Some effects of selenium deficiency on the hepatic microsomal cytochrome P-450 system in the rat

Serum Iron Increases with Acute Inductionof Hepatic Heme Oxygenase-1 in Mice

Heme oxygenase (HO)-1 is induced by oxidative stress and protects against oxidant injury. We examined the effect of rapid induction of hepatic HO-1 on serum iron level. Serum iron was approximately doubled within 6 h when HO-1 was induced by phenobarbital treatment of selenium-deficient mice. Blocking heme synthesis with diethyl 1,4-dihydro-2,4,6-trimethyl-3,5-pyridinedicarboxylate (DDC) prevented the induction of HO-1 and the rise in serum iron. DDC did not block HO-1 induction by hemin. Inhibition of HO activity by tin protoporphyrin prevented a rise in serum iron that occurred following phorone treatment. These results indicate that heme synthesis or an exogenous source of heme is needed to allow induction of HO-1. Further, they link HO-1 induction with a rise in serum iron, suggesting that the iron resulting from catabolism of heme by HO-1 is released by the liver.

Dietary selenium supplementation prolongs pentobarbital induced hypnosis

The present studies characterized the influence of dietary selenium (Na2SeO3) on the duration of pentobarbital (PB) induced hypnosis (sleep) in the rat. Rats were fed semipurified diets varying from 0.01 to 2.0 mg Se/kg for up to 4 weeks. Consumption of diets containing 1.0 and 2.0 mg Se/kg significantly prolonged PB induced hypnosis. Hepatic selenium, but not hepatic glutathione peroxidase activity, correlated with the length of PB induced hypnosis. The prolongation of hypnosis caused by diets containing 1.0 mg Se/kg was substantially reduced or eliminated by repeated exposure to PB. Although single exposure to increasing quantities of PB (60-100 mg/kg body weight) led to a progressive increase in sleep duration, the proportional increase caused by supplemental selenium (2.0 vs 0.1 microg Se/g) remained relatively constant (approximately 25%). Increasing maturity was inversely related to the duration of PB induced hypnosis, regardless of dietary selenium provided. Consumption of the 2.0 mg Se/kg diet prolonged PB induced hypnosis to a greater degree in immature than in mature rats (P < 0.05). Consumption of the selenium enriched diet (2 microg Se/g) resulted in an increase in cytochrome 2B, but had no effect on cytochrome 1A compared to controls (0.1 microg Se/g). Pretreatment of rats with P450 enzymes activators (i.e., PB, Aroclor 1254, or 3-methylcholanthrene) shortened the duration of PB induced sleep and masked the effects of dietary selenium. The current studies document that dietary selenium can influence the response to pentobarbital induced hypnosis and likely relates to changes in drug detoxification enzymes.

Selective induction of liver parenchymal cell heme oxygenase-1 in selenium-deficient rats

Liver heme oxygenase (HO) activity is higher in selenium-deficient rats than in control animals under basal conditions and is further increased in them, but not in controls, by phenobarbital treatment. In the present study we characterized liver HO induction by selenium deficiency using molecular methods. Severe selenium deficiency in rats caused a doubling of liver HO activity without affecting spleen, kidney, brain, or testis HO activities. HO-1 protein and mRNA were increased to accompany the increased HO activity, but HO-2 protein and mRNA were not increased. Fractionation of the liver into hepatocyte and Kupffer cell/endothelial cell fractions revealed that the increased HO activity resides in the hepatocyte fraction. Immunohistochemical localization of HO-1 protein confirms the induction of HO-1 taking place solely in hepatocytes and throughout the liver lobule. Phenobarbital treatment sharply increased HO-1 mRNA and protein expression in selenium-deficient liver and HO activity in hepatocytes, but had no effect in control liver or in the Kupffer cell/endothelial cell fraction of selenium-deficient liver. Electrophoretic mobility shift assays showed increased AP-1 binding activity, suggesting an involvement of this redox-sensitive transcription factor in the induction by phenobarbital of HO-1 in selenium deficiency. We speculate that selenium deficiency affects hepatic antioxidant selenoproteins, resulting in an up-regulation of HO-1.

