Purification of cytochrome P-450, NADPH-cytochrome P-450 reductase, and epoxide hydratase from a single preparation of rat liver microsomes

Composition of cytochrome P-450 isozymes from hepatic microsomes of C57BL/6 and DBA/2 mice assessed by warfarin metabolism, immunoinhibition, and immunoelectrophoresis with anti-(rat cytochrome P-450)

The composition of isozymes of hepatic microsomal cytochrome P-450 (P-450) from C57BL/6 (B6) and DBA/2 (D2) inbred strains of mice, uninduced and treated with phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), and beta-naphthoflavone (BNF), were assessed. The assessment was based on comparisons of the regioselectivity and stereoselectivity of warfarin metabolism, inhibition of metabolism by antibodies raised against highly purified rat P-450 isozymes (the specificity of these antibodies in inhibiting rat microsomal metabolism of warfarin was also determined), and by immunoelectrophoresis using the anti-(rat P-450). Isozymes were considered to be the same or very similar if they exhibited the same substrate specificity, antigenicity, and molecular weight. Untreated D2 and B6 mice had similar compositions of P-450 isozymes which were independent of the Ah receptor. Both strains contained an isozyme very similar to rat P- 450UT -A. PB or PCN induction of the two mouse strains proceeded independently of the Ah receptor and yielded isozymes which are the same as or very similar to the rat P- 450UT -A, P- 450PB -B, P- 450PB -C, and P- 450PB /PCN-E isozymes. D2 and B6 mice had similar isozyme compositions when treated with PB or PCN, but isozyme compositions varied in differently induced mice. BNF treatment of the D2 mice was without effect, but the B6 mice produced an isozyme(s) which was different from rat P- 450BNF -B, the major BNF-induced isozyme in rat liver.

Effects of metyrapone and norharmane on microsomal mono-oxygenase and epoxide hydrolase activities

This study was undertaken to examine the possibility that metyrapone and norharmane stimulate epoxide hydrolase and inhibit mono-oxygenase activities by binding to a cytochrome P-450 component of a stable complex containing the two enzymes. The concentration of metyrapone and norharmane which inhibited mono-oxygenase activities of hepatic microsomes from untreated and diethylnitrosamine treated rats was lower than that required to stimulate epoxide hydrolase of the same microsomes. The ability of metyrapone and norharmane to stimulate epoxide hydrolase in these microsomes was not inhibited by the addition of carbon monoxide and reductant. Epoxide hydrolase activity was inhibited by detergents but the enzyme was still stimulated by metyrapone and norharmane under conditions of total membrane disaggregation. When microsomes were solubilized, epoxide hydrolase could be quantitatively recovered by immunoprecipitation. The immunoprecipitate contained no detectable cytochrome P-450 but was stimulated by metyrapone and norharmane. A purified epoxide hydrolase was stimulated by metyrapone but not by norharmane. The response of the enzyme to norharmane was not restored by the inclusion of cytochrome P-448. These findings suggest that metyrapone and norharmane act at separate sites on both cytochrome P-450 and epoxide hydrolase.

Covalent binding of benzo[a]pyrene to cytochrome P-450 beta NF-B2 and other proteins in reconstituted mixed-function oxidase systems

A reconstituted mixed-function oxidase system, containing the major beta-naphthoflavone-induced isozyme of rat liver cytochrome P-450 bound benzo[a]pyrene covalently in the presence of NADPH. NADPH-cytochrome P-450 reductase was required for binding and a maximum rate of adduct formation was obtained at 8 units of reductase per nmol cytochrome P-450. Phosphatidylcholine inhibited this reaction. Benzo[a]pyrene was bound to the cytochrome, but not to the reductase, as shown by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Approximately 6 molecules of benzo[a]pyrene bound to each molecule cytochrome P-450 during prolonged incubations. No binding occurred when the beta-naphthoflavone-induced isozyme of cytochrome P-450 was replaced by the major isozyme induced by phenobarbital, but both cytochromes incorporated benzo[a]pyrene to approximately the same extent when they were incubated together in the presence of the reductase and NADPH. Metabolically activated benzo[a]pyrene also bound covalently to purified epoxide hydrolase, when this enzyme was added to the reconstituted mixed-function oxidase system.

Metabolism of cyclophosphamide by purified cytochrome P-450 from microsomes of phenobarbital-treated rats

Incubation of [3H]-sidechain-labeled and [14C]-C(4)-ring-labeled cyclophosphamide (CPA) with purified cytochrome P-450 from liver microsomes of rats treated with phenobarbital resulted in the production of a major metabolite that contained both labels, was unaffected by diazomethane, possessed high polarity, was identical in TLC and HPLC behavior to a synthetic standard, didechlorodihydroxy -CPA, and was converted to CPA and bis(2-chloroethyl)amine by thionyl chloride . These results indicate that phenobarbital-inducible cytochrome P-450 is able to dechlorinate CPA and may account, in part, for the inability of phenobarbital to enhance the therapeutic activity and toxicity of this important anticancer and immunosuppressive agent.

