Genetics of aryl hydrocarbon hydroxylase induction in mice: Response of the lung to cigarette smoke and 3-methylcholanthrene

When mice from different inbred strains are injected intraperitoneally with 3-methylcholanthrene (MC), the activity of aryl hydrocarbon hydroxylase (AHH) rapidly increases in livers of some strains but not others. AHH plays a role in the metabolism of polycyclic hydrocarbons. Alleles at a small number of loci account for most of the variation in inducibility of hepatic AHH among mice, when MC is used as the inducing agent. Cigarette smoke is a common source of carcinogenic polycyclic hydrocarbons in the environment. Since some of the hydrocarbons in cigarette smoke are metabolized by AHH, the activity of AHH in tissues may affect the carcinogenicity of smoke in those tissues. The purpose of these experiments was to see whether induction of AHH in lung in response to cigarette smoke is regulated by the same genes that regulate induction of AHH in liver in response to MC. Mouse strains AKR/J and C57L/J and six recombinant inbred (RI) lines derived from them were tested for the response of AHH in lung and liver to parenteral MC or inhalation of cigarette smoke. Inducibility (the ratio of MC-induced AHH activities to basal AHH activities) in liver from MC-treated RI lines is bimodal and compatible with Mendelian segregation of genes at a small number of loci. Increased activities of AHH in MC-treated liver are associated with increased ability to metabolize BP and whole smoke condensates to mutagens detected by Salmonella typhimurium TA1538. Inducibility of AHH in lung in response to MC is not bimodal, and no definite conclusion about the number of loci can be made. When actual levels of AHH activity are considered, following the administration of MC as inducing agent, there is a correlation (r=0.89, p<0.01) between AHH levels in liver and lung, suggesting that some genes affecting liver also affect lung. Basal and MC-induced AHH levels in lung are also correlated (r=0.86, p<0.01). Mice with high basal activities have two to threefold higher levels of AHH after MC treatment than do mice with low basal activities. Induction of AHH in pulmonary tissues occurs in all mice after either parenteral MC or smoke inhalation. In contrast to MC treatment, AHH activities in lungs following smoke inhalation are not correlated with AHH levels in liver after MC (r=0.49) and are only weakly correlated with basal (r=0.66, 0.05相似文献   

Comparison of the in vitro mutagenicity and metabolism of dimethylnitrosamine and benzo[a]pyrene in tissues from inbred mice treated with phenobarbital, 3-methylcholanthrene or polychlorinated biphenyls.

Homogenates of liver, lung, kidney, stomach, small intestine and colon from 8 strains of mice were compared for their ability to metabolize benzo[a]pyrene (BP) and dimethylnitrosamine (DMN) to mutagens. Females of strains CF1, AKR/J, AU/SsJ, DBA/2J, SWR/J, A/J, C3H/HeJ, and C57BL/6J were either untreated or received phenobarbital (PB), 3-methylcholanthrene (MC) or polychlorinated biphenyls (AR) to induce drug-metabolizing enzymes. The effects of these drugs on organ weight and on the amounts of DNA, S-10 protein, and microsomal protein per unit weight of tissue are reported. Salmonella typhimurium TA92 and TA98 were used as indicators of the formation of mutagens. For each organ there was an optimal balance between amount of tissue homogenate and concentration of test compound for maximal yield of revertants. A sensitive radiometric assay of DMN demethylase (DMND) is described which permits measurement of the enzyme in liver, lung and kidney. DMN at 1 mM is used as substrate. Aryl hydrocarbon hydroxylase (AHH) was measured in all tissue using BP as substrate. AR and MC are very good inducers of AHH activity in livers of mice classified as aromatic hydrocarbon responsive, but not in those classified as hydrocarbon nonresponsive. Responsiveness is strain-specific and genetically regulated. Metabolism of BP to mutagens by liver homogenates was correlated with extent of AHH induction. This dimorphism of response of AHH to inducers was present, but less pronounced, in non-hepatic tissues. Basal activities of AHH and DMND were correlated in livers and lungs from untreated mice. DMND activities were increased less than 2-fold by PB, MC or AR treatments. Metabolism of DMN to mutagens was not closely correlated with DMND activities. Strain of mouse, type of tissue and test substance are important variables in assessing the potential effect of microsomal enzyme-inducing agents on the metabolism of mutagenic substances.     