The chemical form of selenium influences 3,2'-dimethyl-4-aminobiphenyl-DNA adduct formation in rat colon

There is increasing evidence that selenium can protect against tumorigenesis or preneoplastic lesion development induced by chemical carcinogens. This study examined whether selenite, selenate or selenomethionine would be protective against 3, 2'-dimethyl-4-aminobiphenyl (DMABP)-DNA adduct formation in the liver and colon of rats and sought to delineate the mechanism for the protective effects of the different chemical forms of selenium against aberrant crypt formation, a preneoplastic lesion for colon cancer. After injection of DMABP, two DNA adducts were identified in the liver and colon of rats. Supplementation with either 0.1 or 2.0 mg selenium/kg diet as either selenite or selenate but not selenomethionine resulted in significantly fewer (53-70%; P < 0.05) N-(deoxyguanosin-8-yl)-(deoxyguanosin-8-yl)-3, 2'-dimethyl-4-aminobiphenyl (C8-DMABP)-DNA adducts in the colon but not the liver than in rats fed a selenium-deficient diet. Rats supplemented with selenomethionine had greater (P < 0.05) plasma and liver selenium concentrations and glutathione peroxidase activity than those supplemented with selenite or selenate; however, they also had more DMABP-DNA adducts. The protective effect of selenite and selenate against DMABP-DNA adduct formation apparently is not a result of alterations in plasma or liver selenium concentrations or altered glutathione peroxidase or glutathione transferase activities but may be related to differences in the metabolism of the different forms of selenium.

Reduction of the ascorbyl free radical to ascorbate by thioredoxin reductase

Recycling of ascorbic acid from its oxidized forms is required to maintain intracellular stores of the vitamin in most cells. Since the ubiquitous selenoenzyme thioredoxin reductase can recycle dehydroascorbic acid to ascorbate, we investigated the possibility that the enzyme can also reduce the one-electron-oxidized ascorbyl free radical to ascorbate. Purified rat liver thioredoxin reductase catalyzed the disappearance of NADPH in the presence of low micromolar concentrations of the ascorbyl free radical that were generated from ascorbate by ascorbate oxidase, and this effect was markedly stimulated by selenocystine. Dehydroascorbic acid is generated by dismutation of the ascorbyl free radical, and thioredoxin reductase can reduce dehydroascorbic acid to ascorbate. However, control studies showed that the amounts of dehydroascorbic acid generated under the assay conditions used were too low to account for the observed loss of NADPH. Electron paramagnetic resonance spectroscopy directly confirmed that the reductase decreased steady-state ascorbyl free radical concentrations, as expected if thioredoxin reductase reduces the ascorbyl free radical. Dialyzed cytosol from rat liver homogenates also catalyzed NADPH-dependent reduction of the ascorbyl free radical. Specificity for thioredoxin reductase was indicated by loss of activity in dialyzed cytosol prepared from livers of selenium-deficient rats, by inhibition with aurothioglucose at concentrations selective for thioredoxin reductase, and by stimulation with selenocystine. Microsomal fractions prepared from rat liver showed substantial NADH-dependent ascorbyl free radical reduction that was not sensitive to selenium depletion. These results suggest that thioredoxin reductase can function as a cytosolic ascorbyl free radical reductase that may complement cellular ascorbate recycling by membrane-bound NADH-dependent reductases.

Effect of sex and time of sampling on selenium and glutathione peroxidase activity in tissues of mature rats

Sprague-Dawley rats were used to investigate variations in measures of glutathione peroxidase (GSH-Px) and selenium (Se) concentration resulting from diurnal cycles and sex. Mature rats (equal numbers of males and females) were killed at 4 h intervals over a 48 h period (0200, 0600, 1000, 1800 and 2200 h each day). Selenium and GSH-Px were measured in plasma, erythrocytes, and liver and kidney cytosols. Selenium concentrations did not vary diurnally, but plasma GSH-Px activities were higher during the light than dark periods. Males had greater plasma GSH-Px activities and Se concentrations (42 EU and .45 mg/kg, respectively) than females (35 EU and .41 mg/kg respectively). GSH-Px activities were also higher in male kidney cytosols than females (117 and 76 EU, respectively). Selenium and GSH-Px activities, however, were lower in male liver cytosols (.48 mg/kg and 272 EU) than females (1.19 mg/kg and 795 EU, respectively). These data suggest that Se is distributed differently in male and female rats and the difference in Se distribution is accomplished by differences in GSH-Px activities.