Purification of cytochrome b-245 from human neutrophils

The low potential cytochrome b (b-245) of the microbicidal oxidase of phagocytic cells has been purified from neutrophils from patients with chronic myeloid leukaemia. Cells were homogenized in the presence of proteinase inhibitors and centrifuged to remove the cytoplasm. The pellets containing membranes, granules and other organelles (15 mg/ml) were then washed with buffered sodium cholate (5 mg/ml). Residual pellets were subsequently solubilized with the non-ionic detergent Triton N 101 (10 mg/ml) which extracted about 60% of the cytochrome b. About 10% of the cytochrome b was of mitochondrial origin which was removed on a column of n-amino-octyl-Sepharose that did not adsorb cytochrome b-245. Cytochrome b-245 was chromatographed on a column of heparin-agarose and eluted with NaCl to give a peak specific content of 11-16 nmol of cytochrome b-245/mg of protein, representing a 140-200-fold purification with a recovery of 15%. This technique results in the purification of approx. 100-150 nmol of highly purified cytochrome b-245 from (3-5) X 10(11) cells within 4 days. The most purified material gave a broad band with an apparent Mr of between 68 000 and 78 000 on sodium dodecyl sulphate/polyacrylamide gel electrophoresis, but gel filtration indicated an aggregated form of the protein in Triton N101 . Purified protein (14 nmol of haem/mg of protein) did not contain FAD or FMN and had no NADPH-dependent O2--generating activity.

Oxidative activation and inactivation of the carcinogen 2-acetylaminofluorene by various purified cytochromes P-450 from 3-methylcholanthrene pretreated rats

Ring- and N-hydroxylations of 2-acetylaminofluorene (AAF) have been examined in a reconstituted system with 4 purified hepatic microsomal cytochromes P-450 isolated from 3-methylcholanthrene (MC)-pretreated rats. Among these 4 isozymes, P-450D showed the most activity whereas P-450C was devoid of any activity; two other P-450s exhibited moderate activity. These and Ouchterlony's double diffusion analyses suggest involvement of multiple cytochromes P-450 in AAF oxidations.

Detection of phenobarbital-induced cytochrome P-450 in rat hepatic microsomes using an enzyme-linked immunosorbent assay

The major phenobarbital-inducible form of cytochrome P-450 (cytochrome P-450 PB) was purified to homogeneity from rat liver microsomes and rabbit antibodies prepared against the purified enzyme. Using these antibodies, an enzyme-linked immunosorbent assay (ELISA) was developed for the detection of cytochrome P-450 PB in microsomes which was sensitive at the nanogram level. The content of cytochrome P-450 PB was determined in hepatic microsomes from rats treated with various xenobiotics. Phenobarbital and Aroclor 1254 pretreatments resulted in several-fold increases in immunoreactive cytochrome P-450 PB over control levels. ELISA measurements of cytochrome P-450 PB were also carried out over a 48-h time course of phenobarbital induction in liver microsomes. Significant increases over control levels were seen at 16 h and beyond. Measurements of ELISA-detectable cytochrome P-450 PB were made in microsomes following the administration of CCl4 to phenobarbital-pretreated rats. Immunoreactive cytochrome P-450 PB was observed to decrease less rapidly than the spectrally detectable enzyme in the microsomal membranes. Inhibition of heme synthesis was carried out by the administration of 3-amino-1,2,4-triazole (AT) to rats. Concomitant pretreatment with phenobarbital and AT resulted in levels of ELISA-detectable cytochrome P-450 PB which were significantly increased over control levels, while spectrally detectable levels of total holoenzyme remained unchanged. These results support the idea that this cytochrome P-450 may exist, at least partly, in the microsomal membrane in an inactive or apoprotein form.

Multiple forms of hepatic cytochrome P-450. Purification, characterisation and comparison of a novel clofibrate-induced isozyme with other major forms of cytochrome P-450

