The metabolism of 3-methylcholanthrene by liver and lung microsomes: effect of enzyme inducing agents

The metabolism of 3-methylcholanthrene (3-MC) by rat liver and lung microsomes was investigated. The animals were pretreated with various inducing agents to determine the effect which microsomal induction has on metabolic profile. Metabolites were separated by high pressure liquid chromatography. Major metabolism was associated with three peaks: 2-hydroxy-3-MC, 1-hydroxy-3-MC and a peak that co-chromatographs with 3-MC 11,12-oxide and probably contains phenols. Pretreatment of animals with various inducing agents caused an increase in total metabolism as well as in specific metabolites. However, lung microsomes from phenobarbital treated animals were less efficient at metabolizing 3-MC than control microsomes. Liver and lung microsomes converted 3-MC to qualitatively similar products. No metabolites were formed by lung microsomes that were not also formed by those from liver.

The isolation and characterization of specific 3-methylcholanthrene-binding proteins from rat liver cytosol

The major proteins to which 3-methylcholanthrene specifically binds have been purified over 480-fold with a 45% yield compared to a rat liver 100,000g supernate. The procedure involved a batch ion-exchange technique together with hydrophobic gel filtration and chromatofocusing chromatography. The multiple, specific 3-methylcholanthrene-binding proteins obtained from this protocol had apparent isoelectric points of pH 6.3, 6.0, 5.7, and 5.5 on elution from a chromatofocusing column. They all shared a common sedimentation coefficient as determined by sucrose gradient analysis of 4.4 S. Gel filtration on Sephadex G-75 gave a common Stokes radius of 27 A. An analysis of these chromatofocusing peaks by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed those which eluted at pH 6.3 and 6.0 to contain two major protein bands of Mr 32,000 and 34,000, together with several contaminating proteins. In contrast, the peaks from chromatofocusing which eluted at pH 5.7 and 5.5 contained three major proteins of Mr 40,000, 25,000, and 14,000. The specific binding capability of these chromatofocusing peaks was found to be unstable to temperatures of -30 degrees C and below. Competition studies showed that these proteins were not steroid receptors, and that only polycyclic aromatic hydrocarbons which could induce cytochrome P-450c were able to displace 3-methylcholanthrene from the binding site. A marked preference was noted for polycyclic aromatic hydrocarbons with four to five benzene rings arranged in a nonlinear fashion, suggesting the stereochemical requirements of the protein binding site. The stability of the noncovalent interaction between the proteins and 3-methylcholanthrene was in the range of pH 7 to 9.

An assay for the detection of specific binding of 3-methylcholanthrene to rat liver cytosolic proteins using DEAE-cellulose

A method for the detection of the specific binding of 3-methylcholanthrene to rat liver cytosolic proteins is described. The separation of the protein-bound 3-methylcholanthrene from the free 3-methylcholanthrene was achieved using a batch DEAE-cellulose technique. Extraction of the DEAE-cellulose with 0.3 M KCl allowed the selective release and measurement of the amount of protein-bound 3-methylcholanthrene. The assay was optimized for the following parameters: time of incubation with DEAE-cellulose, time required for salt extraction, protein concentration, the concentration of KCl required to elute the specific binding proteins, the amount of DEAE-cellulose required to bind the specific binding proteins, and ligand specificity. The sedimentation properties of those 3-methylcholanthrene-binding proteins which were extracted with salt from DEAE-cellulose were examined on 5 to 20% sucrose gradients; the major binding species sedimented as a broad peak at 4.5 S.

Highly persistent polycyclic aromatic hydrocarbon-DNA adducts in mouse skin: detection by 32P-postlabeling analysis

A 32P-postlabeling method for carcinogen-DNA adduct analysis recently developed in our laboratory was applied to skin DNA from mice treated topically with polycyclic aromatic hydrocarbons (PAHs). After application of 4 doses of 1.2 mumol each of benzo[alpha]pyrene (BP), 3-methylcholanthrene (MC) and 7,12-dimethylbenz[alpha]anthracene (DMBA), respectively, total covalent adduct binding in mouse skin DNA initially amounted to 1 adduct in 6.0 X 10(4) - 1.3 X 10(5) nucleotides. Four weeks after treatment, these levels had declined to 1 adduct in 1.4 X 10(6) - 2.7 X 10(6) nucleotides. Substantial removal of DNA adducts occurred during the first 2 weeks after carcinogen application while adducts remaining thereafter underwent little or no repair between 2 and 4 weeks after treatment. These results raise the possibility that the persistent adducts occupy specific genomic sites in quiescent cells where they may not be amenable to repair because of localized conformational alterations of DNA or shielding by associated proteins.

