Biomedical subjects

J M Sayer

Publications and source records attributed to J M Sayer.

At least 55 records · Page 3Linked to original sources

PubMed1986

Mechanism of inactivation of rat liver microsomal cytochrome P-450c by 2-bromo-4'-nitroacetophenone.

The mechanism by which 2-bromo-4'-nitroacetophenone (BrNAP) inactivates cytochrome P-450c, which involves alkylation primarily at Cys-292, is shown in the present study to involve an uncoupling of NADPH utilization and oxygen consumption from product formation. Alkylation of cytochrome P-450c with BrNAP markedly stimulated (approximately 30-fold) its rate of anaerobic reduction by NADPH-cytochrome P-450 reductase, as determined by stopped flow spectroscopy. This marked stimulation in reduction rate is highly unusual in that Cys-292 is apparently not part of the heme- or substrate-binding site, and its alkylation by BrNAP does not cause a low spin to high spin state transition in cytochrome P-450c. Under aerobic conditions the rapid oxidation of NADPH catalyzed by alkylated cytochrome P-450c was associated with rapid reduction of molecular oxygen to hydrogen peroxide via superoxide anion. The intermediacy of superoxide anion, formed by the one-electron reduction of molecular oxygen, established that alkylation of cytochrome P-450c with BrNAP uncouples the catalytic cycle prior to introduction of the second electron. The generation of superoxide anion by decomposition of the Fe2+ X O2 complex was consistent with the observations that, in contrast to native cytochrome P-450c, alkylated cytochrome P-450c failed to form a 430 nm absorbing chromophore during the metabolism of 7-ethoxycoumarin. Alkylation of cytochrome P-450c with BrNAP did not completely uncouple the catalytic cycle such that 5-20% of the catalytic activity remained for the alkylated cytochrome compared to the native protein depending on the substrate assayed. The uncoupling effect was, however, highly specific for cytochrome P-450c. Alkylation of nine other rat liver microsomal cytochrome P-450 isozymes with BrNAP caused little or no increase in hydrogen peroxide formation in the presence of NADPH-cytochrome P-450 reductase and NADPH.

Acetophenones↗

PubMed1986

Chemical modification and inactivation of rat liver microsomal cytochrome P-450c by 2-bromo-4'-nitroacetophenone.

The alkylating agent 2-bromo-4'-nitroacetophenone (BrNAP) binds covalently to each of 10 isozymes of purified rat liver microsomal cytochrome P-450 (P-450a-P-450j) but substantially inhibits the catalytic activity of only cytochrome P-450c. Regardless of pH, incubation time, presence of detergents, or concentration of BrNAP, treatment of cytochrome P-450c with BrNAP resulted in no more than 90% inhibition of catalytic activity. Alkylation with BrNAP did not cause the release of heme from the holoenzyme or alter the spectral properties of cytochrome P-450c, data that exclude the putative heme-binding cysteine, Cys-460, as the major site of alkylation. Two residues in cytochrome P-450c reacted rapidly with BrNAP, for which reason maximal loss of catalytic activity was invariably associated with the incorporation of approximately 1.5 mol of BrNAP/mol of cytochrome P-450c. Two major radio-labeled peptides were isolated from a tryptic digest of [14C]BrNAP-treated cytochrome P-450c by reverse-phase high performance liquid chromatography. The amino acid sequence of each peptide was determined by microsequence analysis, but the identification of the residues alkylated by BrNAP was complicated by the tendency of the adducts to decompose when subjected to automated Edman degradation. However, results of competitive binding experiments with the sulfhydryl reagent 4,4'-dithiodipyridine identified Cys-292 as the major site of alkylation and Cys-160 as the minor site of alkylation by BrNAP in cytochrome P-450c.

Acetophenones↗

PubMed1986

Reactivity and tumorigenicity of bay-region diol epoxides derived from polycyclic aromatic hydrocarbons.

