Characterization of CYP2A6 involved in 3'-hydroxylation of cotinine in human liver microsomes

Nicotine is primarily metabolized to cotinine, and cotinine is further metabolized to trans-3'-hydroxycotinine in human liver, which is a major metabolite of nicotine in humans. We studied the formation of trans-3'-hydroxycotinine from cotinine in human liver microsomes. trans-3'-Hydroxycotinine formation demonstrated single enzyme Michaelis-Menten kinetics (Km, 234.5 +/- 26.8 MicroM; Vmax, 37.2 +/- 2.4 pmol/min/mg protein). Significant correlation (r = .967, P < .001) between cotinine 3'-hydroxylase activities at low (50 microM) and high (1 microM) cotinine concentrations in 20 human liver microsomes suggested the contribution of a single enzyme to cotinine 3'-hydroxylation. The cotinine 3'-hydroxylase activity correlated significantly with immunoreactive cytochrome P450 (CYP)2A6 contents (r = .756, P < .01) and coumarin 7-hydroxylase activity (r = .887, P < .001). The cotinine 3'-hydroxylase activity was inhibited by coumarin, alpha-naphthoflavone, chlorzoxazone and anti-rat CYP2A1 antibodies. Microsomes of B-lymphoblastoid cells expressing human CYP2A6 exhibited cotinine 3'-hydroxylase activity. The Km value of the expressed CYP2A6 (264.7 microM) was almost identical to that of human liver microsomes. In conclusion, cotinine 3'-hydroxylation appears to be catalyzed solely by CYP2A6 in humans. Cotinine is a candidate for a new substrate for CYP2A6 in humans.

Sterol 14-demethylase P450 (P45014DM*) is one of the most ancient and conserved P450 species

To determine the orthology of sterol 14-demethylase (P45014DM), the only known P450 enzyme distributed widely in eukaryotes with a conserved metabolic role, the full-length amino acid sequences of rat and human P45014DMs were determined from the cloned cDNA sequences, and compared with those of the corresponding fungal proteins (CYP51). The amino acid identity value between given pairs of P45014DMs ranged from 93% (human/rat) to 39% (human or rat/Saccharomyces cerevisiae). All the P45014DMs formed a single cluster in a phylogenetic tree constructed from representative P450 protein sequences currently available. The nearest neighbors to the P45014DM cluster in the phylogenetic tree were CYP7 (cholesterol 7 alpha-hydroxylase) and CYP8 (prostacyclin synthase), and the divergence point of fungal and mammalian P45014DMs was clearly more recent than that of P45014DM and CYP7/CYP8. These lines of evidence show that fungal and mammalian P45014DMs are really orthologous. This is the first example of orthologous P450s occurring in distinct kingdoms. P45014DM may be an ancient P450 which arose before the divergence of major eukaryotic branches and has been conserved throughout evolution. The amino acid identity value (93%) between human and rat P45014DMs was comparable to those observed for some housekeeping enzymes. In addition, a processed pseudogene of P45014DM was found in a rat genomic DNA library, suggesting the expression of P45014DM in germ line cells. These facts suggest that P45014DM may be a housekeeping enzyme essential for the viability of mammals.

Multiple forms of human P450 expressed in Saccharomyces cerevisiae. Systematic characterization and comparison with those of the rat

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

Cytochrome P450s of isolated rat hepatocytes in spheroid and monolayer cultures

The maintenance of cytochrome P450s (P450s) and NADPH-cytochrome P450 reductase (P450 reductase) in the monolayer and spheroid cultures of hepatocytes from male rats was examined. The content of total P450 in monolayer culture decreased to almost none after 144 hr, whereas the level in spheroid culture remained within 6-13% of initial values during an incubation period of 144-192 hr. P450 2C11, a major P450 in male rat, in monolayer cells rapidly decreased in 144 hr, while the level in spheroid cells after 144 hr and 192 hr maintained 25% and 15%, respectively, of initial level. On the other hand, P450 2A1 and P450 2E1 in both monolayer and spheroid cells rapidly decreased. P450 reductase in both cells showed a gradual decline reaching a level of 43-44% of the initial level at 96 hr, and remained within 16-17% of the initial value during an incubation period of 192 hr. These results indicate that P450 2C11 in spheroid cells maintained more stable than in the monolayer cells, and that P450 reductase in both cultures declined only moderately, compared with P450s.