Modulation of the induction of rat hepatic cytochromes P-450 by selenium deficiency

The induction by phenobarbital of liver microsomal cytochrome P-450 has been demonstrated to be impaired in rats fed a selenium-deficient diet. Cytochrome P-450 isozyme specific immunologic and molecular techniques were used in the present study to better define the role of selenium in the induction of cytochrome P-450 by phenobarbital. Phenobarbital treatment of the selenium-deficient rats resulted in an increase in the level of total cytochrome P-450 50% of that observed with control rats and in a 10-fold increase in microsomal heme oxygenase. Quantitative immunoblot analyses demonstrated that the levels of cytochromes P-450b + e and P-450p in the phenobarbital-treated selenium-deficient rats were approximately 50% of those found in the phenobarbital-treated control rats. Finally, RNA hybridization studies using cDNA probes to cytochromes P-450b + e or P-450p demonstrated that the accumulations of the RNAs encoding these cytochromes P-450 were unaffected by the selenium status of the rats. These studies suggest that the impaired phenobarbital induction of the cytochromes P-450 in the selenium-deficient rats is the result of an increase in the degradation of the cytochromes P-450 or a decrease in the translation of the mRNAs coding for them.

Has selenium a beneficial role in human exposure to inorganic mercury?

Laboratory experiments have indicated that selenium acts as a powerful antagonist to mercury intoxication. The literature is reviewed and from this it is concluded that mercury and selenium react in various ways. 1) The mercuric ion binds to selenium to form a biologically inert complex leading to increased body burden of both elements. This reaction seems to take place only when a threshold of mercury exposure is exceeded. 2) Selenium influences the oxidation rate of elemental mercury and as such the distribution pattern. This reaction is influenced by the nature of the antioxidative system. In species with low glutathione peroxidase activity, mercury oxidation seems decreased which can lead to an increased brain uptake. In this process there is no threshold. 3) Selenium can, as part of the antioxidative defence system, counteract mercury induced lipid peroxidation. Other antioxidants e.g. vitamin E might be just as effective. Based upon the literature it is hypothesised that to man selenium is of no benefit in cases of exposure to mercury either as mercuric mercury or as vapours. The only protection against mercury will still be a low exposure level.

Comparison of whole blood selenium values and erythrocyte glutathione peroxidase activities of normal individuals on supplementation with selenate, selenite, L-selenomethionine, and high selenium yeast

The selenium levels and the glutathione peroxidase activity GSH-PX of whole blood and of erythrocytes, respectively, were determined in 139 normal Danes and related to sex and smoking habits. No differences were found in relation to sex apart from a higher GSH-PX activity of females when assayed with tertiary butyl hydroperoxide. Smokers showed significantly lower selenium values than non-smokers (p less than 0.05), but the two groups had identical GSH-PX activities. Individuals from the above-mentioned group were divided into four groups, receiving daily oral doses of 200 micrograms of selenium in the form of selenite, selenate, L-selenomethionine, and selenium as contained in yeast. Whole blood selenium values and the erythrocyte glutathione peroxidase activities were determined during three months of supplementation followed by a withdrawal period of four months. Both the inorganic selenium compounds and the organic derivatives gave rise to steady state levels of GSH-PX after one month of supplementation. However, the selenium levels in the groups receiving organic selenium showed a steady rise during the whole period, whereas those supplemented with inorganic selenium leveled off after a period of one to three months. The data for smokers and nonsmokers revealed identical results when organic selenium was supplemented. However, selenite gave rise to significantly higher selenium levels and GSH-PX activities in smokers than in non-smokers. Less significant (p less than 0.08) elevations of both parameters were also observed among the smokers in the selenate group. By taking both the selenium level and the GSH-PX activity into consideration, organic selenium (i.e., L-(+) selenomethionine) was judged to be more bioavailable than selenite and selenate.

The effect of selenium on the hepatic drug metabolism and inducibility in rat

1. Rats were fed either a normal or selenium-deficient diet for 4 weeks. The subgroup on selenium deficient diet had selenium supplementation as 3 ppm Se in the drinking water. Benzo(a)pyrene was given intraperitoneally as an inducer. 2. Se deficiency decreased glutathione peroxidase and cytochrome c-reductase activities while other activities were unchanged as compared to normal diet. 3. Selenium deficiency was a prerequisite for the induction of glutathione peroxidase, S-reductase and S-transferase enzymes. 4. Benzo(a)pyrene increased hepatic microsomal cytochrome P-450 content in rats on normal and selenium supplemented diet but not in the selenium deficient group. 5. The 7-ethoxyresorufin and 7-ethoxycoumarin deethylase, aryl hydrocarbon hydroxylase and cytochrome c-reductase activities were increased by benzo(a)pyrene in all the dietary groups. 6. The UDP-glucuronosyltransferase activity was also increased by benzo(a)pyrene in all the experimental groups and this was true with p-nitrophenol and phenolphthalein as aglycons.

