Liver Microsomal Drug-metabolizing Enzymes Research Articles

Effect of praziquantel treatment on the activities of some liver microsomal enzymes in mice infected with Schistosoma mansoni.

One of the most commonly used drugs for clinical management of schistosomiasis is praziquantel (PZQ, CAS 55268-74-1). Now, PZQ is recognized, world wide, as a powerful therapeutic agent for the control of schistosomiasis. Previous studies have shown decreased activities of some of the microsomal drug-metabolizing enzymes in the livers of S. mansoni-infected mice. Consequently, this work was initiated in order to investigate the effect of PZQ treatment (in a total dose of 1000 mg/kg given on two consecutive days each of 500 mg/kg body weight) administered to mice with or without previous S. mansoni infection on selected liver microsomal drug-metabolizing enzymes. The effect of these factors on liver function was also studied. The drug was given orally seven weeks after infection with 80 Egyptian strains of S. mansoni cercariae/mouse. The activities of aminopyrine-N-demethylase, aniline hydroxylase and the hepatic glutathione content as well as the concentrations of gamma-glutamyl transferase were measured after different time intervals following the second dose of PZQ treatment. The results indicated a meaningful decrease in the activities of aminopyrine-N-demethylase and aniline hydroxylase and a high elevation in the concentrations of gamma-glutamyl transferase and the contents of hepatocellular glutathione in mice infected with S. mansoni compared with their corresponding control groups. After two weeks following the praziquantel treatment, there was an improvement in the activities of these enzymes towards the control values. Collectively, the present findings point to the importance of initiating more studies in this discipline and careful deliberation of results in order to fully understand the possible interactions of antibilharzial drugs with the liver microsomal drug-metabolizing enzymes.

Interaction ofN-(2-Hydroxypropyl)methacrylamide Copolymer-Doxorubicin Conjugates with Human Liver Microsomal Cytochromes P450: Comparison with Free Doxorubicin

Interaction of nine forms of human hepatic cytochromes P450 (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4) with two N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-based doxorubicin (DOX) conjugates designed for passive tumor targeting was studied using pooled human microsomes. The compounds used in this study were two high-molecular-weight HPMA copolymers bearing doxorubicin attached to the polymeric carrier by 1) hydrazone bond enabling intracellular pH-controlled drug release or 2) amide bond through enzymatically cleavable tetrapeptide GlyPheLeuGly spacer. Both polymeric conjugates differing in mechanism of their antitumor activity and the free doxorubicin as the control were tested for potential inhibition activity. Among nine cytochrome P450 forms studied, no HPMA copolymer with bound DOX caused an inhibition of potential clinical significance. The extent of inhibition of enzymatic activities of the cytochrome P450 forms studied was negligible with the exception of CYP2B6 and was apparently caused by DOX as no inhibition was observed with polymers alone, and the extent of inhibition by the complex corresponded to this of the free DOX at the same concentration. In conclusion, the polymers and their conjugates with DOX seem to be relatively safe, at least in this respect, i.e., of inhibition of the liver microsomal drug-metabolizing enzymes.

Metabolism, antitumor activity, and acute toxicity of 5-fluoro-1,3-bis(tetrahydro-2-furanyl)-2,4-pyrimidinedione by oral administration to animals.

The metabolism, antitumor activity, and acute toxicity of 5-fluoro-1,3-bis-(tetrahydro-2-furanyl)-2,4-pyrimidinedione (FD-1) were investigated in animals, compared with 5-fluoro-1-(tetrahydro-2-furanyl)-2,4-pyrimidinedione (FT). It was found that after oral administration of FD-1, the level of 5-fluorouracil (5-FU) was maintained higher and longer than after administration of FT, and that a large amount of 5-FU was released from FD-1 by liver microsomal drug-metabolizing enzymes or spontaneous hydrolysis via 5-fluoro-3-(tetrahydro-2-furanyl)-2,4-pyrimidinedione (3-FT) and FT. FD-1 had a significant activity against the solid form of Ehrlich carcinoma, sarcoma-180, hepatoma AH130, Yoshida sarcoma, Walker carcinosarcoma-256, and leukemia L1210 and P388, but not the ascitic forms, and it produced greater inhibition of tumor growth than FT. The acute toxicity of FD-1 was less than that of FT.