During the past decade substantial progress has been made in elucidating factors that determine the tumorigenic activity of bay-region diol epoxides, major ultimate carcinogenic metabolites derived from polycyclic aromatic hydrocarbons. Neither high nor low chemical reactivity of the diol epoxides (as measured by rates of uncatalyzed solvolysis) is required for high tumorigenic response. In contrast, aspects of molecular structure such as conformation and absolute configuration strongly influence tumorigenic activity. The role of conformation is illustrated by the observation that those diol epoxides whose hydroxyl groups are pseudoaxial are weak or inactive as tumorigens. Absolute configuration is an important determinant of biological activity of bay-region diol epoxides: in all cases studied to date, the predominantly formed (R,S)-diol-(S,R)-epoxides are generally the most tumorigenic of the four metabolically possible configurational isomers. In the course of investigating the effects of structural factors on tumorigenic activity, we identified the (4R,3S)-diol-(2S,1R)-epoxide of benzo(c)phenanthrene as the most potent tumorigen (in initiation-promotion experiments on mouse skin) of the diol epoxides studied to date. Studies of all four configurationally isomeric diol epoxides derived from benzo(c)phenanthrene led to the striking observation that these diol epoxides exhibit an exceptionally high efficiency of covalent binding, relative to hydrolysis, when allowed to react with calf thymus DNA in aqueous solution. Thus, these diol epoxides should provide an excellent tool for the detailed study of such binding. When the four isomeric benzo(c)phenanthrene diol epoxides are compared, there appears to be no simple correlation between tumorigenic response and either the extent of binding to DNA or the major types of deoxyribonucleoside adducts formed. Deoxyribonucleoside adducts of benzo(c)phenanthrene diol epoxide have also been identified from the DNA of cultured rodent embryo cells after treatment of the cells with tritium-labeled benzo(c)phenanthrene. The distribution of adducts is consistent with predominant metabolic formation of the (4R,3S)-diol-(2S,1R)-epoxide; deoxyadenosine is the major site in the cellular DNA attacked by this epoxide, just as it is in DNA in solution. Further experiments are in progress which we hope will identify more subtle aspects of the DNA binding of benzo(c)phenanthrene diol epoxides that may be uniquely correlated with their tumorigenic activity.

Animals↗

PubMed1986

Disposition of the naturally occurring antimutagenic plant phenol, ellagic acid, and its synthetic derivatives, 3-O-decylellagic acid and 3,3'-di-O-methylellagic acid in mice.

The effect of ellagic acid and some of its more lipophilic derivatives on the mutagenicity of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenz[a]pyrene was examined in Salmonella typhimurium TA100. Ellagic acid, 3,3'-di-O-methylellagic acid, 4,4'-di-O-methylellagic acid and 3-O-decylellagic acid were found to have approximately equal antimutagenic activity. The tissue distribution and elimination of ellagic acid, 3,3'-di-O-methylellagic acid and 3-O-decylellagic acid were examined in CD-1 mice. Little or no ellagic acid (less than 1 nmol/g) was found in blood, lung or liver after the oral administration by gavage of 300 mumol of ellagic acid per kg body weight of after feeding 1% of ellagic acid in the diet for 1 week. Following the i.p. administration of 120 mumol/kg of ellagic acid, the blood and lung levels of ellagic acid were 15-20 nmol/g at 30 min after the dose, and the concentrations of ellagic acid decreased to 1-3 nmol/g at 6-8 h after the dose. A portion of the administered i.p. dose precipitated in the abdominal cavity. After i.v. administration, ellagic acid was eliminated very rapidly from blood, lung and liver, and approximately 70% of the administered dose was recovered in the urine and feces as free ellagic acid and its conjugates. At 2 h after an i.v. injection of 60 mumol/kg of ellagic acid, 46% of the dose was recovered in the urine as ellagic acid and its conjugates. Of this amount, about half was excreted as free ellagic acid and half was excreted as conjugates. An additional 25% of the dose was recovered in the feces (mostly as free ellagic acid) after 7 h. The disposition of 3,3'-di-O-methylellagic acid or 3-O-decylellagic acid after i.v. administration (32 mumol/kg) was examined and compared to the disposition of the same i.v. dose of ellagic acid. The concentrations of ellagic acid, 3,3'-di-O-methylellagic acid and 3-O-decylellagic acid decreased rapidly in the blood, liver and lung, but the concentrations of 3-O-decylellagic acid in the lung throughout the experimental period (2-360 min) was on average 20- to 40-fold higher than the corresponding average concentrations of ellagic acid or 3,3'-di-O-methylellagic acid.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1986

Effect of ellagic acid and 3-O-decylellagic acid on the formation of benzo[a]pyrene-derived DNA adducts in vivo and on the tumorigenicity of 3-methylcholanthrene in mice.

The effect of ellagic acid and its more lipophilic derivative, 3-O-decylellagic acid, on the amount of DNA-bound adducts in the epidermis or lung of CD-1 mice treated with [3H]benzo-[a]pyrene ([3H]B[a]P) was evaluated using several different treatment protocols. The i.v. administration of 50 mumol/kg of ellagic acid or 3-O-decylellagic acid either together with or 5 min before a 0.2 mumol/kg i.v. dose of [3H]B[a]P did not inhibit the formation of pulmonary DNA-bound adducts. Feeding mice a diet that contained 1% ellagic acid for 10 days or the i.p. administration of 120 mumol/kg of ellagic acid 30 min before the i.v. administration of 0.2 mumol/kg of [3H]B[a]P did not inhibit the formation of DNA-bound adducts in the lung. The application of 2,500 nmol of ellagic acid or 3-O-decylellagic acid to mouse skin 5 min before the application of 2, 10 or 50 nmol of [3H]B[a]P had little or no effect on the covalent binding of [3H]B[a]P metabolites to epidermal DNA. Feeding mice a diet containing 1% ellagic acid for 10 days did not inhibit the formation of epidermal DNA-bound adducts after a topical dose of 2 nmol of [3H]B[a]P. Similarly, the topical application of 2,500 nmol of ellagic acid at 2 h, 1 h and 5 min before and at 10 min after the application of 2 nmol of [3H]B[a]P did not inhibit the formation of DNA-bound adducts, but the same dosing regimen of 3-O-decylellagic acid (total dose of 10,000 nmol) resulted in a modest inhibition in the formation of DNA-bound adducts. The topical application of 1,500 nmol of ellagic acid 1 h before the application of 1,500 nmol of 3-methylcholanthrene (3-MC) to CD-1 or BALB/c mice twice weekly did not inhibit the development of skin tumors. Our results indicate that ellagic acid and 3-O-decylellagic acid are not effective in inhibiting [3H]B[a]P DNA adduct formation in mouse skin and lung and that ellagic acid does not inhibit 3-MC-induced skin tumorigenesis in BALB/c or CD-1 mice.