In the present studies, a novel form of highly purified cytochrome P-450 (cytochrome P-452) isolated from the hepatic microsomes of clofibrate-pretreated rats has been compared to the major isozymes isolated from the hepatic microsomes of rats pretreated with phenobarbital (cytochrome P-450) and 2-naphthoflavone (cytochrome P-447) using a number of biochemical criteria. The results show that these three isozymes exhibit marked structural differences from each other as judged by a complete lack of immunochemical cross-reactivity between the isozymes and the heterologous rabbit serum antibodies using Ouchterlony double diffusion, and non-identity between the limited proteolytic digestion maps of the three isozymes obtained in the presence of chymotrypsin, papain and Staphylococcus aureus V8 proteases. Furthermore, the three isozymes exhibited clear differences in their monomeric molecular weights determined on calibrated sodium dodecyl sulphate/polyacrylamide gel electrophoresis in gels of varying acrylamide concentration. Substantial differences were also observed in the substrate specificities of the isozymes, which were reflected in differences in the turnover rates and positional selectivities of the hemoproteins for some model substrates. In addition, the isozymes differed in their substrate binding affinities and their ability to interact with purified hepatic microsomal cytochrome b5, as judged using difference spectrophotometry. Finally, subtle differences were detected in the ultraviolet visible absorbance spectra of the hemoproteins in the ferric, ferrous, and carbonmonoxyferrous states. Taken collectively, the above data provides compelling evidence that fundamental differences exist between these cytochrome P-450 isozymes, further establishing the uniqueness of the major form of cytochrome P-450 induced by clofibrate pretreatment.

Presence of NADPH-cytochrome P450 reductase in central catecholaminergic neurones

The cytochrome P450-containing mixed function oxidases metabolize a variety of endogenous and exogenous compounds including drugs, carcinogens, fatty acids and steroids. Mixed function oxidases have been detected in several tissues, including brain. The enzyme system consists of a lipid fraction (phosphatidylcholine), cytochrome P450 and NADPH-cytochrome P450 reductase. NADPH-cytochrome P450 reductase has been purified to apparent homogeneity and demonstrated to supply reducing equivalents from NADPH to cytochrome P450 (refs 5-7). Detection of NADPH-cytochrome P450 reductase thus represents an indirect means of demonstrating the presence of cytochrome P450. Although the role of cytochrome P450 in the central nervous system (CNS) is not known, it may include such different functions as metabolism of xenobiotics, aromatization of androgens to oestrogens and the formation of catecholoestrogens. Despite the potentially very important role(s) of cytochrome P450 in brain function, its exact regional distribution remains essentially unknown. Using a specific antibody against rat liver NADPH-cytochrome P450 reductase in combination with immunohistochemical techniques, we have now localized this enzyme to define catecholamine (CA)-containing structures of the rat and monkey brain.

Cloning and sequence analysis of a rat liver cDNA coding for a phenobarbital-inducible microheterogenous cytochrome P-450 variant: regulation of its messenger level by xenobiotics

A rat liver cDNA library was prepared from total polyribosomal poly(A)+ RNA extracted from phenobarbital-treated animals. A cDNA clone coding for a phenobarbital-inducible cytochrome P-450 (PB P-450) was identified by differential colony hybridization to cDNAs synthesized from liver poly(A)+RNAs isolated from phenobarbital-treated rats for positive selection and cDNAs from either untreated rats or beta-naphthoflavone-treated rats as negative controls, followed by hybrid-selected translation and analysis of the translation products by immunoprecipitation. As the cloning and screening strategies involve no prior enrichment for specific mRNAs, they also permit the identification of sequences coding for phenobarbital-induced proteins other than cytochromes P-450. This relatively straightforward approach is generally applicable to the molecular cloning of sequences coding for other inducible cytochromes P-450. Nucleic acid sequencing data indicated that the cloned PB P-450 cDNA codes for a cytochrome P-450 variant [designated P-450e(U.C.)] that is very similar, but not identical, to P-450e. Sequence analysis of the section of cDNA specifying the 3'-non-coding region of the mRNA revealed that it lacked the usual poly(A) addition site signal sequence but contained three inverted repeat structures. Solution hybridization analysis demonstrated that PB P-450 mRNA is increased 20-fold by phenobarbital treatment and decreased 3-fold by beta-naphthoflavone treatment.

High pressure liquid chromatographic separation of multiple forms of cytochrome P-450

The major form of cytochrome P-450 isolated and purified from the hepatic microsomes of phenobarbital pretreated rats by sequential chromatography on n-octylamino-Sepharose 4B and DEAE-cellulose columns was found to be homogeneous by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. However, this cytochrome P-450 was resolved into three bands by high pressure liquid chromatography on an Anpac ion-exchange column. High pressure liquid chromatography isolated forms had similar molecular weights of 55,000 with lambda max of the CO-reduced difference spectrum at 450 nm and were found to be in the low spin state. The results demonstrate the effectiveness of high pressure liquid chromatography in the resolution of cytochrome P-450s of similar molecular weights but different net charges.