Expression and zonal distribution of CYP2D16 in the guinea pig adrenal cortex: relationship to xenobiotic metabolism

We recently cloned a CYP2D subfamily member (CYP2D16) from a guinea pig adrenal cDNA library and investigated the expression of CYP2D16 in the guinea pig adrenal cortex and its relationship to adrenal xenobiotic metabolism. A modified sodium dodecyl sulfate-polyacrylamide gel electrophoresis technique revealed three major bands in the molecular mass range of cytochromes P450 in guinea pig adrenal microsomes. Two of the bands were immunoreactive with anti-CYP17 (54 kDa) or anti-CYP21 (52 kDa) antibody. The third band (50 kDa) was immunoreactive with antibody raised against CYP2D1 and with anti-CYP1A1/1A2 antibody. Microsequencing of the 50-kDa band yielded an amino-terminal sequence of 38 amino acids identical to that deducted from the CYP2D16 cDNA. In addition, Northern blot analyses indicated the CYP1A1 was not expressed in the adrenal gland, suggesting that only CYP2D16 composed the microsomal 50-kDa band. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analyses demonstrated greater expression of CYP2D16 in microsomes from the inner zone (zona reticularis) of the adrenal cortex than from the outer zones, coinciding with the major site of adrenal xenobiotic metabolism. Bufuralol-1'-hydroxylase activity, a marker for CYP2D isozymes, was also greater in inner- than in outer-zone microsomal preparations and was highly correlated with CYP2D16concentrations. Northern blot analysis with a full-length CYP2D16 cDNA as the probe gave strong bands with adrenal inner zone RNA preparations and relatively weak bands with outer zone RNA. CYP2D16 mRNA was also detectable in liver and kidney RNA preparations, but at lower levels than in the adrenal inner zone, and it was not detectable in testes, lung, intestines, or heart. Overall, the results demonstrate that CYP2D16 is expressed at highest levels in the inner zone of the guinea pig adrenal cortex and suggest a major role for this isozyme in adrenal xenobiotic metabolism.

Metabolism of acetaldehyde to acetate by rat hepatic P-450s: presence of different metabolic pathway from acetaldehyde dehydrogenase system

NADPH-dependent activity of acetaldehyde oxidation was investigated in microsomes by assaying [14C]acetic acid produced from [14C]acetaldehyde with ion-exchange column. Rat hepatic microsomes exhibited acetaldehyde oxidation activity in the presence of NADPH. This activity was induced 2-fold by the treatment of rats with ethanol. We designated this NADPH-dependent oxidation system as microsomal acetaldehyde-oxidizing system (MAOS), to distinguish from the NAD-dependent acetaldehyde oxidation system by acetaldehyde in mitochondria and cytsol. We further investigated essential enzymes contributing to MAOS activity. Acetaldehyde oxidation activity was investigated in eight forms of purified P-450 in a reconstituted system. Cytochrome P-450 (CYP) 2E1 had the highest oxidation activity and CYP1A2 and CYP4A2 had the next highest activity. Other forms had low activity. To assess the contribution of these forms to MAOS activity, immunoblot was done. CYP2E1 was induced 2-fold by ethanol treatment, but CYP1A2 and CYP4A2 were not reflecting the MAOS activity increased by ethanol treatment. These results suggest that CYP2E1 is the essential enzyme in the MAOS of rats.

Induction of CYP isoenzymes in various organs of rats by 3-methylcholanthrene or beta-naphthoflavone

Induction of cytochrome P450 (CYP) isoenzymes in various organs of rats treated with 3-methylcholanthrene (3-MC) and beta-naphthoflavone (BNF) was immunohistochemically and biochemically investigated. Fifteen male F344 rats were divided into three equal groups. Group 1 was untreated as a control. On days 3, 4, 13 and 14, group 2 animals received 3-MC (20 mg/kg body wt i.p.) dissolved in corn oil while group 3 animals were given BNF (50 mg/kg body wt i.p.) dissolved in corn oil, at days 1, 2, 3, 4, 11, 12, 13 and 14. On days 4 and 14, two or three animals in each group were sacrificed, 15 h after administration of the last test compound. Induction of CYP 1A1, 2C11, 2D1, 2E1, 3A2 and 4A1 in various organs was immunohistochemically examined and the levels of CYP 1A1 protein were measured by Western blotting. 3-MC and BNF induced CYP isoenzymes not only in the liver, but also in the small intestine, large intestine, prostate and seminal vesicles. The results indicate that xenobiotic metabolism can occur in various organs.

