Comparative effects of endrin on hepatic lipid peroxidation and DNA damage,and nitric oxide production by peritoneal macrophages from C57BL/6J and DBA/2 mice

1. Endrin is a polyhalogenated cyclic hydrocarbon which produces hepatic and neurologic toxicity. In order to further assess the mechanism of toxicity ofendrin, the dose-dependent effects of endrin on hepatic lipid peroxidation and DNA damage, and nitric oxide (NO) production by peritoneal exudate cells (primarily macrophages) were investigated in C57BL/6J and DBA/2 mice which vary at the Ah receptor genetic locus. C57BL/6J mice are dioxin-responsive, while DBA/2 mice are dioxin-insensitive.2. Mice of both strains were treated with 0, 1, 2 or 4 mg endrin kg⁻¹ as a single oral dose in corn oil, and the animals were killed 24 hr post-treatment. At doses of 1,2 and 4 mg endrin kg⁻¹ in C57BL/6J mice, hepatic mitochondrial lipid peroxidation increased 1.2-, 2.2- and 3.2-fold, respectively, and 1.8-, 2.3- and 3.5-fold with microsomes, respectively. At these same doses in DBA/2 mice, hepatic mitochondrial lipid peroxidation increased 1.3-, 2.0- and 2.6-fold, respectively, and 1.5-, 1.9- and 2.5-fold with microsomes, respectively.3. Increases of 2.3-, 2.4- and 4.9-fold were observed in hepatic DNA damage (elution constants) in C57BL/6J mice at doses of 1, 2 and 4 mg endrin kg⁻¹, respectively, while at these same three doses, increases of 1.9-, 2.1- and 2.3-fold were observed for DBA/2 mice, respectively.4. Nitric oxide production by peritoneal macrophages from C57BL/6J increased by 1.3-, 1.7- and 2.0-fold with doses of 1, 2 and 4 mg endrin kg⁻¹, respectively, while in macrophages from DBA/2 mice at these same doses, increases of 1.7-, 1.7- and 1.8-fold, respectively, were observed.5. The results indicate that the responsiveness of peritoneal macrophages with respect to both DNA damage and nitric oxide production are more dose-dependent in C57BL/6J mice as compared to DBA/2 mice, while similar results are observed with the lipid peroxidation of hepatic mitochondria and microsomes of the two mouse strains. The results suggest that the toxicity of endrin is less reliant on a mechanism which may involve the Ah receptor system as compared to dioxins as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).     

Expression of hepatic pyruvate carboxylase mRNA in C57BL/6J Ahb/b and congenic Ahd/d mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The activity and level of hepatic pyruvate carboxylase (PC) has been reported to be altered by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment in mice. If alteration in PC level/activity by TCDD influences TCDD toxicity, one would expect to observe an early post-exposure reduction in PC mRNA. To examine the molecular events responsible for the alteration of PC activity in livers of TCDD-treated mice, we designed a synthetic DNA oligonucleotide probe specific for PC mRNA. Northern blot analysis on RNA extracts from hepatic tissue at various times and doses post-TCDD exposure were done. Furthermore, to elucidate the role of the dioxin Ah locus on alterations of PC activity by TCDD, we utilized C57BL/6J (Ah^b/b, Ah high TCDD affinity) mice and a congenic (Ah^d/d, Ah low TCDD affinity) mouse strain. At 8 days post TCDD treatment, a dose-dependent reduction of hepatic PC mRNA levels was observed in Ah^b/b mice. The response, reduction in PC mRNA levels, in the Ah^b/b strain was about 10-fold greater than that of comparably exposed congenic Ah^d/d mice. These results indicate that previously reported reductions in PC activity/level by TCDD treatment of mice is a consequence of a reduction in PC mRNA levels and that the effect requires a competent Ah receptor. © 1996 John Wiley & Sons, Inc.     

