Separation and characterization of highly purified forms of liver microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls, phenobarbital, and 3-methylcholanthrene

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.

Comparisons of highly purified hepatic microsomal cytochromes P-450 from Holtzman and Long-Evans rats

The present study describes the purification and characterization of strain variant forms of a major phenobarbital-inducible microsomal hemoprotein, cytochrome P-450b, from Holtzman and Long-Evans rats. The strain variant hemoproteins cannot be resolved by sodium dodecyl sulfate gel electrophoresis, but can be partially separated in two-dimensional isoelectric focusing SDS gels. If, however, sodium tetradecyl sulfate is incorporated into the one-dimensional gel system, separation of the cytochromes P-450b is achieved. Minor structural differences are detected in the peptides of the cytochromes P-450b following limited proteolysis by Staphylococcus aureus V8 protease, cleavage by cyanogen bromide, or reverse-phase high-pressure liquid chromatography of tryptic peptides. The strain variant cytochromes P-450b are immunochemically and spectrally indistinguishable. The optical spectra of the ferric and ferrous hemoproteins are identical, as are the CO- and ethylisocyanide-reduced difference spectra. Ferrous cytochromes P-450b from both rat strains effectively bind metyrapone with equivalent affinities. In addition, the cytochromes P-450b do not differ in their catalytic activities toward benzphetamine, hexobarbital, benzo [a]pyrene, zoxazolamine, 7-ethoxycoumarin, estradiol-17 beta and testosterone. Cytochrome P-450c, the predominant isozyme inducible in rat liver by 3-methylcholanthrene, was purified from Holtzman and Long-Evans rats. Cytochromes P-450c from both rat strains are indistinguishable based on electrophoretic, immunological, spectral and catalytic properties. Minor structural differences in the cytochromes P-450c were revealed in the reverse-phase high-pressure liquid chromatographic profiles of the tryptic peptides of these hemoproteins, but not in the peptides generated by limited proteolysis or cleavage with cyanogen bromide.

Complexes of cytochrome P450 with metyrapone. A convenient method for the quantitative analysis of phenobarbital-inducible cytochrome P450 in rat liver microsomes

Phenobarbital-inducible cytochrome P450 variants have a higher affinity (app. Ks = 0.6 microM). The difference in affinity has enabled us to quantitate, by metyrapone-induced difference spectral analysis, phenobarbital-inducible cytochromes P450 in liver microsomal membranes. The result obtained by this simple, rapid technique compare well with those found by more sophisticated techniques such as radioimmunoassay and immunoprecipitation.

Binding of metyrapone to dithionite-reduced cytochrome P-450 from rats treated with xenobiotics

The in vitro binding of metyrapone to dithionite-reduced cytochrome P-450 in hepatic microsomes from rats treated in vivo with thirteen different xenobiotics was studied spectrophotometrically. The proportion of cytochrome P-450 that bound metyrapone increased 1.8-fold to about 78% following treatment with phenobarbitone (PB) and PB-type inducers (trans-stilbene oxide, 2,2',4,4'-tetrachloro-, 2,2',4,5,5'-pentabromo- and 2,2',4,4',5,5'-hexachlorobiphenyl) but remained unaltered following treatment with 3-methylcholanthrene (MC) and MC-type induces (benzo[a]pyrene, beta-naphthoflavone and 3,3',4,4'-tetrabromobiphenyl). The simultaneous induction of the PB-inducible and MC-inducible forms of cytochrome P-450 by administering Aroclor 1254 or by coadministering PB with MC increased the proportion of cytochrome P-450 that bound metyrapone to 74 and 78% respectively. PB treatment increased whereas MC treatment decrease the binding affinity for metyrapone by approximately 20-fold. Treatment with isosafrole or metyrapone itself failed to stimulate metyrapone binding. In contrast, pregnenolone-16 alpha-carbonitrile was indistinguishable from PB in its ability to increase the binding capacity and binding affinity for metyrapone. Our results indicate that metyrapone binding is not specific for cytochrome P-450b, the major PB-inducible hemoprotein, as has been proposed [V. Luu-The, J. Cumps and P. Dumont, Biochem. biophys. Res. Commun. 93, 776 (1980)].

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.

