Selenium and drug metabolism--I. Multiple modulations of mouse liver enzymes

Male albino mice were raised on diets containing less than 10 ppb selenium (Se-) or supplemented with 0.5 ppm selenium (Se+) for 6 months. In the (Se-) group total liver selenium was less than 10% of the control, liver selenium-dependent glutathione peroxidase (GSH-Px) less than 2%. The specific activities of catalase and superoxide dismutase showed essentially no differences between the dietary groups. Several phase I-related specific enzyme activities were measured in liver microsomes. No significant differences between the two animal groups were found for cytochrome P-450 and b 5 content, NADH-cytochrome b 5 reductase, as well as for aniline hydroxylation and aminopyrine dealkylation rates. In (Se-) microsomes, NADPH-cytochrome P-450 reductase activity was about half that found in (Se+) microsomes. An increase in microsomes from (Se-) mice was found for 7-ethoxycoumarine deethylation rate (460%), cytochrome P-450 hydroperoxidase activity (170%), and heme oxygenase (276%). The N-oxidation rate of the flavin-containing monooxygenase decreased by 35%, the N-demethylation rate by 50% in (Se-) animals. Stopped-flow measurements of the reduction rates of microsomal pigments did not support evidence for limitations in microsomal electron supply during selenium deficiency. Among the phase II reactions examined, sulfotransferase activity towards 4-nitrophenol was 47% of the controls in Se-deficient liver cytosols while UDP-glucuronyl transferase activity towards this substrate increased to 215%. Glutathione-S-transferase activity was much higher in (Se-) livers than in (Se+): 310% with 1,2-dichloro-4-nitrobenzene, 255% with 1-chloro-2,4-dinitrobenzene and 120% with ethacrynic acid as substrate. The data indicate that in addition to GSH-Px many other enzyme activities in mouse liver are affected by prolonged dietary selenium deficiency. These effects might be useful in assessing the severity of selenium deficiency. A microsomal selenium-dependent metabolic modulator is discussed as a possible mechanism.     

Selenium and drug metabolism--III. Relation of glutathione-peroxidase and other hepatic enzyme modulations to dietary supplements

Male mice were fed a torula yeast-based diet containing different amounts of added selenium for a period of 4 months. Liver glutathione peroxidase activity assayed with H2O2 showed a logarithmic dependence on dietary selenium with a saturation plateau above 2 ppm Se and an extrapolated zero of 0.02 ppm Se. In contrast, liver selenium content and GSH-Peroxidase activity showed a linear correlation. Glutathione peroxidase activity became undetectable at a liver Se content of about 90 ng Se/g liver wet wt. Thus, about 10% of liver selenium is not related to GSH-Px activity. Five dietary groups were supplemented, respectively, with 0, 0.05, 0.5, 5.0 and 10 ppm Se in the form of Na2SeO3. Some changes in drug metabolism enzymes were observed with the high Se diets. An increase occurred in Non-Se-GSH activity as well as in ethacrynic acid-assayed GSH transferase, these are interpreted as early signs of Se toxicity. The diet containing 0.01 ppm Se with no supplementary Se produced the multiple hepatic enzyme modulations which were previously reported. The animals raised on this very low Se diet had normal hepatic contents of glutathione, alpha-tocopherol, calcium, magnesium, iron, zinc, copper and manganese compared to controls supplemented with 0.5 ppm Se. However, significant changes in the microsomal fatty acid pattern were observed while the total phospholipid content as well as membrane fluidity showed no differences between the two dietary groups.     