Expression of aryl hydrocarbon hydroxylase induction in mouse tissues in vivo and in organ culture

The elevation of aryl hydrocarbon hydroxylase by various microsomal enzyme inducers in mouse tissues from five inbred strains was examined in vivo and in fetal liver expiants. The magnitude of 3-methylcholanthrene- or β-naphthoflavone-inducible AHH activities in the intact animal varied greatly with the tissue and strain—from no induction in the liver and less than a 2- to 3-fold increase in the lung of DBA/2+ and AKR mice to 4- to 5- and 6- to 7-fold elevation, respectively, in the liver and lung of C57BL mice. Treatment of At or C3H⁺ mice with these inducers increased AHH activity in liver and lung to levels which were intermediate between those observed with tissues from DBA/2+ and C57BL mice. These strain-specific differences in the expression of AHH induction in response to polycyclic hydrocarbons and flavones were also present in fetal liver expiants and were measurable as early as 6 days before parturition. In expiants derived from polycyclic hydrocarbon-“responsive” strains, the extent of enzyme induction was greatest with 4′-bromoflavone, less with β-naphthoflavone and least with 3-methylcholanthrene. Trans-1, 2-dihydroxy-3-methylcholanthrene was about twice as effective in this regard as the parent compound 3-methylcholanthrene. Among expiants from 3-methylcholanthrene-“resistant” strains (DBA/2+, AKR), a disparity in the effects of different classes of compounds was apparent: the flavone derivatives induced aryl hydrocarbon hydroxylase activity from DBA/2+ and AKR expiants by 2- to 3-fold despite the absence of polycyclic hydrocarbon induction in these cultures. Furthermore, although phenobarbital was a comparatively weak inducer under the conditions used in these experiments, this substance stimulated aryl hydrocarbon hydroxylase activity from 3-methylcholanthrene-“responsive” and -“resistant” explants by similar degrees (i.e., about 30%). The results are discussed in the light of previous suggestions on the genetically determined regulation of aryl hydrocarbon hydroxylase induction in mouse tissues.     

Mutagenicity and metabolism of dimethylnitrosamine and benzo[alpha]pyrene in tissue homogenates from inbred Syrian hamsters treated with phenobarbital, 3-methylcholanthrene or polychlorinated biphenyls.

There are significant differences between mice and hamsters in polycyclic hydrocarbon and nitrosamine metabolism. Homogenates of liver, lung and intestinal mucosa from 6 strains of Syrian golden hamster were compared for their ability to metabolize benzo[alpha]pyrene (BP) and dimethylnitrosamine (DMN) to mutagens. Females of strains MHA/SSLak, LSH/SlLak, CB/SsLak, PD4/Lak LHC/Lak and Lak:LVG (SYR) were either untreated or received phenobarbital (PB), 3-methylcholanthrene (MC) or polychlorinated biphenyls (AR) to induce drug-metabolizing enzymes. Salmonella typhimurium TA92 and TA98 were used as indicators of the formation of mutagans. Dimethylnitrosamine demethylase (DMND) was assayed using 1 mM DMN as substrate. Aryl hydrocarbon hydroxylase (AHH) was measured using benzo[alpha]pyrene as substrate. MC does not induced AHH activity in hamster liver, but is an excellent inducer of enzymes converting BP to mutagens. This lack of correlation between increased AHH activity and increased metabolism of BP to mutagen in liver is in marked contrast to correlations seen in mice. MC induces AHH in hamster lung and intestinal mucosa. AR induces AHH in liver, lung and intestinal mucosa. Activity of DMND in liver is not affected by treatment of hamsters with BP or AR, but is repressed approx. 30% by treatment with MC. Activity of DMND and conversion of DMN to mutagen are correlated (r = 0.59) in hamster liver. Microsomes of hamster liver are more effective than those from mouse in converting DMN to mutagen, despite similar DMND activities in livers from the two species.     