Inhibition and induction of cytochrome P450 isozymes after repetitive administration of imipramine in rats

Repetitive oral administration of imipramine (100 mg/kg/day for 5 days) caused a decrease in rat liver microsomal debrisoquine 4-hydroxylase activity, a characteristic reaction catalyzed by cytochrome P450 (CYP) 2D1. Other CYP2D-dependent reactions (such as bunitrolol 4-hydroxylation, lidocaine 3-hydroxylation, and propranolol 4-, 5- and 7-hydroxylations) were also impaired by the treatment, but not those catalyzed by other CYP isozymes. Imipramine pretreatment did not change the immunochemically determined content of the CYP2D protein, suggesting that CYP2D is inactivated. Imipramine pretreatment also resulted in an increase in total CYP content and in formation of a ferrous CYP metabolic intermediate (MI)-complex absorbing at 454 nm. Although the total CYP content was increased by the treatment of these microsomes with ferricyanide to dissociate the MI-complex, the CYP2D-dependent activities were not restored, suggesting that the MI-complex was not the primary cause of CYP2D inhibition. This pretreatment regimen caused marked increases in immunochemically determined levels of CYP2A1, CYP2B1, CYP2B2, CYP2C6, and CYP3A2, and in the activities of 2 alpha-, 2 beta-, 6 beta-, 7 alpha-, 16 alpha-, and 16 beta-hydroxylation and 17-oxidation of testosterone. These results indicate that imipramine has two actions on the liver CYP system (i.e. as an inhibitor of the CYP2D enzyme and as a phenobarbital-type inducer).

Mutagenic activation of 3-methoxy-4-aminoazobenzene by mouse renal cytochrome P450 CYP4B1: cloning and characterization of mouse CYP4B1

A new P450 responsible for mutagenic activation of 3-methoxy-4-aminoazobenzene (3-MeO-AAB) which is a potent procarcinogen was purified from renal microsomes of male mice using an index of umu gene expression. The purified P450 had high bioactivation toward 3-MeO-AAB and also 2-aminofluorene and 2-aminoanthracene. The antibody against this P450 completely inhibited mutagenic activation of 3-MeO-AAB of mouse renal microsomes. With immunoblotting, this form was present abundantly in renal microsomes of male mice but not in those of female mice. This P450 was also present in pulmonary microsomes of male and female mice but not in hepatic microsomes. The NH2-terminal amino acid sequence analysis indicated that this form belonged to the CYP4B subfamily. Thus, mouse kidney cDNA library was screened with rat CYP4B1 probe. The cDNA-deduced amino acid sequence of isolated cDNA consisted of 511 amino acids and bore 90, 86, and 84% similarities to rat, rabbit, and human CYP4B1, respectively. The NH2-terminal amino acid sequence of the purified renal P450 and amino acid sequence of BrCN-digested peptides from the purified P450 agreed with the cDNA-deduced amino acid sequence. These results suggest that CYP4B1 is a major form in renal microsomes of male mice and plays a major role in mutagenic activation of 3-MeO-AAB. In extrahepatic tissue, CYP4B1 may contribute to chemical carcinogenesis.

Induction of hepatic microsomal P450 by 4-phenylalkylpyridines in rat: chain length-dependent and sex-related differential induction of P450s

The effects of 4-phenylalkylpyridines (chain length of 0-5, 7, 9 and 11 carbon atoms) on the induction of hepatic microsomal P450 and its multiple forms (1A1/2 (1A1, 1A2), 3A2, 2B1/2, 2C6 and 2E1) in the male and female rat have been investigated. P450 induction gradually declined with increasing chain length of the 4-phenylalkylpyridines. Immunoblot analysis revealed that three pyridine compounds having methylene units of 0, 1 and 2 only induced P4501A2, whereas those having 4, 5 or 7 methylene units only induced 1A1 in the male rat. In the female rat, however, we could not observe such a chain length-dependent differential induction of the P4501A subfamily. Induced levels of P4503A2 and 2E1 were dependent on total P450 specific content, but 2C6 was increased in a chain length-dependent manner in both sexes. These results provide new information on the differential effects of pyridine-containing compounds on P450 induction in the rat.