Purification, characterization and regulation of five rat hepatic microsomal cytochrome P-450 isozymes

1. Five hepatic microsomal cytochrome P-450 isozymes (cytochromes P-450a, P-450b, P-450c, P-450d, P-450e) have been purified to apparent homogeneity from immature male rats treated with various xenobiotics. 2. The unique electrophoretic properties, substrate specificities and spectral characteristics of these haemoproteins have been compared and contrasted. 3. Structural studies of these cytochrome P-450 isozymes have included NH2-terminal amino acid sequence analyses, as well as electrophoretic profiles of limited proteolytic digests and cyanogen bromide fragments of the haemoproteins. 4. Specific antibodies have been prepared against four of the isozymes and used to evaluate immunochemical relationships among these cytochrome P-450s by Ouchterlony double-diffusion analyses. 5. The levels of some of these cytochrome P-450 isozymes have been quantified immunologically in hepatic microsomal preparations from untreated rats and following induction by phenobarbital, 3-methylcholanthrene or Aroclor 1254. 6. Antibodies directed against cytochromes P-450a and P-450b have been used to establish the presence of more than one 7 alpha- and 16 alpha-testosterone hydroxylase in rat hepatic microsomes. The relative contributions of cytochromes P-450c and P-450d to the overall microsomal metabolism of benzo[a]pyrene have been evaluated using antibodies to these haemoproteins.

Effect of diabetes on rat liver cytochrome P-450. Evidence for a unique diabetes-dependent rat liver cytochrome P-450

Detergent-solubilized hepatic microsomal fractions from alloxan diabetic rats exhibited a 52,000 molecular weight hemeprotein band that was not present in the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) protein profiles of identically solubilized hepatic microsomal fractions from normal, 3-methylcholanthrene- or phenobarbital-treated rats. This 52,000 mol. wt hemeprotein band disappeared from the protein profile of insulin-treated diabetic rat liver to yield the SDS-PAGE profile of normal rat liver. When P-450 hemeproteins were purified by lauric acid affinity and hydroxylapatite chromatography from solubilized microsomes, only the diabetic rat had a 52,000 mol. wt P-450. This distinct 52,000 mol. wt diabetes-induced P-450 interacted with type II compounds to yield a 2-fold greater absorbance change than was observed with the purified P-450s from either the normal or the chemically induced rats. The properties of this unique 52,000 mol. wt P-450 suggest that it may be the catalytic component responsible for the increased rate of type II substrate (aniline) metabolism observed in the diabetic rat.

Difference in liver homogenates from Donryu, Fischer, Sprague-Dawley and Wistar strains of rat in the drug-metabolizing enzyme assay and the Salmonella/hepatic S9 activation test

Comparison studies for detecting differences between liver microsome and S9 preparations from 4 strains (Donryu, Fischer, Sprague-Dawley, Wistar) of young male rats were carried out with pretreatment of the animals by inducers such as PCBs and PB plus 5,6-BF. Each microsome fraction was assayed for the enzymic activity of metabolism of model substrates such as aniline, benzophetamine, BP, DMN and 7-ethoxycoumarin. The hepatic S9 sample was also compared, as regards its metabolizing ability to activate 9 pre-mutagens (2AA, AAF, o-AAT, BP, DAB, DMBA, DMN, m-PDA, quinoline) to directly acting mutagens in the Salmonella/hepatic S9 activation test by using TA98, TA100 and TA1537 strains with or without cytochrome P450 inhibitors (SKF-525A, metyrapone, 7,8-benzo-flavone). In the enzymic assay with PCBs-induced microsomes, BP hydroxylation a strain-specific difference: the microsomes from Fischer and Wistar rats were more effective for metabolizing BP than those from the other strains of rat. The effect of induction by BP plus 5,6-BF for Fischer rats showed relatively higher enzymic activity in the same induction group. Other microsomes prepared from rats with and without induction by PB plus, 5,6-BF did not show a clear-cut strain dependency in the enzymic activities assayed. In the mutation experiments with hepatic S9 samples, the examination of DAB and quinoline revealed a marked strain difference when S9 samples prepared from PCBs-pretreated and PB-plus-5,6-BF-induced rats were used: the S9 sample from Fischer rats was available for activating the two pre-mutagens to directly acting mutagens. No marked difference in the metabolic activation of the remaining 7-pre-mutagens was observed on other S9 preparations. In examinations of mutagenicity activities with the use of three inhibitors, the two S9 preparations made with the two induction methods showed inhibition profiles closely similar to each other. However, there were minor differences in the profiles by these inhibitors. From these findings it was concluded that Fischer rat-liver S9 is useful for detecting mutagens in the metabolic activation test, when induction by PB plus 5,6-BF was used in the Ames Salmonella test.

