A constitutive form of guinea pig liver cytochrome P450 closely related to phenobarbital inducible P450b(e)

Human relevance of rodent liver tumour formation by constitutive androstane receptor (CAR) activators

A large number of nongenotoxic chemicals have been shown to increase the incidence of liver tumours in rats and/or mice by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with the model CAR activator phenobarbital (PB) and its sodium salt (sodium phenobarbital; NaPB) have demonstrated that the key and associative events for rat and mouse liver tumour formation include CAR activation, increased hepatocyte replicative DNA synthesis (RDS), induction of cytochrome P450 CYP2B subfamily enzymes, liver hypertrophy, increased altered hepatic foci and hepatocellular adenomas/carcinomas. The key species difference between the rat and mouse compared to humans, is that human hepatocytes are refractory to the mitogenic effects of PB/NaPB and other CAR activators. While PB/NaPB and other CAR activators stimulate RDS in rat and mouse hepatocytes in both in vitro and in vivo studies, such compounds do not stimulate RDS in cultured human hepatocytes and in in vivo studies performed in chimeric mice with humanised livers. In terms of species differences in RDS, unlike the rat and mouse, humans are similar to other species such as the Syrian hamster and guinea pig in being nonresponsive to the mitogenic effects of CAR activators. Overall, the MOA for rat and mouse liver tumour formation by PB/NaPB and other CAR activators is considered qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies, which demonstrate that chronic treatment with PB does not increase the incidence of liver cancer in humans.

Comparative study on nobiletin metabolism with liver microsomes from rats, Guinea pigs and hamsters and rat cytochrome p450

In vitro metabolism of nobiletin, a polymethoxy-flavonoid abundantly present in citrus peels, was studied using liver microsomes of rats, hamsters and guinea pigs and ten cDNA-expressed rat cytochrome P450 (P450). The effects of P450 inducers on nobiletin metabolism were also investigated. Aerobical incubation with NADPH and animal liver microsomes transformed nobiletin to five metabolites, M-1, M-2, M-3, M-4 and M-5. From LC-MS and (1)H-NMR data and a time-course study, these were assumed to be 4'-hydroxy (OH)-, 7-OH-, 6-OH-, 3',4'-diOH- and 6,7-diOH-metabolites, respectively. Pretreatment of animals with phenobarbital increased M-2 and M-3 to about 2-fold that in untreated animals. Pretreatment with 3-methylcholanthrene (MC) resulted in remarkable increases of both M-1 and M-4 (3 to 9-fold that of untreated). Males had 2-3 times higher M-2 and M-3 formation activities in rats, and for M-2 in hamsters than did females. Immunoinhibition study using antiserum against P450 revealed the involvement of hamster CYP1A2 in the formation of M-1 and M-4 in hamster liver. Of ten rat P450s, CYP2C11, CYP3A1, CYP3A2 and CYP2D1 had high activities for the formation of M-1, M-2 and M-3. Another P450s (CYP1A1, CYP2C12 and CYP1A2) also showed activity for the formation of M-1. Only CYP1A1 produced 3',4'-diOH-metabolites (M-4). However, CYP2A1, CYP2B1 and CYP2E1 had no activity for nobiletin. These results suggested that constitutive P450s such as CYP2C11, CYP2D1, CYP3A1, CYP3A2 and CYP2C12 are responsible for the demethylation at the 6-, 7-, 3'- and 4'-positions; whereas, MC-inducible P450s, CYP1A1 and CYP1A2, preferentially catalyzed demethylation at the 3'-and 4'-positions.

In vitro metabolism of 2,2',3,4',5,5',6-heptachlorobiphenyl (CB187) by liver microsomes from rats, hamsters and guinea pigs