Evaluation of monooxygenase induction as a means of enhancing the yield of the rat liver cytochrome P-450 isoenzyme with high affinity for cimetidine

The binding of cimetidine to liver microsomes prepared from untreated rats and rats pretreated with phenobarbitone or 3-methylcholanthrene has been investigated by difference spectroscopy and equilibrium partition methods. In M/15 phosphate buffer, pH 7.9, microsomes from each group of rats yielded markedly biphasic spectral binding curves, which have been interpreted in terms of two independent classes of cytochrome P-450 site with widely differing binding affinities for the drug. Support for such interpretation was provided by the finding that the spectral binding curve for a purified sample of the principal cytochrome P-450 isoenzyme from liver microsomes of phenobarbitone-pretreated rats could be described adequately by a single rectangular hyperbolic relationship, the spectral dissociation constant being indistinguishable experimentally from that for the weaker class of cytochrome P-450 binding site in the corresponding microsomes. The spectral dissociation constants were 2 microM and 80 microM for microsomes from untreated rats; 44 microM and 540 microM for those from phenobarbitone-pretreated rats; and 34 microM and 540 microM for microsomes from rats pretreated with 3-methylcholanthrene. On this basis, both classes of P-450 site in the microsomes from rats subjected to either pretreatment exhibited lower affinity for cimetidine than their counterparts in microsomes from untreated rats. Equilibrium partition studies of the higher-affinity class of microsomal binding site for cimetidine showed that the twofold increase in the cytochrome P-450 content of microsomes effected by 3-methylcholanthrene pretreatment was more than offset by a diminished proportion of the total cimetidine-binding capacity present as the higher-affinity, pharmacologically significant, receptor (18%, cf. 48% in control microsomes); and that phenobarbitone pretreatment resulted in replacement of the high-affinity receptor by one with a threefold weaker cimetidine-binding affinity. Thus the use of these monooxyginase inducers to enhance the cytochrome P-450 content of liver microsomes would seem to offer little potential in the isolation of the isoenzyme with high affinity for cimetidine.

Pyrrolizidine alkaloid-induced alterations in benzo[alpha]pyrene metabolism and binding of benzo[alpha]pyrene metabolites to deoxyribonucleic acid

Pretreatment of rats by oral administration of jacobine, a pyrrolizidine alkaloid and inducer of epoxide hydrolase, produced a marked shift in hepatic microsomal metabolism in vitro of benzo[alpha]pyrene. The formation of 9-hydroxybenzo[alpha]pyrene and 7,8-dihydroxy-7,8-dihydrobenzo[alpha]pyrene was decreased whereas the formation of 4,5-dihydroxy-4,5-dihydrobenzo[alpha]pyrene was increased following jacobine treatment. This shift in the ratio of benzo[alpha]pyrene metabolites was accompanied by a significant reduction in DNA binding. Addition of purified epoxide hydrolase to control or jacobine microsomes produced a similar decrease in total DNA binding. Chromatography of benzo[alpha]pyrene metabolite-DNA nucleoside adducts showed a marked reduction in four peaks and the elimination of one peak with microsomes from jacobine-treated rats.

Purification and comparative properties of NADPH-cytochrome P-450 reductase from rat and rainbow trout: differences in temperature optima between reconstituted and microsomal trout enzymes

NADPH-cytochrome P-450 reductase has been purified to apparent homogeneity from liver microsomes of beta-naphthoflavone-treated rats and rainbow trout. The apparent monomeric molecular weights were 75,000 and 77,000 for the rat and trout, respectively. Differences in amino acid composition were observed, particularly for lysine, glycine, threonine, and tyrosine. Analysis of the flavin composition showed that there were 0.97 mol of FAD and 0.92 mol of FMN per mol of rat reductase, whereas the values for the trout enzyme were 1.06 and 0.76 for FAD and FMN, respectively. Trout NADPH-cytochrome c reductase was inhibited by anti-rat antibody, but not to the same extent as was the rat enzyme. No precipitin lines between the trout reductase and rat antibody were observed on Ouchterlony plates. Peptide patterns, on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, following limited proteolysis were also markedly different. The trout enzyme was as effective, catalytically, as the rat enzyme in a reconstituted system that contained purified rat cytochrome P-448 and lipid. Comparison of ethoxyresorufin-O-deethylase temperature profiles with various combinations of purified trout and rat P-448, reductase, and lipid, in membranous and nonmembranous reconstitution systems, demonstrated that the lower temperature optimum in trout microsomes could only be reproduced when all three trout components were incorporated into liposomes. These results suggest that it is the structural organization of the mixed-function oxidase enzymes and lipid within trout microsomes which were responsible for the lower temperature optimum compared to rat.