Cytochrome P450 isozymes involved in aromatic hydroxylation and side-chain N-desisopropylation of alprenolol in rat liver microsomes

Alprenolol 4-hydroxylation and N-desisopropylation in liver microsomes from male Wistar rats were kinetically analyzed to be biphasic. In the 4-hydroxylation at a low substrate concentration (5 microM), significant strain [Wistar > Dark Agouti (DA)] and sex (male > female) differences were observed, and the differences decreased at a high substrate concentration (1 mM). In the N-desisopropylation, only a strain difference (Wistar > DA) was observed at the low substrate concentration. Cytochrome P450BTL (P450BTL, corresponding to CYP2D2) in a reconstituted system with 5 microM alprenolol had high 4-hydroxylase activity, which was about 10 times that of P450ml corresponding to CYP2C11, and N-desisopropylase activity at a similar extent to P450ml. The two microsomal activities at 5 microM alprenolol were efficiently decreased by antibodies against P450BTL and by sparteine, a typical substrate of the CYP2D subfamily. Polyclonal antibodies against P450ml and P450PB-1 (corresponding to CYP3A2) partially suppressed only N-desalkylation at 5 microM, whereas they reduced the two activities at 1 mM. P450ml showed a high N-desisopropylase activity at a substrate concentration of 1 mM, where the sex difference was not observed. Furthermore, P450PB-2 corresponding to CYP2C6, which is one of the major P450 isozymes in female rats, also had 4-hydroxylase and N-desalkylase activities. These results suggest that a CYP2D isozyme(s) is the primary enzyme in alprenolol 4-hydroxylation and N-desisopropylation in a lower substrate concentration range, and that the involvement of some male-specific P450 isozyme(s) other than CYP2C11 or CYP3A2 may cause the sex difference in the 4-hydroxylation. In a higher substrate concentration range, CYP2C11 is thought to play a major role particularly in N-desisopropylation in male rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of CYP2C in the metabolism of cannabinoids by human hepatic microsomes from an old woman

The hepatic microsomal metabolism of cannabinoids was studied using the liver from an old woman. delta 8-Tetrahydrocannabinol, delta 9-tetrahydrocannabinol and cannabinol were biotransformed to their respective 11-hydroxy metabolites by a microsomal fraction with specific activities (pmol/min/mg protein) of 29.1, 47.1 and 27.9, respectively. In addition, both 11-oxo-delta 8-tetrahydrocannabinol and 11-oxo-delta 9-tetrahydrocannabinol were metabolized to the corresponding carboxylic acids with the microsomes. An antibody against mouse CYP2C29 almost completely inhibited 11-hydroxylation of the cannabinoids and microsomal aldehyde oxygenase (MALDO) activity for 11-oxo-delta 8-tetrahydrocannabinol and 11-oxo-delta 9-tetrahydrocannabinol, used as substrates, whereas an antibody against rat CYP3A2 conversely stimulated the 11-hydroxylation of delta 8-tetrahydrocannabinol and MALDO activity for 11-oxo-delta 8-tetrahydrocannabinol. The results indicate that a member of CYP2C is primarily responsible for the metabolism of the above cannabinoids in the human hepatic microsomes.

Metabolism of highly persistent PCB congener, 2,4,5,2',4',5'-hexachlorobiphenyl, by human CYP2B6

Metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl was studied with cDNA-expressed human P450 2B isoform, CYP2B6. 3-Hydroxy-2,4,5,2',4',5'-hexachlorobiphenyl was identified as a major metabolite, and the formation activity was compared with that of dog CYP2B11 and guinea pig P450GP-1. The activity of 3-hydroxylation was comparable with that of P450GP-1, but one-tenth of CYP2B11. These results indicate that P450 2B in humans as well as other animal species can metabolize 2,4,5,2',4',5'-hexachlorobiphenyl, and the reason why this PCB congener remained most abundantly in human bodies is discussed.