The metabolism of 2,2',3,4',5,5',6-heptachlorobiphenyl (heptaCB) (CB187) was studied using liver microsomes of rats, hamsters and guinea pigs, and the effect of cytochrome P450 (CYP) inducers, phenobarbital (PB) and 3-methylcholanthrene (MC), was also investigated. In untreated animals, guinea pig liver microsomes formed three metabolites which were deduced to be 4'-hydroxy-2,2',3,5,5',6-hexachlorobiphenyl (M-1), 4'-hydroxy-2,2',3,3',5,5',6-heptaCB (M-2) and 4-OH-CB187 (M-3) from the comparison of GC/MS data with some synthetic authentic samples. The formation rate of M-1, M-2 and M-3 was 18.1, 36.6, 14.7 pmol h-1 mg protein-1, respectively. Liver microsomes of untreated rats and hamsters did not form CB187 metabolites. In guinea pigs, PB-treatment increased M-1 and M-2 significantly to 1.9- and 3.4-fold of untreated animals but did not affect the formation of M-3. In rats, PB-treatment resulted in the appearance of M-2 and M-3 with formation rates of 87.1 and 13.7 pmol h-1 mg protein-1, respectively, but M-1 was not observed. In hamsters, PB-treatment formed only M-2 at a rate of 29.4 pmol h-1 mg protein-1. On the other hand, MC-treatment of guinea pigs decreased the formation of M-1 and M-2 to less than 50% of untreated animals. MC-microsomes of rats and hamsters produced no metabolites. Preincubation of antiserum (300 microl) against guinea pig CYP2B18 with liver microsomes of PB-treated guinea pigs produced 80% inhibition of M-1 and the complete inhibition of M-2 and M-3. These results suggest that PB-inducible CYP forms, especially guinea pig CYP2B18, rat CYP2B1 and hamster CYP2B, are important in CB187 metabolism and that CB187 metabolism in guinea pigs may proceed via the formation of 3,4- or 3',4'-oxide and subsequent NIH-shift or dechlorination.

Comparison of genomic and cDNA sequences of guinea pig CYP2B18 and rat CYP2B2: absence of a phenobarbital-responsive enhancer module in the upstream region of the CYP2B18 gene

Potential mechanisms were investigated whereby CYP2B18, a cytochrome P450 gene exhibiting high constitutive expression but only low levels of phenobarbital-inducibility in the guinea pig liver, may be differentially regulated versus the highly inducible rat CYP2B2 gene. To comparatively assess potential regulatory sequences associated with CYP2B18, a guinea pig genomic library was screened enabling isolation of the CYP2B18 gene. The genomic screening process resulted in the identification of at least four closely-related CYP2B18 genes, designated here as CYP2B18A-D. Of these isolates, CYP2B18A exhibited sequence identical to that of the CYP2B18 cDNA. Further, the deduced amino acid sequence of the CYP2B18 cDNA was identical to that of N-terminal and internally-derived peptide sequences obtained in this investigation from CYP2B18 protein isolated from guinea pig liver. Genomic structural sequences were derived for CYP2B18A, together with the respective 5'-upstream and intronic regions of the gene. Comparison of the CYP2B18A and CYP2B2 gene sequences revealed the lack of repetitive LINE gene sequences in CYP2B18A, putative silencing elements that effect neighboring genes, although these sequences were present in both 5'-upstream and 3'-downstream regions of CYP2B2. We determined that the phenobarbital-responsive enhancer module was absent from the 5'-upstream region as well as the intronic regions of CYP2B18A gene. We hypothesize that the compromised phenobarbital inducibility of CYP2B18A stems from its lack of a functional phenobarbital responsive enhancer module.

N-oxidative transformation of free and N-substituted amine functions by cytochrome P450 as means of bioactivation and detoxication

Indirect evidence for the participation of cytochrome P450 (P450) in the microsomal N-oxygenation of primary and N-substituted amine functions is presented by studies employing diagnostic modifiers of the hemoprotein system as well as immunochemical approaches. Experiments with recombinant hemoproteins or isozymes purified from the tissues of various animal species support the results obtained by the inhibitor assays. Amine substrates and the redox proteins of the microsomal electron transfer chain reveal to be mutually beneficial in interactions with P450s. Numerous N-substituted amines undergo P450-catalyzed N-oxidative transformation despite the presence of accessible alpha-C hydrogens in these structures rather thought to favor N-dealkylation. In these instances, stabilization of the initially formed aminium radicals by the specific active site orientation of the particular P450s obviously permits oxygen rebound. Apart from common iron-oxenoid chemistry involving a (FeO)3+ species, iron-bound hydroperoxide, (FeO2H)3+, appears to act as an electrophilic oxidant with certain N-substituted amines and P450 subforms. Generally, P450-mediated N-oxygenation of amines can produce cytotoxic and mutagenic metabolites, but equally can well yield hydrophilic products, that are readily excreted and thus promote detoxication.