Aryl hydrocarbon hydroxylase activity levels in the hepatic microsomal fraction from MC- or TCDD-treated mice: a comparison between aromatic hydrocarbon responsive and non-responsive strains

The effects of in vivo administration of polycyclic aromatic hydrocarbons on the levels of aryl hydrocarbon hydroxylase (AHH) activity in aromatic hydrocarbon (Ah) responsive and non-responsive strains of mice were studied using the hepatic microsomal fraction. Injection of 3-methylcholanthrene (MC; 42 mg/kg body wt.) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 120 micrograms/kg body wt.) into both strains produced marked enhancement of AHH activity except for MC treatment of Ah non-responsive strains. Addition of 7,8-benzoflavone (BNF) to the microsomal AHH assay mixture prepared from mice previously injected with vehicle (olive oil) alone caused an increase in activity when the mice were responsive, while BNF lowered the activity in non-responsive strains. With regard to MC-injected mice, BNF and 3-methyl-sulphonyl-4,5,3',4'-tetrachlorobiphenyl (3-MSF-TCB) decreased microsomal AHH activity in Ah-responsive mice, whereas these drugs enhanced the activity in Ah-non-responsive strains. 3-MSF-TCB also had inhibitory potency on AHH activity, but the mechanism of inhibition seems to be somewhat different from that of BNF. It may also suggest that cytochrome P-450 isozymes inhibited by BNF are different from those inhibited by 3-MSF-TCB.     

The effects of buthionine sulphoximine (BSO) on glutathione depletion and xenobiotic biotransformation

Buthionine sulphoximine (BSO) is an inhibitor of gamma-glutamylcysteine synthetase (gamma-GCS) and, consequently lowers tissue glutathione (GSH) concentrations. In fed male C3H mice, liver and kidney GSH levels were depleted by BSO in a dose dependent manner with maximum effect (35% of initial levels) occurring with doses between 0.8 and 1.6 g/kg, i.p. At these doses maximum effects on gamma-GCS and GSH were observed 2-4 hr after BSO administration; initial gamma-GCS activity and GSH content were restored approximately 16 hr post BSO. BSO, either in vivo or in vitro, had no effect on hepatic microsomal cytochrome P-450 levels, a range of cytochrome P-450 dependent enzyme activities or p-nitrophenol glucuronyl transferase activity. Similarly, BSO had no effect on phenol sulphotransferase and two GSH-transferase activities in the 105,000 g supernatant fraction. BSO had no effect on the duration of hexobarbitone induced narcosis in mice. Consistent with specific inhibition of GSH synthesis, BSO pretreatment of mice decreased the proportion of a 50 mg/kg dose of paracetamol excreted in the urine as GSH-derived conjugates but did not affect paracetamol clearance through the glucuronidation or sulphation pathways. Since BSO does not affect cytochrome P-450 or conjugating enzyme activity, its use as a specific depletor of tissue GSH in the investigation of mechanisms of xenobiotic-induced toxicities is preferable to the standard GSH-depleting agents as these have other enzymic effects.     

The influence of selenium intake on chronic adriamycin toxicity and lipid peroxidation in rats

This paper reports on the influence of selenium intake on antioxidant protective systems during chronic adriamycin (AM) treatment in rats. Rats were kept for 14 weeks on a selenium deficient (Se-) diet or a diet containing selenium (Se+). No significant differences were found in any group with regard to the cardiac content of total and reduced glutathione (GSH) and heart superoxide dismutase specific activity. AM treatment did not modify lipid peroxidation as measured by cardiac malondialdehyde (MDH) formation in rats receiving either the Se- or the Se+ diet. In the Se+ rats AM had no effect on the exhalation of ethane or pentane but decreased the exhalation of ethane and increased that of pentane in the SE- rats. In Se- AM-treated rats mortality was higher. Since this did not seem to be correlated with modifications of any of the biochemical parameters taken into consideration, it is suggested that the better resistance of Se+ animals to AM treatment is related to some factors not yet identified.     

Role of selenium-dependent glutathione peroxidase in protecting against t-butyl hydroperoxide-induced damage in hepatocytes

