Aryl Hydrocarbon Hydroxylase Activity In Liver Research Articles

Monoclonal Antibody-Directed Analysis of Benzo[

Monoclonal antibody (MAb) 1-7-1 against 3-methylcholanthrene (MC)-induced forms of cytochrome P-450 (CYP) was used to characterize benzo[a]pyrene (B[a]P) metabolism in rat liver and extrahepatic tissues and its modulation by phenolic antioxidants, propyl and octyl gallates. Male Wistar rats were treated with these food additives (50 mg/kg body wt ip) twice weekly for 14 days alone or in combination with MC. Immunochemical inhibition of aryl hydrocarbon hydroxylase (AHH) and [<O*>14C]B[a]P metabolism (analyzed by high-performance liquid chromatography) were measured in liver, kidney, and lung microsomes. Organ-specific changes in levels of MAb-mediated inhibition of microsomal metabolism of B[a]P were observed. In liver microsomes from untreated rats, AHH was not affected by MAb, but in kidney and lung, there was 70% and 50% inhibition, respectively. In MC-treated rats, MAb reduced AHH activity by 43% in liver. Kidney and lung AHH was inhibited up to 80% by this MAb. Formation of B[a]P metabolites in MC-induced microsomes from liver and kidney was affected by MAb in a similar way. In lung, the total metabolism was inhibited by 50% by MAb treatment, but significant differences in inhibition of individual metabolites were observed. Treatment with propyl or octyl gallate alone had no effect on MAb inhibition of AHH activity in liver and lung but decreased the level of inhibition in kidney. Combined treatment with MC and propyl or octyl gallate slightly reduced the effect of MAb on AHH activity in liver and significantly reduced the level of inhibition in kidney but did not affect AHH activity in lung. The same treatment regimen dramatically reduced MAb inhibition of B[a]P metabolism in kidney but had no effect on B[a]P metabolite formation in liver. Inhibition by MAb of renal 3-hydroxy-B[a]P, 7,8-B[a]P-dihydrodiol, and 1,6-quinone-B[a]P was the most affected. In lung, treatment with gallates affected only formation of 7,8-B[a]P-dihydrodiol. These results suggest that treatment with gallates affects the CYP 1A and may change the CYP isozyme composition and, thus, alter the tissues' susceptibility to tumor induction by B[a]P.

Retinoids: From metabolism to environmental monitoring

Adult herring gulls, Larus argentatus, were captured on their nests at various colonies on the Great Lakes and retinoids (vitamin A) analyzed in the liver by reverse-phase HPLC. All- trans-retinol and all- trans-retinyl palmitate concentrations showed significant differences between colonies. Colonies having the highest concentrations of 2,3,7,8-tetrachlorodibenzo- p-dioxin had the lowest retinoid concentrations. Particularly low levels of hepatic retinoids occurred in Lake Ontario gulls in 1987; and all- trans-retinol was negatively correlated with liver aryl hydrocarbon hydroxylase activity. Liver retinoids were identified in the white sucker, Catostomus commersoni, using reverse-phase HPLC with diode array detection. All- trans-retinol, all- trans-retinyl palmitate, and 3,4-didehydroretinyl esters were detected. In the case of adult white suckers sampled from a contaminated river near Montreal and a control site, significantly lower levels of all- trans-retinol and all- trans-retinyl palmitate were found in the contaminated population.

Organ-selective induction of cytochrome P-450-dependent activities by indole-3-carbinol-derived products: Influence on covalent binding of benzo[ a]pyrene to hepatic and pulmonary DNA in the rat