Comparisons of warfarin metabolism by liver microsomes of rats treated with a series of polybrominated biphenyl congeners and by the component-purified cytochrome P-450 isozymes

R- and S-warfarin metabolite profiles (regio- and stereoselectivity) has been determined with hepatic microsomes from untreated rats and rats treated with nine individual polybrominated biphenyl (PBB) congeners, a commercial mixture of PBBs, and, for comparison with phenobarbital and 3-methylcholanthrene. The metabolic rates have been correlated with cytochrome P-450 (P-450) isozyme concentrations in the microsomes determined by immunochemical quantitation techniques (G.A. Dannan, F.P. Guengerich, L.S. Kaminsky, and S.D. Aust, (1983) J. Biol. Chem. 258, 1282-1288). The warfarin hydroxylase activities of the P-450 isozyme components of the various microsomal preparations (F.P. Guengerich, G.A. Dannan, S.T. Wright, M.V. Martin, and L.S. Kaminsky (1982) Biochemistry 21, 6019-6030) were multiplied by the corresponding isozyme concentrations to obtain an assessment of the potential warfarin hydroxylase capacity of the microsomes, and the results were compared with actual activities. The results of these studies and comparisons indicate that substrate regio- and stereoselectivities of microsomal-bound P-450s are essentially retained on purification of the isozymes to homogeneity and reconstitution, that warfarin metabolism by microsomal preparations can be used to predict microsomal P-450 isozyme compositions, and that microsomal warfarin hydroxylase activity is greater than would be predicted based on the approx 20:1 ratio of P-450 to NADPH-P-450 reductase in the microsomes and on the known activities of constituent isozymes. Two P-450 isozymes which are induced by treatment of rats with phenobarbital appear to be more tightly linked to NADPH-P-450 reductase than does an isozyme induced by beta-naphthoflavone.

Induction of cytochrome P-450 in the rat liver after exposure to xylenes, dose-response relationship and dependence on endocrine factors

Male rats were exposed to different concentrations of xylenes for 3 days. Small but statistically significant increases in cytochrome P-450 content, NADPH-cytochrome c reductase activity and O-deethylation of 7-ethoxyresorufin in liver microsomes were detected already at an exposure level of 75 ppm. Morphological studies of livers from rats exposed to relatively high concentrations of xylenes for 5 days showed marked proliferation of smooth ER with little changes of the rough ER. No pathological alterations were observed. Castration of male rats influenced the response to xylene exposure only to a minor extent. Hypophysectomy alone was shown to cause significant increases in cytochrome P-450 and cytochrome b5 content and epoxide hydrolase activity. Induction of cytochrome P-450 dependent enzymatic activities after exposure to xylenes was reduced but qualitatively similar to that obtained with normal male rats whereas the induction of epoxide hydrolase activity was prevented. The difference in response to exposure to xylenes between male and female rats was mainly quantitative in nature. Polyacrylamide gel electrophoresis of liver microsomes from animals exposed to xylenes in the presence of SDS showed increases in protein bands comigrating with cytochrome P-450 PB-B2 and epoxide hydrolase purified from phenobarbital treated rats. It is concluded that in the rat exposure to xylenes mimics the effects of phenobarbital treatment as judged by both biochemical and morphological criteria and that the pituitary seems to have a regulatory function with regard to the induction of several drug-metabolizing enzymes.

The role of highly purified forms of rat liver cytochrome P-450 in the dimethylation of dimethylnitrosamine and its activation to mutagens

Highly purified NADPH-cytochrome P-450 reductase and the major phenobarbital (PB) and beta-naphthoflavone (beta NF) forms of cytochrome P-450 were used in reconstituted systems to study the demethylation and subsequent activation of dimethylnitrosamine (DMN) to mutagenic intermediates. Both forms of cytochrome P-450 were active in the demethylation of DMN, cytochrome P-450 from PB-treated animals being more efficient, generating nearly twice as much formaldehyde per nmol of haemoprotein. Neither form of the cytochrome could activate DMN to mutagens in the Ames test. These findings indicate that DMN demethylation does not lead to its activation to mutagenic products.

Quantification of NADPH: cytochrome P-450 reductase in liver microsomes by a specific radioimmunoassay technique

We have developed a specific radioimmunoassay to quantify NADPH: cytochrome P-450 reductase. The assay is based on the use of 125I-labelled NADPH: cytochrome P-450 reductase as the radiolabelled antigen and can detect quantities of this protein in amounts as low as 30 pg. The results of the radioimmunoassay demonstrates that the 2.7-fold increase in enzyme activity in rat liver microsomal membranes after phenobarbital treatment is due to increased amounts of the protein. beta-Naphthoflavone treatment, however, did not alter the activity or the quantity of this enzyme in microsomes. The quantification of NADPH: cytochrome P-450 reductase in the microsomes isolated from control and phenobarbital- and beta-naphthoflavone-treated animals permits the calculation of the ratio of this protein to that of total cytochromes P-450. A molar ratio of 15:1 (cytochromes P-450/NADPH: cytochrome P-450 reductase) was calculated for control and phenobarbital-treated animals. This ratio increased to 21:1 after beta-naphthoflavone treatment. Thus the molar ratio of these proteins in liver microsomes can vary with exposure of the animals to particular xenobiotics.