Purification and characterization of the hepatic CYP2C and 3A isozymes from phenobarbitone pretreated rhesus monkey

Hepatic P450s, named M-3 and M-4 were purified from phenobarbitone pretreated rhesus monkey. These demonstrated polypeptide molecular mass of 50 and 52.5 kDa and specific content of 12 and 20 nmol P450/mg protein, respectively. Both the isozymes demonstrated low spin state of heme. Antibodies raised against M-3 inhibited the activity of aminopyrine, erythromycin and ethylmorphine N-demethylase in the microsomes obtained from PB pretreated rhesus monkey by 76, 40 and 35%, respectively. M-4 did the same by 69, 85 and 79%, respectively. These observations indicated M-3 and M-4 to be the members of CYP2C and 3A subfamilies, respectively. These results were substantiated by the observations that M-3 metabolized aminopyrine whereas M-4 metabolized aminopyrine, erythromycin and ethylmorphine in the reconstituted system. Microsomal lipids and cytochrome b5 enhanced the rate of these reactions. Further confirmation to the identity of these isozymes was provided by N-terminal amino acid sequences. The first 10 N-terminal amino acid residues of M-3 were 90% similar to CYP2C20 and 2C9 and that of M-4 were 100 and 90% similar to CYP3A8 and 3A5, respectively. In conclusion, two isozymes of hepatic P450 purified from PB pretreated rhesus monkey belong to CYP2C and 3A subfamilies.

Molecular modelling of CYP2B6, the human CYP2B isoform, by homology with the substrate-bound CYP102 crystal structure: evaluation of CYP2B6 substrate characteristics, the cytochrome b5 binding site and comparisons with CYP2B1 and CYP2B4

1. Molecular modelling studies of CYP2B isoforms from rat (CYP2B1), rabbit (CYP2B4) and man (CYP2B6) are reported, with particular emphasis on substrate interactions with the human CYP2B isoform, CYP2B6. 2. The findings represent an advance on our previous study that focused primarily on the rat CYP2B isoform, CYP2B1, and involved homology modelling with substrate-free CYP102. 3. The current work utilizes the recently published substrate-bound CYP102 crystal structure as a template for construction of the CYP2B subfamily isoforms and shows, in particular, that known CYP2B6 substrate specificity and regioselectivity can be rationalized by putative active site interactions.

Metabolism of 2,3',4',5-tetrachlorobiphenyl by cytochrome P450 from rats, guinea pigs and hamsters

The metabolism of 2,3',4',5-tetrachlorobiphenyl (TCB) was compared using liver microsomes and six isoforms of cytochrome P450 purified from rats, guinea pigs and hamsters. In microsomal study, the following species differences were observed: 1) Untreated guinea pigs and hamsters but not rats can metabolize this TCB to 3-hydroxy- or 4-hydroxy-2,3',4',5-TCB, 2) Guinea pig microsomes showed only 3-hydroxylating activity, whereas hamster microsomes showed higher activity of 4-hydroxylation than that of 3-hydroxylation. In common with three species, the 3-hydroxylation was accelerated by phenobarbital. The 4-hydroxylation in rats and hamsters was increased by pretreatment with 3-methylcholanthrene and 3,3',4,4',5-pentachlorobiphenyl. The hydroxylation activities of liver microsomes from the three species could be explained by an involvement of different isoforms of cytochrome P450. In addition, it is apparent that hamster CYP1A2 as well as hamster CYP2A8 is involved in the 4-hydroxylation of 2,3',4',5-TCB although it has no activity for 2,2',5,5'-TCB.

Microsomal alcohol oxygenase: purification and characterization of a cytochrome P450 responsible for oxidation of 7-hydroxy-delta 8-tetrahydrocannabinol to 7-oxo-delta 8-tetrahydrocannabinol in guinea pig liver

Guinea pig hepatic enzyme, microsomal alcohol oxygenase, was able to oxidize both 7 alpha- and 7 beta-hydroxy-delta 8-tetrahydrocannabinol (7 alpha- and 7 beta-hydroxy-delta 8-THC) to 7-oxo-delta 8-THC. A cytochrome P450, named P450GPF-B, which mediates this oxidative metabolism was purified from hepatic microsomes of untreated female guinea pigs. The purified enzyme showed a single protein band of molecular mass 50,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The NH2-terminal amino acid sequence of P450GPF-B is highly homologous with those of several cytochrome P450s belonging to the CYP3A subfamily. 18O derived from atmospheric oxygen was incorporated into 31 and 6%, respectively, of 7-oxo-delta 8-THC formed from 7 alpha- and 7 beta-hydroxy-delta 8-THC when the substrates were incubated with P450GPF-B under 18O2. The antibody against P450GPF-B significantly suppressed the oxidative activities of 7 alpha- and 7 beta-hydroxy-delta 8-THC to 7-oxo-delta 8-THC in hepatic microsomes of guinea pig. These results indicate that P450GPF-B is a major enzyme responsible for the hepatic microsomal alcohol oxygenase activities in the guinea pig.