Indole-3-carbinol (I3C) is a dietary modulator of carcinogenesis that can reduce the level of carcinogen binding to DNA. I3C-derived products are potent inducers of certain cytochrome P-450(CYP)-dependent enzyme activities. To investigate whether the protective effects of I3C against carcinogen damage to DNA are associated with increased activities of CYP1A1 enzymes, we examined the relationship of I3C-mediated organ-specific CYP enzyme induction with total levels of benzo[ a]pyrene (BP) binding to hepatic and pulmonary DNA of rats. Oral intubation (PO) of I3C (500 μmol/kg body wt.) in 10% DMSO in corn oil produced after 20 h, increases in ethoxyresorufin O-deethylase (EROD) activities (associated with CYP1A1 isozyme) of 700-fold, 245-fold and 36-fold in small intestine, lungs and liver, respectively, compared with activities in untreated controls. Hepatic aryl hydrocarbon hydroxylase (AHH) activity was increased 4-fold under these conditions. Pentoxyresorufin O-depentylase (PROD) activity (associated with CYP2B isozyme) was increased 6-fold in the liver but was unaffected in lung and small intestine. Intraperitoneal injection (IP) of I3C (500 μmol/kg body wt.) produced no significant change in EROD or PROD activities in lung, liver, or small intestine. PO administration of the acid reaction mixture (RXM) of I3C increased hepatic AHH activity (5-fold) and EROD activities in small intestine (650-fold), lung (100-fold) and liver (18-fold). IP administration of RXM (equivalent to 500 μmol I3C/kg body wt.) significantly increased only EROD activity in lung and liver, but did not affect EROD activity in small intestine, AHH activity in liver, or PROD activity in any of the organs examined. Twenty hours after inducer treatment, half of the rats were treated PO with 0.2 μmol [ 3H]BP in corn oil. Analysis of tissues 5 h after BP administration indicated that compared with untreated controls, administration of I3C and RXM by either route reduced by 30–50% the level of BP binding to hepatic DNA, an effect that was not correlated to CYP1A1 enzyme induction in any of the organs examined. However, PO administration of I3C and RXM produced a 50–70% decrease in carcinogen binding to pulmonary DNA, while IP administration of inducers had no effect on DNA binding in this organ. These results with the lung are consistent with an increased presystemic clearance of BP in the intestine and are discussed in terms of the role of induction of intestinal CYP1A1 activity in the decreased lymphatic and venous transport of unmetabolized BP to the lung.

Monocyte aryl hydrocarbon hydroxylase (AHH) activity mimics Kupffer cell and hepatocyte AHH activity in an animal model of liver disease

We have previously reported that monocyte aryl hydrocarbon hydroxylase (AHH) activity is depressed in patients with liver disease and is decreased more in cirrhosis than in early stage liver disease. To determine if monocyte AHH activity reflects liver AHH activity, we studied an animal model of cirrhosis, i.e., yellow phosphorus induced cirrhosis in the pig. AHH activity was detectable in monocytes isolated from peripheral blood of normal pigs (0.32 +/- 0.13 nmol.mg-1 P.h-1, n = 11) and was comparable to the level of AHH activity in hepatic Kupffer cells isolated from wedge or needle biopsies of livers of normal pigs (0.38 +/- 0.21, n = 7). The AHH level in pig Kupffer cells was approximately 10% of the AHH level in hepatocytes and microsomes. To induce liver disease, pigs were administered yellow phosphorus (0.6 mg/kg) 5 days per week for 16 weeks. At 4 weeks of treatment, monocyte AHH activity was not different from control and liver histology was normal. Depression of monocyte AHH activity was evident at 8 weeks of treatment when liver fibrosis was seen histologically. At 12 weeks of treatment when histology revealed extensive liver fibrosis and collagen levels were elevated, the level of monocyte AHH activity was decreased 67% compared with controls. Similar changes were observed at 12 weeks in Kupffer cell AHH activity (86% decrease) and hepatocyte AHH activity (70% decrease) compared with controls. These results suggest that monocyte AHH activity reflects liver AHH activity and may be a good indicator of change in liver enzyme function in liver disease in the pig model of cirrhosis.

Effects of 3-methylsulphonyl-4,5,3′,4′-tetrachlorobiphenyl and 7,8-benzoflavone on mouse liver aryl hydrocarbon hydroxylase activity in vitro

