Conversion of benzo[a]pyrene-3,6-quinone to quinol glucuronides with rat liver microsomes or purified NADPH-cytochrome c reductase and UDP-glucuronosyltransferase

DT-Diaphorase Affects the Mutagenic Activity of Pyrene 1,6-Quinone in Chinese Hamster Epithelial Liver (CHEL) Cells

The mutagenic potential of pyrene 1,6-quinone (P 1,6-Q) has been studied in a wide range of in vitro genetic assays including the use of mammalian cell lines. P 1,6-Q has been shown to induce gene mutations and micronuclei in V79 cells, whereas, in Chinese hamster epithelial liver (CHEL) cells, a cell line which retains activities of various xenobiotic-metabolising enzymes, a non-specific pattern of structural and numerical chromosomal aberrations has been observed. In this study, we have evaluated the mutagenic activity of P 1,6-Q on V79 and CHEL cells both with and without dicoumarol, a potent inhibitor of DT-diaphorase. In V79 cells, dicoumarol (100μM) did not affect the mutagenic response, whereas in CHEL cells, the mutation frequency significantly increased. This suggests that DT-diaphorase, which is expressed in liver cells at high levels, has a possible role in the detoxification of P 1,6-Q to redox-stable hydroquinone.

Radiation inactivation analysis of microsomal UDP-glucuronosyltransferases catalysing mono- and diglucuronide formation of 3,6-dihydroxybenzo(a)pyrene and 3,6-dihydroxychrysene

Indirect evidence has suggested that multiple subunits of microsomal UDP-glucuronosyltransferases (UGTs) are involved in diglucuronide formation of diphenols of polycyclic aromatic hydrocarbons (Bock et al., Mol Pharmacol 42: 613-618, 1992). To substantiate this suggestion functional target sizes of UGTs catalysing these reactions were determined in microsomes in situ by radiation inactivation analysis. Target sizes of UGTs catalysing the glucuronidation of 1-naphthol and 6-hydroxychrysene were found to be 91 +/- 29 and 120 +/- 27 kDa, respectively. However, target sizes for mono- and diglucuronide formation of 3,6-dihydroxybenzo(a)pyrene were 118 +/- 33 and 218 +/- 24 kDa, respectively. Similarly, using 3,6-dihydroxychrysene as substrate target sizes of 109 +/- 21 and 101 +/- 23 kDa were found for 6-O-monoglucuronide and 3-O-monoglucuronide formation and a target size of 192 +/- 34 kDa observed for diglucuronide formation. Based on subunit molecular masses of 50-60 kDa for UGTs, these results suggest that UGTs involved in monoglucuronide formation of phenols may function as dimers. In contrast, UGTs involved in diglucuronide formation of diphenols of polycyclic aromatic hydrocarbons may function as tetramers in microsomes in situ.

Reactive intermediates and their toxicological significance

Many chemicals that cause toxicity do so via metabolism to biologically reactive metabolites. However, the nature of the interaction between such reactive metabolites and various cellular components, and the mechanism(s) by which these interactions eventually lead to cell death are poorly understood. The relative importance of macromolecular alkylation (covalent binding), lipid peroxidation, alterations in thiol, calcium and energy homeostasis are discussed with reference to specific toxicants. It is concluded that the cytotoxic effects of reactive metabolites are a consequence of simultaneous and/or sequential alterations in several cellular processes. Further studies are required to determine the relationship between these alterations and cell death.

Metabolism and cytotoxicity of benzo(a)pyrene in the human lung tumour cell line NCI-H322

1. The human lung tumour cells NCI-H322 metabolized benzo(a)pyrene (BP) at a rate of 160 pmol/10(6) cells/h for at least 8 h. About 30% of the total metabolites were water-soluble, 30% of which were conjugates with glutathione. The water-soluble fraction also contained BP sulphates but no BP glucuronides. 2. The cytotoxic potency of BP and its metabolites, 3-hydroxybenzo(a)pyrene (3-OH-BP) and (+/-) anti-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene (7,8-diol-BP), differed by an order of magnitude with the ranking 7,8-diol-BP greater than BP greater than 3-OH-BP. The cytotoxicity of BP was not detected at the end of a 24 h treatment period but became increasingly apparent at later times. In contrast, the cytotoxicity of 3-OH-BP was observed immediately after the treatment period and did not increase greatly thereafter. 7,8-Diol-BP caused both 'immediate' and 'late' effects. 3. The time course and concentration dependence suggested that the cytotoxicity of BP in NCI-H322 cells was not attributable to the formation of 3-OH-BP, but more likely resulted from the formation of other products such as 7,8-diol-BP.