Purification of detergent-solubilized form and membrane-binding domain of rat gamma-glutamyltransferase by immuno-affinity and hydrophobic chromatography

A new method to purify papain- or detergent-solubilized form (papain or detergent form) of gamma-glutamyltransferase from rat hepatomas as well as from rat kidney by immuno-affinity column chromatography is presented. The antibody-column was prepared by coupling the anti-kidney papain form antibody, which had been purified by using a kidney papain form-Sepharose column, to CNBr-activated Sepharose 4B. The enzyme bound to the antibody-column was eluted with 0.04 M NH4OH. By this method, detergent forms were purified 300 and 1600-fold in approx. 50% yields from rat kidney and rat ascites hepatoma AH 13, respectively, and the papain form was also purified 16 000-fold in a similar yield from primary hepatoma which has a very low activity of this enzyme. Preparations thus obtained apparently did not contain any peptide other than heavy and light subunit peptides of this enzyme on SDS-polyacrylamide gel electrophoresis. The detergent form of kidney enzyme was preferentially absorbed to a hydrophobic column of aminooctyl-Sepharose, while the papain form was not, suggesting that the detergent form might be adsorbed to the column through hydrophobic interaction of the membrane-binding domain. The domain peptide was also purified by the hydrophobic column after release from the detergent form by papain treatment. The molecular weight of the peptide was estimated to be about 16 000 on SDS-polyacrylamide gel electrophoresis. On double immunodiffusion, the domain peptide reacted with anti-detergent form antibody but not with anti-papain form antibody. The domain-specific antibody was also purified from the anti-detergent form antibody.

Induction and repression of the major phenobarbital-induced cytochrome P-450 measured by radioimmunoassay

Two independent radioimmunoassay techniques for the major phenobarbital-inducible cytochrome P-450 (PB P-450) of rat liver microsomal membranes are described. The first technique employs as the source of radiolabelled antigen the products of translation in vitro labelled with [35S]methionine. The second technique employs purified antigen labelled with 125I and is quicker, less expensive and more precise. Both assays are highly specific for PB P-450 and can detect quantities of this variant as small as 1 ng. This is several orders of magnitude more sensitive than any method described previously for the quantification of cytochromes P-450, and consequently the technique is particularly well suited for the quantification of so-called constitutive cytochrome P-450 variants that are present in very low amounts. The results of the radioimmunoassays demonstrate that the apparent 2.6-fold induction of total cytochromes P-450 after phenobarbital treatment is due to a 43-fold increase in Pb P-450. Although beta-naphthoflavone increases the total content of cytochrome P-450 of microsomal membranes 1.4-fold, it actually causes a 55% decrease in the amount of PB P-450. Thus different xenobiotics can have differential effects on the expression of the genes for specific cytochrome P-450 variants.

Selectivity of 1-phenylimidazole as a ligand for cytochrome P-450 and as an inhibitor of microsomal oxidation

Equilibrium dialysis studies established that 1-[4'-(3H)-phenyl]imidazole (PI) was bound to hepatic microsomal suspensions from control, phenobarbital (PB)- and 3-methylcholanthrene (3MC)-treated rats and that the binding was directly related to the cytochrome P-450 content. Computer-assisted Scatchard plot analysis of the binding data indicated the existence of two major types of microsomal binding sites in both control and induced rats, one with a high affinity (Ka approximately 1.5 X 10(7) M-1) and the other with a low affinity (Ka approximately 5 X 10(5) M-1) for PI. The binding of PI to the highly purified, individual cytochrome P-450s that constituted the major forms from the PB- and beta-naphthoflavone (beta NF)-induced rats exhibited affinities similar to the high and low affinity binding sites observed in microsomal suspensions. The two types of PI binding sites were characteristic of two classes of cytochrome P-450, and the major cytochrome induced by PB and 3MC (or beta NF) were each associated with one of these two classes. In concurrence with this, it was shown that, although PI was an excellent inhibitor of aromatic hydrocarbon hydroxylase (AHH) activity in PB-induced rats, it exhibited little or no inhibitory activity towards AHH activity in 3MC-induced animals.

A rapid one-step purification of NADPH-cytochrome c (P-450) reductase from rat liver microsomes

NADPH-cytochrome c (P-450) reductase from liver microsomes of phenobarbital-treated rats has been purified in a single step by affinity chromatography on agarose-hexane-adenosine 2',5'-diphosphate. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, enzyme assay, and radioimmunoassay the protein obtained by this single step procedure is as pure as that isolated by multicolumn procedures.

Comparative properties of purified cytochrome P-448 from beta-naphthoflavone treated rats and rainbow trout

1. Cytochrome P-448 from beta-naphthoflavone treated rainbow trout (Salmo gairdnerii) liver was purified and compared to purified P-448 from beta-naphthoflavone treated rats (Rattus rattus) and purified P-450 from phenobarbital induced rats. 2. The two P-448 forms had similar spectral properties, substrate specificity, sensitivity to inhibitors and regioselectivity in the metabolism of benzo(a)pyrene and testosterone. 3. Rat and trout P-448 differed in apparent monomeric mol. wt (Mr) by at least 2000 daltons, and did not share identical antigenic determinants. Both rat and trout P-448 were shown to be quite different from rat P-450 using all of the above criteria for distinguishing multiple forms.