Stimulation of aniline, p-nitrophenol and N-nitrosodimethylamine metabolism in kidney by pyridine pretreatment of rabbits.

Pyridine has been shown to cause liver and kidney damage in animals and in humans. In a previous study we examined the effects of pyridine on rabbit liver and lung microsomal drug-metabolizing enzymes. In this study, in vivo i.p. administration of pyridine to rabbits caused a significant 3.4-fold increase in kidney N-nitrosodimethylamine (NDMA) N-demethylase activity as compared to the activity in control rabbits. The same treatment also significantly stimulated the activity of other cytochrome P4502E1-associated enzymes. The activities of p-nitrophenol hydroxylase and aniline 4-hydroxylase in kidney microsomes were increased 4.9-and 4.5-fold, respectively. Pyridine treatment increased the P450 content of the kidney 1.6-fold (P<0.05). SDS-PAGE of both kidney and liver microsomes of pyridine-treated rabbits showed a protein band of enhanced intensity at 51,000 Mr migrating in the region of cytochrome P4502E1. p-Aminophenol, a 4-hydroxylation product of aniline, has been shown to be nephrotoxic and NDMA, a procarcinogen, has been shown to be carcinogenic following bioactivation by NDMA N-demethylase in a number of tissues including the kidney. Since pyridine was shown to be nephrotoxic, it is expected that pyridine potentiates the toxic and/or carcinogenic effects of aniline, p-nitrophenol and NDMA through induction of their metabolism by the cytochrome P450-dependent drug-metabolizing enzymes.

Effect of Chloroquine on Rat Liver Microsomes.

Alterations in liver microsomal drug-metabolizing enzymes and phospholipids following oral administration of chloroquine to rats for 14 days were investigated. An increase in cytochrome P450 and cytochrome b5 contents and an increase in the activities of NADPH cytochrome c reductase and NADH microsomal cytochrome reductase were observed. A decrease in cholesterol and an increase in phospholipids were noted in the microsomal membranes, which altered the membrane fluidity, which may be of importance in controlling different membrane functions. The increase in different fractions of microsomal phospholipid is probably due to an increased incorporation of acetate into phospholipids.

Activities of Liver Microsomal Drug-Metabolizing Enzymes by Oral Administration of Thiabendazole(TBZ) to Male and Female Mice.

Enzyme activities of liver microsomal cytochromes P-450 induced by thiabendazole (TBZ) which has been used as a food additive were investigated in 5 and 11 week-old mice (male and female). TBZ was suspended in olive oil and mice were given orally at 350, 700 or 1000 mg/kg/d for 3 d. Cytochromes P-450 were purified from mice liver microsomes by EAH Sepharose 4B and DEAE Sephadex A-50 chromatography. The catalytic activities of mice liver microsomes and purified cytochromes P-450 fractions on TBZ, aniline, aminopyrine and biphenyl were examined. TBZ hydroxylase, aniline hydroxylase and biphenyl 4-hydroxylase of liver microsomes were induced by TBZ in mice, but aminopyrine N-demethylase was not induced. TBZ hydroxylase activity was higher in female than male in 11 week-old mice. In a reconstituted system of cytochromes P-450, the fractions from TBZ-treated mice showed higher activities of TBZ and aniline hydroxylases than the corresponding fractions from untreated mice.

Suppression of liver microsomal drug-metabolizing enzyme activities in adult female rats pretreated with cannabidiol.

The suppression by cannabidiol (CBD) of the liver microsomal drug-metabolizing enzyme activities in female rats was demonstrated and its mechanism was examined. Pretreatment of rats with CBD (10 mg/kg, i.p.) caused temporary decreases in contents of cytochrome P450 (P450) and b5 and NADPH-cytochrome c (P450) reductase activity compared with values from the vehicle control group. p-Nitroanisole O-demethylase, aniline hydroxylase, d-benzphetamine N-demethylase and delta 9-tetrahydrocannabinol 11-hydroxylase were also decreased by the CBD pretreatment. The latter two activities took a longer time to return to control levels than the former two. However, the CBD pretreatment, which reduced the protein level of P450 UT-2 (CYP2C11) in adult male rats, did not decrease the protein level of P450 F-1 (CYP2C6) or F-2 (CYP2C12) in liver microsomes from female rats. These results suggest that the mechanisms by which CBD suppresses liver microsomal drug-metabolizing enzyme activities are different in male and female rats.