Effects of superoxide generated in vitro on glucuronidation of benzo[a]pyrene metabolites by mouse liver microsomes

Glucuronidation of benzo[a]pyrene (B[a]P) metabolites, generated in situ by oxidative pathways, was studied using mouse liver uninduced microsomes. No coupling was evident between UDP-glucuronyltransferase and oxygenation of B[a]P. UDPGA protected microsomal macromolecules against binding of reactive B[a]P metabolites. Superoxide, and other reactive oxygen species decreased both the overall B[a]P metabolism and glucuronidation of some B[a]P metabolic products, and caused more extensive binding to macromolecules; UDPGA was less protective in this condition. Peroxidation of microsomes differentially affected glucuronidation of various metabolites of B[a]P, and of various model substrates, indicating that multiple glucuronyltransferases are involved in the conjugation of hydroxylated metabolites of B[a]P.

Interaction between vitamin K3 and benzo(a)pyrene metabolism in uninduced microsomes

1. Relationship between quinone recycling, glucuronidation and benzo(a)pyrene (BaP) oxygenation was investigated in uninduced mouse liver microsomes--native and modified by Fe3+.FeEDTA and/or superoxide (O2-.)-initiated lipid peroxidation. 2. A functional coupling between glucuronidation of reduced quinones and BaP metabolism, not discernible during BaP metabolism by native uninduced microsomes, was demonstrable in the presence of a model quinone, vitamin K3 (menadione). 3. Menadione inhibited BaP oxygenation in microsomal preparations, by siphoning off electrons from cytochrome P-450, while addition of UDPGA reversed this effect by glucuronidation of menadiol. 4. Fe3+.FeEDTA and/or O2-.-initiated lipid peroxidation decreased, to different extent, the microsomal enzymatic activities involved in quinone metabolism. The most sensitive was quinone reductase activity, which was reduced by 77%. Under peroxidative conditions menadione was a less effective inhibitor of BaP metabolism. 5. The important role of the balance between quinone reductase and UDP-glucuronyltransferase activities in the coupling with BaP oxygenation is discussed. A mechanism by which vitamin K3 could exert a regulatory effect on BaP metabolism is proposed.

Dicoumarol-sensitive glucuronidation of benzo(a)pyrene metabolites in rat liver microsomes

The effect of dicoumarol on glucuronidation of 3-OH-benzo(a)pyrene (BP) appears to be due to inhibition of UDPglucuronosyltransferase (UDPGT) and not to an inhibited DT-diaphorase (NAD(P)H:quinone oxidoreductase); to date the only enzyme known to be inhibited by dicoumarol. This dicoumarol-sensitive form of UDPGT does not seem to be identical to the major form catalyzing the glucuronidation of p-nitrophenol or methylumbelliferone, nor to the isozyme involved in the formation of phenolphthalein glucuronides. These conclusions are based on the following observations: In solubilized microsomes, devoid of DT-diaphorase, a 3-OH-BP glucuronidation activity is found which is very similar to that observed in microsomes before passing through an azodicoumarol Sepharose 6B column that binds more than 98% of DT-diaphorase; in the eluate from this column the inhibition by dicoumarol of 3-OH-BP glucuronidation is the same as in microsomes containing DT-diaphorase; other coumarin derivatives, which are either modified or substituted in the methylene bridge between the two coumarin entities in dicoumarol, are potent inhibitors of DT-diaphorase but not of UDPGT; a concentration of 10(-6) M dicoumarol is sufficient to inhibit 3-OH-BP glucuronidation 50%. In contrast, to inhibit glucuronidation of p-nitrophenol or methylumbelliferone the concentration of dicoumarol must be raised to the substrate level: i.e., 10(-4) M. Phenolphthalein glucuronidation is almost unaffected even by this high concentration of dicoumarol. The present investigation also reveals that DT-diaphorase and NADPH-cytochrome P-450 reductase can both catalyze the reduction of BP-3,6-quinone for the formation of BP-3,6-quinol glucuronides. In the eluate from the azodicoumarol Sepharose 6B column, no NADH-supported glucuronidation of BP-3,6-quinone can be detected unless DT-diaphorase is added. However, NADPH-supported formation of BP-3,6-quinol glucuronides can still be observed. The rate of the latter reaction is sufficient enough to allow studies on the effect of dicoumarol on BP-3,6-quinone glucuronidation. These results show that glucuronidation of BP-3,6-quinols is also catalyzed by a dicoumarol-sensitive UDPGT. However, not only is the formation of BP-3,6-quinol monoglucuronides inhibited by dicoumarol, but the conversion of monoglucuronides to diglucuronides is inhibited as well. The former reaction is inhibited 50% by 3.5 X 10(-6) M dicoumarol (close to the I50 for 3-OH-BP glucuronidation), whereas 10 times less dicoumarol (2 X 10(-7) M) is sufficient for 50% inhibition of the latter reaction.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of glutathione S-transferase and phenol sulfotransferase, but not of UDP-glucuronosyltransferase, in the human lung tumor cell lines NCI-H322 and NCI-H358