The genetics of aryl hydrocarbon hydroxylase induction in mice: A single gene difference between C57BL/6J and DBA/2J

Twenty-one inbred strains of mice were surveyed for inducibility of hepatic aryl hydrocarbon hydroxylase (AHH) activity by the carcinogen 3-methylcholanthrene (MC). In 11 strains given MC, AHH activity increased 1.3- to 5-fold (inducible), whereas ten strains responded with a less than 0.5-fold increase (noninducible). Neither the inducible nor the noninducible class was homogeneous, and in each considerable variation was found in both the basal activity of AHH and the response to MC. Strains DBA/2J and C57BL/6J were chosen to represent the noninducible and inducible classes, respectively. In the crosses (C57BL/6 × DBA/2)F₁ × DBA/2 and (C57BL/6 × DBA/2)F₂, inducibility segregated as a single autosomal dominant gene. The gene symbols Ahh ⁱ and Ahh ⁿ are proposed for the alleles present in C57BL/6J and DBA/2J, respectively. No genetic linkage was found between the Ahh locus and the following loci: b, d, Es-1, Es-3, Gpd-1, Hbb, Id-1, Pgm-1, and sex. Some implications of this work in the study of mammalian enzyme induction and chemically induced carcinogenesis are discussed. There is a positive correlation between AHH inducibility and the development of an inflammatory response to the topical application of the carcinogen 7,12-dimethylbenzanthracene.     

Studies on pulmonary aryl hydrocarbon hydroxylase activity in inbred strains of mice.

Pulmonary and hepatic levels of aryl hydrocarbon hydroxylase (AHH) were studied in inbred strains of mice following intratracheal (i.t.) instillation of 3-methylcholanthrene (MCA). I.t. instillation of 188 mug MCA in sterile 0.2% gelatin in saline resulted in preferential induction of pulmonary AHH. After treatment with this dose of MCA, the pulmonary AHH levels of strains C57BL/6Cum, C57BL/6J, BALB/cMai, C3H/fMai, and C57L/J were observed to be induced within 24 h after treatment. Strains DBA/2Cum, AKR/J, SJL/J, DBA/2J and RF/J expressed no such increase. At a dose of 500 mug MCA, the pulmonary tissue of DBA/2 mice did express a 4-fold increase. This increase in AHH was determined to be quite different from the increase observed in C57BL/6 mice by: (1) specific activity of the enzymes, (2) genetic regulation, (3) susceptibility to inhibition by 7,8-benzoflavone, and (4) spectral properties of the associated cytochromes. It was of major importance that induction of pulmonary AHH was observed to be regulated by a single dominant gene in crosses involving the C57BL/6Cum and DBA/2Cum strains of mice. Results were discussed with the view in mind that these genetically regulated levels of AHH may play a role in susceptibility to cancers induced by polycyclic aromatic hydrocarbon carcinogens.     

Selection of inducers: An important factor in characterizing genetic differences to induction of aryl hydrocarbon hydroxylase in strains of mice

The effect of various microsomal enzyme inducers such as DDT, benzpyrene, 3-MC, TCDD or phenobarbital on liver microsomal mixed-function oxidases and cytochrome P450 content in mice genetically responsive (C57B1/6J) and resistant (DBA/2J) to induction of aryl hydrocarbon hydroxylase (AHH) was studied. 3-MC and benzpyrene administration stimulated liver AHH activity 6–8 fold in C57B1/6J mice but had no effect in DBA/2J mice. However, intraperitoneal administration of TCDD increased AHH activity in both C57BL/6J and DBA/2J mice. This increase was accompanied by shift in the peak of cytochrome P450 difference spectrum from 450 to 448 nm. It is concluded that genetic resistance to AHH stimulation in DBA/2J mice is influenced by the type of inducer used.     

Normal tissue alloantigens and genetic control of susceptibility to tumors: microcytotoxicity studies on resistant C3Hf and susceptible (A X C3Hf) F1 mice inoculated with transplacentally induced C3Hf lung tumor.