Microsomal hydroxylation of 3-methylcholanthrene: analysis by computerized gas chromatography-mass spectrometry

Microsomal metabolism of 3-methylcholanthrene (MC) was examined by computerized gas chromatography-mass spectrometry. Five mono-, four di- and thirteen trihydroxylated metabolites were found after incubation of MC in mouse liver microsomal fraction for 15 min, in the presence of NADPH. Among these metabolites, three mono- and three dihydroxylated metabolites were identified by means of authentic samples. The chemical structures of the other metabolites were deduced from their characteristic mass spectral fragmentations. This is the first description of trihydroxylated metabolites in MC metabolism in vitro and in vivo.

Defective binding of 3-methylcholanthrene to the Ah receptor within C3H/1OT1/2 clone 8 mouse fibroblasts in culture

C3H/1OT1/2 clone 8 mouse fibroblasts (C3H/1OT1/2 cells) exhibit induction of aryl hydrocarbon hydroxylase (cytochrome P1-450) when exposed in culture to benzo(a)pyrene, benz(a)anthracene or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but do not display the induction response when treated with 3-methylcholanthrene (MCA), the classical inducer of cytochrome P1-450. Induction of cytochrome P1-450 is regulated by the Ah receptor which initially binds inducing chemicals in the cytoplasm, after which the inducer x receptor complex translocates into the nucleus. Cytosolic and nuclear forms of the Ah receptor can be detected in C3H/1OT1/2 cells using [3H]TCDD as the radioligand in culture, but specific Ah receptor binding is not detectable within C3H/1OT1/2 cells incubated with [3H]MCA. In contrast, in Hepa-1c1 cells, which exhibit cytochrome P1-450 induction when treated with MCA, cytosolic and nuclear Ah receptor can be detected by incubation of the cells either with [3H]MCA or with [3H]TCDD. Nonradioactive MCA is able to compete with [3H]TCDD for Ah receptor sites in C3H/1OT1/2 cells, but the relative potency of MCA as a competitor is lower within C3H/1OT1/2 cells than in C3H/1OT1/2 cytosol during extracellular incubation. Specific binding of [3H]MCA to Ah receptor can be detected by incubation of [3H]MCA with C3H/1OT1/2 cytosol outside the cell. The selective loss of response to MCA as a cytochrome P1-450 inducer (while retaining response to other inducers) appears to be due to defective interaction of MCA with the Ah receptor within the intracellular environment. The specific molecular alteration which makes the MCA x receptor complex ineffective within C3H/1OT1/2 cells is unknown. Some fibroblast lines other than C3H/1OT1/2 also selectively fail to respond to MCA; thus, this variation in Ah receptor function may not be due to a mutational change in the Ah regulatory gene which codes for the Ah receptor.

Benz[j]aceanthrylene: a novel polycyclic aromatic hydrocarbon with bacterial mutagenic activity

Initial studies on the mutagenicity and metabolism of a novel cyclopenta-PAH, benz[j]aceanthrylene, are reported in the Salmonella bacterial system. The spectrum of activity of benz[j]aceanthrylene over the 5 Ames tester strains is similar to that of benzo[a]pyrene, and the dose-response curves for strain TA98 are comparable. Like other biologically active PAH, benz[j]aceanthrylene is a frame-shift mutagen requiring metabolic activation. An interesting feature of the S9 dependence of activity is the low concentration (congruent to 10-fold smaller than for benzo[a]pyrene) at which optimal activity is observed. The 1,2-dihydro-1,2-diol (product of metabolism of the cyclopenta-ring) appears to be the predominant metabolite, and implicates the 1,2-oxide as the ultimate mutagenic species.