Characterization of the microsomal cytochrome P-450 species induced in rat liver by trans-stilbene oxide

trans-Stilbene oxide differs from the classical inducers of drug-metabolizing enzymes, phenobarbital and 3-methylcholanthrene, in that it induces the so-called phase II activities, epoxide hydrolase and glutathione S-transferase, to a much larger extent than it induces cytochrome P-450. Nonetheless, the level of cytochrome P-450 in liver microsomes from rats treated with trans-stilbene oxide is increased significantly to twice the control value. The existence of a number of different isozymes of cytochrome P-450 has now been clearly demonstrated and in the present study we have posed the question. What form(s) of cytochrome P-450 is induced by trans-stilbene oxide? A number of criteria including substrate specificity, pattern of benzo(a)pyrene metabolism, sensitivity to inhibitors, substrate binding spectra, ethylisocyanide binding spectra, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and crossed immunoelectrophoresis were used to answer this question. It seems clear that trans-stilbene oxide induces the same form(s) of cytochrome P-450 as phenobarbital.

Purification and characterization of microsomal cytochrome P-450s

1. Eight different forms of cytochrome P-450 have been purified to electrophoretic homogeneity. Electrophoretic, spectral and catalytic properties of these cytochrome P-450s are presented and comparison is made with preparations presented elsewhere in the literature. 2. The levels of these forms of cytochrome P-450 present in liver microsomes of rats treated with various compounds have now been quantified. Several forms of cytochrome P-450 are induced, in a more or less coordinate manner, while levels of other cytochrome P-450s are lowered, during administration of commonly used inducing agents. 3. The role of cytochrome P-450 purification and characterization studies in the understanding of the total field is discussed, along with directions in which future research is needed.

Metabolism of arachidonic acid by isolated rat hepatocytes, renal cells and by some rabbit tissues. Detection of vicinal diols by mass fragmentography

Purified cytochromes P-450 (LM2 and PB-B2) in a reconstituted system and epoxide hydrolase were recently found to metabolize arachidonic (eicosatetraenoic) acid to four vicinal dihydroxyeicosatrienoic acids. These metabolites were chemically synthetized from octadeuterated arachidonic acid and employed as internal standards for mass fragmentography. Isolated rat hepatocytes and renal cells were incubated with arachidonic acid (0.1 mM; 37 degrees C, 15 min) and, following extractive isolation and reversed-phase HPLC, formation of 11,12-dihydroxy-5,8,14-eicosatrienoic acid and 14,15-dihydroxy-5,8,11-eicosatrienoic acid was demonstrated by mass fragmentography using a capillary GC column. Furthermore, these diols were also detected in rabbit liver and renal cortex and they therefore appear to be formed endogenously. Formation of vicinal diols was also studied in cell free systems. Rabbit liver and renal cortical microsomes were incubated with NADPH (1 mM) and arachidonic acid (0.15 mM) for 15 min at 37 degree C and, besides 11,12-dihydroxy- and 14,15-dihydroxyeicosatrienoic acid, small amounts of 8,9-dihydroxy- and 5,6-dihydroxyeicosatrienoic acid could be detected by mass fragmentography. Renal as wall as hepatic monooxygenases can thus epoxidize each of the four double bonds of arachidonic acid. In contrast, rabbit lung microsomes and NADPH metabolized arachidonic acid mainly to prostaglandins and 19-hydroxy- and 20-hydroxyarachidonic acid, while only small amounts of 11,12-dihydroxyeicosatrienoic acid could be found. Monooxygenase metabolism of arachidonic acid by epoxidation might therefore be a significant pathway for the metabolism of this essential fatty acid in isolated rat renal cells and hepatocytes but presumably not in the lung.

Purification of cytochromes P-448 from beta-naphthoflavone-treated rainbow trout

Rainbow trout were treated with beta-naphthoflavone and the hepatic microsomal cytochrome P-450 solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. Chromatography on tryptamine-Sepharose 4B gave a single cytochrome P-450 peak which was further resolved into three components by elution from DEAE-Sepharose. The two main peaks were then chromatographed on hydroxyapatite and a total of four fractions obtained. Two of these fractions had similar properties and significantly metabolized [14C]benzo[a]pyrene in a reconstituted system containing rat cytochrome P-450 reductase. This activity was inhibited by alpha-naphthoflavone but not by metyrapone of SKF-525A. Purified cytochromes P-448 from 3-methylcholanthrene-treated rat had similar spectral properties and activity towards [14C]benzo[a]pyrene suggesting similarities between these forms.