The role of the selenoenzyme glutathione peroxidase (Se-GSHPx) in protecting against oxidative injury was studied in hepatocytes isolated from rats fed either a low-selenium (Se-) or a selenium-adequate (Se+, control) diet. In rats fed Se- diet for eight weeks the selenium content of plasma and liver was lowered to 15 and 8%, respectively. No Se-GSHPx and only 5% of total GSHPx activity was detected in Se- hepatocytes. However, the Se- hepatocytes were as resistant as the Se+ cells to oxidative injury by 0.8 mM tert-butyl hydroperoxide (t-BuOOH), or 0.2 mM t-BuOOH plus 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of oxidized glutathione (GSSG) reductase. Only at 1.5 mM t-BuOOH or at 0.5 mM t-BuOOH with BCNU were cell damage and lipid peroxidation more evident in Se- cells. At all t-BuOOH concentrations used the depletion of cellular glutathione (GSH) was similar in magnitude in Se- and Se+ cells, but Se+ cells released more glutathione (mainly GSSG), obviously due to their higher Se-GSHPx activity. These results suggest that hepatocytes devoid of Se-GSHPx activity maintain a high capacity to resist peroxidative attack, either via residual (non-Se)GSHPx activity or other compensatory GSH-associated detoxication mechanisms.     

A novel biologically active seleno-organic compound--II. Activity of PZ 51 in relation to glutathione peroxidase

The anti-inflammatory compound 2-phenyl-1,2-benzoisoselenazol-3(2H)-on (PZ 51) catalysed GSSG formation from GSH in the presence of hydroperoxides in an NADPH/GSSG reductase system with the following rates (delta log GSH/min per molar selenium): 1.1 X 10(6) with H2O2, 1.2 X 10(6) with butylhydroperoxide, 1.7 X 10(6) with cumenehydroperoxide. The reaction catalysed by the sulphur analogue of PZ 51 was negligible. Similar results were obtained in a direct assay of GSH-Px activity based on GSH estimation by dithionitrobenzoate. The activation energy of the reaction was determined as 55 kJ/mol . deg in the presence of 30 mumol/1 PZ 51 compared to 36.5 kJ/mol . deg obtained in the presence of 1 nmol/1 pure GSH-Px isolated from bovine red blood cells. In mouse liver microsomes, NADPH-dependent aminopyrine dealkylation was totally inhibited in the presence of 50 mumol/1 PZ 51. In vivo experiments with Se-deficient mice showed that the Se-moiety of PZ 51 is not available for the synthesis of the selenoenzyme GSH-Px after dietary treatment or i.p. doses up to 25 mg Se as PZ 51 per kg body wt. After oral administration of labelled PZ 51, unlike with selenite, no radioactivity was incorporated into GSH-Px within 48 hr. The data suggest that several similarities between PZ 51 and the active site of GSH-Px exist, resulting in the capability of the compound to catalyse the GSH-Px reaction. An extracellular pharmacodynamic action of the drug seems likely.     

Effects of ethanol ingestion on the hepatotoxicity and metabolism of paracetamol in mice

The influence of ethanol on paracetamol-induced liver damage was studied in mice and related to changes in microsomal monooxygenases and plasma paracetamol metabolites in the same group of animals. Paracetamol (400 mg/kg body wt, p.o.) was administered alone or simultaneously with ethanol (3 g/kg, p.o.) to mice fed either a chow diet or pretreated for 4 weeks with a liquid diet containing ethanol (20% of energy). Acute ethanol administration protected against paracetamol hepatotoxicity, but this protection was complete only in mice not fed ethanol previously. Acute ethanol administration also appeared to reduce paracetamol monooxygenation in vivo, but ethanol (50 mM) added to microsomal incubations in vitro had no significant effect on paracetamol activation and covalent binding. The chronic ingestion of ethanol in the diet increased paracetamol-related liver damage, but there appeared to be no induction of paracetamol monooxygenation in these animals. We are unable to confirm current concepts that the potentiation of paracetamol hepatotoxicity by chronic ethanol ingestion and its reduction by acute ethanol administration result solely from contrasting effects of ethanol on cytochrome P-450, and alternative explanations are proposed.     