Drug metabolism in adult white leghorn hens--response to enzyme inducers

1. Adult hens were given a mixture of polychlorinated biphenyls (0.5 and 2 g/kg Aroclor 1254), 3-methylcholanthrene (80 mg/kg) or beta-napthoflavone (80 mg/kg). 2. beta-Napthoflavone elevated activities of both microsomal and nonmicrosomal enzymes 48 hr after dosing, with cytochrome P-450 p-nitroanisole O-demethylase, aniline hydroxylase, and glutathione S-epoxytransferase at 319% +/- 25, 157% +/- 12, 410% +/- 26 and 120% +/- 3 of control values, respectively. 3. Aroclor 1254 and 3-methylcholanthrane also elevated microsomal enzyme activities, but did not increase the activity of glutathione S-epoxytransferase. 4. Drug metabolizing capability in control and experimental hens differed from that in rats and mice.

Drug residue formation from ronidazole, a 5-nitroimidazole. II. Involvement of microsomal NADPH-cytochrome P-450 reductase in protein alkylation in vitro

Purified liver microsomal NADPH-cytochrome P-450 reductase is able to catalyze the activation of [14C]ronidazole to metabolite(s) which bind covalently to protein. Like the reaction catalyzed by microsomes, protein alkylation catalyzed by the reductase is (1) sensitive to oxygen, (2) requires reducing equivalents, (3) is inhibited by sulfhydryl-containing compounds and (4) is stimulated several fold by either flavin mononucleotide (FMN) or methytlviologen. A cytochrome P-450 dependent pathway of ronidazole activation can be demonstrated as judged by the inhibition of the reaction by carbon monoxide, metyrapone and 2,4-dichloro-6-phenylphenoxyethylamine but the involvement of specific microsomal cytochrome P-450 isozymes has not been definitively established. Milk xanthine oxidase is also capable of catalyzing ronidazole activation. Polyacrylamide sodium dodecyl sulfate (SDS)-gel electrophoresis reveals that the reactive intermediate(s) of ronidazole does not alkylate proteins selectively.

Benzo(a)pyrene phenol production by perfused rat liver and its inhibition by ethanol

A rapid and inexpensive method has been developed to estimate rates of benzo(a)pyrene phenol production by perfused rat liver. This method is based on the measurement of benzo(a)pyrene phenols utilizing a simple fluorometric procedure. Within 2 to 3 min after infusion of benzo(a)pyrene bound to serum albumin, phenols are excreted into the perfusate, primarily as glucuronide and sulfate conjugates. Maximal rates of phenol release were 8 to 10 nmol/g/hr in livers from control rats and 40 to 42 nmol/g/hr in livers from 3-methylcholanthrene-treated rats. Fasting of 3-methylcholanthrene-treated rats for 24 hr prior to perfusion experiments did not affect either the rate of phenol production or the extent of their conjugation. Ethanol (20 mM) inhibited rates of phenol formation by 50% in livers from fasted, 3-methylcholanthrene-treated rats but had no effect on benzo(a)pyrene hydroxylase activity in isolated hepatic microsomes. These data indicate that ethanol inhibits phenol formation from benzo(a)pyrene in intact liver, probably by diminishing the supply of the cofactor reduced nicotinamide adenine dinucleotide phosphate.

Induction of 6-thioguanine-resistant mutations by rat-liver homogenate (S9)-activated promutagens in human embryonic skin fibroblasts

Most normal human fibroblasts grown in culture do not metabolize promutagens/procarcinogens. Thus screening assays employing normal human fibroblasts have only been successful for direct-acting chemical mutagens and various radiations. In this report we describe a mutation assay (HGPRT locus) employing a normal human embryonic skin fibroblast and a rat-liver homogenate (S9) mixture. 3 model promutagens, benzo[a]pyrene (B[a]P), 3-methylcholanthrene (3MC), and dimethylnitrosamine (DMN) have been utilized in these studies. In addition to discussing conditions for optimizing the response of this assay, our results indicate that at constant amount of S9 protein concentration, there exists a linear correlation between mutagenicity and dose. At 50% survival, the mutant frequencies induced by B[a]P and 3MC (5 micrograms/ml) are 60 and 30 times the background mutant frequency, respectively. Similarly, at 50% survival, DMN (5 mg/ml) induced 6-TGr mutant frequencies are 25-fold over the background frequency. The increase in cytotoxicity resulting from exposure of cells to these 'activated' chemicals is also a linear dose response. At high S9 concentrations a deactivation or detoxification phenomenon occurs. However, the mutagenic efficiency of S9-activated chemicals when plotted as the number of induced mutations versus log survival is unaffected by the deactivating capacity of S9 proteins. This study demonstrates a quantitative mutation assay using an early passage human culture with an exogenous rat-liver microsomal preparation providing activating enzymes.