The expression of xenobiotic-metabolizing enzymes was studied in the human lung tumour cell lines NCI-H322 and NCI-H358. These cells are derived from adenocarcinomas and exhibit features of Clara cells and alveolar type II cells, respectively. Examination of the in vitro activities showed that both cell lines lack UDP-glucuronosyltransferase against the substrates 3-hydroxybenzo[a]pyrene (3-OH-BaP) and 4-hydroxybiphenyl (4-OH-Bph) and that in vitro conjugation of sulfate with 3-OH-BaP was only just detectable. In contrast, both cell lines showed fairly high levels of glutathione-S transferase activity with the substrate 1-chloro-2,4-dinitro-benzene (54.4 and 83.0 nmol/min X mg protein, respectively) and of glutathione (81 and 41 nmole/mg protein, respectively). The metabolic capacity of intact NCI-H322 and NCI-H358 cells was examined using benzo[a]pyrene (BaP) and 3-OH-BaP as substrates. The cell lines formed sulfate conjugates from 3-OH-BaP (4.5 and 0.4 pmol/min X mg protein, respectively) but did not produce any detectable glucuronides. When cultures of the two cell lines were exposed to BaP, phenolic products accumulated in the growth medium. NCI-H322 cells also formed some sulfate conjugates, whereas such conjugates were barely detectable in the medium of NCI-H358 cells. In contrast A549, a human pulmonary adenocarcinoma cell line known to contain UDP-glucuronosyltransferase activity, efficiently conjugated 3-OH-BaP to glucuronic acid and converted the primary phenolic products formed from BaP to glucuronides. Thus the NCI-H322 and NCI-H358 cells are exceptional in that they possess no or very little glucuronosyltransferase activity but exhibit appreciable monooxygenase activity. The cell lines may therefore be of interest for examining the biological effects of potentially toxic chemicals which are otherwise detoxified by glucuronic acid conjugation. The cells may also be useful as test systems for evaluating the potential cytotoxicity and genotoxicity of chemicals to human lung.

Formation of benzo[a]pyrene-3,6-quinol mono- and diglucuronides in rat liver microsomes

The formation of benzo[a]pyrene (BP)-3,6 quinol glucuronides in liver microsomes in the presence of UDP-glucuronic acid and NAD(P)H appears to occur by a sequence of three reactions: BP-3,6-quinone----BP-3,6 hydroquinone----BP-3,6-quinol monoglucuronide----BP-3,6-quinol diglucuronide. This conclusion is based on the following results. Incubations with [14C]BP-3,6-quinone or UDP-[14C]glucuronic acid and analysis of the samples by TLC established the existence and identity of the two BP-3,6-quinol glucuronides which exhibit different fluorescence spectra. The nature of the monoglucuronide, i.e., a quinol and not a semiquinone glucuronide, was suggested by the finding that the rate of diglucuronide formation was the same with or without NAD(P)H provided that a sufficient amount of monoglucuronide had been formed prior to oxidation of the nucleotides. Furthermore, BP-3,6-quinol monoglucuronides can serve as substrates in the formation of diglucuronides. The ratio between the decrease in monoglucuronides and the formation of diglucuronides was found to be close to 1, suggesting that the conversion of the monoglucuronide of BP-3,6-quinol to the diglucuronide is also catalyzed by UDP-glucuronosyltransferase. However, great differences in the pattern of induction of mono- and diglucuronide formation indicate that two different UDP-glucuronosyltransferases are involved. The yield of BP-3,6-quinol glucuronides with NADH relative to NADPH and the increase in glucuronide formation observed in the presence of cytosolic DT-diaphorase (NAD(P)H-quinone oxidoreductase) are discussed with regards as to whether DT-diaphorase plays an important role as a BP-3,6-quinone reductase in the formation of BP-3,6-quinol glucuronides compared to other NAD(P)H-oxidizing flavoproteins.