The activity of aryl hydrocarbon hydroxylase (AHH) in mouse liver microsomes was assayed in the presence of 3-methylsulphonyl-4,5,3′,4′-tetrachlorobiphenyl (3-MSF-TCB). The mice had been previously injected with a fixed amount of the AHH inducer 3-methylcholanthrene (MC) or 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) or with the vehicle (olive oil) alone. The dose-effect patterns for 3-MSF-TCB were markedly different according to the genetic responsiveness of the mice towards aromatic hydrocarbons ( Ah responsiveness); in Ah responsive strains 3-MSF-TCB inhibited the MC-induced AHH activity as did 7,8-benzoflavone (ANF), which is known to be a potent inhibitor of AHH, whereas in Ah non-responsive strains 3-MSF-TCB (and ANF) greatly enhanced the same activity. On the other hand, the dose-response patterns were similar for both types of mice for TCDD-induced activity or basal activity (mice injected with the vehicle alone). The dose-effect curves for 3-MSF-TCB were quite distinct from those for ANF, particularly with respect to the basal AHH activity, for both Ah responsive and non-responsive strains. These results indicate that both ANF and 3-MSF-TCB can have either an inhibitory or an activating effect on AHH, depending on the dose, the Ah phenotype and previous induction with MC or TCDD.

Differential induction of fetal mouse liver and lung cytochromes P-450 by -naphthoflavone and 3-methylcholanthrene

Previous studies have shown that the incidences of liver and lung tumors in mice exposed transplacentally to 3-methyl-cholanthrene (MC) were significantly influenced by the sensitivity of both mothers and fetuses to induction of cytochrome(s) P-450 by polycyclic aromatic hydrocarbons. In order to delineate further the biochemical and molecular processes underlying the observed biological effects, the inductive effect of MC and beta-naphthoflavone (beta NF) on cytochrome P-450 was determined at the biochemical and molecular levels. C57BL/6 females were mated with DBA/2 males and treated i.p. on day 17 of gestation with olive oil alone, 150 mg/kg of beta NF or different doses of MC. At various times after injection the mothers were sacrificed and the fetuses removed for biochemical and molecular studies. MC caused maximal induction of aryl hydrocarbon hydroxylase (AHH) activity by 8 h in both the liver and lung. beta NF caused nearly maximal induction of AHH activity by 8 h in the lung but had little effect on liver AHH activity at this time. Maximal induction with beta NF occurred by 24 h in both organs. Addition of monoclonal antibody 1-7-1, specific for the MC-inducible forms of cytochrome P-450 (P-450IA1 and A2), to the incubation mixtures resulted in a 55-70% inhibition of AHH activity in both lung and liver assays, regardless of the inducing agent used, while having no effect on AHH activity from oil-treated mice. RNA blot analysis carried out in parallel with enzyme assays demonstrated that the levels of enzyme activity correlated very well with the levels of steady-state RNAs. MC caused maximal induction of P-450IA1 RNA levels 4 h after injection in both organs and a biphasic secondary increase was observed in the lung. Maximal levels of P-450IA1 RNA were seen at 12-16 h following injection of beta NF. However, the ratio of P-450IA1 RNAs present at 16 versus 2 h in the beta NF-treated liver appeared greater than that in the lung. P-450IA2 was also induced in fetal liver and lung, but at low levels relative to P-450IA1. The results indicate that the increase in functional AHH activity was primarily due to induction of cytochrome P-450IA1. The differences in induction kinetics observed for cytochromes P-450IA1 and A2 suggest that these enzymes exhibit both tissue- and inducer-dependent specificity.

Effect of chronic cysteamine treatment on mouse liver aryl hydrocarbon hydroxylase activity.

Patients receive chronic cysteamine in the management of nephropathic cystinosis. In a previous report our results indicated that acute cysteamine treatment inhibited cytochrome P-450. Cysteamine (85 mg/kg i.p.) was administered daily to female Swiss mice for 1.5 and 8.5 months. Cysteamine treatment (8.5 months) did not affect hepatic microsomal aryl hydrocarbon hydroxylase (AHH) activity compared with controls. A small decrease in liver AHH activity was seen after 1.5 months of treatment with cysteamine. Liver histology, body weight, liver and spleen weights, and serum aminotransferase activity after chronic and subchronic treatment did not differ from controls. Chronic in vivo cysteamine treatment, unlike acute in vitro treatment did not decrease AHH activity. Incubation of isolated murine hepatocytes with cysteamine significantly inhibited AHH activity compared with controls. The inhibition occurred in a concentration-related manner, with 65% inhibition at 8.8 mM (1 mg/mL) (equivalent to the predicted plasma concentration using the maximally tolerable human dose), and 100% inhibition at 44 mM (5 mg/mL). The concentrations used in vitro were not cytotoxic. This suggests that chronic cysteamine treatment may not result in drug interactions and that in vitro results are not always good indicators of in vivo effects.