Characterization of a chemically reactive propranolol metabolite that binds to microsomal proteins of rat liver

We have characterized a chemically reactive propranolol (PL) metabolite which binds to proteins in rat liver microsomes. During incubation with rat liver microsomes (1 mg of protein) fortified with an NADPH-generating system, 4-hydroxypropranolol (4-OH-PL) quickly disappeared from the reaction medium, but none of the possible metabolite peaks was detected under the high-performance liquid chromatographic conditions used. The consumption of 4-OH-PL depended on microsomes and NADPH. The reaction was not affected by inhibitors of cytochrome P450 or FAD monooxygenase, but was markedly diminished in the presence of cytosol and ascorbic acid. The effect of cytosol was inhibited by potassium cyanide but not by sodium benzoate or dimethyl sulfoxide, and was also not affected by heating at 60 degrees C for 30 min, suggesting that superoxide (SO) ion was involved in the reaction and that it was blocked by superoxide dismutase (SOD) present in the cytosol. Cu,Zn-SOD, purified from cytosol, effectively mimicked the suppressive effect of cytosol. Incubation of 4-OH-PL in an SO-generating system of xanthine and xanthine oxidase generated 1,4-naphthoquinone (1,4-NQ), which was identified by TLC, HPLC, and GC/MS. 1,4-NQ was also formed in microsomal incubates containing NADPH and small amounts of microsomes (below 0.1 mg of protein). These results indicate that 4-OH-PL is converted by SO, or some reactive oxygen species derived from it, to 1,4-NQ which binds to proteins and is one of the reactive metabolites of PL.

Cytochrome P4502D isozymes catalyze the 4-hydroxylation of methamphetamine enantiomers

The 4-hydroxylation of S(+)- and R(-)-methamphetamine by rat liver microsomes was examined in Sprague-Dawley and Dark Agouti strains to determine the role of cytochrome P4502D (CYP2D) subfamily isozymes in catalyzing the reaction. In the study, anti-P450-BTL IgG, bufuralol, and quinine, a substrate and inhibitors of CYP2D isozymes, respectively, were found to block approximately 90% of the reaction as catalyzed by microsomes from Sprague-Dawley rats. Reconstituted systems of CYP2D isozymes purified from rat liver microsomes also mediated the reaction. These observations and the minimal activity found in microsomes from Dark Agouti rats support the notion that methamphetamine, like other phenylisopropylamine compounds, is oxidized on the 4-position of the aromatic ring by CYP2D isozymes.

Induction of hepatic P450s in rat by essential wood and leaf oils

1. The effects of essential oils extracted from the wood and leaves of Chamaecyparis obtusa, Cryptomeria japonica and Thujopsis dolabrata on P450s in the hepatic microsomes of the male rat have been investigated. 2. Essential oils did not affect the content of total P450s measured photometrically. However, some metabolic activities and the levels of some forms of P450, including 2B1, were significantly increased, indicating their induction by essential oils. 3. The effects of components derived from essential oils (alpha-pinene, 1,8-cineole, cadinene and borneol) on P450s in rat hepatic microsomes were investigated. The activities of testosterone 2 beta-, 6 beta-, 16 alpha- and 16 beta-hydroxylation and the levels of P4502B1 and 3A2 were increased by 1,8-cineole and cadinene. The induction of P450s by essential oils is thought to be caused by cadinene, a major component. The P450 isoform induced by cadinene is similar to that induced by phenobarbital. However, the magnitude of induction by cadinene was less than that by phenobarbital at the dose levels studied. 4. P4502B1 and 3A2 were induced by essential oils in the perivenous area in the hepatic lobe.