Experimental study on protection of vitamin B6 on TCDD-induced palatal cleft formation in the mice

BACKGROUND: 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin (TCDD) can cause a high percentage of cleft palate in fetuses when administered during organogenesis in certain strains of mice including the C57BL/6J. In this study, vitamin B₆ (B₆) was tested for antiteratogenic effects on TCDD-induced cleft palate in fetal mice. METHODS: The pregnant C57BL/6J mice were dosed with 24 µg TCDD/kg and/or 5, 10, 20, and 40 mg B₆/kg body weight on gestation day (GD) 10. The control group mice were dosed with 50 ml sesame oil/kg body weight on GD10. The mice were sacrificed on GD12.5, GD13.5, GD14.5, GD15.5, and GD17.5, respectively. The harvested embryos were examined to detect the incidence of cleft palate and the developing palatal shelves in a different phase were investigated morphologically and histologically among different groups. RESULTS: Total frequency of clefts is 55.56% in the TCDD group and 31.81% (5 mg), 44.44% (10 mg), 40.90% (20 mg), and 32.00% (40 mg) in the TCDD+ B₆ groups. There were no statistically significant differences among the TCDD and TCDD+ B₆ groups (p=0.743>0.05). CONCLUSIONS: It was demonstrated in this study that B6 could not antagonize 2, 3, 7, 8-TCDD-indued cleft palate. Birth Defects Res (Part B) 86:357–361, 2009. © 2009 Wiley-Liss, Inc.     

Ah receptor involvement in mediation of pyruvate carboxylase levels and activity in mice given 2,3,7,8-tetrachlorodibenzo-p-dioxin

The arylhydrocarbon receptor (AhR) plays a central role in mediating 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity in animals. The investigations described here provide evidence that support a role for the AhR in TCDD-mediated pyruvate carboxylase (PC) level/activity reductions in mice. Pyruvate carboxylase plays a pivotal role in gluconeogenesis and in supplying carbon units for the citric acid cycle. Delivered ip in a corn oil carrier, TCDD suppresses PC activity/amount at doses as low as 1 μg/kg in responsive C57BL/6J(Ah^b/b) mice. Corn oil alone injected ip into mice at 4 mL/kg appears to be an inducer that increases the amount and activity of PC. However, TCDD suppresses this induction. In the Ah^b/b mouse, PC levels and activity are reduced to 10% of control values at a dose of 75 μg/kg. A time-course experiment shows that the PC reductions are apparent within 16 hours post-TCDD exposure. Here we report investigations on the PC/TCDD response using a congenic C57BL/6J(Ah^d/d) mouse strain having an AhR with a low affinity for TCDD. If the PC/TCDD response is AhR mediated, the congenic mouse strain (Ah^d/d) would require much higher doses of TCDD to suppress PC. In the Ah^d/d mice, we observe that an approximately 60-fold increase in TCDD dose is necessary to produce a PC/TCDD effect. We also find that in Ah^d/d mice, corn oil does not induce an increase in PC activity/amounts, as reported for Ah^b/b mice.     

Identification of the site of inhibition of mitogenic signaling by oncogenic ras-p21 by a ras effector peptide

We have previously found that a ras switch 1 domain peptide (PNC-7, residues 35–47) selectively blocks oocyte maturation induced by oncogenic (Val 12–containing) ras-p21 protein and also blocks c-raf–induced oocyte maturation. We now find that oncogenic ras-p21 does not inhibit oocyte maturation of a constitutively activated raf protein (raf BXB) that is lacking most of the first 301 amino terminal amino acids, including the major ras binding domain and accessory ras-binding regions. We also find that a dominant negative raf that completely blocks c-raf–induced maturation likewise does not block raf-BXB–induced maturation. We conclude that PNC-7 blocks ras by binding to the amino-terminal domain of raf and that raf BXB must initiate signal transduction in the cytosol.     

