Epoxides in polycyclic aromatic hydrocarbon metabolism and carcinogenesis

Comparison of the effect of butylated hydroxytoluene on N-nitrosomethylurea and 7,12-dimethylbenz[a]-anthracene-induced mammary tumors

The antioxidant butylated hydroxytoluene (BHT) when fed at a level of 0.3% in a defined semi-purified diet was found to decrease mammary tumor incidence in female Sprague-Dawley rats induced by 7,12-dimethylbenz[a]-anthracene (DMBA). however, no effect of BHT on tumor incidence was seen in animals consuming the same diet, under identical experimental conditions, but treated with the carcinogen nitrosomethylurea (NMU). Differences in effectiveness of BHT as a tumor inhibitor in the 2 model systems, and thoughts as to a possible mechanism of action with regard to BHT are discussed.

Polycyclic aromatic hydrocarbons and possible metabolites: convertogenic activity in yeast and tumor initiating activity in mouse skin

The diploid respiratory-deficient strain of yeast D4-RDII was used to assay PAH and urethane as well as some oxygenated derivatives of PAH and the (aliphatic) epoxide hydrolase inhibitor TCPO for convertogenic (mutagenic) activity. As a positive control, the convertogenic ultimate rat liver carcinogen NOAcAAF was used. PAH and urethane were found inactive as convertogens, TCPO was weakly active, whereas oxygenated electrophilic derivatives of PAH, such as K-region oxides, were found strong convertogens. For comparison, some convertogenic key compounds were assayed for their tumor-initiating activity in mouse skin in the standardized system using TPA as a promotor. PAH were stronger initiators than all oxygenated derivatives of PAH tested. TCPO alone exhibited very weak, if any, initiating activity. It was unable to modify initiation to any significant extent, if administered 5 min prior to administration of an initiator. In the absence of correlation between convertogenic and initiating activity the question of the chemical nature of "ultimate initiators" of mouse skin carcinogenesis awaits further investigation.

Metabolic study of 7-methylbenzo[a]pyrene with rat liver microsomes: separation by reversed-phase and normal-phase high performance liquid chromatography and characterization of metabolites

The 7-methylbenzo[a]pyrene (7-MBaP) was incubated with liver microsomes of rats pretreated with polychlorinated biphenyls (Aroclor 1254) (PCBs). Metabolites of 7-MBaP were isolated by both reversed-phase and normal-phage high performance liquid chromatography (HPLC) and were characterized by nuclear magnetic resonance, UV-visible and mass spectral analyses. The predominant metabolite of 7-MBaP was found to be 3-hydroxy-7-methylbenzo[a]pyrene (3-hydroxy-7-MBaP). Other identified metabolites include 7-MBaP 4,5-, 7,8-, and 9,10-trans-dihydrodiols, 7-hydroxymethyl-BaP, 7-hydroxymethyl-BaP trans-9,10-dihydrodiol, 9-hydroxy-7-MBaP, 3-hydroxy-7-hydroxymethyl-BaP, 7-MBaP 1,6- and 3,6- quinones, and a hydroquinone which is also formed by further metabolism of the 3-hydroxy-7-MBaP. Comparative metabolic studies of 7-MBaP and BaP indicated that, relative to that of BaP, the methyl substituent of 7-MBaP slightly increases the formation of 3-hydroxy-7-MBaP and decreases the metabolism at other regions of the 7-MBaP molecule. The finding that a 7,8-dihydrodiol is a metabolite indicates that, like BaP, 7-MBaP may also be activated to the potentially reactive 7,8-dihydrodiol 9,10-epoxides although their formations are significantly reduced.