Purification and characterization of a form of cytochrome P450 from bear liver microsomes

A form of P450 [termed P450(b-1)] was purified from male bear liver microsomes. The specific content of the final P450(b-1) preparation was 11.26 nmol/mg protein, and recovery was 0.20% of the microsomal P450. The apparent molecular weight of P450(b-1) was 54,000. The absorption spectrum of P450(b-1) indicated that this protein was a low- and high-spin mixed type P450 in the oxidized form. The carbon monoxide complex of reduced P450(b-1) showed an absorption peak at 450.5 nm. The reconstituted system containing P450(b-1) catalyzed the metabolism of aminopyrine, benzo[a]pyrene, 7-ethoxycoumarin, imipramine and propranolol, of which P450(b-1) most strongly catalyzed aminopyrine N-demethylation and imipramine N-demethylation. The N-terminal amino acid sequence of P450(b-1) was highly homologous to that of P450-D1 from liver microsomes of male beagle dogs. P450(b-1) showed similarities in spectral properties, N-terminal amino acid sequence, and catalytic activities to rat P450 2C11. P450(b-1) was immunochemically cross-reactive with anti-P450 2C11 antibody and very weakly cross-reactive with anti-P450 2E1 antibody, but did not react with anti P450 1A1 or 2B1 antibodies. On the basis of these results, we suggest that P450(b-1) belongs to the P450 2C subfamily.

Mepyramine, a histamine H1 receptor antagonist, inhibits the metabolic activity of rat and human P450 2D forms

The interaction of antihistaminics, including mepyramine, with rat hepatic cytochrome P450s (P450s) was investigated. We first investigated mepyramine binding to eight forms of rat hepatic P450s. Mepyramine bound specifically to P450 2D1, which suggests that it inhibits P450 2D activity. Histamine H1 receptor antagonists (mepyramine, diphenhydramine, chlorpheniramine and triprolidine) inhibited the lidocaine 3-hydroxylation activity catalyzed by P450 2D1 but did not inhibit the testosterone hydroxylation activities catalyzed by P450s other than P450 2D forms. The Ki values of these antagonists for the catalytic activity of P450 2D1 were low and were similar to those of quinine and quinidine, which are specific inhibitors of P450 2D forms. The Ki value of mepyramine was especially low, at 34 nM. Furthermore, the effects of mepyramine on human P450 2D6 were investigated. Among the ten forms of human P450 expressed in yeast, mepyramine bound specifically to P450 2D6 in a binding assay. In human hepatic microsomes, mepyramine inhibited the debrisoquine 4-hydroxylation activity catalyzed by P450 2D6. These results indicate that histamine H1 receptor antagonists such as mepyramine are potent inhibitors of P450 2D forms because of their high affinity for these enzymes.

Maintenance of phenobarbital-inducible Cyp2b gene expression in C57BL/6 mouse hepatocytes in primary culture as spheroids

Expression of the Cyp2b-9 and Cyp2b-10 genes was investigated in primary cultured adult C57BL/6NCrj mouse hepatocytes in monolayers or during the formation of spheroids (multicellular aggregates). Both the constitutive and phenobarbital-inducible expression of Cyp2b-9 and Cyp2b-10 mRNA decreased rapidly after transferring the hepatocytes to monolayer culture. The decrease was dependent on cell-density, and became more rapid in more densely seeded cells. However, in spheroid culture, the Cyp2b-9 and Cyp2b-10 mRNAs were induced by phenobarbital at high levels for at least 4 days either with continuous exposure from the start of cultivation or for 24 h before harvesting. The expression of both Cyp2b-9 and Cyp2b-10 species was confirmed by either reverse transcriptase-polymerase chain reaction or Western blotting. Although more Cyp2b-9 than Cyp2b-10 was expressed in the liver of control mice, the amounts of the latter became relatively overwhelming in untreated hepatocytes because of a faster decline of Cyp2b-9 species in culture. The level of mRNA induced by phenobarbital was concentration-dependent, the highest being at 2 or 4 mM, which was equivalent to in vivo treatment levels. There was more Cyp2b-10 species than those of Cyp2b-9 after exposure to phenobarbital both in vivo and in vitro. Phenobarbital also induced CYP1A2 mRNA, which was again peculiar to spheroid culture. Although the expression levels of both Cyp2b-9 and Cyp2b-10 species was very low in hepatocytes cultured without dexamethasone even in the presence of phenobarbital, the addition of 10(-7) or 10(-6) M dexamethasone caused an increase in the mRNA. When given concomitantly with phenobarbital, the expression was greatly enhanced. Nicotinamide or isonicotinamide similarly enhanced the expression of the two mRNA species, but the levels in monolayer cultures were still far lower than those in vivo. In contrast to the findings for mRNA expression, the protein levels in the presence of nicotinamide were similar to those in vivo under both monolayer and spheroid conditions. Since previous efforts to maintain expression of the CYP2B1 and CYP2B2 genes in primary cultured rat hepatocytes, orthologous to mouse Cyp2b-9 and Cyp2b-10 genes, required special cell attachment factors, our spheroid culture system, in which mouse hepatocytes are simply seeded onto noncoated dishes, has advantages for mechanistic studies.