Effects of Maternal Dietary Supplementation with Selenite on the Postnatal Development of Rat Offspring Exposed to Methyl Mercury in Utero

Abstract: Female Sprague-Dawley rats were fed a control standard diet or a selenite (Se) supplemented diet (1.3 p.p.m. Se) for 8 weeks before mating and during gestation and lactation. Blood glutathione peroxidase activity (GSH-Px) was measured as a biomarker of Se in dames. After mating, the females from two dietary groups were divided into three subgroups (6 groups with 10 animals in each) given 0 (vehicle), 2 or 6 mg/kg methyl mercury (MeHg) by gavage on days 6–9 of gestation. Day 2 post parturition all litters were standardized to 6 pups per litter and remaining pups were used for determination of blood and brain total Hg contents. Behavioural testing was performed at two months of age. The results of the study showed that supplementing the diet with Se partly antagonized some adverse effects of the MeHg such as hypoactivity especially in the high MeHg dose group. There were no changes in physical development or body weight except a tendency to decreased body weight in offspring of mothers exposed to 6 mg Hg/kg. The GSH-Px activity was significantly increased in animals fed on Se supplemented diet. The dietary Se supplementation resulted in considerably increased concentrations of mercury in the blood of the offspring despite milder signs of CNS toxicity and no increase in brain concentrations of mercury.     

Effect of acute and repeated selenium treatment on hepatic monooxygenase enzyme activity in male rats

The effect of sodium selenite administered acutely or repeatedly on the biochemical components of the hepatic microsomal monooxygenase enzyme system was examined in male rats. 72 h following acute administration of selenium (2.4 mg Se/kg, i.p.), there was a significant decrease in ethylmorphine-N-demethylase activity and cytochrome P-450 levels but no change in aniline hydroxylase or NADPH cytochrome c reductase activity. Following repeated administration of selenite in the drinking water (1, 2, or 4 ppm Se) for 30 days, there was no alteration in any of the parameters measured. Following the in vitro additions of selenite to microsomes obtained from untreated rats, ethylmorphine-N-demethylase and aniline hydroxylase activities were inhibited at selenium concentrations of 10(-4) M or greater, but the inhibition achieved was less than 50%. No alterations in cytochrome P-450 levels were observed. These results indicate that selenium is a rather weak, indirect, and substrate-specific inhibitor of the hepatic monooxygenase enzyme system.     

Differential effect of dietary selenium on the long-term neurotoxicity induced by MDMA in mice and rats

We examined the effect of dietary selenium (Se) on the long-term effect of 3,4-methylenedioxymethamphetamine (MDMA) on dopamine (DA) and 5-hydroxytryptamine (5-HT) containing neurons in the brain of mice and rats. Animals were fed either a Se-deficient (<0.02 ppm) or Se-replete (0.2 ppm) diet for 8 weeks. On the seventh week mice received three injections of MDMA (15 mg/kg, i.p. 3 h apart) or saline and rats a single dose of MDMA (12.5 mg/kg i.p.) or saline. All animals were sacrificed 7 days later. MDMA administration to mice depleted striatal DA concentration in both dietary groups, although depletion was considerably larger in the Se-deficient mice (64%) than Se-replete mice (30%). In addition, a decrease in 5-HT (17-32%) occurred in brain regions of Se-deficient but not Se-replete mice. In rats, MDMA decreased cortical [(3)H]-paroxetine binding (62%) and 5-HT content, the depletion being similar in the Se-deficient and Se-replete groups. No DA loss occurred in either group. There was no difference in the hyperthermic response induced by MDMA in Se-deficient or Se-replete animals. The Se-deficient diet decreased glutathione peroxidase (GPx) activity by 30% in mouse striatum and cortex and increased the degree of lipid peroxidation in cortical synaptosomes. Se-deficient rats also showed a decrease in brain GPx activity compared with the Se-replete group, but the degree of lipid peroxidation in synaptosomes was similar in both dietary groups. These results suggest that the antioxidant capacity of rats and mice differ leading to a differential susceptibility to the oxidative stress caused by MDMA in situations of low dietary Se.     