Heterocyclic polycyclic aromatic hydrocarbon carcinogenesis: 7H-dibenzo[c,g]carbazole metabolism by microsomal enzymes from mouse and rat liver

The metabolism of dibenzo[c,g]carbazole (DBC), was studied in vitro using microsomal fractions of mouse and rat liver from animals, which were treated with 3-methylcholanthrene (MC). The separation of extractable metabolites by high pressure liquid chromatography (HPLC) and thin-layer chromatography (TLC) as well as identification of most of them by nuclear magnetic resonance, mass spectrometry and comparison with synthetically obtained products are described. The microsomes of both species produced the same twelve compounds of which the following have been identified: five monohydroxylated derivatives (phenols), the product of further oxidation of one of them, and a dihydrodiol. The 5-OH-DBC (60% including its spontaneously-formed dimer) and the 3-OH-DBC (14%) are the main metabolites. Three minor metabolites cochromatographed with synthetically prepared 2-OH-DBC, 4-OH-DBC and 6-OH-DBC. The dihydrodiol detectable in small quantity (4-6%) was tentatively identified as 3,4-dihydroxy-3,4-dihydro-DBC by the sensitivity of its formation to very low concentrations of the inhibitor of microsomal epoxide hydrolase, 1,1,1-trichloropropene oxide, by its molecular ion and major fragment in mass spectrometry and by its dehydration product 3-OH-DBC. No other dihydrodiols were detected. The qualitative and quantitative effects of various modulators of metabolism (enzyme inhibitors, apparently homogeneous epoxide hydrolase, glutathione, supernatant fraction) were investigated. The results are discussed with respect to possible ultimate carcinogens.

Regio-selectivity of purified forms of rabbit liver microsomal cytochrome P-450 in the metabolism of benzo(a)pyrene, n-hexane and 7-ethoxyresorufin

The specificity of electrophoretically homogeneous preparations of rabbit liver microsomal cytochrome P-450LM2-4 towards oxygenation of n-hexane, 7-ethoxyresorufin and benzo(a)pyrene was examined using a reconstituted system consisting of cytochrome P-450, NADPH-cytochrome P-450 reductase and dilauroylphosphatidylcholine. Epoxide hydrase was included when benzo(a)pyrene was used as substrate. Cytochrome P-450LM2 was most active in n-hexane and benzo(a)pyrene oxygenation especially with regard to the formation of 2-hexanol, B(a)P-4,5-dihydrodiol and B(a)P-phenol metabolites. 7-Ethoxyresorufin was, however, a very poor substrate for cytochrome P-450LM2. Cytochrome P-450LM3 had less activity towards the investigated substrates while cytochrome P-450LM4 preferentially formed 2- and 3-hexanol, resorufin and B(a)P-9,10-dihydrodiol. Cytochrome P-450LM4 isolated after pretreatment with 3-methylcholanthrene or phenobarbital showed roughly the same characteristics except in the formation of 1-hexanol where cytochrome P-450LM4 isolated after phenobarbital treatment was the most effective. The formation of B(a)P-4,5- and -9,10-dihydrodiols was greatly increased by incorporation of epoxide hydrase. Our results indicate a certain specificity of the different forms of cytochrome P-450 in the liver microsomes although some overlap in activities was observed.

Arylhydrocarbon hydroxylase activity and cytochrome P-450 in human tissues

The stability and distribution of arylhydrocarbon hydroxylase activity in four human tissues has been examined. Two tissues, liver and lung, were obtained from autopsy samples while lymphocytes and placenta were obtained from cell lines and donors. Marked differences in arylhydrocarbon hydroxylase activity were observed between tissues and individuals, with liver being the richest source. Activity in all tissues was stable at 4 degrees C for 24 h, but freeze-thawing markedly reduced hydroxylase activity in liver. Using gel exclusion chromatography, the molecular weight of a non-dissociated form of arylhydrocarbon hydroxylase was estimated to be about 400000. A heme staining band corresponding to a molecular weight of 50000 was observed after polyacrylamide gel electrophoresis of liver microsomal preparations. This appears to be a cytochrome P-450 subunit based on correlations between staining intensity and hydroxylase activity in tissues and partially purified preparations examined.

