR were also diminished by ACTH treatment. Immunohistochemical analyses indicated that CYP17 protein was found in both the ZF and ZR in control animals, but with greater immunostaining intensity in the ZF. The enzyme protein distribution corresponded with higher 17alpha-hydroxylase activity in the ZF than in the ZR. After ACTH treatment, the intensity of staining and enzyme activities in the two zones were similar, attributable largely to increases in the ZR. In situ hybridization-and immunohistochemistry showed that in control animals CYPD216 was highly expressed in the ZR but not in the ZF. ACTH treatment dramatically reduced the intensity of CYP2D16 mRNA and protein staining in the ZR. Bufuralol 1'-hydroxylase activity, a marker for CYP2D subfamily members, was also decreased significantly in the ZR by ACTH treatment. The data indicate that administration of ACTH to guinea pigs has opposite effects on the expression of CYP17 and CYP2D16 in the ZR, and diminishes or eliminates some of the structural and functional differences between the ZF and ZR. The results suggest a role for ACTH in establishing and maintaining adrenocortical zonation.

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.

Cloning and characterization of the CYP2D1-binding protein, retinol dehydrogenase

A CYP2D1-binding protein, 29 k-protein (p29), has been isolated and its N-terminal amino acid sequence has been reported (Ohishi et al. (1993) Biochim. Biophys. Acta 1158, 227-236). In this study, p29 cDNA was isolated by PCR with oligonucleotide probes designed from the N-terminal amino acid sequence and p29 was found to be a microsomal retinol dehydrogenase, a member of the short-chain alcohol dehydrogenase family which metabolize hydroxysteroids and prostaglandins. CYP2D1 and p29 were expressed in Saccharomyces cerevisiae to characterize these proteins. CYP2D1 had an absorption maximum at 448 nm in a CO-reduced form. Expressed p29 in yeast cells was detected with anti-p29 antibody. Solubilized CYP2D1 and p29 from yeast microsomes were mixed and applied to an anti-CYP2D1 antibody-binding column. Both proteins were retained in the column and eluted with glycine buffer (pH 2.8). However, when applied alone, p29 was not retained in the column. The findings indicated that CYP2D1 bound tightly with p29. Catalytic activities of p29 expressed in yeast were investigated. p29 had retinal reductase activity in the presence of NADPH. Addition of CYP2D1 and NADPH-P450 reductase increased the retinal reductase activity of p29. These findings suggest that the complex of CYP2D1, p29, and NADPH-P450 reductase has an important role in the metabolism of retinoids.

Tissue distributions of CYP2D1, 2D2, 2D3 and 2D4 mRNA in rats detected by RT-PCR

The tissue distributions of four isoforms (CYP2D1/5, 2D2, 2D3 and 2D4/18) in rat CYP2D subfamily were investigated. Twelve kinds of tissue (liver, kidney, brain, lung, heart, spleen, adrenal gland, small intestine mucosa, bladder, testis, ovary and gonecystis) were removed from Sprague-Dawley male and female rats. The expression of CYP2D mRNA in these tissues was detected by RT-PCR. Specific primers were designed to recognize the four isoforms individually. In liver, kidney and small intestine mucosa, the mRNA expression of all four CYP2D isoforms was detected as high-intensity PCR products. mRNA of CYP2D1/5 was expressed in all tissues used in this study except the brain, although the intensity of PCR products varied among tissues. mRNAs of CYP2D2 and CYP2D3 were mainly expressed in liver, kidney and small intestine mucosa, which were exposed to xenobiotics such as drugs, food components and environmental contaminations. mRNA of CYP2D4/18 was expressed in liver, kidney, small intestine mucosa and brain. In brain, only mRNA of CYP2D4/18 was expressed. CYP2D4/18 mRNA was also expressed in ovary, testis and gonecystis. The tissue distributions help to clarify the differences in physiological and pharmacological functions between CYP2D isoforms.

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

We cloned four cDNAs belonging to the CYP2D subfamily to express these enzymes in yeast cells and to compare their catalytic activities simultaneously. Three are believed to be alleles of CYP2D1, 2D2, and 2D3, respectively, based on high nucleotide sequence similarity, while CYP2D4 had both sequences of CYP2D4 and CYP2D18. Expression plasmids carrying CYP2D cDNAs were transformed into Saccharomyces cerevisiae. Typical P450 CO-difference spectra with absorbance maximum at 448 nm were recorded with microsomal preparations from the yeast cells expressing the four CYP2D forms. A catalytic study of these CYP2D forms was done with debrisoquine, bufuralol, and lidocaine. CYP2D2 had the highest debrisoquine 4-hydroxylation (2.2 nmol/min/nmol P450) activity, similar to that (2.2 nmol/min/nmol) of human CYP2D6 expressed in yeast cells. CYP2D3 had high lidocaine N-deethylation (43 nmol/min/nmol P450) activity, and both CYP2D3 and 2D2 exhibited high lidocaine 3-hydroxylation (2.4 and 1.6 nmol/min/nmol P450, respectively) activity. Bufuralol 1'-hydroxylation catalytic capabilities were comparable among the four isoforms. The activity of CYP2D1 was relatively low toward the three substrates (debrisoquine, 0.091; bufuralol, 1.5; lidocaine 3-hydroxylation, 0.019; lidocaine N-deethylation, 2.8 nmol/min/nmol P450). These findings indicate that debrisoquine, a typical substrate for CYP2D forms, was mainly metabolized by CYP2D2 but not CYP2D1 in rat liver and that the CYP2D forms have different substrate specificity.