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.

Structure-activity relationships of chlorinated benzenes as inducers of different forms of cytochrome P-450 in rat liver

Hexachlorobenzene (HCB) produced increases in ethoxyresorufin (ERR) O-deethylase, aryl hydrocarbon hydroxylase (AHH) and aminopyrine N-demethylase activities in rat liver phenobarbital which were intermediate between those produced by phenobarbital and 3,4-benzpyrene (BP), alpha-Naphthoflavone (ANF) selectivity inhibited ERR activity in BP and HCB-induced microsomes (94% and 88%). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of liver microsomes indicated that HCB did not produce a detectable increase in a polypeptide with electrophoretic properties similar to those of purified cytochrome P-448 (Mr = 56,000). However, HCB did induce a polypeptide with Mr = 53,000 corresponding to one of two polypeptide bands induced by BP. This polypeptide may represent a second form of cytochrome P-448. Purification of HCB to remove possible dibenzo-p-dioxin impurities did not alter the 'mixed-type' induction produced by HCB. In contrast to HCB, all other chlorinated benzenes tested resembled phenobarbital as inducers.

Induction of liver microsomal cytochrome P-450 and associated monooxygenases by octachlorostyrene in inbred strains of mice. Lack of correlation with the murine Ah locus

Single intraperitoneal injections of octachlorostyrene (OCS) and hexachlorobenzene in genetically polycyclic aromatic hydrocarbon "responsive' C57/BL/6 (B6) mice led to a time- and dose-dependent increase in the levels of liver microsomal cytochromes P-450 and b5 as well as in the activities of NADPH cytochrome P-450 (cytochrome c) reductase, ethylmorphine (EM) N-demethylase, 4-nitroanisole (PNA) O-demethylase and acetanilide 4-hydroxylase (AcA hydroxylase). No, or only a very moderate, increase in the activity of aryl hydrocarbon hydroxylase was seen after OCS and HCB, respectively. Pretreatments with phenobarbital (PB) or 3-methylcholanthrene (MC) both increased AcA hydroxylase activity to a similar degree, whereas pretreatment with polychlorinated biphenyls (Aroclor 1254) had an effect equal to the sum of PB and MC. Judged from sodium dodecylsulfate polyacrylamide gel electrophoresis studies, OCS and HCB predominantly increased a microsomal polypeptide of apparent mol. wt 52,000, similar to PB. A reduced response was seen after OCS or HCB treatment of aromatic hydrocarbon "non-responsive' DBA/2 (D2) mice compared to B6 mice, both with respect to AcA hydroxylase as well as EM demethylase and PNA demethylase activities. OCS treatment of B6D2F1 mice resulted in a doubling of AcA hydroxylase activity, but in mice of the (B6D2)D2 backcross no distinct subgrouping of individual AcA hydroxylase activities were apparent. These results demonstrate that OCS is an inducer of the PB-type in mice and that induction of AcA hydroxylase by OCS is not regulated by the Ah locus.

Metabolic activation of carbon tetrachloride: induction of cytochrome P-450 with phenobarbital or 3-methylcholanthrene and its effect on covalent binding

Anaerobic incubation of [14C]carbon tetrachloride with normal rat liver microsomes and microsomes from rats treated with the inducers phenobarbital (PB) and 3-methylcholanthrene (MC) reveals distinct differences in metabolic activation. While the increase in CO-binding pigment is comparable for both inducers, metabolism of CC14 is enhanced only by PB-induction; MC-induced microsomes are equivalent to microsomes from untreated animals. Sodium dodecyl sulphate (SDS)-electrophoresis of microsomal proteins confirms the expected increase at 52 000 daltons (cytochrome P-450 PB) on PB-induction, at 56 000 daltons (cytochrome P-450MC) on MC-induction; after anaerobic incubation with [14C]CC14 the electrophoretic pattern is largely unchange. The highly reactive radical intermediates of CC14-metabolism should attack the closest possible partner. Most of protein-bound radioactivity is located in the mass range between 47 000 and 54 000 daltons, indicating that cytochrome P-450PB is the isoenzyme mainly responsible for CC14-activation; cytochrome P-450MC plays virtually no role in metabolic activation. The direct participation of NADPH-cytochrome P-450 reductase appears unlikely, since the specific binding to proteins in the corresponding mass range is not elevated. A significant percentage of label is attached to proteins at 120 000 daltons and above, presumably oligomers of cytochrome P-450 apoprotein.