Different Sensitivity of Complement to Salmonella typhimurium Accounts for the Difference in Natural Resistance to Murine Typhoid between A/J and C57BL/6 Mice

The difference in natural resistance to Salmonella typhimurium between S. typhimurium-resistant A/J mice and S. typhimurium-susceptible C57BL/6 mice was analyzed. In both strains, the growth of S. typhimurium was controlled in the spleen until 48 hr of infection, while serum C3b levels were increased in A/J mice immediately after infection but not in C57BL/6 mice. Incubation of A/J mouse serum with S. typhimurium or its lipopolysaccharide (LPS) generated sufficient amounts of C3b, but that of C57BL/6 mouse serum with them did not. A/J macrophages had higher intracellular killing activity in vitro than did C57BL/6 cells against S. typhimurium pre-opsonized with each corresponding fresh serum. However, the cells from both mice exhibited a similar level of killing activity against S. typhimurium pre-opsonized with fresh A/J serum or rabbit complement. The resistance of C57BL/6 mice was significantly increased by opsonizing S. typhimurium with fresh A/J serum or rabbit complement before inoculation. The serum level of interferon-γ (IFN-γ) in A/J mice was 2.7 times as high as in C57BL/6 mice at 48 hr post-infection. Recombinant murine IFN-γ enhanced the intracellular killing activity of macrophages from both mice when S. typhimurium was pre-opsonized with fresh A/J serum but not with fresh C57BL/6 serum. These findings suggest that A/J macrophages exhibit maximal killing activity against A/J serum-opsonized S. typhimurium in vivo when the cells are activated with IFN-γ. Therefore, the rapid and sufficient activation of complement by Salmonella LPS may render A/J mice more resistant against murine typhoid.     

Regulation of resistance to leprosy by chromosome 1 locus in the mouse

Mice of different inbred strains vary in their resistance to intravenous infection with Mycobacterium lepraemurium (MLM). The mean survival time of MLM-infected A/J and DBA/2 mice is significantly longer than that of similarly infected C57BL/6 and BALB/c mice. The typing of AXB/BXA recombinant inbred strains (A = A/J, B = C57BL/6) for the trait of relative resistance/susceptibility to MLM revealed a perfect match with the strain distribution pattern of resistance/susceptibility to Mycobacterium bovis (BCG), the trait which is controlled by the Bcg (Ity, Lsh) locus on chromosome 1. The control, by this gene, of response to MLM was further confirmed by the demonstration that BALB/c-Bcg ^r congenic mice,which carry the DBA/2-derived Bcg ^r (resistant) allele on chromosome 1, are significantly more resistant to MLM infection than their BALB/c (Bcg ^s , susceptible) counterparts.     

Comparison of lipids in total brain tissue from five mouse genotypes

Brain tissue from adult male and female mice of the C57BL/6J, C57BL/6J-A^w—J, BALB/cJ, SJL/J, and DBA/2J genotypes was examined for brain weight, total protein, total lipid, cholesterol, phospholipid, plasmalogen, sulfatide, nonganglioside—glycolipid sphingosine, and ganglioside N-acetyl neuraminic acid, fatty acid, and sphingosine. No significant differences were found between sexes for any of these constituents. When compared to the overall average obtained for other animals, the DBA/2J, C57BL/6J-A^w−J, and BALB/cJ mice contained lower quantities of nonganglioside—glycolipid sphingosine. The DBA/2J and C57BL/6J-A^w−J mice also contained lower quantities of plasmalogen and sulfatide compared to the overall averages obtained for the other genotypes. In addition, the sterol content in DBA/2J mice was significantly higher than the overall average value obtained for the other animals.     

Undaria pinnatifida soluble fiber regulates Angptl3-LPL pathway to lessen hyperlipidemia in mice

Angiopoietin-like protein 3 (Angptl3)–lipoprotein lipase (LPL) pathway may be a useful pharmacologic target for hyperlipidemia. The present study was conducted to test the effect of soluble fiber extracted from Undaria pinnatifida (UP), on hyperlipidemia in apolipoprotein E-deficient (ApoE^?/?) mice. Forty mice were divided into four groups (n?=?10): control group (C57BL/6J mice), ApoE^?/? mice group, and two groups of ApoE^?/? mice treated with UP fiber (5 or 10 % per day). UP soluble fiber treatment significantly decreased plasma and hepatic total cholesterol, triglycerides levels, plasma low-density lipoprotein cholesterol, and malondialdehyde concentrations and increased plasma high-density lipoprotein cholesterol level and downregulated protein expression of Angptl3 concomitantly with upregulated protein expression of LPL. In addition, T0901317 caused elevated expression of hepatic Angptl3 protein, and the effect of T0901317 was also abrogated by UP soluble fiber in C57BL/6J mice. The present results suggest that the UP soluble fiber regulates Angptl3-LPL pathway to lessen hyperlipidemia in mice.     