Erythrocyte and liver characteristics in low and high glutathione Finn sheep

Selenium-uptake, glutathione peroxidase (GSH-Px), hepatic microsomal cytochrome P-450 and hepatic glutathione contents were studied in Finnsheep. The erythrocytes of low-GSH sheep had higher Selenium uptake and GSH-Px activity than did those in high-GSH animals. The low Se uptake in high-GSH sheep could be due to continuous pumping of Se from the cells into the erythrocytes, which reflects the active role of these cells, compared to those in low-GSH sheep. Significant differences in hepatic microsomal cytochrome P-450 content were found in the two groups of sheep. The greater activity among the low-GSH sheep suggests that they may have more active detoxification mechanisms than the high-GSH animals. The glutathione content in the liver was similar in both high and low-GSH sheep.     

Measurement of contents in organs of selenium- or vitamin E-deficient rat using instrumental neutron activation analysis

The contents of iron (Fe), cobalt (Co), zinc (Zn), and selenium (Se) in the organs (liver, kidney, spleen, heart, lung, and brain) and the liver cell fractions (nuclear, mitochondrial, microsomal, and cytosolic fractions) of Se- or vitamin E (VE)-deficient rats were measured using instrumental neutron activation analysis (INAA). The contents of Fe in the liver of Se-deficient rats, and in the liver and the spleen of VE-deficient rats were increased compared with those in normal rats. Fe contents increased mainly in the microsomal fraction. Contents of Co in the organs and liver cell fractions of Se- and VE-deficient rats were markedly low, reflecting the Co contents in both diets. Contents of Zn in the organs and liver cell fractions of Se- and VE-deficient rats decreased to 60-80% of the contents in normal rats. The Se contents in Se-deficient rat organs except for the kidney, spleen, and brain were below the detectable level under the present conditions. Se contents in VE-deficient rat decreased to 50-80% of those in normal rats in all organs and fractions. It is suggested that oxidative stress due to Se- or VE-deficiency affects the dynamics of Fe and Zn.     

Dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in C57BL/6J and DBA/2J mice

The dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in B6D2F1/J (B6D), C57BL/6J (C57), and DBA/2J (DBA) mice. A 14-fold difference in lethality was observed in C57 and DBA mice, based upon 30-day LD50 values of 182 and 2570 micrograms TCDD/kg body wt, respectively. The 30-day LD50 for B6D mice was 296 micrograms TCDD/kg body wt. A progressive loss of body weight in all strains of mice was observed during the 30-day LD50 studies, with maximal weight losses of 24.7, 34.0, and 33.4% prior to death of C57, B6D, and DBA mice, respectively. In separate experiments, it was found that decreased feed consumption did not contribute to weight loss in C57 mice exposed to lethal or sublethal doses of TCDD until the animals were moribund. Time-course studies in C57 mice treated with 200 micrograms TCDD/kg body wt indicated that decreases in serum glucose and triglyceride concentrations and increases in hepatic triglyceride content occurred within 4 to 8 days of exposure, and were maximally altered within 17 to 21 days postexposure, concomitant with a 25% body weight loss. C57 mice fasted for 24 to 96 hr lost 18% of body weight and also exhibited alterations in glucose and lipid parameters; however, these changes were substantially different than the effects of TCDD exposure. In concert, these observations demonstrate that decreased feed consumption (hypophagia) does not account for weight loss and changes in carbohydrate and lipid metabolism in TCDD-treated C57 mice. Dose-response experiments resulted in comparable changes in glucose and lipid parameters when DBA mice were exposed to 10-fold higher doses of TCDD than C57 mice. Parallel LD50 responses and parallel changes in carbohydrate and lipid metabolism, at 10- to 15-fold differences in dose range, are indicative of a common mechanism of toxicity in TCDD-treated C57 and DBA mice.     