Mechanism of protection against pneumotoxicity caused by O,S,S-trimethyl phosphorodithioate.

O,S,S-Trimethyl phosphorodithioate (OSS-TMP) is a pneumotoxic impurity present in various organophosphorus insecticides. It produces morphological alteration of bronchiolar epithelial Clara cells accompanied by an increase in relative wet weight of the lung and severe body weight loss in rats. The present investigation demonstrates that rats pretreated with small doses of OSS-TMP develop a tolerance against body weight loss caused by a large dose of the compound. Pretreatment for three to four days gave better protection than that of one to two days with a concomitant decrease in relative lung weight to control level, although the protective effect against morphological change in lung parenchyma was not as striking; all the pretreatment regimens (one to four days) gave similar and rather moderate protection. Microsomal enzyme activities were measured in order to examine a possible biochemical mechanism of tolerance, and it was found that the small multiple doses of OSS-TMP for three and four days decreased arylhydrocarbon hydroxylase activity in liver and lung. Pretreatment of rats with phenobarbital or SKF 525-A protects against body weight loss and increase in relative lung wet weight induced by OSS-TMP. Light microsopic observation of lung tissue also indicates that these pretreatments abolish the abnormality of lung parenchyma. We postulate that pretreatment with phenobarbital or SKF 525-A induces the detoxification process(es) and inhibits the activation process(es) of OSS-TMP, respectively, resulting in a protective effect against OSS-TMP-induced toxicity. These findings further support the view that the lungs may be a target organ of delayed toxicity produced by OSS-TMP, and a metabolite(s) of OSS-TMP is involved in the manifestation of the toxicity.

Effect of polybrominated biphenyls, β-naphthoflavone and phenobarbital on arylhydrocarbon hydroxylase activities and chloroform-induced nephrotoxicity and hepatotoxicity in male C57BL/6J and DBA/2J mice

Administration of chloroform (CHCl 3) to male C57/6J (C57) and DBA/2J (DBA) mice produced dose-dependent hepatic and renal damage. Hepatic arylhydrocarbon hydroxylase (AHH) activity was higher in C57 than DBA mice; in kidney, AHH activity was higher in DBA than in C57 mice. CHCl 3 caused the same degree of liver damage in both strains of mice; however, nephrotoxicity of CHCl 3 was greater in DBA than in C57 mice. Pretreatment of C57 and DBA mice with phenobarbital (PB) markedly increased hepatic AHH activity and hepatotoxicity of CHCl 3 in both strains but did not affect renal AHH or nephrotoxicity of CHCl 3. Similarly, β-naphthoflavone (BNF) enhanced AHH activity and CHCl 3 hepatotoxicity in C57 mice, but had little effect on nephrotoxicity. BNF did not affect hepatic AHH nor CHCl 3-induced hepatic injury in male DBA mice. Pretreatment with polybrominated biphenyl (PBB) enhanced AHH activity in liver and CHCl 3 hepatotoxicity in both strains. After PBB, nephrotoxicity of CHCl 3 and renal AHH activity were increased in C57 mice whereas PBB did not alter nephrotoxicity or renal AHH in DBA mice. These results suggest that CHCl 3-nephrotoxicity is independent of hepatoxicity.

Effect of steroid hormone treatment on aryl hydrocarbon hydroxylase activity in the Syrian hamster kidney