Biochemical toxicology and disposition of Therminol 66 heat transfer fluid after inhalation or after dietary administration to male Sprague-Dawley rats.

The objectives of this study were to determine the disposition of Therminol 66 in rats and to determine the effects of this heat-transfer fluid on liver and kidney microsomal drug-metabolizing enzymes. Therminol 66 was administered to male Sprague-Dawley rats at various doses as either a single oral administration at 0, 100, or 300 mg/kg, or as a single 6-h inhalation exposure at 0 or 350 mg/m3. Animals were killed 48 h after gavage or after termination of inhalation exposure. Additional groups of animals were exposed to Therminol 66 via the diet at 0, 100, 500, or 5000 ppm for 14 d, or via repeated inhalation exposure at 0, 25, 250, or 1200 mg/m3 for 6 h/d for 14 d. These exposure scenarios represent approximately equivalent doses of Therminol 66 by the different routes of administration. No change in body weight was observed after acute oral or inhalation exposure, and little change in body weight was observed in animals administered Therminol 66 via the diet except at the highest dose. There was no change in kidney weight, and liver weights were increased only at the higher doses of Therminol 66. The body weight gain of animals exposed to Therminol 66 via inhalation decreased in a dose-dependent manner over the 2-wk exposure period. Results from the disposition study indicated that Therminol 66 did not appear to accumulate in the tissues examined and did not appear to be extensively absorbed after a single oral dose of 300 mg/kg. The whole-body elimination half-life was approximately 14 h and occurred primarily via the feces. There was no significant induction of hepatic aryl hydrocarbon hydroxylase (AHH) activity after single oral or inhalation exposures to Therminol 66. Ethoxycoumarin O-deethylase (ECOD) was significantly induced only in animals exposed to 350 mg/m3 via inhalation. Repeated dietary and inhalation exposures resulted in AHH and ECOD induction only at the highest doses, and the kidney appeared to be less sensitive than the liver. Animals exposed via inhalation demonstrated a greater hepatic inductive effect than did animals exposed via the diet, which may be due to absorption differences.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of supplementing methionine and threonine to a non-protein diet on the induction of enzymes by polychlorinated biphenyls in rats.

The N-sparing action of methionine and threonine in rats fed with a non-protein diet seems to indicate that methionine and threonine are the endogenously most-limiting amino acids. The effect of a dietary supplementation of methionine and threonine to a non-protein diet on enzyme induction by polychlorinated biphenyls (PCB) was investigated. Liver microsomal drug-metabolizing enzymes were further induced by the methionine and threonine supplement to the non-protein diet in PCB-receiving rats. Although feeding with PCB did not increase the urinary ascorbic acid excretion in rats of the non-protein diet group, the supplementation of methionine and threonine to the non-protein diet markedly increased this excretion. From these observations, dietary methionine and threonine might promote the synthesis of drug-metabolizing enzymes and the enzymes related to ascorbic acid biosynthesis in PCB-receiving rats.

Studies on the metabolic fate of azuletil sodium. (III): Absorption, distribution, metabolism and excretion after repeated oral administration to rats, transfer into the fetus and milk, and induction of the liver microsomal drug-metabolizing enzymes by the repeated oral administration to rats.