Animals↗

PubMed1986

Inhibitory effect of 3-hydroxybenzo(a)pyrene on the mutagenicity and tumorigenicity of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene.

The 12 isomeric phenols of benzo(a)pyrene were tested for their ability to inhibit the mutagenic activity of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene [B(a)P 7,8-diol-9,10-epoxide-2], an ultimate mutagenic and carcinogenic metabolite of benzo(a)pyrene. 3-Hydroxybenzo(a)pyrene [3-HO-B(a)P], a major metabolite of benzo(a)pyrene, was the most potent antagonist tested. Approximately 3 nmol of 3-HO-B(a)P, 14 nmol of 10-HO-B(a)P, and 5-8 nmol of 1-, 2-, 4-, 5-, 6-, 7-, 8-, 9-, 11-, and 12-HO-B(a)P inhibited the mutagenic activity of 0.05 nmol of B(a)P 7,8-diol-9,10-epoxide-2 by 50% in Salmonella typhimurium strain TA 100. The importance of the phenolic group for antimutagenic activity was indicated by the lack of antimutagenic activity of benzo(a)pyrene itself. 3-HO-B(a)P also inhibited the mutagenic activity resulting from the metabolic activation of benzo(a)pyrene and (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene by rat liver microsomes. This inhibition may have resulted from an effect of 3-HO-B(a)P on the metabolic activation of these carcinogens and/or from a direct effect on the action of B(a)P 7,8-diol-9,10-epoxide-2. In a mammalian cell culture system utilizing Chinese hamster V79 cells, 3-HO-B(a)P (8 microM) inhibited the mutagenicity of B(a)P 7,8-diol-9,10-epoxide-2 (0.2 microM) by 50%. Although 3-HO-B(a)P was a potent inhibitor of the mutagenic activity of bay-region diol epoxides of benzo(a)pyrene, dibenzo(a,h)pyrene, and dibenzo(a,i)pyrene in S. typhimurium strain TA 100, higher concentrations of 3-HO-B(a)P were needed to inhibit the mutagenicity of the chemically less reactive benzo(a)pyrene 4,5-oxide and the bay-region diol epoxides of benz(a)anthracene, chrysene, and benzo(c)phenanthrene. Both 3-HO-B(a)P and 10-HO-B(a)P accelerated the disappearance of B(a)P 7,8-diol-9,10-epoxide-2 from 1:9 dioxane-water solutions at pH 7 and 25 degrees C. 3-HO-B(a)P, the most effective antimutagen of the B(a)P phenols tested, was much more reactive with the diol epoxide than 10-HO-B(a)P, the least effective antimutagen. The rate constant for the reaction of 3-HO-B(a)P with the diol epoxide exhibited a nonlinear (greater than first-order) dependence on the concentration of the phenol. Evidence was obtained for covalent adduct formation between the diol epoxide and each of the two phenols.(ABSTRACT TRUNCATED AT 400 WORDS)

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1985

Differential stereoselectivity of cytochromes P-450b and P-450c in the formation of naphthalene and anthracene 1,2-oxides. The role of epoxide hydrolase in determining the enantiomer composition of the 1,2-dihydrodiols formed.

As is the case for cytochrome P-450c, arene 1,2-oxides have been identified as initial metabolites when naphthalene and anthracene are oxidized by cytochrome P-450b in a highly purified, reconstituted system. Overall rates of metabolism by cytochrome P-450b are greater than 3-fold and greater than 50-fold lower than the respective rates of metabolism by cytochrome P-450c. For both hydrocarbons, the (-)-(1S,2R)-oxide predominates (74%) with cytochrome P-450b as the terminal oxidant, based on trapping the labile arene oxides as N-acetyl-L-cysteine S-conjugates of known absolute configuration. This result is in marked contrast to data obtained with cytochrome P-450c where the (+)-(1R,2S)-oxides predominate (73-greater than 95%). In the absence of added epoxide hydrolase, the metabolically formed arene oxides rapidly isomerize to phenols. Addition of increasing amounts of epoxide hydrolase to the incubation medium results in the formation of trans-1,2-dihydrodiols at the expense of phenols from the common arene oxide intermediates. Evaluation of the kinetic parameters (Km and kcat) for the hydration of the (+)- and (-)-enantiomers of both arene oxides by epoxide hydrolase has indicated that the (+)-(1R,2S)-enantiomers exhibit lower values of Km (approximately 1 microM) whereas the values of kcat are similar for both enantiomers of a given arene oxide. These parameters have allowed construction of a mathematical model which predicts the enantiomer composition of the dihydrodiols formed from naphthalene in reconstituted systems containing specific epoxide hydrolase concentrations. The data reported argue against a selective functional coupling mechanism between cytochrome P-450c and epoxide hydrolase in the metabolism of naphthalene and anthracene to the 1,2-dihydrodiols.