Low serum selenium concentration and glutathione peroxidase activity in intrahepatic cholestasis of pregnancy

Serum selenium concentrations were found to be significantly lower in women with intrahepatic cholestasis of pregnancy than in women with normal pregnancies during the last trimester of pregnancy and post partum. The activity of the selenoenzyme glutathione peroxidase had a significant positive correlation with selenium concentration and it was also significantly lower in women with the disease. These findings suggest that selenium deficiency and reduced glutathione peroxidase activity are associated with the aetiopathogenesis of intrahepatic cholestasis of pregnancy.

Hyperbaric oxygen protection against carbon tetrachloride hepatotoxicity in the rat. Association with altered metabolism

Because reports have appeared suggesting that hyperbaric O2 treatment protects against the hepatotoxicity of CCl4 in humans, studies were undertaken in rats to assess this possibility and to explore its mechanism. Treatment with 2 atm of O2 for 6 h after intragastric administration of CCl4 (2.6 mmol/100 g) improved survival from 31% to 96%. The same treatment administered 24 h after the CCl4 improved survival from 36% to 50%. The protective effect does not appear to be mediated through changes in excretion or distribution because exhalation of CCl4 was not increased, and hepatic CCl4 concentration was not decreased by hyperbaric O2. Changes in CCl4 metabolism were observed, which could explain the protection. Hyperbaric O2 treatment inhibited in vivo conversion of CCl4 to its volatile metabolites CHCl3 and CO2 by 52% in the 10 h following CCl4 dosing. The predominant effect was on CO2, which is quantitatively the more significant metabolite. Studies with hepatic microsomes isolated from these rats demonstrated that hyperbaric O2 treatment diminished their capacity to metabolize CCl4 to CO2, and O2-dependent process. It had no effect on the metabolism of CCl4 to CHCl3, an O2-inhibited process. These results indicate that hyperbaric O2 suppresses the microsomal mechanism for metabolizing CCl4 in the presence of O2. Because the mechanism of CCl4 hepatotoxicity is thought to be the same in the rat and in humans, hyperbaric O2 therapy is recommended for treatment of CCl4 poisoning in humans.

Serum selenium, glutathione peroxidase, lipids, and human liver microsomal enzyme activity : A double-blind controlled trial of selenium supplementation

This study evaluated selenium status in relation to lipid peroxidation, liver microsomal function, and serum lipids in humans. Serum selenium concentration, glutathione peroxidase (GSH-Px) activity, liver microsomal enzyme activity, assessed by plasma antipyrine clearance (AP-CL) rate, and serum lipids were determined in 23 healthy subjects in a double-blind placebo-controlled trial of selenium supplementation. The low selenium concentration (74.0±14.2 μg/L, mean±SD) is attributable to the low selenium content of the diet. Subjects with the lowest selenium levels (n=11) had reduced serum GSH-Px activity, AP-CL rate, high-density lipoprotein cholesterol (HDL-C), and total cholesterol (T-C) as compared with subjects with higher selenium concentrations (n=12). Low AP-CL rates were associated with low HDL-C: T-C ratios. Selenium supplementation, 96 μg/d for 2 wk, increased serum selenium, GSH-Px activity, and the HDL-C: T-C ratio. The results suggest that a low serum selenium level is associated with a decrease in liver microsomal enzyme activity and serum HDL-C and T-C concentrations. Selenium supplementation in subjects with low serum selenium may favorably influence relations between serum lipoproteins connected with the development of atherosclerotic vascular disease.