Discordant susceptibility of inbred C57BL/6 versus outbred CD1 mice to experimental fungal sepsis

Individual susceptibility differences to fungal infection following invasive and/or immunosuppressive medical interventions are an important clinical issue. In order to explore immune response‐related factors that may be linked to fungal infection susceptibility, we have compared the response of inbred C57BL/6J and outbred CD1 mouse strains to different experimental models of fungal sepsis. The challenge of animals with the zymosan‐induced generalised inflammation model revealed poorer survival rates in C57BL/6J, consistent with lower Th1 cytokine interferon (IFN)‐γ serum levels, compared with CD1 mice. Likewise, ex vivo exposure of C57BL/6J splenocytes to zymosan but also bacterial lipopolisaccharide or lipoteichoic acid, resulted in lower IFN‐γ secretion compared with CD1 mice. C57BL/6J susceptibility could be reverted by rescue infusion of relative low IFN‐γ doses (0.2 μg/kg) either alone or in combination with the ß‐glucan‐binding CD5 protein (0.7 mg/kg) leading to improved post zymosan‐induced generalised inflammation survival. Similarly, low survival rates to systemic Candida albicans infection (2.86 × 10⁴ CFU/gr) were ameliorated by low‐dose IFN‐γ infusion in C57BL/6J but not CD1 mice. Our results highlight the importance of strain choice in experimental fungal infection models and provide a susceptibility rationale for more specific antifungal immunotherapy designs.     

Spatiotemporal changes in Ha-ras p21 expression through the hepatocyte cell cycle during liver regeneration

The protein product of the ras oncogene, Ha-ras (p21), is thought to be an important regulator of cell growth. The cytoplasmic relocalization of p21 in the cell during the cell cycle suggests a transient signaling role for this protein in association with its signal transduction function. Because of the importance of this role we examined spatial patterns in vivo of p21 expression at the protein and mRNA levels in hepatocytes during compensatory growth in rat liver following partial hepatectomy. A low level of p21 was immunolocalized on the cytoplasmic membrane of nonregenerating hepatocytes. The level of hepatic p21 increased significantly and without spatial restriction within the liver from 36 to 60 hr after partial hepatectomy (PH). p21 was localized in the cytoplasm of dividing hepatocytes and on the hepatic cytoplasmic membrane. The elevated p21 level decreased and was found mainly on hepatocyte plasma membranes by 96 hr after PH. Immunogold electron microscopy showed p21 localized over mitochondrial membranes and nuclei in nondividing regenerating hepatocytes. Approximately 50% of nonregenerating hepatocytes show nuclear localization of p21. This percentage changes with time following PH. The decrease in nuclear localization was accompanied with an increase in the low number of hepatocytes which demonstrated cytoplasmic localization in nondividing hepatocytes in regenerating liver. Flow cytometric analysis revealed a significant increase of p21 at 36 hr after PH which was 12 hr after the initial induction of ras mRNA. ras mRNA level increased 1.5-fold at 24 hr after PH and a maximum twofold induction was observed at 48 hr. Cell-cycle analysis of regenerating hepatocytes indicated a synchronized first peak of cell division 36–40 hr after PH. Dual parameter flow cytometry revealed that the level of p21 in hepatocytes in S phase and G₂/M phase of the cell cycle was significantly higher than that in G₀/G₁ phase during regeneration. These findings suggest that p21 is important for the progression of regenerating hepatocytes to S phase and then to G₂/M phase.     