Animals↗

PubMed1985

Stereoselectivity of microsomal epoxide hydrolase toward diol epoxides and tetrahydroepoxides derived from benz[a]anthracene.

We have examined the selectivity of rat liver microsomal epoxide hydrolase (EC 3.3.2.3) toward all of the possible positional isomers of benzo-ring diol epoxides and tetrahydroepoxides of benz[a]anthracene, as well as the 1,2-diol 3,4-epoxides of triphenylene. This set includes compounds with no bay region in the vicinity of the benzo-ring, a bay-region diol group, a bay-region epoxide group, and (for the triphenylene derivatives) both a bay-region diol and a bay-region epoxide. In all cases where both the tetrahydroepoxides and the corresponding diol epoxides were examined, there is a large retarding effect of hydroxyl substitution on the rate of the enzyme-catalyzed hydration. When the tetrahydroepoxides are fair or poor substrates (epoxide group in the 1,2-, 8,9-, or 10,11-position), the additional retardation introduced by adjacent hydroxyl groups causes the enzyme-catalyzed hydrolysis of the corresponding diol epoxides to be insignificantly slow or nonexistent. In contrast, a benz[a]anthracene derivative with an epoxide group in the 3,4-position, (-)-tetrahydrobenz[a]anthracene (3R,4S)-epoxide, has been identified as the best substrate known for epoxide hydrolase, with a Vmax at 37 degrees C and pH 8.4 of 6800 nmol/min/mg of protein, and the two diastereomeric (+/-)-benz[a]anthracene 1,2-diol 3,4-epoxides, unlike all the other diol epoxides examined to date, are moderately good substrates for epoxide hydrolase. This novel observation is accounted for by the fact that the very high reactivity of the tetrahydrobenz[a]anthracene 3,4-epoxide system towards epoxide hydrolase is large enough to overcome a kinetically unfavorable effect of hydroxyl substitution. The enantioselectivity and positional selectivity of the enzyme have been determined for the tetrahydro-1,2- and -3,4-epoxides of benz[a]anthracene as well as the 1,2-diol 3,4-epoxides. When the epoxide is located in the 3,4-position, the benzylic carbon is the preferred site of attack, whereas for the enantiomers of the bay-region tetrahydro-1,2-epoxides, the chemically less reactive non-benzylic carbon is preferred. The regio- and enantioselectivity of epoxide hydrolase are discussed in terms of a possible model for the hydrophobic binding site of this enzyme.

Animals↗

PubMed1985

Inhibition of the mutagenicity of bay-region diol-epoxides of polycyclic aromatic hydrocarbons by tannic acid, hydroxylated anthraquinones and hydroxylated cinnamic acid derivatives.

Tannic acid and several hydroxylated anthraquinone and cinnamic acid derivatives inhibited the mutagenic activity of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene (B[a]P 7,8-diol-9,10-epoxide-2), an ultimate mutagenic and carcinogenic metabolite of benzo [a]pyrene. The mutagenic activity of 0.05 nmol of B[a]P 7,8-diol-9,10-epoxide-2 towards strain TA 100 of Salmonella typhimurium was inhibited 50% by incubation of the bacteria and the diol-epoxide with tannic acid (0.5 nmol), anthraflavic acid (7 nmol), rufigallol (7 nmol), quinalizarin (10 nmol), alizarin (30 nmol), purpurin (60 nmol), and danthron (88 nmol). Dose-dependent, but weaker antimutagenic activity was observed for quinizarin, and a number of hydroxylated cinamic acid derivatives. Gallic acid and m-digallic acid, major components of tannic acid, possessed less than 1% of the anti-mutagenic activity of tannic acid, although m-digallic acid was over 3 times more active than gallic acid. The anti-mutagenic activity of tannic acid was a result of its interaction with B[a]P 7,8-diol-9,10-epoxide-2 since the rate of disappearance of the diol-epoxide from cell-free solutions in 1:9dioxane:water was markedly stimulated by the polyphenol. Tannic acid was a highly potent inhibitor of the mutagenic activity of the bay-region diol-epoxides of benzo[a]pyrene, dibenzo[a,h]pyrene and dibenzo[a,i]pyrene, but higher concentrations of tannic acid were needed to inhibit the mutagenicity of the chemically less reactive benzo[a]-pyrene 4,5-oxide and the bay-region diol-epoxides of benz[a]-anthracene, chrysene and benz[c]phenanthrene.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1985

Effect of ellagic acid and hydroxylated flavonoids on the tumorigenicity of benzo[a]pyrene and (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene on mouse skin and in the newborn mouse.