Some aspects of the role of cytochrome P-450 isozymes in the N-oxidative transformation of secondary and tertiary amine compounds

Indirect evidence of the participation of cytochrome P-450 (P-450) in the microsomal N-oxygenation of secondary and tertiary nitrogen functions is presented by studies employing diagnostic modifiers of the hemoprotein system as well as antibodies directed toward the diverse P-450 isoforms and NADPH-cytochrome P-450 reductase. Experiments with recombinant hemoproteins or P-450 isozymes directly purified from the tissues of various animal species support the results obtained by the inhibitor assays. Although the intermediacy of aminium radicals is thought to be restrictive to P-450-catalyzed N-oxygenation of secondary and tertiary amine groups bearing accessible hydrogens on the alpha-carbon, numerous exceptions to this rule are documented. It is proposed that aminium radicals partition between oxygen rebound and alpha-hydrogen abstraction to yield a finite level of N-oxygenated product in all P-450-mediated amine oxidations, the partition ratio depending on the amine structure and particular P-450 isozyme operative. In some instances, N-oxygenation appears to proceed by peroxidatic mechanisms. The relative contribution of P-450 to the N-oxygenation of secondary and tertiary amines in crude preparations or live animals, where competition with the flavin-containing monooxygenase (FMO) occurs, seems to be a function of the relative amounts and catalytic capacities of the two enzyme systems. Both parameters are species and tissue dependent. Accordingly, the extent to which P-450 contributes to total N-oxidative turnover of the amine substrates varies from minor to major.

A cytochrome P4502B form is the major bioactivation enzyme for the pyrrolizidine alkaloid senecionine in guinea pig

1. We have purified three P450s from the liver of the phenobarbital (PB)-treated guinea pig in order to evaluate the role of these enzymes in pyrrolizidine alkaloid (PA) metabolism. 2. PB treatment of guinea pig increased the hepatic microsomal conversion of the PA senecionine (SN) to the pyrrolic metabolite (+/-)6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP), an activation product, and SN N-oxide, a detoxification product by 224 and 70% respectively. 3. Reconstitution of a PB-inducible guinea pig P4502B isoform (M(r) = 57,512 by MALDI-TOF mass spectrometry) in a reconstituted system metabolized SN to DHP and SN N-oxide at rates of 1.98 and 1.45 min-1 respectively. A second purified guinea pig P450, a 2C-type isoform (M(r) = 56,496 by MALDI-TOF mass spectrometry), produced SN N-oxide from SN at the rate of 13.3 min-1 but catalyzed little DHP formation. The third guinea pig P450, an apparent 3A type (M(r) = 54-56,000 by SDS-PAGE), lost its catalytic activity towards SN during the final purification process. 4. Immunoinhibition of microsomal SN metabolism by rabbit antibodies raised against the guinea pig P4502B, 2C and 3A isoforms indicated that the 2B played the most important role (> 70% of the total metabolism) in bioactivation of SN in both the untreated or PB-treated guinea pig, whereas 2C and 3A seemed to exhibit little (around 13%) PA metabolism. P4502B, along with flavin-containing monooxygenase, also contributed to the detoxification of SN in both the untreated (34%) and PB-treated (40%) guinea pig. 5. This study suggests that the putative P4502B form plays the most important role in SN bioactivation in guinea pig.

Regulatory mechanisms in the activation of nitrogenous compounds by mammalian cytochrome P-450 isozymes