Relationship of oxygen and glutathione in protection against carbon tetrachloride-induced hepatic microsomal lipid peroxidation and covalent binding in the rat. Rationale for the use of hyperbaric oxygen to treat carbon tetrachloride ingestion

CCl4 exerts its toxicity through its metabolites, including the free radicals CCl3. and CCl(3)00.. Oxygen strongly inhibits the hepatic cytochrome P-450-mediated formation of CCl3. from CCl4 and promotes the conversion of CCl3. to CCl(3)00.. Both these free radicals injure the hepatocyte by causing lipid peroxidation and binding covalently to cell structures. A reduced glutathione (GSH)-dependent mechanism can protect the liver microsomal membrane against CCl4-induced damage under aerobic conditions but not under anaerobic conditions (Burk, R.F., K. Patel, and J.M. Lane, 1983, Biochem. J., 215:441-445). Experiments were carried out using rat liver microsomes to examine the effect of O2 tensions found in the liver and of GSH on CCl4-induced covalent binding and lipid peroxidation. An NADPH-supplemented microsomal system was used. CCl4 or 14CCl4 was added to the sealed flask that contained the system, and after 20 min CHCl3 production, thiobarbituric acid-reactive substances (an index of lipid peroxidation), and covalent binding of 14C were measured. O2 tensions of 0, 1, 3, 5, and 21% were studied. Increases in O2 tension caused a fall in CHCl3 production, which indicated that it decreased CCl3.. GSH had no significant effect on CHCl3 production at any O2 tension. Lipid peroxidation and covalent binding of 14C fell progressively as O2 tension was increased from 1 to 21%. The addition of GSH decreased both lipid peroxidation and covalent binding, but did so better at the higher O2 tensions than at the lower ones. These results indicate that low O2 tensions such as are found in the centrilobular areas of the liver favor conversion of CCl4 to free radical products which cannot be detoxified by the GSH-dependent mechanism. They suggest that hyperbaric O2 might decrease free radical formation in the liver in vivo and promote formation of CCl(3)00. from CCl3.. This should result in diminished CCl4-induced lipid peroxidation and liver damage. Rats given CCl4 (2.5 ml/kg) were studied in metabolic chambers. Production of CHCl3 and ethane, the latter an index of lipid peroxidation, were measured. Rats in two atmospheres of 100% O2 produced much less CHCl3 and ethane than rats in air. This strongly suggests that hyperbaric O2 is decreasing free radical formation from CCl4 and/or promoting the formation of CCl(3)00. from CCl3.. These results provide the rationale for the use of hyperbaric O2 in the treatment of CCl4 ingestion.

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.

Regulation of intestinal cytochrome P-450 and heme by dietary nutrients. Critical role of selenium

The intestinal cytochrome P-450 (I-P-450)-dependent mixed function oxidase (MFO) system is regulated to a remarkable extent by various ingested xenobiotics, including drugs and carcinogens, as well as dietary nutrients. Accordingly, acute dietary iron deprivation is found to result in a marked decrease in I-P-450 content and activity. This decrease is most pronounced in the villous tip cells, the very cells committed to absorption of ingested materials. We investigated the mechanistic basis for such acute reduction and report that iron was not only required as a co-substrate for I-P-450 heme formation, but also as a regulator of two key heme-synthetic enzymes, delta-aminolevulinic acid synthetase and ferrochelatase. In addition, our studies revealed that dietary deprivation of selenium for a single day dramatically reduced I-P-450-dependent MFO activity. This prompt reduction apparently reflects impaired I-P-450 formation resulting from lowered ferrochelatase activity and consequently decreased intestinal heme availability, and was not a consequence of intracellular peroxidation presumably enhanced by concomitant lowering of the seleno-dependent glutathione peroxidase. Thus, we report the novel observation that dietary selenium also appears to be a critical modulator of intestinal cytochrome P-450-dependent metabolism of ingested drugs, carcinogens, and toxins that are absorbed by the intestinal mucosa.

Decrease of flux through pyruvate dehydrogenase and branched-chain 2-oxo-acid dehydrogenase by nitrofurantoin in perfused rat liver

Addition of nitrofurantoin to isolated perfused rat liver leads to an inhibition of 14CO2 production from [1-14C]pyruvate, 2-oxo-[1-14C]isocaproate and 2-oxo-[1-14C]isovalerate, indicating a decreased flux through the corresponding mitochondrial 2-oxo-acid dehydrogenases. This is in agreement with a decreased tissue level of the active (dephospho)form of pyruvate dehydrogenase in presence of nitrofurantoin. 2) Evidence is presented for an inhibition of the monocarboxylate translocator in the mitochondrial membrane during addition of nitrofurantoin by comparing the effects of alpha-cyanocinnamate as a transport inhibitor with those of nitrofurantoin. 3) It is concluded that nitrofurantoin-induced hepatocyte toxicity may include mitochondrial effects due to decreased oxo-acid dehydrogenase flux.