The immune response of mice to a transplacentally induced lung tumor was investigated with the microcytotoxicity (MC) assay. The tumor, originally induced in C3Hf mice, does not grow readily when transplanted to normal syngeneic C3Hf recipients. It grows readily, however, in (A C3Hf)F1 hybrids and in strain C3H mice, which express in their normal lung tissue a component which constitutes a strong lung tumor-associated transplantation antigen (TATA) in C3Hf mice. Both lung tumor-immunized C3Hf and tumor-bearing (A X C3Hf)F1 and C3H mice possessed lymphoid cells reactive against cultured lung tumor cells in the MC assay. Reactivity was also observed against cells cultured from normal lungs of (A X C3Hf)F1 and C3H mice, but not against cells similarly cultured from C3Hf of C57BL/6 mice. Anti-tumor MC was inhibited by serum-blocking factors present in some but not all tumor-bearing and tumor-immunized mice. The MC assay and detection by it of serum-blocking factors does not distinguish the effective anti-C3Hf lung tumor immune response of immunized C3Hf mice from the ineffective immune response of tumor-bearing (A X C3Hf)F1 and C3H mice. Furthermore, in lung tumor-bearing mice cells reactive in the MC assay may be directed against a normal tissue antigen rather than a tumor-associated antigen.     

Benzo[a]pyrene-DNA-adducts and monooxygenase activities in mice treated with benzo[a]pyrene, cigarette smoke or cigarette smoke condensate

Synchronous fluorescence spectrophotometry (SFS), developed to study benzo[a]pyrene-7,8-diol-9,10-epoxide(BPDE)-DNA, was used to measure the in vivo formation of DNA-adducts in genetically responsive C57BL/6 (B6) and non-responsive DBA/2 (D2) mice. Treatment with cigarette smoke by inhalation for 3-16 days, or i.p. injection of cigarette smoke condensate or neutral fraction did not lead to detectable levels of BPDE-DNA-adducts in either lungs or liver, although aryl hydrocarbon hydroxylase (AHH) activity, an indicator of benzo[a]pyrene (BP) metabolism, was clearly induced in lungs of B6 mouse. A dose-dependent amount of BPDE-DNA-adducts in lung and somewhat less in liver was found after i.p. injection with BP (20-80 mg/kg). Mice treated with vehicle or 4 mg/kg of BP were negative for adducts by SFS. In B6 mice AHH was induced both in lungs and livers while there was no AHH induction in D2 mice although the levels of BPDE-DNA-adducts were somewhat higher than in B6 mice. Thus, no clear correlation seems to exist between AHH activity and the formation of BPDE-DNA-adducts. Also, according to our results SFS can be used to quantitate adduct-formation in in vivo animal studies.     

Suppression of aryl hydrocarbon hydroxylase activity in human primary lung carcinoma x mouse hepatoma somatic cell hybrids

Variants of the mouse hepatoma cell clone inducible for aryl hydrocarbon (benzo(a)pyrene) hydroxylase (AHH) (EC 1. 14. 14.1) activity and deficient in hypoxanthine guanine phosphoribosyl-transferase (EC 2.4.2.8), and human primary lung carcinoma cell clone noninducible for AHH activity and deficient in thymidine kinase (EC 2.7.1.21) were isolated. The variant lines characterized for AHH inducibility and drug resistant phenotype were utilized to study somatic cell hybrids for the expression of AHH induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In two hybrids AHH activity was not expressed. In view of these results we conclude that aryl hydrocarbon hydroxylase activity is suppressed in AHH noninducible human lung carcinoma x AHH inducible mouse hepatoma cell hybrids.     

Experimental Models for Prevention of Graft-versus-Host Reaction in Bone Marrow Transfusion

Induction and suppression of splenomegaly and cytotoxicity against C57BL/6 cells were studied in (AKR × C57BL/6) F1 hybrid adult mice after the transfer of AKR lymphoid and bone marrow cells. 1) Splenomegaly and cytotoxicity were dissociated in the developmental stages of the graft-versus-host reaction. When lymphoid and bone marrow cells of normal AKR mice were injected into F1 recipients, splenomegaly was prominent on days 5 and 7, but cytotoxicity of spleen cells was not detected. Splenomegaly became less prominent but the cytotoxicity became detectable on day 14 after the injection. 2) Cytotoxic activity of spleen cells of F1 recipients was suppressed by the treatment of AKR donors with C57BL/6 lymphoid cells in Freund's complete adjuvant. Splenomegaly, however, was substantially enhanced by such a treatment of the donors. On the other hand, induction of the cytotoxic activity was facilitated by the treatment of donors with C57BL/6 skin grafts. 3) F1 hybrid mice could be protected from the graft-versus-host reaction by the injection of AKR anti-C57BL/6 serum or pretreatment of AKR donors with sonicated cellular antigens of C57BL/6.     