Metabolic activation of nitrogenous compounds by the cytochrome P-450 system is a highly complex process. Inherent substrate factors, such as basicity, electronic state, lipophilicity, and conformation control binding of the diverse classes of amines to cytochrome P-450. Accommodation of these compounds in the enzyme cavity and proper orientation of the molecules are governed by intrinsic properties of the peptide structure of cytochrome P-450, which may be subject to modification by the action of effectors. On the membrane level, phospholipid might have some impact on substrate binding. On the other hand, bound amine substrate is beneficial to the productive interaction of the electron transport chains with the terminal acceptor, improving economy of the system. Certain amines appear to regulate O2 association with cytochrome P-450 and stabilize the various oxy species formed. Considering the selective prerequisites for oxidative attack by cytochrome P-450 at vulnerable nitrogen centers, many cytotoxic amines belonging to the category of relatively rigid, planar molecules undergo N-oxidative activation by the cytochrome P-450IA subfamily, while more bulky amines with flexible conformation are N-oxygenated preferentially by phenobarbital-inducible cytochromes P-450. Small differences in protein structure between the various cytochrome P-450 subforms might serve to stabilize aminium radicals to permit oxygen rebound. Collectively, the selective regulatory mechanisms operative in the bioactivation of nitrogen-containing compounds appear to be determined largely by the type of substrate used and the isozyme involved in catalysis. With respect to the latter, the interplay of the multiple cytochromes P-450 in the various organs of animal species thus serves to rationalize the differences in the particular selectivities for amine substrates. These are responsible for the mode and/or extent to which activation of nitrogenous compounds, including promutagens and procarcinogens, occurs, and this may explain the tissue-specific response to the tumorigenic action of these agents.

Molecular cloning of guinea pig CYP1A1: complete primary structure and fast mobility of expressed protein on electrophoresis

Guinea pig CYP1A1 cDNA was isolated from a liver cDNA library of guinea pig treated with 3-methylcholanthrene. The cDNA, named GPc1, was 2674 bp long and contained an entire coding region for 516 amino acids. The amino acid sequence of guinea pig CYP1A1 shared 74-78% identity with those of the other mammalian CYP1A1s. RNA blot and immunoblot analyses revealed that CYP1A1 was constitutively expressed and was induced by 3-methylcholanthrene in guinea pig liver. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, guinea pig CYP1A1 expressed in yeast had a significantly smaller apparent molecular mass than expressed mouse CYP1A1. An alignment of the amino acid sequences of mammalian CYP1A1s demonstrated that guinea pig CYP1A1 was several residues shorter than the counterparts in the N-terminal region. Thus, to clarify the contribution of the N-terminal sequence of guinea pig CYP1A1 to the fast mobility on the electrophoresis, mouse-guinea pig chimeric CYP1A1 was prepared through cDNA-directed expression in yeast. The chimeric CYP1A1 protein had an intermediate molecular mass between mouse and guinea pig CYP1A1s indicating that the anomalous mobility of guinea pig CYP1A1 is in part due to the shortened N-terminal amino acid sequence of the protein.

Species differences of testosterone 16-hydroxylases in liver microsomes of guinea pig, rat and dog

1. In hepatic microsomes, remarkable species differences in the activity of testosterone 16-hydroxylase was observed in guinea pig, dog, and rat. The activity of testosterone 16 beta-hydroxylase was higher than that of 16 alpha-hydroxylase in guinea pig, whereas 16 alpha-hydroxylated testosterone was predominant as the metabolite in dog and rat. 2. Since P4502B isoenzyme has been shown to be a catalyst for testosterone 16-hydroxylations, we compared the catalytic properties of the P4502B subfamily (P450GP-1, P450b and P450PBD-2) purified from liver microsomes of guinea pig, dog, and rat, respectively. P450GP-1, P450b and P450PBD-2 showed different stereoselectivities for hydroxylation of testosterone at the 16-position. 3. P450GP-1, P450b and P450PBD-2 together comprised 47, < 0.1 and 23% of total P450 in liver microsomes of untreated guinea pig, rat and dog, respectively, indicating that the amounts of the P4502B isoenzyme in untreated animals were clearly different in these three animal species. Both 16 alpha- and 16 beta-hydroxylations of testosterone in liver microsomes of phenobarbital-treated guinea pig, rat and dog were inhibited by anti-P450GP-1, anti-P450b and anti-P450PBD-2 antibodies, respectively. 4. These and other results indicate that the species difference observed in testosterone 16-hydroxylation may be, in part, due to differences in the amounts of P450 of the P4502B subfamily, and their stereoselectivities for 16-hydroxylation.