The effects of dietary selenium on the biotransformation of 7,21-dimethylbenz[a]anthracene

The influence of dietary selenium on the mutagenic activation of 7,12-dimethylbenz[a]anthracene (DMBA) by rat liver S9 was studied using the Ames test. Rats received supplemental selenium, as sodium selenite, in the drinking water or in the diet. All rats additionally received 0, 20, 50, 100, or 500 mg Aroclor 1254 per kg body weight. Revertant counts decreased 72 and 31% at the 20- and 100-mg/kg induction levels, respectively, with S9 preparations from rats given selenium supplementation, compared to controls. No significant effects of selenium on S9 preparations was observed in rats treated with 500 mg/kg Aroclor. Preparations of S9 from rats receiving 2.5 ppm Se in their diet produced 46, 84 and 70% less revertants than controls at the 20-, 50- and 100-mg/kg induction levels. Increasing the selenium concentration in the diet to 5 ppm reduced the revertant counts by 71, 68 and 65%, at the 20-, 50- and 100-mg/kg induction level of Aroclor, respectively. Dietary selenium supplementation was shown to decrease the mutagenic activation of DMBA by liver microsomes. These studies indicate that in vivo selenium supplementation may reduce susceptibility to the action of various carcinogens.

Cross-linking studies of the protein topography of rat liver microsomes

A cleavable cross-linking reagent, dithiobis(succinimidyl propionate), DSP, was used to study the topography of the proteins in the endoplasmic reticulum membrane of rat liver. Reaction of untreated (control), phenobarbital- or 3-methylcholanthrene-induced microsomes with 0.5 mM DSP for 30 min at 0 degrees C resulted in the cross-linking of a protein with a molecular weight of about 52 000 to form an apparent dimer. In phenobarbital microsomes, a smaller amount of a 52 000-dalton protein also appeared in a dimer in the absence of DSP if N-ethylmaleimide was not included during homogenization. In phenobarbital and 3-methylcholanthrene microsomes, a 48 000-dalton protein was cross-linked by DSP to a protein of about 57 000. In all three types of microsomes, a protein with an Mr of about 52 000 was also cross-linked to a protein of about 79 000. In phenobarbital and control microsomes, cross-linking resulted in an oligomeric protein of approximate molecular weight 180 000 which contained three proteins, two with Mr of about 52 000 and one about 79 000. Under the cross-linking conditions, little or no denaturation of cytochrome P-450 and NADPH-cytochrome c reductase was observed. The aryl hydrocarbon hydroxylase activity was significantly inhibited by the bifunctional cross-linking reagent, DSP, but not by the monofunctional reagent N-succinimidyl-3-(4-hydroxyphenyl) propionate. However, attempts to regenerate the aryl hydrocarbon hydroxylase activity by cleavage of the disulfide linkage with 2-mercaptoethanol or dithiothreitol were not successful.

Induction of liver microsomal cytochrome P-450 and associated monooxygenases by octachlorostyrene in the rat

The inducing effects of octachlorostyrene (OCS) and hexachlorobenzene (HCB), chlorinated hydrocarbon contaminants found in fish, on the microsomal cytochrome p-450 system were studied in rats. Administration of single doses of OCS and HCB at 50mg/kg intraperitoneally and higher led to increases in microsomal protein an cytochrome p-450 content, cytochrome p-450 reductase, ethylmorphine N-demethylase, 4-nitroanisole O-demethylase, acetanilide 4-hydroxylase, but not in benzo(a)pyrene hydroxylase activities. Similar patterns of induction were seen after 14 days repeated intraperitoneal or oral dosing. Siodium dodecyl sulfate polyacrylamide gel electrophoresis of microsomes from OCS and HCB treated rats showed increases in proteins similar to those found after phenobarbital (PB) induction. Under the present experimental conditions OCS and HCB were found to be PB-type inducers.