A twin study of aryl hydrocarbon hydroxylase (AHH) inducibility in cultured lymphocytes

Aryl hydrocarbon hydroxylase (AHH) inducibility was studied in cultured lymphocytes from 28 monozygotic (MZ) and 19 dizygotic (DZ) twin pairs. The results indicate that the induced level of AHH activity as well as inducibility (expressed as the ratio between levels in induced and non-induced cells) are inherited. The best (h2) estimate of heritability is 0.7. There was no suggestion that non-induced AHH activity level is an inherited trait. Inducibility of AHH was not normally distributed and the distribution observed in this limited series might even be trimodal. The results of the study appear to confirm previous reports that AHH inducibility is an inherited trait, and do not exclude the possibility that the major part of the variation is controlled by one locus.

Activation of phenanthrene to mutagenic metabolites and evidence for at least two different activation pathways

Phenanthrene, generally considered to be a non-carcinogen, was converted by mammalian tissue preparations to products that were mutagenic for Salmonella typhimurium TA100 and TA1537. In TA100 the mutagenic response was highly dependent on the activation system used. High amounts of 9000 x g supernatant fraction from the liver of rats induced by Aroclor 1254 were required. Equivalent amounts of microsomal or cytosolic fraction alone did not activate phenanthrene to an observable extent. Furthermore, this activation was only observed when the rats had been treated with Aroclor. Liver preparations from control rats and from rats treated with phenobarbital, beta-naphthoflavone, a mixture of both, and transstilbene oxide failed to activate phenanthrene to mutagens for TA100. Interestingly, liver microsomes and 9000 x g supernatant fractions of Aroclor-treated mice also failed significantly to activate phenanthrene to mutagens for this strain. Addition of pure epoxide hydrolase to the S9 mix had no influence on this activation. Glutathione (GSH) decreased the mutagenicity, but uridine diphosphate glucuronic acid (UDPGA) had only minor effects. An adenosine-3'-phosphate-5'-sulfate phosphate (PAPS) generating system, however, increased the number of his+ revertants from TA100 (2.7-fold). TA1537 was reverted by mutagens produced from phenanthrene by liver microsomes or 9000 x g supernatant fraction, when the microsomal epoxide hydrolase was inhibited by 1,1,1-trichloropropene oxide. This activation pathway exists in Aroclor-treated rats and mice. The results show that at least 2 different pathways for metabolic activation of phenanthrene exist which were observed in 2 differentially sensitive tester strains and distinguished by their different metabolic requirements. Furthermore, the study shows that earlier suggestions do not hold that equivalent results can be obtained by inducing animals with a combination of phenobarbital and beta-naphthoflavone instead of the environmentally persistent Aroclor 1254. Moreover, the study provides a striking example that the use of 9000 x g supernatant in amounts corresponding to standard practice but sub-optimal for a particular compound only impede the detection of a weak mutagen and that the rapid inactivation of active metabolites by inactivating enzymes may be responsible for negative results in mutagenicity testing.

Mutagenicity of structurally related oxiranes: derivatives of benzene and its hydrogenated congeners

The mutagenicities of 17 closely related oxiranes were determined in 4 tester strains (Salmonella typhimurium TA98, TA100, TA1535, TA1537). The test compounds comprised all possible oxides of benzene and its partially hydrogenated congeners. In TA100 and TA1535, 12 of the tested oxiranes were weak to moderate mutagens. 4 of these were also active in TA98. No mutagenicity was observed with the remaining 5 compounds in any of the 4 strains. The presence of a double bond in formal conjugation with the epoxide ring increased the mutagenicity relative to that of the saturated oxirane. Interestingly, additional epoxide rings within the same molecule did not markedly increase the mutagenic activity, and for the oxiranes that are not activated by a double bond, the relationship between mutagenic activity and the number of epoxide rings in the molecule was even inverse. The influence of bromo and hydroxyl substitution on oxirane mutagenicity is discussed. Most notably, a compound having a 4-hydroxyl group in syn position to a 1,2-epoxide ring fused to the cyclohexane ring, a structure which has been suggested to increase the electrophilic reactivity of dihydrodiol epoxides through hydrogen bonding, was almost inactive.