Ellagic acid, quercetin and robinetin were tested for their ability to antagonize the tumor-initiating activity of benzo[a]pyrene (B[a]P) and (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P 7,8-diol-9,10-epoxide-2), the ultimate carcinogenic metabolite of benzo[a]-pyrene. Ellagic acid, robinetin or quercetin (2500 nmol) had no tumor-initiating activity on mouse skin, but the topical application of 2500 nmol of ellagic acid 5 min before a tumor-initiating dose of 200 nmol of B[a]P 7,8-diol-9,10-epoxide-2 caused a 59-66% inhibition in the number of skin tumors per mouse that were observed after 15-20 weeks of promotion with 12-O-tetradecanoylphorbol-13-acetate. Similar treatment with 2500 nmol of robinetin or quercetin caused a statistically insignificant 16-24% inhibition in the tumor-initiating activity of 200 nmol of B[a]P 7,8-diol-9,10-epoxide-2 applied 5 min later. Treatment of mice with 2500 nmol of ellagic acid 5 min before the application of 50 nmol of B[a]P inhibited the mean number of skin tumors per mouse by 28-33% after 15-20 weeks of promotion, but these decreases were not statistically significant. Robinetin and quercetin had little or no effect on the tumor-initiating activity of B[a]P on mouse skin. Treatment of preweanling mice with 1/7, 2/7 and 4/7 of the total dose of ellagic acid (300 nmol), robinetin (1400 nmol), myricetin (1400 nmol) or quercetin (1400 nmol) i.p. on their first, eighth and fifteenth day of life, respectively, did not cause the formation of tumors in animals that were killed 9-11 months later. Similar treatment of preweanling mice with the above doses of the phenolic compounds 10 min before the i.p. injection of a total dose of 30 nmol of B[a]P 7,8-diol-9,10-epoxide-2 during the animal's first 15 days of life caused a 44-75% inhibition in the number of diol-epoxide-induced pulmonary tumors per mouse. Similar treatment with these plant phenols had little or no effect on B[a]P-induced pulmonary tumors.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1984

Effects of a 6-fluoro substituent on the metabolism of benzo(a)pyrene 7,8-dihydrodiol to bay-region diol epoxides by rat liver enzymes.

Metabolism of trans-7,8-dihydroxy-7,8-dihydro-6-fluorobenzo(a)pyrene by liver microsomes from 3-methylcholanthrene-treated rats and by a highly purified monooxygenase system, reconstituted with cytochrome P-450c, has been examined. Although both the fluorinated and unfluorinated 7,8-dihydrodiol formed from benzo(a)pyrene by liver microsomes share (R,R)-absolute configuration, the fluorinated dihydrodiol prefers the conformation in which the hydroxyl groups are pseudodiaxial due to the proximate fluorine. The fluorinated 4,5- and 9,10-dihydrodiols are also greater than 97% the (R,R)-enantiomers. For benzo(a)pyrene, metabolism of the (7R,8R)-dihydrodiol to a bay-region 7,8-diol-9,10-epoxide in which the benzylic hydroxyl group and epoxide oxygen are trans constitutes the only known pathway to an ultimate carcinogen. With the microsomal and the purified monooxygenase system, this pathway accounts for 76-82% of the total metabolites from the 7,8-dihydrodiol. In contrast, only 32-49% of the corresponding diol epoxide is obtained from the fluorinated dihydrodiol and this fluorinated diol epoxide has altered conformation in that its hydroxyl groups prefer to be pseudodiaxial. Much smaller amounts of the diastereomeric 7,8-diol-9,10-epoxides in which the benzylic hydroxyl groups and the epoxide oxygen are cis are formed from both dihydrodiols. As the fluorinated diol epoxides are weaker mutagens toward bacteria and mammalian cells relative to the unfluorinated diol epoxides, conformation appears to be an important determinant in modulating the biological activity of diol epoxides. One of the more interesting metabolites of 6-fluorinated 7,8-dihydrodiol was a relatively stable arene oxide, probably the 4,5-oxide, which is resistant to the action of epoxide hydrolase.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1984

Stereoselectivity of cytochrome P-450c in the formation of naphthalene and anthracene 1,2-oxides.