Differential induction of rat liver microsomal UDP-glucuronosyltransferase activites by various inducing agents

The selectivity of various inducers of UDP-glucuronosyltransferase was investigated in rat liver microsomes and compared with their effect on monooxygenase reactions. (1) Similar to 3-methyl-cholanthrene beta-naphthoflavone selectively stimulated the glucuronidation of 1-naphthol and 4-methylumbelliferone (GT1 substrates). (2) In contrast, DDT preferentially enhanced the glucuronidation of morphine, 4-hydroxybiphenyl (GT2 substrates) and bilirubin, similar to phenobarbital. (3) Colfibric acid and bezafibrate selectively enhanced bilirubin glucuronidation without affecting GT1 and GT2 reactions. (4) Similar to ethoxyquin and Aroclor 1254, trans-stilbene oxide enhanced both GT1 and GT2 activities but not bilirubin glucuronidation. (5) In contrast to 3-methylcholanthrene-type inducers which induce both cytochrome P-450MC and GT1, probably through a common receptor protein, ethoxyquin and trans-stilbene oxide markedly induced GT1 reactions without affecting benzo[a]pyrene monooxygenase.

Toxic drug effects associated with oxygen metabolism: redox cycling and lipid peroxidation

Various endogenous and exogenous compounds exert cytotoxic effects via oxygen reduction. In general, these are reduced by intracellular enzymes (reductases of various kinds) in one-electron transfer reactions, before they in turn reduce O2 to O2, the superoxide anion radical. Thus, a cycle is formed of O2 uptake at the expense of cellular reducing equivalents, notably NADPH, generating further active oxygen species (figs 1,2). Structures capable of 'redox cycling' include catechols and other quinone compounds, iron chelates, and aromatic nitro compounds. Several anticancer agents, and also some mutagens, operate on this principle, and their toxic effects may be explained by redox cycling. The particular importance of hypoxic conditions for deleterious O2 effects is given by the concomitant flux through reductive as well as oxidative pathways. Toxic effects include membrane damage resulting from peroxidative reactions of polyunsaturated fatty acids (lipid peroxidation), as well as the attack of reactive oxygen species on proteins (enzymes) and nucleic acids; thus O2 metabolism is linked to carcinogenicity and mutagenicity. Lipid peroxidation is also induced by various halogenated compounds such as carbon tetrachloride. Again, hypoxic conditions are particularly critical because, on the one hand, metabolic activation leading to the free radical is enhanced and, on the other hand, oxygen required for the maintenance of lipid peroxidation is still available. - Powerful antioxidant systems of the cell maintain low steady state concentrations of oxygen metabolites, and toxic effects may, in part, also be explained by the constant drain of reducing equivalents resulting from redox cycling.

Release of mutagenic metabolites of benzo[a]pyrene from the perfused rat liver after inhibition of glucuronidation and sulfation by salicylamide

The role of glucuronide and sulfate conjugation in presystemic inactivation of benzo[a]pyrene (BP) metabolites was investigated with rat livers perfused with BP (12 mumol). Comparisons were made between metabolite profiles and mutagenicity of medium from perfusions with and without salicylamide, a selective inhibitor of glucuronide and sulfate conjugation. After 4 h perfusion in the presence of salicylamide, certain BP metabolites (diols, quinones, phenols, and metabolites more polar than BP-9,10-diol) were significantly increased at the expense of quinones and phenols in the glucuronide fraction. Mutagenicity of medium (detected by the Ames test, using tester strains TA98 and TA100) was low in perfusion without salicylamide. Mutagenicity detected with tester strain TA98 was significantly increased in perfusions with salicylamide. Involvement of glucuronidation in BP inactivation was also observed at the subcellular level; when cofactors of glucuronidation were added to liver homogenates along with the NADPH regenerating system in the Ames test, BP mutagenicity was markedly decreased. Both the activation of BP to mutagenic metabolites and the inactivation of BP metabolites by glucuronidation was much more pronounced with liver homogenates from 3-methylcholanthrene-treated rats than with those from phenobarbital-treated animals or untreated controls. The results suggest an important role for glucuronidation and sulfation in the inactivation and elimination of polycyclic aromatic hydrocarbons.