Effect of phenobarbital on the level of translatable rat liver epoxide hydrolase mRNA

Liver poly(A)+RNA isolated from untreated and phenobarbital-treated rats has been translated in the rabbit reticulocyte cell-fre system in order to determine the level of translationally active epoxide hydrolase (EC 3.3.2.3) mRNA. The in vitro translation systems were immunoprecipitated with rabbit IgG prepared against purified epoxide hydrolase, and the amount of epoxide hydrolase synthesized by the lysate programmed with control and phenobarbital poly(A)+RNA was quantitated. The level of translatable epoxide hydrolase mRNA is increased 3-fold after chronic phenobarbital administration. This level of induction correlates well with the 5-fold induction in catalytic activity of epoxide hydrolase (using styrene 7,8-oxide as substrate) in microsomes isolated from phenobarbital-treated rats. Therefore, we suggest that chronic phenobarbital administration increases the amount of functional epoxide hydrolase in rat liver microsomes by way of an increase in the translatable mRNA level encoding for the enzyme. We do not know whether the increase in mRNA is the result of increased transcription or messenger stability.

Significant variation in mouse-skin aryl hydrocarbon hydroxylase inducibility as a function of the hair growth cycle

An easy, rapid and improved technique for homogenizing whole skin is described. This technique consists of reducing skin to a powder in liquid N2 by using a metallic mortar, and homogenizing the powder in a Potter-Elvehjem tube. Using this homogenizing method, we have shown that skin AHH activity in C57BL/6K and C3H/Ico mice can be induced by i.p. injected or topically applied methylcholanthrene during a defined period of the hair growth cycle, i.e. between the 8th and 14th days after depilation (Stage 6 of the anagen period). In each experimental model, there is an optimal methylcholanthrene concentration which yields a maximum induction. Topical methylcholanthrene is also responsible for a smaller aryl hydrocarbon hydroxylase (AHH) induction when the chemical is applied the same day that the club hairs are plucked. On the other hand, skin AHH activity is never induced by methylcholanthrene in DBA/2J mice, a genetically non-responsive strain. No clear-cut segregation of skin AHH inducibility levels is found among the offspring from the back-cross between (C57BL/6J X DBA/2J)F1 and non-inducible DBA/2J mice.

Effect of environmental temperature on naphthalene metabolism by juvenile starry flounder (Platichthys stellatus)

Juvenile starry flounder (Platichthys stellatus) maintained at 4 degrees or 12 degrees C were forced-fed 3H-1-naphthalene. At 24 hr, after the initiation of exposure, significantly (p less than 0.05) higher concentrations (2 to 15 times) of naphthalene were present in tissues of starry flounder at 4 degrees C than those present in fish held at 12 degrees C. The influence of lowering of water temperature on naphthalene retention was even more marked after one week. At this time, muscle and liver of fish at 4 degrees C contained 26 and 34 times, respectively, more naphthalene than did muscle and liver of fish at 12 degrees C. Concentrations of total metabolites, in most tissues were not substantially higher at the lower temperature either 24 or 168 hr after the naphthalene-exposure. Thin-layer chromatographic separation of the metabolites revealed that at 24 hr, 1,2-dihydro-1,2-dihydroxynaphthalene (dihydrodiol) was the major component in liver (40 to 50% of extracted metabolites) and muscle (approximately 80% of extracted metabolites) regardless of the temperature. Bile contained, primarily conjugates (e.g., glucuronides), which yielded the dihydrodiol as the principal metabolite on enzymatic hydrolysis. From 24 to 168 hr, the concentrations of each metabolite class did not vary directly with the concentrations of total metabolites. Accordingly, at 168 hr, the ratio of total metabolite concentrations in liver of fish at 4 degrees C compared to 12 degrees C was 1.6, whereas the ratios for the dihydrodiol, sulfate/glucoside conjugates and glucuronide conjugates were 4.5, 0.6 and 3.8 respectively. Generally, lowered water temperature increased tissue concentrations of the parent hydrocarbon and its metabolites. However, the magnitude of the increase was dependent upon the compound, the tissue, and the time after the initiation of the exposure. The results emphasize the importance of determining concentrations of individual metabolites together with parent hydrocarbons in tissues of fish when assessing effects of environmental parameters on xenobiotic toxicity.