Experimental models for prevention of graft-versus-host reaction in bone marrow transfusion. I. Selective suppression and augmentation of splenomegaly and cytotoxicity.

Induction and suppression of splenomegaly and cytotoxicity against C57BL/L cells were studied in (AKR X C57BL/6) F1 hybrid adult mice after the transfer of AKR lymphoid and bone marrow cells. 1) Splenomegaly and cytotoxicity were dissociated in the developmental stages of the graft-versus-host reaction. When lymphoid and bone marrow cells of normal AKR mice were injected into F1 recipients, splenomegaly was prominent on days 5 and 7, but cytotoxicity of spleen cells was not detected. Splenomegaly became less prominent but the cytotoxicity became detectable on day 14 after the injection. 2) Cytotoxic activity of spleen cells of F1 recipients was suppressed by the treatment of AKR donors with C57BL/6 lymphoid cells in Freund's complete adjuvant. Splenomegaly, however, was substantially enhanced by such a treatment of the donors. On the other hand, induction of the cytotoxic activity was facilitated by the treatment of donors with C57BL/6 skin grafts. 3) F1 hybrid mice could be protected from the graft-versus-host reaction by the injection of AKR anti-C57BL/6 serum or pretreatment of AKR donors with sonicated cellular antigens of C57BL/6.     

Modulation of F1 cytotoxic potentials by GvHR: role and mode of action of non-MHC genes that determine the hybrid resistance to GvHR-associated suppression of F1 cytotoxic potential

The resistance of unirradiated F1 mice against graft-vs-host reaction (GvHR) induced by lymphocytes from certain parental strains is apparently a violation of the basic law in classical transplantation immunity. To explore genetic mechanisms of this peculiar phenomenon, GvHR-associated immunosuppression was examined on various kinds of F1 mice undergoing GvHR induced by parental lymphocytes. In F1 mice raised by crossing DBA/2 mice with various H-2-congeneic B10-series strains, parental lymphocytes having non-H-2 genetic background of DBA (DBA/2 and DBA/1) invariably could not induce GvHR-associated immunosuppression, irrespective of the H-2 haplotype incompatibility involved, whereas lymphocytes of the partner parental strain induced the immunosuppression. The number of the relevant loci in the DBA non-H-2 was assessed to be three recessive loci by examination of the capability to induce the GvHR-associated immunosuppression on lymphocytes from individual (B 10.D2 X DBA/2)F1 X DBA/2 backcross mice. On the other hand, in F1 mice raised by crossing C3H/He or AKR/J mice with various H-2-congeneic B10-series strains, parental lymphocytes of H-2k haplotype, irrespective of their non-H-2 haplotype, invariably could not induce the GvHR-associated immunosuppression. Furthermore, it was revealed that non-H-2 genes of parental C3H or AKR incorporated in the F1 mice determine the resistance of the F1 mice against the H-2k-induced GvHR. The results of examination of the resistance on individual (B10 X [B10.BR X C3H/He]F1) and (B10 X [B10.BR X AKR/J]F1) mice suggested that three non-H-2 loci of C3H/He or two non-2 loci of AKR/J incorporated in F1 hybrids could determine the resistance of the respective F1 mice.     