Aryl hydrocarbon hydroxylase (AHH) activity was determined in castrate and intact male Syrian hamster kidney and liver microsomes following in vivo treatment with either diethylstilbestrol (DES) or 17β-estradiol as well as other steroid hormones. After 1 month of estrogen treatment, there was a 5-fold decline in AHH activity in castrated hamster kidneys compared with untreated castrate levels. the amount of AHH activity in the kidney was depressed more than 75% of untreated castrate levels even after the estrogen had been withdrawn for 6 days. Consistent with a nearly 2.5-fold higher renal AHH activity observed in intact male hamsters compared to castrates was the finding of a 1.7-fold elevation in the activity of this enzyme after treatment of castrated animals with androgen[5α-dihydrotestrosterone (5α-DHT)] for 1 month. Moreover, following withdrawal of estrogen from intact hamsters, the increase in AHH activity in the kidney essentially paralleled the rise in serum testosterone levels. In castrated animals, the depression of AHH activity by estrogen was partially reversed by concomitant 5α-DHT treatment. However, no appreciable changes were seen in liver AHH activity with androgen treatment in the presence or absence of estrogen. Similarly, the level of AHH activity, which was nearly 7- and 14-fold higher than intact and castrate kidney levels, respectively, was not altered by estrogen treatment. Neither progesterone nor cortisone had any effect on the levels of AHH activity in either the kidney or liver. Therefore, AHH activity in the male hamster kidney, but not the liver, is responsive to both estrogens and androgenic hormone.

Induction of Neonatal Rat Skin and Liver Aryl Hydrocarbon Hydroxylase by Coal Tar and Its Constituents

Topical application of coal tar solution (USP) to neonatal rats resulted in the induction of skin and liver aryl hydrocarbon hydroxylase (AHH) activities. Furthermore indirect exposure of the animals to coal tar vapors resulted in induction of the enzyme in skin and liver. Cutaneous application of coal tar to pregnant rats resulted in induction of skin and liver AHH activity in both mothers and prenatal rats. Among several defined constituents of coal tar tested benzo(a)pyrene (BP), anthracene and acridine were found to have measurable induction effects on neonatal rat skin and liver AHH. These studies indicate that therapeutic coal tar solution as well as selected defined chemical constituents of coal tar are capable of altering the activity of AHH in skin and liver.

Murine colonic mucosal metabolism and cytotoxicity of benzo [alpha] pyrene.

Homogenates of colonic mucosa from seven mouse strains metabolized 14 C-benzol [alpha] pyrene (BP) to alkali-soluble and water-phase products. In those strains in which liver aryl hydrocarbon hydroxylase activity is inducible by beta-naphthoflavone (beta-NF), the C57Bl/6, BALB/c, C3H and random-bred Swiss CD-1, colonic metabolism of BP was also induced by intraperitoneal treatment with 150 mg/kg beta-NF 24 h before assay. No increase in the colonic metabolism of BP was seen after beta-NF treatment of the noninducible strains, AKR, DBA and SWR. A detailed study of the products formed was carried out with C57BL/6 male mice. High-performance liquid-chromatography analysis of the products of BP metabolism revealed that phenolic derivatives predominated, but that a small percentage of the BP was converted into the proximate carcinogen, the BP-7,8-diol. Addition of uridine 5'-diphosphoglucuronic acid to the incubation mixture caused an increase in total product formation and a shift from alkali-soluble to water-phase products. Of the water-phase products formed by beta-NF-induced colonic mucosal homogenates, about 40% were nondialyzable and associated with precipitable material. In a study of the cellular effects of BP on the colonic mucosa, intrarectal administration of 1 mg BP had a significant cytotoxic effect, as indicated by the number of pyknotic cells and 3H-thymidine-labeling index, and prior treatment with beta-NF afforded significant protection against this cytotoxicity.

Effects of indole and tryptophan on cytochrome P-450, dimethylnitrosamine demethylase, and arylhydrocarbon hydroxylase activities

The effects of indole and l-tryptophan feeding on the activities of two liver microsomal enzymes, dimethylnitrosamine demethylase (DMN-d) and arylhydrocarbon hydroxylase (AHH), were studied in rats and hamsters. Both indole and tryptophan increased cytochrome P-450 concentration and DMN-d activity in rats, but only indole was effective in hamsters. AHH activity in both species was induced by indole. Both indole and tryptophan increased the ratio A 392–500nm/ A 410–500nm obtained by n-octylamine difference spectroscopy of rat liver microsomes but not of hamster liver microsomes. Indole increased the absorbance difference ratio A 455–500nm/ A 430–500nm of rat liver cytochrome P-450 obtained by ethyl isocyanide. The opposite was true for hamsters, which showed a low band at 455 nm. Indole gave a type II binding spectrum with rat liver microsomes, but the values for absorption minimum and maximum for tryptophan were somewhat outside the reported values for type II binders. Indole which induced liver AHH activity, protected against 7,12-dimethylbenz[a]anthracene (DMBA)-produced mammary gland carcinogenicity, whereas tryptophan was without effect.