Relationship between the murine Ah phenotype and the hepatic uptake and metabolism of 2,3,7,8-tetrachlorodibenzo-p-dioxin

The influence of the Ah locus on the hepatic uptake and metabolism of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied using isolated hepatocytes from Ah responsive C57BL/6J (C57) and nonresponsive DBA/2J (DBA) mice. Hepatocytes from control and TCDD-pretreated C57 and DBA mice were incubated with purified [14C] TCDD (2.2 microM) for 8 hr in the metabolism studies or 2 hr in the uptake studies. Mice were pretreated 7 days prior to hepatocyte isolation with TCDD at doses that maximally induce aryl hydrocarbon hydroxylase activity (C57: 3 micrograms/kg, ip; DBA: 30 micrograms/kg, ip) or at doses approaching the LD50 value (C57: 150 micrograms/kg, ip; DBA: 600 micrograms/kg, ip). Hepatocytes isolated from untreated C57 and DBA mice had similar uptake of [14C]TCDD, and, at all doses, TCDD pretreatment increased [14C]TCDD uptake. The rates of hepatic TCDD metabolism over the first 2 hr of incubation were similar for control C57 and DBA mice, although some qualitative differences in metabolites were detected by HPLC. TCDD pretreatment at doses of 3 and 30 micrograms/kg for C57 and DBA mice, respectively, produced no detectable quantitative or qualitative changes in TCDD metabolism, despite increases in cytochrome P-450 content, 7-ethoxyresorufin O-deethylase (EROD) activity, and benzo[a]pyrene (BaP) metabolism. Pretreatment of C57 and DBA mice with the respective LD50 doses of TCDD decreased the rate of TCDD metabolism by hepatocytes, although cytochrome P-450 content, EROD activity, and BaP metabolism were increased. These results suggest that the uptake and the rate of hepatic metabolism of TCDD do not correlate with genetic differences at the murine Ah locus.     

Changes in the rat liver drug metabolizing system during a short thiobenzamide feeding cycle

The changes in the hepatic drug metabolizing enzymes induced by the liver tumor promoter thiobenzamide (TB) were investigated. Feeding of TB to rats at a promoting regimen (1 g/kg of diet for 2 weeks) resulted in a significant decrease in the amount of liver microsomal cytochrome P-450 and of total heme. Also, the activity of cytochrome P-450 dependent monooxygenases (aminopyrine demethylase, arylhydrocarbonmonooxygenase and ethoxycoumarindeethylase) and FAD-containing monooxygenase (N,N-dimethylaniline N-oxidase and TB S-oxidase) were depressed. By contrast, phase II enzymes such as epoxide hydrase, UDP-glucuronyl transferase and GSH-transferase were significantly induced. This overall change in the drug metabolizing system was associated with tolerance of the liver towards a high necrogenic dose of TB itself as well as with an increase of mitoses and apoptosis of the hepatocytes. The findings suggest a possible relationship between this TB-induced adaptive response and the promoting activity of the compound on liver carcinogenesis.     

Absorption, distribution and elimination of selenium as L-selenomethionine in non-human primates.

20 adult female macaques (Macaca fascicularis) were given oral doses of L-selenomethionine (L-SeMet) equivalent to 0, 25, 150, 300 and 600 micrograms selenium (Se)/kg body weight, and plasma, erythrocyte, hair, faecal and urine Se concentrations were determined. The macaques were scheduled for 30 daily oral doses of L-SeMet, but systemic toxicity necessitated dose reduction in several animals; two macaques given 600 micrograms Se/kg body weight/day for 10-15 days died, and the concentration of Se in their tissues was determined and compared with Se concentrations in tissues collected from one untreated animal. Circulating and urinary Se concentrations in control macaques were within the normal human ranges. Plasma, erythrocyte, hair and urinary Se concentrations were generally dependent on the dose of L-SeMet administered. Plasma Se reflected more immediately exposure to L-SeMet, whereas erythrocyte Se concentrations increased and decreased more slowly. In some cases, erythrocyte Se was still increasing or showed a plateau after L-SeMet treatment was discontinued. Plasma Se concentrations of 6.7-7.3 ppm were observed in the two animals that died due to acute toxicity to L-SeMet. Neither plasma nor erythrocyte GPx activity was influenced by a single L-SeMet dose, but an increase in erythrocyte GPx activity occurred with continuous exposure. Total tissue Se increased 13-28-fold in macaques given 600 micrograms Se/kg body weight/day for 10-15 days, with the liver and kidneys containing the the highest Se concentrations.     