Similarity and difference in the mechanisms of neonatally induced tolerance and cyclophosphamide-induced tolerance in mice

The mechanisms of cyclophosphamide (CP)-induced tolerance were investigated by comparing with those of neonatally induced tolerance. When C3H/He Slc (C3H; H-2k, Mls-1b) mice were given i.v. either AKR/J Sea (AKR; H-2k, Mls-1a) or (AKR x C3H)F1 (AKC3F1; H-2k, Mls-1a/b) spleen cells and treated i.p. with CP 2 days later, a long-lasting skin allograft tolerance to AKR was induced in each case without any signs of graft-vs-host disease (GVHD). However, typical signs of GVHD were observed in the C3H mice neonatally tolerized with AKR spleen cells, but not in those tolerized with AKC3F1 spleen cells. The expression of TCR V beta 6, which is strongly correlated with the reactivity to Mls-1a Ag (of donor AKR origin), in the periphery was quite different between the two types of tolerant C3H mice. Namely, in the lymph nodes of the C3H mice tolerized with AKR spleen cells and CP, only CD4(+)-V beta 6+, but not CD8(+)-V beta 6+, T cells selectively disappeared, whereas both of them were abrogated in the lymph nodes of the C3H mice neonatally tolerized of AKR. By contrast, in the thymus of the two types of tolerant C3H mice, both CD4+CD8- and CD4-CD8+ single-positive thymocytes expressing TCR V beta 6 were clonally deleted, suggesting that the thymic involvement was the same in each type of tolerance. These results suggest that the preferential disappearance of the CD4(+)-V beta 6+ T cells (of host origin) and the effector T cells of GVHD (of donor origin) occurred only in the periphery of the C3H mice tolerized with AKR spleen cells plus CP and was attributable to the destruction of Ag-stimulated T cells by the CP treatment. In contrast, the intrathymic clonal deletion of immature V beta 6+ T cells was a common mechanism for both of the tolerance induction systems.     

Strain differences in aryl hydrocarbon hydroxylase induction by 3-methylcholanthrene in rabbits.

We determined both basal and induced AHH activity in livers of six partially inbred strains of rabbits. Strain III rabbits had the highest enzyme activity upon induction by 3-MCA, i.e., four to five times that in strain WH (noninducible), which has the lowest enzyme activity. AHH induction was also "low" in strains X, OS, ACEP, and AC. F1 hybrids between strains III and WH revealed a differential response to the induction of liver AHH activity by MCA: the levels of induced hydroxylase activity were consistently higher in (III X WH)F1 rabbits than in the reciprocal (WH X III)F1 hybrids. All possible crosses between these two "extreme" strains are now being analyzed to estimate the number of genes involved in their response difference to MCA.     

AKR/J gene(s) unlinked to H-2 determines dominant inheritance of lymphocyte hyporesponsiveness to acetylcholine receptor

Mice with the H-2b major histocompatibility complex haplotype are high immune responders to nicotinic acetylcholine receptors (AChR), whereas mice with the H-2k haplotype are generally low responders. F1 progeny of C57BL/6 (H-2b) mice crossed with mice of most H-2k strains are high responders to AChR in standard conditions of testing helper T cell proliferation in vitro (4 X 10(5) lymph node cells/microwell, 1 wk after primary challenge in vivo). In contrast, the F1 progeny of AKR/J (H-2k) crossed with high responder (H-2b) strains (B6, A.BY, or C3H.SW) were all hyporesponsive to AChR when lymphocytes were tested at 4 X 10(5) cells/well. However, at a density of 1 X 10(6) or greater/well, a high level of antigen-specific responsiveness was demonstrable in the F1 hybrid lymphocytes. A shift from low to high responsiveness to AChR at high cell densities was observed also in the H-2b strain AKR.B6. Other strains previously demonstrated to be low responders to AChR did not become responsive to AChR when lymphocyte numbers were increased to 1.4 X 10(6)/well. The N2 generation yielded by backcrossing (AKR X B6)F1 mice to AKR/J were all low responders, whereas N2 progeny derived by backcrossing F1 to B6 were high or low responders in a ratio of approximately 1:1 (independent of their H-2 phenotype). Results consistent with this observation were obtained in (AKR X B6) F2 mice. These data suggest that at least one AKR/J gene outside of the H-2 complex exerts a hyporesponsive influence on the I-A-dependent helper T cell response to AChR in H-2b mice.     