Regio- and stereoselectivity of hepatic cytochrome P-450 toward polycyclic aromatic hydrocarbon substrates

Although the cytochrome P-450 system shows broad substrate specificity toward the PAH, it also has regio- and stereoselectivity. Of the PAH substrates yet studied, phenanthrene is metabolized most efficiently, whereas chrysene and DBA are much more slowly turned over. Interestingly, the cytochrome P-450 system and epoxide hydrolase favor the formation of dihydrodiols with R,R configuration. The predominant enantiomers of the dihydrodiols with the bay-region double bond, as well as bay-region diol epoxides derived from phenanthrene, chrysene, BA and B[a]P, are superimposable when their bay-regions are aligned (Fig. 3). These metabolically favored isomers of the dihydrodiols and bay-region diol epoxides are also the more carcinogenic isomers when derived from carcinogenic polycyclic hydrocarbons.

The metabolism of pyrene by rat liver microsomes and the influence of various mono-oxygenase inducers

1. Pyrene metabolite g.l.c. profiles were recorded and metabolites identified by mass spectrometry. 2. Pyrene is metabolized by liver microsomes of untreated rats to 1-hydroxypyrene, 4,5-dihydroxy-4,5-dihydropyrene, two different diphenols and a triol, tentatively identified as 1,4,5-trihydroxy-4,5-dihydropyrene. 3. Pretreatment with phenobarbital or polychlorinated biphenyls favours oxidation at the K-region, whereas cytochrome P-448 inducers stimulate oxidation at the non-K-region of pyrene. 4. 1-Hydroxypyrene does not inhibit pyrene oxidation. 5. Pyrene diphenols are formed by secondary oxidation of 1-hydroxypyrene. 6. Triols are formed from dihydrodiols by secondary oxidation.

Further characterization of the effects of 3-methylcholanthrene administration upon hepatic ribonucleic acid polymerase activities

The administration of 3-methylcholanthrene (MC) to rats results in a marked increase in the specific activities of hepatic RNA polymerases I and II. In the present study, we were able to show that this increase was not caused by a shift in the ratio of 'free' to 'template-engaged' RNA polymerase. By means of binding studies with [3H]amatoxin, we were unable to demonstrate any increase in the number of RNA polymerase II molecules in liver after MC administration to the rats. RNA polymerase I was purified in excess of 3000-fold from hepatic nuclei isolated both from control and MC-treated rats. The stimulation in activity was demonstrated at each step in the purification scheme until glycerol sedimentation analysis. Results from cation-exchange chromatography on phosphocellulose indicated that the polycyclic hydrocarbon increased the enzyme activity of RNA polymerase Ib somewhat specifically. Subsequent to glycerol gradient centrifugation, this stimulatory advantage was no longer evident. Reconstitution experiments revealed the presence of a stimulatory component, which was demonstrated in low molecular weight fractions from both control and experimental preparations.

The influence of polycyclic aromatic hydrocarbons as inducers of monooxygenases on the metabolite profile of benz[a]anthracene in rat liver microsomes

Microsomal oxidation of benz[a]anthracene (BaA) in rat liver has been shown to occur at various positions (1,2-, 3,4-, 5,6-, 8,9- and 10,11-position) by means of gas chromatography/mass spectrometry (GC/MS) and comparison with synthetic reference substances. In normal rats trans-5,6-, 8,9- and, mainly, 10,11-dihydrodiols have been detected as primary metabolites. The induction of monooxygenases by polycyclic aromatic hydrocarbon (PAH) results in a considerable change in the metabolite profile, since the 5,6- and 8,9-isomers become the main metabolites while the amount of 10,11-isomer is not increased. Simultaneously, the secondary metabolism to form triols and tetrols is induced. Phenobarbital as well as 'moderately inducing' PAH (pyrene, benzo[ghi]perylene, benzo[e]pyrene) induced the oxidation at 5,6- and 8,9-position, whereas almost all other compounds investigated, especially the benzofluoranthenes, additionally induced the oxidation at the 3,4-position forming the precursor of the ultimate carcinogen of BaA, 3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenz[a]anthracene, which was detected as its isomerisation product, the 2,3,4-triol.