Absolute configurations of the arene 1,2-oxides formed from napththalene and anthracene by cytochrome P-450c, the predominant isozyme of cytochrome P-450 found in the livers of rats treated with 3-methylcholanthrene, were determined via two different approaches. The first consisted of trapping the arene oxides with N-acetyl-L-cysteine to form S-conjugates, methylation of the conjugates with diazomethane, and separation of the resulting diastereomeric esters by reversed phase high performance liquid chromatography. Analysis by this procedure of the arene oxides formed from radioactive naphthalene and anthracene by a highly purified and reconstituted monooxygenase system containing cytochrome P-450c indicated that 73 and greater than or equal to 95%, respectively, of the metabolically formed arene oxides consisted of the (+)-(1R,2S)-enantiomer. In the second approach, each hydrocarbon was incubated with a reconstituted system containing both cytochrome P-450c and epoxide hydrolase. Under these conditions, the predominant metabolites are trans-1,2-dihydrodiols formed by epoxide hydrolase catalyzed trans-addition of water to the arene oxide intermediates. In both cases, the (-)-(1R,2R)-dihydrodiols predominated; 92% for naphthalene and 99% for anthracene. Enzyme-catalyzed addition of water to (+)- and (-)-anthracene 1,2-oxide and (+)-napthalene 1,2-oxide occurred exclusively (greater than 99%) at the allylic 2-position. The (-)-(1S,2R)-naphthalene 1,2-oxide, however, is converted to a 40:60 mixture of the (-)-(1R,2R)- and (+)-(1S,2S)-dihydrodiols by benzylic and allylic attack, respectively, resulting in increased enantiomeric purity of the dihydrodiol relative to the oxide. Thus, qualitatively and quantitatively both approaches indicate that the (+)-arene (1R,2S)-oxides predominate. The results are discussed in terms of the steric constraints of a proposed model for the catalytic binding site of cytochrome P-450c.

Animals↗

PubMed1984

Mutagenicity of the enantiomers of the diastereomeric bay-region benzo(c)phenanthrene 3,4-diol-1,2-epoxides in bacterial and mammalian cells.

The mutagenic activities of the enantiomers of the pair of diastereomeric bay-region benzo(c)phenanthrene 3,4-diol-1,2-epoxides were evaluated in histidine-dependent strains of Salmonella typhimurium and in an 8-azaguanine-sensitive Chinese hamster cell line. In strains TA 98 and TA 100 of S. typhimurium, the range in mutagenic activity observed for the four optically active isomers was less than 4- and 2-fold, respectively. The diol-epoxide with (1S,2R,3R,4S) absolute configuration and the benzylic hydroxyl group trans to the epoxide oxygen [(+)-diol epoxide-2] was the most active isomer in both strains. The enantiomeric (-)-diol-epoxide-2 isomer, with (1R,2S,3S,4R) absolute configuration identical to that of the exceptionally tumorigenic (+)-diol-epoxide-2 isomers of benzo(a)pyrene, benz(a)anthracene, and chrysene, was the least active isomer in strain TA 98 (27%) and the second most active isomer in strain TA 100 (90%). In Chinese hamster V79 cells (-)-diol-epoxide-2 was the most active of the four benzo(c)phenanthrene isomers, and a 4- to 5-fold range in mutagenic activity was observed. The differences in mutagenic activity between the four bay-region diol-epoxide isomers of benzo(c)phenanthrene in the three test systems are relatively small when compared with results from similar studies with optically active bay-region diol-epoxide isomers of three other polycyclic aromatic hydrocarbons, and may be explicable, in part, by a tendency of the hydroxyl groups of benzo(c)phenanthrene diol-epoxides to adopt comparable pseudodiequatorial conformations.

Animals↗

PubMed1983

Inhibition of the mutagenicity of bay-region diol-epoxides of polycyclic aromatic hydrocarbons by phenolic plant flavonoids.

Myricetin, robinetin and luteolin inhibited the mutagenic activity resulting from the metabolic activation of benzo[a]-pyrene and (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]-pyrene by rat liver microsomes. These naturally occurring plant flavonoids and seventeen additional flavonoids and related derivatives with phenolic hydroxyl groups inhibited the mutagenic activity of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (B[a]P 7,8-diol-9,10-epoxide-2), which is an ultimate mutagenic and carcinogenic metabolite of benzo[a]pyrene. Several flavonoids without phenolic hydroxyl groups or with methylated phenolic hydroxyl groups were inactive. The mutagenic activity of 0.05 nmol of BP 7,8-diol-9,10-epoxide-2 towards strain TA 100 of S. typhimurium was inhibited 50% by incubation of the bacteria and the diol-epoxide with myricetin (2 nmol), robinetin (2.5 nmol), luteolin (5 nmol), quercetin (5 nmol), 7-methoxyquercetin (5 nmol), rutin (5 nmol), quercetin (5 nmol), delphinidin chloride (5 nmol), morin (10 nmol), myricitrin (10 nmol), kaempferol (10 nmol), diosmetin (10 nmol), fisetin (10 nmol), or apigenin (10 nmol). Considerably less antimutagenic activity was observed for dihydroquercetin, naringenin, robinin, D-catechin, genistein, kaempferide and chrysin. Pentamethoxyquercetin, tangeretin, nobiletin, 7,8-benzoflavone, 5,6-benzoflavone, and flavone, which lack free phenolic groups, were inactive. The antimutagenic activity of hydroxylated flavonoids results from their direct interaction with B[a]P 7,8-diol-9,10-epoxide-2 since the rate of disappearance of the diol-epoxide from cell-free solutions in 1:9 dioxane:water was markedly stimulated by myricetin, robinetin and quercetin. Myricetin was a highly potent inhibitor of the mutagenic activity of bay-region diol-epoxides of benzo[a]pyrene, dibenzo[a,h]pyrene and dibenzo[a,i]pyrene, but higher concentrations of myricetin were needed to inhibit the mutagenicity of the chemically less reactive benzo[a]pyrene 4,5-oxide and bay region diol-epoxides of benz[a]anthracene, chrysene and benzo[c]phenanthrene.