DNA adducts in relation to lung tumour outcome are not markers of susceptibility following a single dose treatment of SWR,BALB/c and C57BL/6J mice with N-nitrosodiethylamine

The formation of DNA adducts by the covalent binding of genotoxic chemicals to DNA represents a valuable marker for assessing exposure to carcinogens but as yet the role of DNA adducts as a biomarker of carcinogenic susceptibility still needs to be clearly ascertained. To address this question an animal study was instigated using mice (SWR (high), BALB/c (intermediate) and C57BL/6J (low)) varying in their susceptibility to lung carcinogenesis. Groups of animals from each strain were dosed with a single intraperitoneal injection of saline or N -nitrosodiethylamine (NDEA) at 15 or 90 mg kg^-1 body weight. Lung and liver tissues were removed at different time points following dosing. Further groups of mice dosed with the same regime had urine samples collected 24 h post dosing and were then left up to 18 months to allow for the development of tumours. Immunoslot-blot analysis was used for the determination of N-7 ethylguanine (N-7EtG) and O⁶ ethylguanine (O⁶EtG) adduct levels in the DNA from the tissues and gas chromatography-mass spectrometry (GC-MS) was used to determine N-3 ethyladenine (N-3EtA) adduct levels in the urine samples. Levels of alkyltransferase (ATase) were also determined in the tissues. The results showed that the DNA adduct levels and persistence were similar across the three strains of mice following dosing with 15 and 90 mg kg^-1 NDEA. High levels of adducts were observed in the urine of the BALB/c strain, implying an increased metabolic or repair capacity in this strain. However there were no differences in the levels of ATase in the lung and liver of the three strains of mice following dosing with 15 mg kg^-1 NDEA. The incidence of tumours in C57BL/6J mice was lower compared with the other two strains and showed a dose dependent increase. The results from this study show that the differences in susceptibility to lung carcinogenesis between the three strains of mice do not appear to be linked to the formation of the two adducts detected. These results imply that dosing with NDEA resulted in toxicity which may have led to cell death and induction of tumours by compensatory cell proliferation. Although these results do not allow decisive conclusions to be drawn concerning the relationship between total levels of DNA adducts and differences in carcinogenic susceptibility for the three strains of mice it is clear that the increased presence of a DNA adduct in the target tissue increases the likelihood of tumour development.     

Production of a truncated human c-myc protein which binds to DNA

Two kinds of truncated human c-myc proteins were produced in Escherichia coli. The human c-myc gene is composed of three exons, exons 2 and 3 having coding capacity for a protein of 439 amino acids. 252 N-terminal amino acids are encoded by exon 2, the C-terminal 187 amino acids being encoded by exon 3. One of the proteins (p42) produced in E. coli corresponds to 342 amino acids from the 98th Gln to the C-terminus, plus 21 amino acids derived from the H-ras gene at the N-terminus. The other (p23) corresponds to 155 amino acids from the 98th Gln to the 252nd Ser, plus five amino acids (Gly-Gly-Thr-Arg-Arg) at the C-terminus, plus 21 amino acids from the H-ras gene at the N-terminus. The p23 protein was produced by using cDNA in which a frame shift occurred at the boundary between exons 2 and 3. We investigated the DNA-binding activity in p42 and p23 proteins. DNA-cellulose column chromatography showed that p42 binds to DNA, whereas p23 does not. This DNA-binding activity of p42 was inhibited by antiserum prepared against p42 but not by antiserum against p23. This indicates that the DNA-binding activity of c-myc protein is localized in the portion encoded by exon 3.     

Purification and Immunological Properties of NAD+-linked Malate Dehydrogenase Isozymes from Euglena gracilis

We investigated the hypolipidemic effects of young persimmon fruit (YP) on apolipoprotein E-deficient C57BL/6.KOR-ApoE^shl mice. These mice exhibited higher plasma cholesterols, except for high-density lipoprotein (HDL), and lower plasma HDL cholesterol than C57BL/6.Cr mice that had the same genetic background as the C57BL/6.KOR-ApoE^shl mice. Male C57BL/6.KOR-ApoE^shl mice (n=5) were fed a diet supplemented with dry YP, Hachiya-kaki, at a concentration of 5% (w/w) for 10 weeks. YP treatment significantly lowered plasma chylomicron, very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) cholesterols, and triglyceride, and this response was accompanied by an elevation of fecal bile acid excretion. In the liver, sterol regulatory element binding protein-2 gene expression was significantly higher in mice fed YP, while the mRNA and protein levels of the LDL receptor did not change. These results indicate that acceleration of fecal bile acid excretion is a major mechanism of the hypolipidemic effect induced by YP in C57BL/6.KOR-ApoE^shl mice.     