Oxidation of trichloroethylene by liver microsomal cytochrome P-450: evidence for chlorine migration in a transition state not involving trichloroethylene oxide

Trichloroethylene (TCE) was metabolized by cytochrome P-450 containing mixed-function oxidase systems to chloral (2,2,2,-trichloroacetaldehyde), glyoxylic acid, formic acid, CO, and TCE oxide. TCE oxide was synthesized, and its breakdown products were analyzed. Under acidic aqueous conditions the primary products were glyoxylic acid and dichloracetic acid. The primary compounds formed under neutral or basic aqueous conditions were formic acid and CO. TCE oxide did not form chloral in any of these or other aqueous systems, even when iron salts, ferriprotoporphyrin IX, or purified cytochrome P-450 was present. Ferric iron salts catalyzed the rearrangement of TCE oxide to chloral only in CH2Cl2 or CH3CN. A 500-fold excess of iron was required for complete conversion. A kinetic model involving the zero-order oxidation of TCE to TCE oxide by cytochrome P-450 and the first-order degradation of the epoxide was used to test the hypothesis that TCE oxide is an obligate intermediate in the conversion of TCE to other metabolites. Kinetic constants fo the breakdown of TCE oxide and for the oxidative metabolism of TCE to stable metabolites were used to predict epoxide concentrations required to support the obligate intermediacy of TCE oxide. The maximum levels of TCE oxide detected in systems using microsomal fractions and purified cytochrome P-450 were 5-28-fold lower than those predicted from the model. The kinetic data and the discrepancies between the observed metabolites and TCE oxide breakdown products support the view that the epoxide is not an obligate intermediate in the formation of chloral, and an alternative model is presented in which chlorine migration occurs in an oxygenated TCE-cytochrome P-450 transition state.

Metabolism of dichlorobiphenyls by highly purified isozymes of rat liver cytochrome P-450

Hepatic mixed-function oxidase metabolism of the ubiquitous pollutant polychlorinated biphenyls (PCBs) is implicated in their toxification and detoxification. We used dichlorobiphenyls (DCBs) as models to investigate the effect of the chloro substituent sites on this metabolism experimentally and by molecular orbital calculations. Reconstituted, purified cytochrome P-450 PB-B and BNF-B, the major terminal oxidase isozymes of this system, from phenobarbital (PB)- and beta-naphthoflavone (BNF)-induced rats were used to investigate this metabolism. Both isozymes are also induced by PCBs. High-performance liquid chromatography (HPLC) was used to detect, quantify, and isolate metabolites. Metabolite structures were identified by mass spectrometry, dechlorination to identifiable hydroxybiphenyls, and HPLC retention times. All DCBs yielded 3- and 4- but no 2-monohydroxylated metabolites (3,3'-DCB also yielded a dihydroxy metabolite). Di-o-chloro-substituted DCBs were metabolized primarily by cytochrome P-450 PB-B, mono-o-chloro substituted DCBs by both isozymes approximately equivalently, and DCBs without o-chloro substituents by BNF-B primarily. Thus PB-B preferentially metabolizes noncoplanar DCBs and BNF-B coplanar DCBs. The cytochrome isozymes exhibited differing regioselectivities for DCB metabolism - PB-B hydroxylated unchlorinated phenyl rings and BNF-B chlorinated rings. Incorporation of epoxide hydrolase yielded DCB dihydrodiols, and hydroxy metabolite patterns were consistent with those calculated from ring-opened arene oxide intermediates. Thus the rates and regioselectivities of metabolism and thus possibly the toxicity and carcinogenicity of DCBs are dependent on the cytochrome P-450 isozymes induced.

Immunological comparison of rat, rabbit, and human microsomal cytochromes P-450

Antibodies were raised in rabbits to electrophoretically homogeneous cytochromes P-450 isolated from rat and human liver microsomes. These antibodies were used to compare various forms of rat, rabbit, and human cytochromes P-450 present in microsomes and in purified preparations by using double-diffusion analysis, immunoelectrophoresis, quantitative microcomplement fixation, competitive radioimmune assay and inhibition of enzyme activity toward d-benzphetamine and benzo[a]pyrene. The results indicate that (1) at least some forms of cytochrome P-450 from the three species share certain common immunological determinants, (2) there are immunological differences between cytochromes P-450 isolated from the three species, (3) some immunological differences exist between cytochromes P-450 isolated from rats of different strains, (4) immunologically distinguishable forms of cytochrome P-450 exist within individual human liver samples, and (5) human liver samples obtained from different individuals contain immunologically different forms of cytochrome P-450. Quantitative microcomplement fixation techniques were used to assign immunological distances to different form of rat, rabbit, and human liver microsomal cytochrome P-450. Cross-reactivity was observed in all systems tested, and the extent of immunological similarity was dependent upon the particular assay used.