The effect of selenium-deficiency on rat fat-cell glucose oxidation

When rats are fed a selenium-deficient diet, the glutathione peroxidase activity of epididymal fat-cells decreases to 5-9% of that of control rats fed the same diet supplemented with 0.5 p.p.m. of selenium as sodium selenite. [1-14C]Glucose oxidation in fat-cells from rats fed a selenium-deficient diet is unresponsive to the action of t-butyl hydroperoxide, which stimulates 14CO2 formation from [1-14C]glucose 4-fold in control rats. Insulin enhances [1-14C]glucose oxidation and incorporation into lipids in fat-cells from both groups of rats; however, the response elicited is reduced in fat-cells prepared from selenium-deficient animals. The 'C-1/C-6 ratio' (ratio of glucose C-1 to glucose C-6 oxidized) is enhanced by insulin to a similar degree in fat-cells from both groups of animals. The stimulatory action of Zn2+ and dithiothreitol on [1-14C]glucose oxidation observed in fat-cells from selenium-supplemented rats is greatly reduced in fat-cells from selenium-deficient rats. [1-14C]Glucose oxidation in fat-cells from both groups of animals is highly sensitive to the stimulatory action of adenosine. It is concluded that the enhanced formation and glutathione-linked destruction of H2O2 plays, at the most, only a minor role in the stimulation of the flux of glucose through the pentose phosphate pathway elicited by insulin, although elimination of glutathione peroxidase activity may influence the action of insulin on glucose oxidation. Production and subsequent destruction of H2O2 may play an important role in the stimulatory action of Zn2+ and dithiothreitol on fat-cell [1-14C]glucose oxidation.

Decreased flux through pyruvate dehydrogenase by thiol oxidation during t-butyl hydroperoxide metabolism in perfused rat liver

Addition of t-butyl hydroperoxide to isolated perfused rat liver leads to a decreased flux through pyruvate dehydrogenase, shown by a decreased 14CO2 release from [1-14C]pyruvate. The effect is observed at rates of infusion of t-butyl hydroperoxide exceeding 0.7 mumol per min per g liver in normal livers and at significantly lower rates in glutathione-depleted livers. The effect is absent in livers from Se-deficient rats in which the hepatic Se-dependent glutathione peroxidase activity is very low, indicating that reduction of t-butyl hydroperoxide by glutathione peroxidase is a necessary prerequisite for the inhibition. With isolated mitochondria, decreased 14CO2 release from [1-14C]pyruvate during t-butyl hydroperoxide metabolism correlates with decreased GSH and increased GSSG contents, respectively. The addition of various disulfide compounds, including GSSG, inhibits activity of the enzyme in mitochondrial extracts. In both mitochondria and perfused liver, t-butyl hydroperoxide-mediated decrease of pyruvate dehydrogenase flux is relieved by thiol reductants. The active (dephospho)form of pyruvate dehydrogenase as measured in freeze-stopped liver samples is actually increased from 46% to 72% during t-butyl hydroperoxide metabolism. The tissue levels of ATP and ADP and perfusate beta-hydroxybutyrate/acetoacetate ratio are not markedly perturbed by addition of the hydroperoxide (10 min). It is concluded that the decreased flux through pyruvate dehydrogenase during t-butyl hydroperoxide metabolism results from oxidation of critical thiol group(s) of the enzyme complex consequential to a decrease in mitochondrial GSH/GSSG.

Defective utilization of haem in selenium-deficient rat liver

We have previously suggested that an inherent defect in hepatic haem utilization was responsible for the rapid stimulation of hepatic microsomal haem oxygenase activity observed in selenium-deficient rats given phenobarbital, a well known inducer of haem formation. To test this hypothesis, hepatic haem content was deliberately raised in selenium-deficient rats by administration of either tryptophan or allylisopropylacetamide, or by injecting haem itself. We now report that selenium-deficient rats are apparently relatively less efficient in utilizing hepatic haem than normal controls. The findings detailed in the present paper thus indicate that stimulation of hepatic microsomal haem oxygenase activity is indeed a manifestation of abnormal haem utilization in selenium deficiency. This suggests a novel role for selenium in hepatic haem metabolism.

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.