Specificity in interaction of benzo[a]pyrene with nuclear macromolecules: implication of derivatives of two dihydrodiols in protein binding

Benzo[a]pyrene (B[a]P), 7,8-dihydroxy-7,8-dihydro-B[a]P, and 9,10-dihydro-B[a]P are metabolized by hamster embryo cells to derivatives that bind to nuclear macromolecules. The selectivity for different classes of macromolecules varies depending on the compound analyzed. The ratio of DNA specific activity to protein specific activity (pmol bound/mg of macromolecules) is high (1.51) for 7,8-dihydroxy-7,8-dihydro-B[a]P, extremely low (0.03) for 9,10-dihydroxy-9,10-dihydro-B[a]P, and intermediate (0.26) for B[a]P. Histones H3 and H2A are the major targets of 7,8-dihydroxy-7,8-dihydro-B[a]P; a protein(s) with a mobility similar to that of histone H1 is heavily labeled by 9,10-dihydroxy-9,10-dihydro-B[a]P, with minor labeling of other (nonhistone) bands. The labeling pattern seen with B[a]P is a combination of the patterns seen with the two dihydrodiol metabolites studied. Analysis of the ethyl acetate-soluble metabolites suggests that hamster embryo cells produce 9,10-dihydroxy-7,8-oxy-7,8,9,10-tetrahydro-B[a]P from 9,10-dihydroxy-9,10-dihydro-B[a]P and raise the possibility that this vicinal diol epoxide is an intermediate in the binding of 9,10-dihydroxy-9,10-dihydro-B[a]P to nuclear proteins. The differences seen suggest that factors other than the intrinsic chemical reactivity of the epoxide group are extremely important in the interaction of potential ultimate carcinogens with biological systems.

The non-covalent binding of benzo[a]pyrene and its hydroxylated metabolites to intracellular proteins and lipid bilayers

The non-covalent interactions of benzo[a]pyrene (BP) and several of its hydroxylated metabolites with ligandin, aminoazodye-binding protein A (Z-protein, fatty acid binding protein) and lecithin bilayers have been studied by equilibrium dialysis, an adsorption technique and fluorescence spectroscopy. Binding affinities expressed as v/c (where v = moles of BP or BP metabolite bound per mole of protein or lipid and c = unbound concentration), were measured at concentrations sufficiently low that there was no self-association of the unbound compounds as judged by their fluorescence characteristics. 3-Hydroxybenzo[a]pyrene (BP-3-phenol), 4,5-dihydro-4,5-dihydroxybenzo[a]pyrene (BP-4,5-dihydrodiol) and 7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-dihydrodiol) bind more strongly (v/c = 10(5)-5 x 10(5) l x mol-1) to all three binders than does BP itself (v/c = 10(4)-7 x 10(4) l x mol-1). 9,10-Dihydro-9,10-dihydroxybenzo[a]pyrene (BP-9,10-dihydrodiol) binds to ligandin with an affinity similar to those of the other BP metabolites studied here, but binds much less strongly to both protein A and lecithin (v/c = 10(4) and 3 x 10(4) x mol-1, respectively). The low affinity of BP-9,10-dihydrodiol for lecithin would account for earlier findings that on incubation of BP with isolated rat hepatocytes, this metabolite egressed from the cells to the extracellular medium much more readily than either BP-4,5-dihydrodiol or BP-7,8-dihydrodiol. Calculations based on these results suggest that within hepatocytes BP and its metabolites, including BP-9,10-dihydrodiol, will be found almost exclusively associated (> 98%) with lipid membranes.

Biotransformation of allylbenzene analogues in vivo and in vitro through the epoxide-diol pathway

1. The epoxidation of allylbenzene, safrole, estragole, eugenol and eugenol methyl ether was investigated in rats pretreated with these compounds and also in vitro using hepatic microsomal preparations and adult rat liver cell cultures. 2. Dihydrodiols were detected in the urine and liver of rats pretreated with allylbenzene compounds. Similarly, incubation of allylbenzene epoxides with hepatic microsomal preparations and adult rat liver cell cultures gave rise to the formation of dihydrodiols.