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature

We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1-11, 1987; Nebert et al., DNA 8, 1-13, 1989; Nebert et al., DNA Cell Biol. 10, 1-14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen should precede the final number in mouse genes. "P" ("p" in mouse) after the gene number denotes a pseudogene. If a gene is the sole member of a family, the subfamily letter and gene number need not be included. We suggest that the human nomenclature system be used for all species other than mouse. The mRNA and enzyme in all species (including mouse) should include all capital letters, without italics or hyphens. This nomenclature system is identical to that proposed in our 1991 update. Also included in this update is a listing of available data base accession numbers for P450 DNA and protein sequences. We also discuss the likelihood that this ancient gene superfamily has existed for more than 3.5 billion years, and that the rate of P450 gene evolution appears to be quite nonlinear. Finally, we describe P450 genes that have been detected by expressed sequence tags (ESTs), as well as the relationship between the P450 and the nitric oxide synthase gene superfamilies, as a likely example of convergent evolution.

Tissue-specific expression, induction, and inhibition through metabolic intermediate-complex formation of guinea pig cytochrome P450 belonging to the CYP2B subfamily

The tissue-specific expression and induction of P450GP-1, a constitutive form of cytochrome P450 of the guinea pig classified into the CYP2B subfamily, were studied. Prior to these studies, a P450 form (P450GP-1 PB) was purified from phenobarbital-treated guinea pigs and the properties were compared with those of the P450GP-1. This form was judged to be the same as P450GP-1 existing in untreated animals by comparisons of their N-terminal amino acid sequences, peptide maps, and affinities toward anti-P450GP-1 antibody. Immunostaining of P450GP-1 revealed that the lung and small intestine as well as the liver of untreated guinea pigs contain P450GP-1, while none or only small amounts of this P450 form were observed in the kidney, heart, spleen, urinary bladder, and testis. The amount of liver P450GP-1 protein expressed in untreated guinea pigs was estimated to be 19.4% of the total cytochrome P450 and this form was increased 1.7-fold by phenobarbital treatment. Similarly, intestinal P450GP-1 was increased by phenobarbital treatment. However, lung P450GP-1 was not increased by the treatment. It was also observed that the liver P450GP-1 is induced with SKF-525A to the same extent as with phenobarbital. On the other hand, dexamethsone, p,p'-dichlorodiphenyltrichloroethane, and isosafrole showed no or only a weak ability to increase the liver P450GP-1 content. The drug-metabolizing activities in the liver microsomes of SKF-525A-pretreated guinea pigs were lower than those in phenobarbital-treated animals, although the P450GP-1 protein was induced equally by these treatments. The low activities of SKF-525A-treated animals in the drug metabolisms were attributed to the formation of the metabolic-intermediate complex between P450GP-1 and SKF-525A metabolite. These results permitted us to conclude that the tissue specificity in the expression of guinea pig P450 belonging to the CYP2B subfamily and the inducibility with chemicals are similar to those of rat CYP2B1, although the constitutive expression of guinea pig liver P450GP-1 is much higher than that of CYP2B1.

Immunochemical characterization of a cytochrome P450 isozyme and a protein purified from liver microsomes of male guinea pigs and their roles in the oxidative metabolism of delta 9-tetrahydrocannabinol by guinea pig liver microsomes

A protein (designated as protein-B) was purified from liver microsomes of adult male guinea pigs by an affinity chromatography with omega-aminooctyl Sepharose 4B, followed by HPLC using DEAE-5PW and hydroxyapatite columns which had been used to purify a cytochrome P450 (P450) isozyme (P450-A) from the same subcellular fraction (Narimatsu et al., Biochem Biophys Res Commun 172: 607-613, 1990). Protein-B had a molecular mass of 49 kDa in SDS-PAGE, but did not show absorbance at 417 nm for heme. Further, it did not show any oxidative activities towards aniline (AN), d-benzphetamine (d-BP), p-nitroanisole (p-NA) or delta 9-tetrahydrocannabinol (delta 9-THC) in a reconstituted system including dilauroylphosphatidylcholine, NADPH-P450 reductase, and cytochrome b5. However, antiserum against protein-B raised in rabbits suppressed liver microsomal oxidative activities towards d-BP and p-NA dose-dependently. The antibody decreased delta 9-THC oxidative activity most effectively, but did not decrease AN hydroxylation activity. Antiserum against P450-A suppressed all the activities towards these four substrates, especially towards delta 9-THC, in liver microsomes of male guinea pigs. Moreover, reconstitution with hemin made it possible for protein-B to produce some oxidative activity toward delta 9-THC. These results suggest that protein-B is also a cytochrome P450 isozyme which has lost a heme moiety during purification steps. Both P450-A and protein-B could have a role as cytochrome P450 isozymes in the oxidative metabolism of drugs, especially that of delta 9-THC by the liver microsomes of adult male guinea pigs.