Interaction of selenium deficiency and fat intake in the regulation of enzymes associated with peroxide metabolism

The present study was designed to examine the effect of selenium deficiency on the activities of heme hydroperoxidase and glutathione peroxidases in the liver of male rats maintained on either a 5% or a 25% corn oil diet and treated with phenobarbital. Our results showed that although the basal levels of cytochrome P-450 and heme hydroperoxidase were unaffected by selenium deficiency, the magnitude of phenobarbital induction was impaired because of the depletion of this trace element. This effect was accentuated especially in rats with a high-fat intake. Selenium deprivation resulted in a virtual disappearance of glutathione peroxidase activity when assayed with hydrogen peroxide, because of depletion of the selenium-dependent enzyme. In contrast, only a 60% reduction in glutathione peroxidase activity was observed when assayed with cumene hydroperoxide. Phenobarbital administration was found to increase the activity of the latter only. Unlike the situation with the hemoprotein, dietary fat had no influence on either the basal or stimulated glutathione peroxidase activities, regardless of the selenium status of the animals.

Rise of coenzyme A-glutathione mixed disulfide during hydroperoxide metabolism in perfused rat liver

The hepatic metabolism of an externally added hydroperoxide, t-butyl hydroperoxide, results in substantial decreases in CoASH and acetyl-CoA and a concomitant increase in CoA-disulfides. The principal CoA-disulfide which is formed under these conditions was identified as CoASSG, the mixed disulfide of coenzyme A and glutathione; CoASSCoA levels were unchanged. The perturbation in the coenzyme A system was absent when the hydroperoxide was infused into livers from rats maintained on a selenium-deficient diet. In these livers. Se-dependent glutathione peroxidase is lowered to less than 5% of control levels; the observed effects can be attributed to flux through Se-dependent GSH peroxidase.

Stimulation of de novo synthesis of cytochrome P-450 by phenobarbital in primary nonproliferating cultures of adult rat hepatocytes

Primary monolayer cultures of nonproliferating parenchymal cells prepared from adult rat liver and maintained in serum-free medium responded to additions of phenobarbital with concentration-dependent increases in synthesis and accumulation of a cytochrome P-450 protein immunochemically and catalytically indistinguishable from that found in the livers of adult rats treated with phenobarbital. Maximal stimulation of the rate of synthesis of this cytochrome protein by phenobarbital, as much as 20-fold higher than in control cultures (1.01% of the rate of synthesis of total cellular protein), could be achieved when the drug was first added to cultures no older than 24 hr and then was maintained in the medium for 96 hr. In addition to phenobarbital, chemicals classified as "phenobarbital-like" inducers in vivo (mephenytoin, mirex, 2,2',4,4',5,5'-hexabromobiphenyl) induced synthesis in culture of this same immunoreactive protein. Supplementation of the medium with 0.1 microM H2SeO3 plus phenobarbital produced an average 2-fold enhancement in the rate of synthesis of this inducible cytochrome protein as compared to that in cultures receiving phenobarbital alone. Inasmuch as there was a decline in selenium content and in the activity of the seleno-enzyme glutathione peroxidase in hepatocyte cultures maintained in standard culture medium for more than 24 hr, the added selenium appears to correct a spontaneously acquired cellular deficiency in selenium. Contrary to the concept that liver cells placed in culture promptly dedifferentiate with general loss of specialized functions such as cytochrome P-450, our data demonstrate that expression of the phenobarbital-inducible form of cytochrome P-450 is not extinguished in culture, but rather it is masked transiently and is attenuated as the cells adapt to the imperfect conditions of the culture environment.

Biointeraction of sodium selenite and aflatoxin B1 in the Mongolian gerbil

The interaction of sodium selenite (Na2SeO3) and aflatoxin B1 (AFB1) was studied in 6-wk-old male Mongolian gerbils. Each of four groups of gerbils were fed one of the following diets during a 12-wk experimental period: control (commercial Chow), 5.0 ppm Na2SeO3, 12.8 ppm AFB1, or 5.09 ppm Na2SeO3 + 12.8 ppm AFB1. Animals receiving Na2SeO3 in the diet, alone and with AFB1, had a significantly lower mean total weight gain during the experiment than did control animals. Animals receiving both compounds together displayed a very high level of physical activity compared to the three other groups. Blood analysis showed no change in total leukocytes, but the relative percentage of lymphocytes increased and the percentage of neutrophils decreased concurrently in the order: control less than AFB1 less than Na2SeO3 + AFB1 les than Na2SeO3. A significant reduction in organ weight relative to body weight was observed in the liver, kidney, and lung of the animals fed AFB1 alone but only in the liver of those fed both Na2SeO3 and AFB1. No similar alterations were observed in the Na2SeO3 group. Histopathological examination revealed considerably less hepatic damage in animals fed Na2SeO3 with AFB1 than in those receiving either compound alone. Renal and intestinal damage, however, was most severe in this double-treatment group. Hepatic protein analysis revealed two protein peaks in the Na2SeO3 + AFB1 group that were absent in all other groups. It was concluded that these proteins may be selenoproteins directly or indirectly involved in the lower incidence of histopathological damage in this group.