Altered drug metabolism in hepatic and extrahepatic tissues in mice as a function of age

An increase in the susceptibility to drugs is known to occur with advanced age. Several possible explanations for this phenomenon exist, but the exact cause has not been determined. Aryl hydrocarbon hydroxylase (AHH), 7-ethoxycoumarin O-deethylase (ECD) and aniline hydroxylase activities of hepatic and extrahepatic tissues as well as hepatic cytochrome P-450 content of female Swiss Webster mice at 1, 3, 6, 9, 12, 15 and 18 months of age were examined. AHH activity in liver and intestine reached a maximum at 6 mo. of age and began to decline thereafter. No change in pulmonary AHH activity was observed with age. ECD activity in liver as well as lungs was maximal at 6 mo. of age and decreased thereafter. Hepatic aniline hydroxylase activity exhibited over a 700% increase between one to six mo. of age, and declined by approximately 52% by 15 mo. of age as compared to peak activity. Hepatic cytochrome P-450 content was highest at 3 mo. of age and significantly decreased by 12 mo. The results demonstrate that decreases in microsomal mixed function monooxygenase activities do occur with advanced age and may contribute to a decreased rate of metabolism and increased susceptibility to drugs and other foreign chemicals with age.

Aryl Hydrocarbon Hydroxylase and Epoxide Hydratase Activities: Age Effects in Mammary Epithelial Cells of Sprague-Dawley Rats&lt;xref ref-type="fn" rid="FN2"&gt;2&lt;/xref&gt;&lt;xref ref-type="fn" rid="FN3"&gt;3&lt;/xref&gt;

By use of techniques developed for rat mammary epithelial cell isolation, studies were conducted to characterize aryl hydrocarbon hydroxylase (AHH) and epoxide hydratase (EH) activities and to determine whether changes in the enzyme activities might contribute to the age-dependent changes in polycyclic aromatic hydrocarbon (PAH)-induced tumorigenesis in the rat mammary gland. Constitutive AHH and EH activities in rat mammary epithelial cell homogenates were substantially lower than those measured in liver (9,500×g) supernatant. The administration of 3-methylcholanthrene (MCA) or 7,12-dimethylbenz[a]anthracene increased mammary epithelial AHH activity 93- and 44-fold, respectively, whereas hepatic AHH levels rose 17- and 6-fold. Neither inducer had any effect on constitutive EH activity in either tissue. The addition (0.1 mM) of 7,8-benzoflavone (7,8-BF), SKF 525-A, or metyrapone to homogenates of mammary epithelial cells from MCA-treated rats inhibited AHH activity 93, 48, and 22%, respectively. In contrast, 7,8-BF addition decreased rat liver AHH activity 83%, whereas SKF 525-A and metyrapone had little effect. EH activities in rat liver and mammary epithelial cells were reduced with addition of 1,1,1-trichloro-2-propene oxide and cyclohexene oxide; however, trans-stilbene oxide had no effect in either tissue preparation. Additional studies determined the age-dependent changes in rat mammary epithelial AHH and EH activities. Constitutive AHH activity remained unchanged in rats between 40 and 100 days of age. In contrast, mammary epithelial EH activity was highest in 40-day-old Sprague-Dawley rats, declined in rats between 40 and 60 days of age, and remained constant in rats between 60 and 100 days of age. The level of MCA-induced AHH activity in rat mammary epithelial cell homogenates was age-dependent. MCA administration induced constitutive AHH activity 85- and 74-fold in mammary epithelial cells isolated from 50- and 60-day-old Sprague-Dawley females. The magnitude of mammary epithelial AHH induction was significantly lower in both younger and older animals. The results demonstrated PAH-metabolizing enzymes in rat mammary epithelial cells and suggested that the age-dependent incidence of mammary tumorigenesis may be attributed in part to the age-dependent inducibility of AHH activity.