Maternal selenium deficiency enhances the fetolethal toxicity of methyl mercury

The effect of maternal selenium deficiency on methyl mercury fetotoxicity was examined in the ICR strain of mice. Pregnant mice were fed either selenium-deficient diets based on torula yeast or selenium-supplemented diets which were identical to the former except that 0.1, 0.2, or 0.4 mg of selenium per kilogram of diet was added as sodium selenite. Fetolethality of methyl mercury was exacerbated by maternal selenium deficiency when mothers were administered sc 15, 25, or 35 mumol/kg/day of methylmercuric chloride (MMC) on the 13, 14, and 15th days of pregnancy. One-tenth part per million of selenium in the diet was sufficient to protect the fetuses against MMC fetolethality when dams were administered 25 mumol/kg/day of MMC. Mercury concentrations in maternal and fetal tissues were independent of the dietary selenium level. Selenium concentration and glutathione peroxidase (GSH-Px) activity in maternal tissues were unaffected by MMC administration. In fetal liver, on the other hand, selenium concentration was increased and GSH-Px activity was decreased concurrently by maternal MMC administration in the selenium-supplemented groups. Therefore, as far as GSH-Px activity was concerned, the bioavailability of selenium was markedly decreased in fetal liver by maternal injection of MMC. The increase in selenium content in fetal liver, which was observed only in the selenium-supplemented groups, may play an important role in protection against fetolethal toxicity of MMC.     

Effects of polychlorinated biphenyls in rat liver: correlation between primary subcellular effects and promoting activity.

In the present study the promoting activity of various PCB and PBB isomers and congeners in rat liver has been studied and compared with a variety of primary xenobiotic-mediated enzymatic changes in this target organ. Female Wistar rats were given diethylnitrosamine (DEN; 10 mg/kg body wt for 10 days) and were subsequently treated once weekly with polychlorinated biphenyls (150 or 15 mumol/kg body wt) for a total of 8 weeks. Additional groups of rats were administered 3,3',4,4'-tetrabromobiphenyl or 3-methylcholanthrene (8 weekly injections of 15 or 150 mumol/kg body wt, respectively) or were given phenobarbital (0.05% in the diet) until the end of the experiment. Reference groups were treated with the various test compounds without prior initiation. One week and 9 weeks after cessation of promoter treatment rats were killed and the volumetric fraction of enzyme-altered foci characterized by changes in adenosine triphosphatase and gamma-glutamyl transpeptidase activity was determined as a means to quantitatively assess the extent of preneoplastic response in this organ. Out of the series of polyhalogenated biphenyls tested, promoting effects were seen with the following compounds: 2,2',4,5'-tetrachlorobiphenyl, 3,3',4,4'-tetrachlorobiphenyl, 2,3,4,4',5-pentachlorobiphenyl, and 3,3',4,4'-tetrabromobiphenyl, whereas no significant effects were obtained with 4-monochlorobiphenyl. In rats not treated with DEN, the two strongly promoting agents 2,3,4,4',5-pentachlorobiphenyl and 3,3',4,4'-tetrachlorobiphenyl also significantly increased the volume fraction of enzyme-altered foci over the respective controls when analyzed at the second time point of investigation. In parallel experiments, induction of liver growth and of microsomal cytochrome P450 content in liver was found to correlate well with the promoting activity of the various xenobiotics, suggesting that these parameters may be used to predict the promoting activity of polyhalogenated biphenyls in a short term assay.     

Association between progesterone binding and cytochrome P-450 content of hepatic microsomes in the rat treated with cobalt-haem

The effect of cobalt protoporphyrin IX (Co-haem) given to male rats in single subcutaneous doses (25-100 mumol/kg body wt.) was studied. Co-haem decreased cytochrome P-450 content and aminopyrine N-demethylase activity, but increased progesterone content and 3H-progesterone binding in a dose-related manner. The effect of a single dose of 50 mumol/kg body wt. was reversible; cytochrome P-450 and progesterone content, and progesterone binding, returned to the normal level 24-40 d after injection but aminopyrine N-demethylase activity was only partially restored. The converse actions of Co-haem on microsomal progesterone and cytochrome P-450 content showed high correlation.