Animals↗

PubMed1983

Regioselectivity and stereoselectivity in the metabolism of trans-1,2-dihydroxy-1,2-dihydrobenz[a]anthracene by rat liver microsomes.

Metabolism of [3H]-(+/-)-trans-1,2-dihydroxy-1,2-dihydrobenz[a] anthracene by liver microsomes isolated from control, phenobarbital-treated, and 3-methylcholanthrene-treated Long-Evans rats and from 3-methylcholanthrene-treated Sprague-Dawley rats was examined. Liver microsomes from both control and phenobarbital-treated rats metabolized the dihydrodiol at a rate of 0.5 nmol/nmole of cytochrome P450 per minute, whereas prior treatment of rats with 3-methylcholanthrene stimulated the rate of metabolism by 4-fold. Prior treatment of the rats caused marked differences in the regio- and stereoselectivity of the metabolism of this pseudo-diaxial dihydrodiol. In each case, the major metabolites were three bis-dihydrodiols and a pair of diastereomeric 1,2-diol-3,4-epoxides in which the benzylic 1-hydroxyl group is either cis or trans to the epoxide oxygen (diol epoxides-1 and -2, respectively). The presence of the diol epoxides in the incubation medium was inferred from the identification of their corresponding tetraols, which arise by hydrolysis of the diol epoxides on chromatography. Hepatic microsomes from control and phenobarbital-treated rats metabolized the 1,2-dihydrodiol predominantly to 1,2-diol-3,4-epoxides (68-85% of the total metabolites) whereas bis-dihydrodiols represented 28% and 13% of the total metabolites, respectively. In contrast, liver microsomes from 3-methylcholanthrene-treated rats of either strain metabolized the 1,2-dihydrodiol primarily to isomeric bis-dihydrodiols (51-56% of total metabolites), with diol epoxides accounting for only 36-38% of the total metabolites. Bis-dihydrodiol-1 (32-35% of the total metabolites) was formed in greater amounts (2- to 4-fold) than either bis-dihydrodiols-2 or -3, which were formed in about equal amounts and have identical absorption spectra. The ratio of the diastereomeric 1,2-diol-3,4-epoxides-1 and -2 was highly dependent upon the preparation used. For microsomes from control and phenobarbital-treated rats, this ratio was between 3:1 and 4:1 whereas microsomes from 3-methylcholanthrene-treated rats (greater than 70% cytochrome P-450c) gave a ratio of between 1:1.5 and 1:2. The basis for this ratio in the latter case was explained by examination of the products formed from the (+)-(1S,2S)-and (-)-(1R,2R)-enantiomers of the dihydrodiol on metabolism by a highly purified system reconstituted with cytochrome P-450c. The (-)-isomer is a 3-fold better substrate than the (+)-isomer and forms only the diol epoxide-2 diastereomer, whereas the (+)-isomer forms much more diol epoxide-1 than diol epoxide-2 diastereomer.

Animals↗

PubMed1982

Mechanism of the inhibition of mutagenicity of a benzo[a]pyrene 7,8-diol 9,10-epoxide by riboflavin 5'-phosphate.

Riboflavin 5'-phosphate (flavin mononucleotide; FMN) inhibits the mutagenicity of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P diol epoxide), the only known ultimate carcinogenic metabolite of benzo[a]pyrene. Coincubation of 10, 25, and 50 nmol of FMN with strain TA100 of histidine-dependent Salmonella typhimurium inhibits the mutagenicity of 0.05 nmol of the diol epoxide by 50, 70, and 90%, respectively. Ribose 5-phosphate and riboflavin show no significant effects at comparable doses. Reaction of B[a]P diol epoxide with FMN in aqueous solution at neutral pH produces only tetraols, with no evidence for covalent adducts. At pH 7 the rate of hydrolysis of B[a]P diol epoxide in dioxane/water, 1:9 (vol/vol), at 25 degrees C is increased more than 10-fold in the presence of 100 muM FMN. Spectrophotometric studies and quantitative rate data for the reaction of the diol epoxide with FMN indicate that a complex is formed between the diol epoxide and the flavin moiety of FMN (Ke = 1,400-3,400 M-1) prior to general acid-catalyzed hydrolysis of the epoxide to tetraols by the phosphate monoanion of FMN. Comparable concentrations of ribose 5-phosphate and riboflavin do not significantly increase the rate of hydrolysis, although evidence for complex formation between riboflavin and the diol epoxide is observed. General acid-catalyzed hydrolysis of bay-region polycyclic hydrocarbon diol epoxides by compounds that have a high affinity for these ultimate carcinogens represents a potentially useful way of inhibiting their carcinogenic activity.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗

PubMed1982

Inhibition of the mutagenicity of bay-region diol epoxides of polycyclic aromatic hydrocarbons by naturally occurring plant phenols: exceptional activity of ellagic acid.