Effects of polyhalogenated aromatic hydrocarbons on benzo(a)pyrene hydroxylase activity in the intestine and liver

Aryl hydrocarbon hydroxylase (AHH) has been measured as benzo(a)pyrene hydroxylase in the intestine and liver of rats and mice treated with a single dose of different polyhalogenated aromatic hydrocarbons. Maximal stimulation of liver AHH activity is reached with a dose of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) half as great as that necessary for maximal stimulation of the intestine. The duration of the effect of TCDD on intestinal AHH differs from the constant increase that occurs in the liver. Although the magnitude of the stimulation by 1,1,1-trichloro-2,2-bis (p-chlorophenyl) ethane (DDT) is less than that of TCDD, the qualitative changes in intestinal and liver AHH are similar. The changes in activity of intestinal and hepatic AHH were not directly correlated in the tissues of rats treated with several other polyhalogenated aromatic hydrocarbons. Liver and intestinal AHH activity were affected differently by fasting. These results suggest that AHH activity in the intestine and liver has different control mechanisms.

Aryl hydrocarbon (benzo[a]pyrene) hydroxylase in rat mammary tissue during pregnancy and lactation

Rats are highly resistant to mammary tumour induction by polycyclic aromatic hydrocarbons during pregnancy and lactation. Since local changes in hydrocarbon metabolism in mammary tissue or altered hepatic metabolism might contribute to the resistance, aryl hydrocarbon hydroxylase (AHH) activity in microsomes from mammary tissue and liver was measured throughout the course of pregnancy and lactation in Sprague–Dawley rats. Basal constitutive AHH activity in mammary tissue and liver of untreated rats did not change significantly during pregnancy or lactation. In rats injected with beta-naphthoflavone (BNF), AHH activity in liver microsomes increased approximately 25-fold over basal levels and the degree of enzyme induction by BNF was relatively constant throughout pregnancy and lactation. In mammary tissue, however, the induction of AHH by BNF increased dramatically from about 10 times basal at the beginning of pregnancy to over 80 times basal on day 3 of lactation. The high inducibility of mammary AHH during late pregnancy and early lactation exists both when activity is expressed per unit microsomal protein or per unit tissue DNA. Increased inducibility of mammary AHH may contribute to the resistance rats show to hydrocarbon carcinogenesis during pregnancy and lactation. Since the activity of hepatic AHH does not change significantly during pregnancy or lactation, it seems unlikely that resistance is due to altered hepatic metabolism of the hydrocarbons. AHH activity can be associated both with 'metabolic activation' and 'detoxification' of aromatic hydrocarbons. The balance between local 'activation' and 'detoxification' of aromatic hydrocarbons by AHH in mammary tissue during different physiological states is important in determining susceptibility to carcinogenesis and requires further study.

Differential inhibition of aryl hydrocarbon hydroxylase in human foetal liver, adrenal gland and placenta*.

Abstract It is known from experiments on rats and mice that control and polycyclic aromatic hydrocarbon‐induced activities of aryl hydrocarbon hydroxylase differ profoundly with regard to their inhibition by several compounds. In order to obtain information about the nature of the human enzyme system from different tissues, the inhibition of aryl hydrocarbon hydroxylase (AHH) by different compounds in the human foetal and adult liver, foetal adrenal gland, as well as the placenta was studied. Substantial AHH activity was present in all the livers and adrenal glands, but only in term placentas from smoking mothers. The placental AHH was inhibited by 7,8‐benzoflavone, but not appreciably by SKF 525 A, aminopyrine or metyrapone. On the other hand, the foetal liver AHH was inhibited by SKF 525A, aminopyrine and metyrapone, whereas 7,8‐benzoflavone apparently activated the enzyme. The adult human liver AHH resembled that of foetal liver. The foetal adrenal AHH displayed a pattern of inhibition differing from both the liver and the placenta. Maternal cigarette smoking did not have a substantial effect either on the enzyme activity or on the inhibitory properties. Foetal hepatic AHH resembled the enzyme from control‐rat liver and the placental AHH was similar to the enzyme from 3‐methylcholanthrene‐treated rat liver. The effect of 7,8‐benzoflavone on control rat liver AHH activity varied greatly from the prenatal to the adult period, whereas AHH activity in MC‐treated rat liver was always strongly inhibited by 7,8‐benzoflavone.

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.