Metabolism of a new local anesthetic, ropivacaine, by human hepatic cytochrome P450

BACKGROUND

Ropivacaine is a local anesthetic with a long duration of action. Although it is less toxic than bupivacaine, local anesthetic toxicity is possible when the plasma concentration is increased. Because ropivacaine is an amide-type local anesthetic, it is metabolized by cytochrome P450 (P450) in the liver, and its elimination and plasma concentration can be dependent on the level of P450. The purpose of this investigation was to elucidate the metabolism of ropivacaine by human hepatic P450.

METHODS

The metabolism of ropivacaine was compared using recombinant human and purified rat hepatic P450 isozymes. An inhibition study using antibodies against rat P450 was performed using hepatic microsomes from human and rat to identify which P450s are involved in ropivacaine metabolism.

RESULTS

Ropivacaine was metabolized to 2',6'-pipecoloxylidide (PPX), 3'-hydroxyropivacaine (3'-OH Rop), and 4'-hydroxyropivacaine (4'-OH Rop) by hepatic microsomes from human and rat. PPX was a major metabolite of both human and rat hepatic microsomes. In a reconstituted system with rat P450. PPX was produced by CYP2C11 and 3A2, 4'-OH Rop by CYP1A2, and 3'-OH Rop by CYP1A2 and 2D1. Formation of PPX in rat hepatic microsomes was inhibited by anti CYP3A2, but not by CYP2C11 antibody, and formation of 3'-OH Rop was inhibited by CYP1A2 and 2D1 antibodies. Anti CYP3A2 and 1A2 antibodies inhibited the formation of PPX and 3'-OH Rop in human hepatic microsomes, respectively. Recombinant human P450s expressed in lymphoblast cells were used for further study. CYP3A4 and 1A2 formed the most PPX and 3'-OH Rop, respectively. Ropivacaine N-dealkylation and 3'-hydroxylation activities correlated well with the level of CYP3A4 and 1A2 in human hepatic microsomes, respectively.

CONCLUSIONS

Ropivacaine was metabolized to PPX, 3'-OH Rop, and 4'-OH Rop by hepatic P450. PPX was a major metabolite in human hepatic microsomes. CYP3A4 was involved in producing PPX. CYP1A2 was involved in the formation of 3'-OH Rop in human hepatic microsomes.

Induction of cytochrome P450 isozymes in rat renal microsomes by cyclosporin A

To examine the effects of cyclosporin A (CsA) on renal cytochrome P450 forms, CsA was administered to rats, and the renal levels of P450 were determined by immunoblotting. CsA treatment for 17 days increased total renal P450 content by 40% with a concomitant elevation of the omega- and (omega-1)-hydroxylation activities of lauric acid. Arachidonic acid omega-hydroxylation activity was also induced 2-fold by treatment with CsA for 17 days. Among the P450 forms, CYP4A2 was induced significantly, whereas CYP2C23, CYP4A1 and CYP4A8 were unaffected. These changes were accompanied by slight but significant increases in blood urea nitrogen and systolic blood pressure. These data suggest that CsA increased arachidonic acid omega-hydroxylation activity by the induction of CYP4A2. The specific induction of CYP4A2 may be related to CsA-induced nephrotoxicity and elevated blood pressure, because omega-hydroxyarachidonic acid is a potent vasoconstrictor.