Localization of CYP2D16 in the guinea pig adrenal cortex by immunohistochemistry and in situ hybridization

Recent reports indicate that the cytochrome P450 isozyme, CYP2D16, is expressed at high levels in the inner regions of the guinea pig adrenal cortex and may contribute to xenobiotic and/or steroid metabolism in the gland. In the present studies, immunohistochemical and in situ hybridization techniques were employed to definitively establish the localization of CYP2D16 within the adrenal cortex. In male guinea pigs of various ages, CYP2D16 protein and mRNA were highly localized to the zona reticularis (ZR); none was detectable in the zona fasciculata (ZF), zona glomerulosa (ZG) or the medulla. In contrast, the steroidogenic P450 isozyme, CYP17, was distributed throughout the ZF and ZR. From the earliest stages of development of the ZR, CYP2D16 staining was intense. As guinea pigs aged, the ZR progressively enlarged and comprised a proportionately greater amount of the cortex. At all ages, CYP2D16 was uniformly distributed throughout only the ZR. Coinciding with the age-related growth of the ZR and increase in adrenal CYP2D16 content was an increase in adrenal xenobiotic-metabolizing activity. The results establish that CYP2D16 has an intraadrenal localization that is unique among P450 isozymes, suggesting novel regulatory mechanisms and indicating that CYP2D16 may serve as a specific marker for ZR cells. The increase in CYP2D16 expression with age probably accounts for increasing levels of xenobiotic metabolism and may also contribute to an increase in intraadrenal cortisol degradation in older animals.

Strain differences in adrenal CYP2D16 expression in guinea pigs. Relationship to xenobiotic metabolism

Experiments were done to determine the mechanisms responsible for differences in adrenal microsomal xenobiotic metabolism between Strain 13 and English Short-Hair (ESH) guinea pigs. The rates of adrenal xenobiotic metabolism (bufuralol 1'-hydroxylase, benzo[a]pyrene hydroxylase, benzphetamine N-demethylase) were 2-3 times greater in microsomes from the Strain 13 animals. In both strains, xenobiotic-metabolizing activities were far greater in the inner zone (zona reticularis) than in the outer zones (zona fasciculata and zona glomerulosa) of the adrenal cortex. Northern blot analyses of total adrenal RNA with a CYP2D16 cDNA as the probe revealed significantly greater amounts of CYP2D16 mRNA in the Strain 13 guinea pigs. In addition, SDS-PAGE and Western blotting of adrenal microsomes demonstrated higher concentrations of CYP2D16 protein in Strain 13 than in ESH animals. Expression of CYP2D16 was predominantly in the inner zone of the adrenal, coinciding with the major site of xenobiotic metabolism. The results demonstrated higher levels of expression of CYP2D16 in adrenal glands from Strain 13 than from ESH guinea pigs, which may account for the strain differences in adrenal xenobiotic metabolism. Strain 13 guinea pigs should serve as a good experimental model for further studies on the regulation of adrenal CYP2D16.

Characterization of the oxidation reactions catalyzed by CYP2D enzyme in rat renal microsomes

Monooxygenase activities in rat renal microsomes were determined with the substrates of hepatic CYP2D enzymes. Seven kinds of CYP2D-mediated monooxygenase activities and immunochemically determined CYP2D contents in kidneys corresponded to approximately 3% of those in livers. Debrisoquine 4-hydroxylase and bunitrolol 4-hydroxylase in renal microsomes were inhibited almost completely by the antibody against a CYP2D enzyme purified from rat liver. A marked strain difference (Wistar > Dark Agouti) in these activities was observed in kidney like in liver. The two hydroxylases were inhibited stereoselectively by quinine and quinidine both in renal and hepatic microsomes. Substrate stereoselectivity in (+)- and (-)-bunitrolol 4-hydroxylase activities in kidneys was also consistent with that in livers. These results suggested that the CYP2D enzyme(s) was expressed in the kidney at levels much less than in the liver but had similar functions to those in the liver.

Role of the CYP2D subfamily in metabolism-dependent covalent binding of propranolol to liver microsomal protein in rats

In vitro covalent binding of a chemically reactive metabolite of propranolol to microsomal macromolecules, which is presumed to cause inhibition of its own metabolism in rats, was diminished in liver microsomes from rats pretreated with propranolol. Covalent binding was suppressed by the addition of an antibody against P450BTL, which is a cytochrome P450 (P450) isozyme belonging to the CYP2D subfamily. SDS-PAGE of microsomal proteins after incubation with [3H]propranolol and NADPH indicated that the binding was non-selective but prominent at the molecular mass of approx. 50 kDa, corresponding to those of the P450 protein. The radioactivity peak was markedly but not completely diminished by the addition of reduced glutathione. In a reconstituted system containing P450BTL, NADPH-cytochrome P450 reductase (fp2) and dilauroylphosphatidylcholine, propranolol 4-, 5- and 7-hydroxylase activities decreased time dependently following preincubation with propranolol in the presence of NADPH, indicating time-dependent inactivation of P450BTL. The covalent binding of a reactive metabolite of [3H]propranolol to the proteins was also observed in this system. SDS-PAGE showed that among the three proteins in the reconstituted system, fp2 and P450BTL consisting of two polypeptides with molecular masses of 49 and 32 kDa, the binding was specific for a polypeptide corresponding to the P450 isozyme with a molecular mass of 49 kDa. In addition, the ratio of the amount of covalently bound radiolabelled materials to that of P450BTL which was estimated from each impaired propranolol hydroxylase activity under the same reconstitutional conditions was calculated to be approx. 1.0. These findings indicate that propranolol is a mechanism-based inactivator of a cytochrome P450 isozyme(s) belonging to the CYP2D subfamily.