Fungal oxidation of 3-methylcholanthrene: formation of proximate carcinogenic metabolites of 3-methylcholanthrene

The filamentous fungus, Cunninghamella elegans, was found to metabolize the potent carcinogen, 3-methylcholanthrene (3-MC) to 1-hydroxy-3-MC, 2-hydroxy-3-MC, 1-keto-3-MC, 2-keto-3-MC and trans-9,10-dihydrodiols of 1-hydroxy-3-MC. In addition several unidentified derivatives of 3-MC were found. The metabolites formed were separated by high pressure liquid chromatography (HPLC) and identified by comparison of retention times, absorbance, fluorescence and mass spectra with those of synthetic standards. Incubation of (+/-)-1-hydroxy-3-MC and (+/-)-2-hydroxy-3-MC with cells of C. elegans indicated that 1-hydroxy-3-MC is metabolized to form diasteromerically related trans-9,10-dihydrodiols of 1-hydroxy-3-MC. Experiments with 3-[14C]MC showed that over a 48-h period, 8.7% of the hydrocarbon was oxidized to organic solvent-soluble metabolic products. Most of the metabolites were polar products, some of which co-chromatographed with trans-9,10-dihydrodiols of 1-hydroxy-3-MC. The results show that C. elegans has the ability to oxidize 3-MC to metabolites that have been implicated as proximate carcinogenic forms of 3-MC in higher organisms.

The importance of short term exposure of C3H 10T1/2 cells to polycyclic hydrocarbons: evidence for hydrocarbon-mediated anticarcinogenic activity

Since it has been shown that transformation frequencies (TF) of cultured mammalian cells exposed to polycyclic aromatic hydrocarbons (PAH) reach a maximum with increasing PAH concentration and then decline, we have examined TF in C3H 10T1/2 CL8 (10T1/2) cells as a function of an additional parameter of treatment, length of exposure. A 15-min exposure to either benzo[alpha]pyrene (BP) or 3-methylcholanthrene (3-MC), at concentrations ranging from 0.3 to 10 micrograms/ml, was sufficient to induce transformation suggesting that in 10T1/2 cells, non-induced enzymes of the cytochrome P-450 system are involved in the metabolic activation of PAH. At lower BP concentrations (0.3-1.25 micrograms/ml), TF generally increased with exposure time; at higher BP concentrations (2.5-10 micrograms/ml) maximal TF were achieved with 3 h of exposure. For 3-MC, maximal TF occurred at 0.5-1 h with a concentration of 10 micrograms/ml and at 1-6 h with lower concentrations. Moreover, low TF were obtained after 12-h and 24-h exposures to 10 micrograms/ml BP or 3-MC. These results show that TF depend on both the length of exposure and concentration of PAH. Since both BP and 3-MC are extensively metabolized to polyoxygenated derivatives and conjugates, we suggest that certain metabolites may be anticarcinogenic or antipromoting agents. The identities of such metabolites are yet to be determined.