Ferulic, caffeic, chlorogenic, and ellagic acids, four naturally occurring plant phenols, inhibit the mutagenicity and cytotoxicity of (+/-)-7beta,8alpha-dihydroxy-9alpha, 10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P 7,8-diol-9,10-epoxide-2), the only known ultimate carcinogenic metabolite of benzo[a]pyrene. The mutagenicity of 0.05 nmol of B[a]P 7,8-diol-9,10-epoxide-2 in strain TA100 of Salmonella typhimurium is inhibited 50% by incubation of the bacteria and the diol epoxide with 150 nmol of ferulic acid, 75 nmol of caffeic acid, 50 nmol of chlorogenic acid or, most strikingly, 1 nmol of ellagic acid in the 0.5-ml incubation mixture. A 3-nmol dose of ellagic acid inhibits mutation induction by 90%. Ellagic acid is also a potent antagonist of B[a]P 7,8-diol-9,10-epoxide-2 in Chinese hamster V79 cells. Mutations to 8-azaguanine resistance induced by 0.2 muM diol epoxide are reduced by 50% when tissue culture media also contains 2 muM ellagic acid. Similar to results obtained with the bacteria, ferulic, caffeic, and chlorogenic acids are approximately two orders of magnitude less active than ellagic acid in the mammalian cell assay. The antimutagenic effects of the plant phenols result from their direct interaction with B[a]P 7,8-diol-9,10-epoxide-2, because a concentration-dependent increase in the rate of diol epoxide disappearance in cell-free solutions of 1:9 dioxane/water, pH 7.0, is observed with all four phenols. In parallel with the mutagenicity studies, ellagic acid is 80-300 times more effective than the other phenols in accelerating the disappearance of B[a]P 7,8-diol-9,10-epoxide-2. Ellagic acid at 10 muM increases the disappearance of B[a]P 7,8-diol-9,10-epoxide-2 by approximately 20-fold relative to the spontaneous and hydronium ion-catalyzed hydrolysis of the diol epoxide at pH 7.0. Ellagic acid is a highly potent inhibitor of the mutagenic activity of bay-region diol epoxides of benzo[a]pyrene, dibenzo[a,h]pyrene, and dibenzo[a,i]pyrene, but higher concentrations of ellagic acid are needed to inhibit the mutagenic activity of the chemically less reactive bay-region diol epoxides of benz[a]anthracene, chrysene, and benzo[c]phenanthrene. These studies demonstrate that ellagic acid is a potent antagonist of the adverse biological effects of the ultimate carcinogenic metabolites of several polycyclic aromatic hydrocarbons and suggest that this naturally occurring plant phenol, normally ingested by humans, may inhibit the carcinogenicity of polycyclic aromatic hydrocarbons.

Animals↗

PubMed1981

Highly tumorigenic bay-region diol epoxides from the weak carcinogen benzo[c]phenanthrene.

In the four years since its inception, the bay-region theory has proved highly successful in predicting which diol epoxide of a polycyclic aromatic hydrocarbon would have the highest tumorigenic activity. The present studies on benzo[c]phenanthrene have shown this hydrocarbon to be unique. It is the first hydrocarbon for which the bay-region diol epoxide that has its benzylic hydroxyl group and epoxide oxygen cis (isomer-1 series) has significant tumorigenic activity. Additionally, its bay-region diol epoxides are the most tumorigenic diol epoxides yet tested on mouse skin despite their expected and observed very low chemical reactivity. Perhaps some unique feature of the shape of benzo[c]phenanthrene can account for the remarkable biological activity of its bay-region diol epoxides. The high degree of crowding in the bay-region of benzo[c]phenanthrene may be such a contributing factor. It is know, for example, that methyl-substitution in the bay-region but not on the critical benzo-ring enhances the tumorigenic activity of 7,12-dimethylbenzo[a]anthracene relative to 7-methylbenzo[a]anthracene (Newman, 1976), of 5-methylchrysene relative to chrysene (Hecht et al., 1974), and of 11-methylbenzo[a]pyrene relative to benzo[a]pyrene (Iyer et al., 1980). Steric crowding in the bay-region of benzo[c]phenanthrene (Hirshfeld, 1963) and 7,12-dimethylbenzo[a]anthracene has been shown by x-ray crystallography to cause out-of-plane deformation of their aromatic ring systems.

Animals↗