Changes in content of P450 isozymes in hepatic and renal microsomes of the male rat treated with cis-diamminedichloroplatinum

1. The changes in the activity of drug-metabolizing enzymes and in the content of P450 isozymes in renal and hepatic microsomes after treatment of the male Sprague-Dawley rat with cis-diamminedichloroplatinum (Cisplatin, CDDP) were examined. 2. NADPH-P450 reductase activity in renal microsomes was significantly increased by treatment with CDDP, but lauric acid omega- and (omega-1)-hydroxylation activities of renal microsomes were not increased. 3. The level of P4502C23 was increased significantly and levels of P4504A2 and 4A8 tended to increase in renal microsomes. 4. In hepatic microsomes, lauric acid omega-hydroxylation activity was increased, but (omega-1)-hydroxylation activity was not. Levels of P4502C11 and 3A2, which are male-specific forms, were decreased, whereas levels of P4502A1, 2C7 and 2E1 were increased in hepatic microsomes. The levels of P4504A2 and 4A3 were increased by CDDP and the level of P4504A1 was not changed. Changes in the protein levels of P450 by CDDP were consistent with those in the mRNA levels reported previously (LeBlanc et al. 1992). 5. Male-specific forms in rat liver such as P4502C11 were decreased by CDDP, but those in the kidney such as P4504A2 was not. Therefore, CDDP has different influences on the regulation of hepatic and renal P450s.

Occurrence of a P450 showing high homology to yeast lanosterol 14-demethylase (P450(14DM)) in the rat liver

The rat liver cDNA library in lambda gt11 was screened with the antisera against rat liver lanosterol 14 alpha-demethylase (P450(14DM)), and one clone (pRT-9) was isolated. The open reading frame of pRT-9 cDNA showed high homology to yeast P450(14DM) (48.9% in nucleotides and 37.7% in amino acids). Homology of the heme-binding region of pRT-9 protein to that of yeast P450(14DM) was 80%. Three additional regions showing more than 60% homology were identified and one of them was assignable as the distal helix. These findings suggest that pRT-9 protein is P450(14DM) and P450(14DMs) of rat and yeast may be orthologous. This is the first evidence suggesting the occurrence of orthologous P450 in widely diverged species.

Differential roles of cytochromes P450 2D1, 2C11, and 1A1/2 in the hydroxylation of bufuralol by rat liver microsomes

Bufuralol hydroxylation activities of liver microsomal cytochrome P450 (P450) enzymes were studied in the rat; the reaction has been used widely in determining levels of liver microsomal P450 2D6, which shows debrisoquine-type genetic polymorphism in humans. Liver microsomes catalyzed the conversion of bufuralol to 1'-hydroxybufuralol and a structurally unidentified metabolite (termed here as M-1) in the presence of an NADPH-generating system and molecular oxygen. Bufuralol 1'-hydroxylation activities catalyzed by the liver microsomes were not increased in rats treated with several P450 inducers, whereas beta-naphthoflavone treatment (and to a lesser extent that of isosafrole) caused a significant induction of M-1 formation. The major role of P450 1A1/2 in M-1 formation was confirmed by catalytic inhibition with anti-P450 antibodies and alpha-naphthoflavone in liver microsomes of beta-naphthoflavone-treated rats, and by reconstitution experiments containing P450 1A1 and 1A2. Among nine forms of purified rat P450 enzymes studied in the reconstituted system, P450 2C11 displayed the highest activities for bufuralol 1'-hydroxylation, followed by P450 1A1 and P450 2D1. A female-specific form of P450 2C12 did not catalyze bufuralol 1'-hydroxylation. In liver microsomes of male rats, however, P450 2D1 was the dominant enzyme because only anti-P450 2D1 antibodies, and not anti-P450 2C11 and anti-P450 1A1, inhibited the bufuralol hydroxylation activities, and a specific P450 2D1 inhibitor, quinine, caused a dramatic decrease in the hydroxylation activities. The major contribution of P450 2D1 in the bufuralol 1'-hydroxylation activities was also supported by a kinetic analysis of the reconstituted system; P450 2D1 enzyme had a very low Km value (8.4 microM) as compared with those of P450 2C11 (Km = 83 microM) and P450 1A1 (Km = 230 microM). Thus, the present results suggested that different P450 enzymes are involved in the hydroxylation of bufuralol in rat liver microsomes, and the kinetic analysis, as well as immunoinhibition and chemical inhibition experiments, may be of great importance for determining the major roles of P450 enzymes in drug hydroxylation reactions.