Polybrominated biphenyls as aryl hydrocarbon hydroxylase inducers: structure-activity correlations

The synthesis of all possible laterally-substituted polybrominated biphenyl (PBB) congeners containing two para bromines is described. Using enzymic, electrophoretic and ligand-binding assays that distinguish between phenobarbitone(PB)- and 3-methylcholanthrene(MC)-type inducers, the synthetic PBBs were evaluated as inducers of liver microsomal drug-metabolizing enzymes in the immature male Wistar rat. 4,4'-Dibromobiphenyl resembled PB in its mode of induction whereas all the meta-brominated derivatives of 4,4'-dibromobiphenyl, namely 3,4,4'-tri, 3,4,4',5-tetra-, 3,3', 4,4'-tetra-, 3,3',4,4',5-penta- and 3,3',4,4',5,5'-hexabromobiphenyl, resembled MC in their mode of induction. The results obtained with 3,4,4'-tribromobiphenyl demonstrate that, in contrast to the polychlorinated biphenyls (PCBs), a single meta halogen substituent is sufficient to abolish the PB-type characteristics of 4,4'-dibromobiphenyl and convert it to a strictly MC-type inducer. PBBs which induce AHH activity must be substituted at both para positions and at one, two, three or four meta positions. Ortho-substitution of PBBs which contain only lateral bromine groups may also give compounds which are aryl hydrocarbon hydroxylase (AHH) inducers. One of the MC-type PBBs, namely 3,3',4,4'-tetrabromobiphenyl, which has been tentatively identified in the commercial PBB mixture, fireMaster BP-6, was at least 50 times more potent as an inducer of AHH activity than the commercial PBB mixture. The induction of AHH by 3,3',4,4'-tetrabromobiphenyl was accompanied by a dose-dependent decrease in both thymus and spleen weights. The thymus and/or spleen weights were decreased in rats treated with the other MC-type PBBs which further supports the correlation between the toxicity of the PBBs and their ability to induce AHH.     

Induction of P-450 isoenzyme activities in Syrian golden hamster liver compared to rat liver as probed by the rate of 7-alkoxyresorufin-O-dealkylation

The activities of 7-ethoxyresorufin-O-deethylase (EROD), 7-pentoxyresorufin-O-deethylase (PROD), 7-ethoxycoumarin-O-deethylase (ECOD) and aromatic hydrocarbon hydroxylase (AHH) were measured in hepatic microsomes from male and female Wistar rats and Syrian golden hamsters in order to probe the basal activity and the inducibility by phenobarbital (PB) and 3-methylcholanthrene (MC) of different P-450 isoenzymes. The basal activities of EROD and ECOD, but not PROD and AHH, were higher in male hamsters than in male rats. No sex-related difference in enzyme activities was observed with hamsters, whereas male rats had a higher ECOD and AHH activity than female rats. Induction by PB led to a 450-fold and 250-fold increase in PROD activity in male and female rat liver microsomes, respectively, while MC had a more pronounced inductive effect on EROD activity in this species. In hamsters, EROD activity was induced by MC but not by PB. Unexpectedly PROD activity in male and female hamster liver microsomes was only moderately induced by PB, the extent being lower than on induction by MC. Therefore, the activity of PROD, which is useful as a specific enzymatic assay for P-450 IIB in the rat liver, cannot be used to probe PB-like inducers in the hamster liver.     

Characterisation of a novel mouse liver aldo-keto reductase AKR7A5

We have characterised a novel aldo-keto reductase (AKR7A5) from mouse liver that is 78% identical to rat aflatoxin dialdehyde reductase AKR7A1 and 89% identical to human succinic semialdehyde (SSA) reductase AKR7A2. AKR7A5 can reduce 2-carboxybenzaldehyde (2-CBA) and SSA as well as a range of aldehyde and diketone substrates. Western blots show that it is expressed in liver, kidney, testis and brain, and at lower levels in skeletal muscle, spleen heart and lung. The protein is not inducible in the liver by dietary ethoxyquin. Immunodepletion of AKR7A5 from liver extracts shows that it is one of the major liver 2-CBA reductases but that it is not the main SSA reductase in this tissue.