Accelerated hepatic haem catabolism in the selenium-deficient rat

1. Hepatic microsomal cytochrome P-450 concentrations are lower in selenium-deficient rats treated with phenobarbital for 4 days than in similarly treated control rats. 2. No defect in haem synthesis was found on the basis of measurements of delta-aminolaevulinate synthase (EC 2.3.1.37), delta-aminolaevulinate dehydratase (EC 4.2.1.24) and ferrochelatase (EC 4.99.1.1) activities, and urinary excretion of delta-aminolaevulinate, porphobilinogen, uroporphyrin and coproporphyrin. 3. No defect in apo-(cytochrome P-450) separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 4. An increase in haem catabolism was found. An 8-fold increase in hepatic microsomal haem oxygenase (EC 1.14.99.3) activity occurred in selenium-deficient rats after phenobarbital treatment, compared with a less than 2-fold increase in control rats. Also excretion of 14CO in the breath after administration of delta-amino[5-14C]laevulinate was greater by phenobarbital-treated selenium-deficient rats than by similarly treated controls. 5. These studies demonstrate that the defective induction of cytochrome P-450 by phenobarbital in selenium-deficient rats is accompanied by increased haem catabolism. This could be due to increased breakdown of cytochrome P-450 or to catabolism of haem before it attaches to the apo-cytochrome. The role of selenium in stabilizing cytochrome P-450 and/or in protecting haem from breakdown remains to be determined.

Selenium regulation of hepatic heme metabolism: induction of delta-aminolevulinate synthase and heme oxygenase

Selenium was found to be a novel regulator of cellular heme methabolism in that the element induced both the mitochondrial enzyme delta-aminolevulinate synthase [succinyl-CoA:glycine C-succinyltransferase (decarboxylating); EC 2-3-1-37] and the microsomal enzyme heme oxygenase [heme, hydrogen-donor:oxygen oxidoreductase(alpha-methene-oxidizing, hydroxylating); EC 1-14-99-3] in liver. The effect of selenium on these enzyme activities was prompt, reaching a maximum within 2 hr after a single injection. Other changes in parameters of hepatic heme metabolism occurred after administration of the element. Thirty minutes after injection the cellular content of heme was significantly increased; however, this value slightly decreased below control values within 2 hr, coinciding with the period of rapid induction of heme oxygenase. At later peroids heme content returned to normal values. Selenium treatment caused only a slight decrease in microsomal cytochrome P-450 content. However, drug-metabolizing activity was severely inhibited by higher doses of the element. Unlike other inducers of delta-aminolevulinate synthase, which as a rule are also porphyrinogenic agents, selenium induction of this enzyme was not accompanied by an increase in the cellular content of prophyrins. When rats were pretreated with selenium 90 min before administration of heme, a potent inhibitor of delta-aminolevulinate synthase production, the inhibitory effect of heme of formation of this mitochondrial enzyme was completely blocked. Selenium, at high concentrations in vitro, was inhibitory to delta-aminolevulinate synthase activity. It is postulated that selenium may not be a direct inducer of heme oxygenase as is the case with trace metals such as cobalt, but may mediate an increase in heme oxygenase through increased production and cellular availability of "free" heme, which results from the increased heme synthetic activity of hematocytes. Subsequently, the increased heme oxygenase activity is in turn responsible for the lack of increase in the microsomal heme content, thus maintaining heme levels at normal values despite the highly increased activities of both heme oxygenase and delta-aminolevulinate synthase. It is further suggested that the increase in delta-aminolevulinate synthase activity is not due to a decreased rate of enzyme degradation or an activation of preformed enzyme, but to increased rate of synthesis of enzyme protein. Although selenium in trace amounts has been postulated to be involved in microsomal electron transfer process, the data from this study indicate that excess selenium can substantially inhibit microsomal drug metabolism.