Benzo[a]pyrene metabolism and macromolecular binding in strains of Ah responsive and Ah non-response mice

Using liver microsomes as the enzyme source in in vitro assays, benzo[a]pyrene (BP) metabolism was studied in eight inbred strains of mice (C57BL/6J, DBA/2HaD, BALB/cCR, AKR/Sn, RF/J, CBA/J, C57L/J and 129/J). BP metabolite formation was monitored by high pressure liquid chromatography (HPLC) and by following aryl hydrocarbon hydroxylase (AHH) activity. Other parameters measured in cluded the formation of alkali-extractable radioactivity due to [3H] BP metabolites, microsomal protein binding, DNA binding and epoxide hydrase activity. The induction of BP metabolism and binding to macromolecules was studied after treatment of animals with phenobarbital (PB) or 3-methylcholanthrene (MC). Four of the mouse strains (C57BL/6J, BALB/cCr, CBA/J and C57L/J) were highly inducible with respect to liver AHH when pretreated with MC. The induction of AHH by MC in these strains correlated well with the radioactive metabolites of [3H] BP remaining in the alkali extract derived from the AHH assay mixture and with the increased binding of [3H] BP to microsomal protein and DNA. In addition to the eight strains listed above, eight recombinant inbred lines showed a positive correlation between AHH induction and induction of DNA-binding metabolites. PB pretreatment resulted in less than two-fold induction of AHH and alkali-extractable radioactivity. However, DNA and microsomal protein binding were induced by PB pretreatment more than AHH. Ratios of MC-induced/basal activity for BP-phenols were very similar to induction ratios of AHH activity determined by the fluorometric method. BP-quinone formation was induced to the same extent as phenols. This relationship did not hold for dihydrodiol formation; dihydrodiol induction was often higher than AHH or phenol induction. For MC-pretreated mice, dihydrodiol induction, as determined by HPLC, did not parallel macromolecular binding induction as closely as did AHH. For PB-pretreated mice, dihydrodiol induction was as poor as indicator of binding induction as AHH. Epoxide hydrase activity, using styrene oxide as substrate, was induced markedly by PB-pretreatment, but very little by MC-pretreatment. Epoxide hydrase induction did not parallel BP-dihydrodiol induction when microsome preparations from MC- or PB-treated mice were used. These data suggest this enzyme is not rate limiting in this system.

Quantum chemical studies of methylbenz[a]anthracenes: metabolism and correlations with carcinogenicity

In the context of the bay region, K-region and radical cation hypotheses for polycyclic aromatic carcinogens, molecular properties were calculated for fourteen methyl derivatives of benz[a]anthracene (BA) related to (1) intrinsic substrate reactivities towards activating and detoxifying metabolism and (2) the stabilities of the putative carbocation ultimate carcinogens. All-valence electron methods were used, avoiding the inherent difficulties found in the pi-electron methods. The calculated substrate reactivities were found to predict major metabolites successfully, supporting the validity of their use in attempted correlations with observed carcinogenic potencies. Positive correlations were found between observed carcinogenic potencies smf (1) the reactivities of the parent polycyclic aromatic hydrocarbons (PAH) towards the initial distal bay region epoxidation and (2) the stabilities of the diol epoxide carbocations. This latter correlation holds when both the methyl derivatives of BA and previously studied unsubstituted PAH are considered together, indicating its potential usefulness as a screening parameter for carcinogenic activity.