Effects of particulate matter on rates of membrane uptake of polynuclear aromatic hydrocarbons

Investigations of the mechanism of particle-enhanced uptake of benzo [a] pyrene into model membranes and microsomes have been extended by studying the effects of particulates on the rates of uptake of four additional polynuclear aromatic hydrocarbons (PAH) into model membranes. Adsorption of dibenzo [a, h] anthracene, benzo [g,h,i] perylene, and 3-methylcholanthrene to the surface of particles resulted in enhanced rates of membrane uptake of these PAH. Adsorption to the fibrous asbestos minerals (chrysotile and anthophyllite) produced the greatest enhancement in membrane uptake rates compared to uptake from microcrystalline dispersions of the PAH. Adsorption of these PAH to nonfibrous minerals resulted in uptake rates intermediate between those of the asbestos-adsorbed and microcrystalline states. In contrast, adsorption of dibenzo-[c,g] carbazole to particles did not result in enhanced uptake. In this case, the rates of uptake in the absence and presence of particles were all extremely high, presumably as a result of the high water solubility of this PAH. The methods described in this paper permit characterization of particles-PAH mixtures in terms of the rate of PAH uptake into membranes.

Effect of diet on fecal excretion and gastrointestinal tract distribution of unmetabolized benzo(a)pyrene and 3- methylcholanthrene when these compounds are administered orally to hamsters

3-Methylcholanthrene (3MC) administered p.o. has induced tumors of the hamster gastrointestinal tract (GIT), including the large intestine. This process may depend on the concentration of unchanged hydrocarbon in the GIT contents. Benzo(a)pyrene (BP) ingestion could be involved in human GIT carcinogenesis. Accordingly, male Syrian golden hamsters were fed diets containing BP or 3MC for 10 days. Feces collected during the last two to three days of feeding were analyzed for the unchanged hydrocarbons by KOH:methanol digestion, Florisil column and paper chromatography, and ultraviolet spectrophotometry. With a semisynthetic diet containing 5% Alphacel, 6% corn oil, and 100 microgram BP per g. fecal BP excretion was 0.45% of the dose. Variation of the corn oil content had little effect. Fecal BP excretion was increased 13 times (to 6% of the dose) when 5% wheat brain was used in place of Alphacel and 4.5 times when a commercial diet was used. This suggests that bran adsorbed or sequestered the BP. Water content of the large-intestine contents was increased when the brain diet was fed. Both these factors could affect mucosal exposure to BP. For 3MC, fecal excretion of unchanged hydrocarbon was 14 times greater than for BP under similar conditions. The GIT contents of hamsters fed BP or 3MC showed hydrocarbon concentrations in the order: stomach greater than lower large intestine greater than other sections.

Correlation of induction of aryl hydrocarbon hydroxylase in cultured rat hepatocytes with saturable high-affinity binding of 3-methylcholanthrene to a 4S cytoplasmic protein

The binding of 3-methylcholanthrene (3-MC), a potent inducer of aryl hydrocarbon hydroxylase activity, to cytoplasmic proteins of a cloned rat hepatocyte culture, RL-PR-C, was studied by sucrose gradient centrifugation. Time course and dose-binding experiments performed on late-passage aryl hydrocarbon hydroxylase-inducible cultures indicate the presence of a saturable pool of high-affinity (average Kd, 3.6 nm) binding sites in the cytosol of these cells. The number of binding sites varied from 20,000 to 80,000 per late-passage hepatocyte with a total capacity of approximately 2.2 pmol of 3-MC bound per mg of cytosolic protein. The complex sedimented at 4.0 +/- 0.2S regardless of the ionic strength of the homogenization buffer or gradient solutions. It was sensitive to denaturation by sodium dodecyl sulfate and trypsin but not by DNase I, RNase A, or the nonionic detergent Nonidet P-40. The binding of 3-MC to the protein was inhibited by 1,2-benzanthracene, benzo(a)pyrene, 5,6-benzoflavone, and 7,8-benzoflavone but not by a series of steroids, aflatoxin B1, phenobarbital, or Aroclor 1254. Elevating the temperature of cultures cells to 37 degrees after the standard ligand-binding incubation at 4 degrees resulted in a rapid decrease in cytoplasmic saturable binding and a concomitant increase in nuclear- and chromatin-associated ligand. A portion of this nuclear-associated ligand was extractable with 400 mM KCl. Adsorption of the [3H]-3-MC binding complex by nuclei in vitro suggested that the 4S binding protein facilitated the entry of 3-MC into the nucleus. The presence of the 4S binding species correlated with the level of inducibility of aryl hydrocarbon hydroxylase throughout its development in RL-PR-C and therefore may be involved in the process of induction of this enzyme.