Susceptibility to a mouse acquired immunodeficiency syndrome is influenced by theH-2

The development of a mouse acquired immunodeficiency syndrome (MAIDS) induced following LP-BM5 MuLV infection depends on host genetic factors. Susceptible mice, such as C57BL/6J mice, develop a profound impairment of lymphoproliferative response to mitogens and hyperplasia of lymphoid organs and succumb to infection within 6 months. These changes do not occur in resistant mice, such as A/J mice. Resistance to MAIDS is a dominant trait since (C57BL/6JxA/J)F₁ hybrid mice did not develop any immune dysfunctions following infection. Genetic regulation of the trait of resistance/susceptibility to MAIDS was determined in AXB/BXA recombinant inbred (RI) mouse strains (derived from resistant A/J and susceptible C57BL/6J progenitors). Two different criteria were used to determine their resistance or susceptibility to developing MAIDS: the gross pathologic evaluation of lymphoid organs at 13–15 weeks of infection, and survival. RI mouse strains segregated into two non-overlapping groups. The first group did not develop any significant pathology, and these mouse strains were considered as resistant to MAIDS. The second group showed the virus-induced pathological changes as well as an immunological dysfunction as seen in C57BL/6J progenitor mice, and these strains were thus considered as susceptible to MAIDS. This bimodal strain distribution pattern of resistance/susceptibility to MAIDS among the RI strains suggests that this phenotype is controlled by a single gene. Linkage analysis with other allelic markers showed a strong association between resistance/susceptibility to MAIDS and theH-2 complex. Possession of theH-2 ^b haplotype derived from C57BL/6J mice was associated with susceptibility to MAIDS, while theH-2 ^a haplotype conferred resistance to the disease. This finding was confirmed by demonstrating thatH-2 ^a congenics on the susceptible C57BL/10 background were as resistant to MAIDS as A/J mice which donated theH-2 ^a locus. Gene(s) within theH-2 complex thus represent the major regulatory mechanism of resistance/susceptibility to MAIDS.     

2,3,7,8-tetrachlorodibenzo-p-dioxin induced alterations of pyruvate carboxylase levels and lactate dehydrogenase isozyme shifts in C57BL/6J male mice

A dose-dependent reduction of hepatic pyruvate carboxylase levels and activity occurs in C57BL/6J male mice given 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) i.p. in a corn oil carrier. The dose range was from 1 to 75 μg/kg body weight and the analysis was done 8 days postinjection. At the maximum TCDD level investigated, we found a 10-fold reduction in pyruvate carboxylase activity. Furthermore, TCDD at a dose of 1 μg/kg body weight blocks corn oil induction of an increase in the amount of pyruvate carboxylase in liver protein extracts. At doses beyond those required to initiate a reduction in pyruvate carboxylase, lactate dehydrogenase isozyme patterns shift. This is accompanied by an increase in blood lactic acid levels. We propose that TCDD-mediated reduction in pyruvate carboxylase and lactate dehydrogenase isozyme shifts may represent a major component in TCDD toxicity.     

Identification of the site of inhibition of oncogenic ras-p21-induced signal transduction by a peptide from a ras effector domain

We have previously found that a peptide corresponding to residues 35–47 of the ras-p21 protein, from its switch 1 effector domain region, strongly inhibits oocyte maturation induced by oncogenic p21, but not by insulin-activated cellular wild-type p21. Another ras–p21 peptide corresponding to residues 96–110 that blocks ras–jun and jun kinase (JNK) interactions exhibits a similar pattern of inhibition. We have also found that c-raf strongly induces oocyte maturation and that dominant negative c-raf strongly blocks oncogenic p21-induced oocyte maturation. We now find that the p21 35–47, but not the 96–110, peptide completely blocks c-raf-induced maturation. This finding suggests that the 35–47 peptide blocks oncogenic ras at the level of raf; that activated normal and oncogenic ras–p21 have differing requirements for raf-dependent signaling; and that the two oncogenic-ras-selective inhibitory peptides, 35–47 and 96–110, act at two different critical downstream sites, the former at raf, the latter at JNK/jun, both of which are required for oncogenic ras-p21 signaling.     

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.