Lipid epoxide hydrolase in rat lung preparations

The activity of rat lung epoxide hydrolase (epoxide hydrolase, EC 3.3.2.3) was studied using two lipid epoxides which can be isolated from lung tissue. These epoxides displayed different Km,app and hydration rates. Methyl cis-9,10-epoxystearate was hydrated 20-times more rapidly than cholest-5 alpha,6 alpha-epoxy-3 beta-ol. The Km for the lung microsomal enzyme was variable and dependent on the microsome concentration in the medium. A soluble epoxide hydrolase was also detected in both lung and liver. This enzyme appears similar to the microsomal enzyme in its activity toward methyl epoxystearate. The measured activities for liver microsomal epoxide hydrolase were over 8-times those for lung microsomes; activity against cholesterol epoxide was 40-times greater for liver. In spite of the slow rates measured with cholesterol epoxide in lung preparations, this compound was an effective competitive inhibitor against methyl epoxystearate over a wide concentration range. This suggests that cholesterol epoxide readily binds to epoxide hydrolase and is an effective competitive inhibitor against a much more actively metabolized substrate, methyl epoxystearate. Such circumstances indicate that cholesterol epoxide binds with a high degree of nonproductivity to lung microsomal epoxide hydrolase. This attribute of lung epoxide hydrolase may relate to the relatively high concentrations of cholesterol epoxide found in lung tissue.

Immunofluorescence of NADPH-cytochrome c (P-450) reductase in rat and minipig tissues injected with phenobarbital

The enzyme NADPH-cytochrome c (P-450) reductase was identified by indirect immunofluorescence in hepatocytes, bronchioles, and proximal tubules of liver, lung, and kidney, respectively, of rats and minipigs that had been injected with phenobarbital or saline. The distribution of this component of the cytochrome P-450-mediated microsomal system may be relevant to sites of drug toxicity and carcinogenesis.

Specificity of mouse liver cytosolic epoxide hydrolase for K-region epoxides derived from polycyclic aromatic hydrocarbons

Mouse liver cytosol epoxide hydrolase, known to be very active for certain alkene oxides, had a specific activity which was 2.1-, 11- and 160-fold lower than that of the microsomal epoxide hydrolase for the arene oxides 7-methylbenz[a]anthracene 5,6-oxide, benz[a]anthracene 5,6-oxide and phenanthrene 9,10-oxide, respectively. For benzo[a]pyrene 4,5-oxide no activity (less than 10 pmol product/mg protein/min) of cytoplasmic epoxide hydrolase was detectable. The specific activity of cytoplasmic epoxide hydrolase was much lower for all K-region epoxides investigated, compared to trans-stilbene oxide used as a positive control and for which a new assay is described. It is concluded from these rates combined with the fact that these lipophilic K-region epoxides are expected to stay preferentially at membranous sites where they are generated, that cytoplasmic epoxide hydrolase plays a minor role for their transformation compared to membrane-bound hydrolase. The data also show that for the substrates investigated the epoxide hydrolase activities in the cytoplasmic and microsomal fractions are complementary to some extent, but there is no quantitative inverse relationship.

Benzo(a)pyrene hydroxylase from Saccharomyces cerevisiae. Substrate binding, spectral and kinetic data

Saccharomyces cerevisiae, brewer's yeast, produces a microsomal benzo(a)pyrene hydroxylase when grown at high glucose concentrations of which the haemoprotein, cytochrome P-450 (RH, reduced-flavoprotein:oxygen oxidoreductase (RH-hydroxylating) EC 1.14.14.1) is a component. We report here kinetic data derived from Lineweaver-Burk plots of benzo(a)pyrene hydroxylation. The Michaelis constant was decreased by growth of the yeast in the presence of benzo(a)pyrene showing the induction of a form of the enzyme more specific for this compound. NADPH or cumene hydroperoxide could be used as cofactors by this enzyme, although with different Km and V values for benzo(a)pyrene. A solubilised and a solubilised, immobilised enzyme preparation were capable of benzo(a)pyrene hydroxylation, using cumene hydroperoxide but not NADPH as the cofactor. Benzo(a)pyrene was found to produce a modified type I spectral change with yeast and rat liver microsomes. The interaction of benzo(a)pyrene with cytochrome P-450 was investigated further by means of an equilibrium gel filtration technique. There appeared to be 20 binding sites per mol ofcytochrome P-450 for benz(a)pyrene, in both yeast and rat liver microsomes.