The absorption, distribution and excretion of 14C-KT1-32 were studied after repeated oral administration to male rats. Transfer of radioactivities into the fetus and milk after oral administration of 14C-KT1-32 to pregnant or lactating rats were studied. The induction of the hepatic drug-metabolizing enzymes was studied after repeated oral administration of KT1-32 to male rats.1. On day 13 and 18 of gestation, the autoradiography revealed that the radioactivity in the fetus was lower than that in the maternal blood. On day18 of gestation, the radioactivities in the fetal liver and kidney were lower than that in the maternal plasma.2. The radioactivity in the milk was lower than that in the maternal blood within 8hr after administration. The disappearance of radioactivity from the milk was slower than that from the blood.3. After repeated administration, the radioactivity in the blood reached the steady state within 24hr following the 1st administration. Cumulative excretion ratios in urine and feces reached a constant value after the 3rd administration. The radioactivities in the tissues at 24 hr reached the steady state after the 7th administration, but the disappearance of radioactivity from the tissues after the 21st administration was quite similar to that after single administartion.4. In the plasma and urine after the 21st administration, the Ml, M2 metabolites and the intact drug were detected as major components and M4, M3, M5 and M6 were found in the urine as the minor ones.5. After repeated oral administration of KT1-32 to male rats, there was no induction of the hepatic drug-metabolizing enzymes.

Inducing effect of albendazole on rat liver drug-metabolizing enzymes and metabolite pharmacokinetics

Albendazole (ABZ), methyl (5-(propylthio)-1H-benzimidazol-2-yl)carbamate, is a broad spectrum anthelmintic drug. S-oxidation to the sulfoxide (SO-ABZ) and the sulfone (SO 2-ABZ) are the first steps of its bioconversion. SO-ABZ is pharmacologically active and embryotoxic in rats. In the present study, rat liver microsomal drug-metabolizing enzymes were assayed after 10 days oral administration with 40 μmol ABZ/kg per day. The activities of 4-nitroanisole O-demethylase, benzo[ a]pyrene hydroxylase, 7-ethoxycoumarin O-deethylase, and 7-ethoxyresorufin O-deethylase increased 6-, 7-, 8-, and 30-fold, respectively. By immunoblotting an increase in cytochrome P-448 was observed. UDP-glucuronosyltransferase (GT) type 1 activities (1-naphthol, 7-hydroxycoumarin, 4-nitrophenol, and 4-methylumbelliferone) were significantly higher than in control microsomes (3- to 4-fold), while GT type 2 activities and bilirubin-GT remained unchanged. Microsomal epoxide hydrolase (benzo[ a]pyrene oxide) increased 2-fold. Microsomal γ-glutamyltransferase activity was unchanged. The in vivo SO-ABZ plasma level was decreased when the SO 2-ABZ plasma level was increased. In vitro sulfoxidation and sulfonation were, however, unchanged. Although a range of imidazole derivatives, including benzimidazole itself, were commonly reported as inhibitors of monooxygenase activities, ABZ behaved as an inducer of cytochrome P-448, GT1, and epoxide hydrolase.

Effects of phenobarbital pretreatment on the in vivo metabolism of carbaryl in rats.

Phenobarbital (PB) pretreatment of animals is known to induce the activity of drug-metabolizing enzymes in liver microsomes. Previous studies showed that incubation of carbaryl with microsomes obtained from livers of untreated or PB-treated rats resulted in little or no oxidative metabolism of the substrate. In addition, no spectral interactions were observed when carbaryl was added to hepatic microsomal suspensions. The present study was carried out to determine the effect of PB pretreatment on the in vivo metabolism of carbaryl in rats.

Effects of vitamin A and/or thyroidectomy on liver microsomal enzymes and their induction in 2,3,7,8-tetrachlorodibenzo- p-dioxin-treated rats

Vitamin A and thyroid hormone status have been shown previously to alter the toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in rats. In the present study, we have examined the effects of a vitamin A-excess and a vitamin A-deficient diet on thyroid hormone levels, on selected drug-metabolizing enzymes in liver microsomes, and on their inducibility by TCDD in male Sprague-Dawley rats. Except for a slight increase in serum T 3 levels, none of these end points was affected by feeding rats the vitamin A-deficient diet. In contrast, excess dietary vitamin A caused a decrease in serum thyroxine (T 4) and triiodothyronine (T 3) levels, although the levels of T 3 remained in the euthyroid range (60–80 ng/dl). The concentration of liver microsomal cytochromes P-450 and b 5 and the basal activity of benzo[ a]pyrene hydroxylase and 7-ethoxyresorufin O-de-ethylase were unaffected by excess dietary vitamin A. This result is consistent with our previous observation that the basal activity of these enzymes is dependent more on T 3 than on T 4 levels. Vitamin A excess markedly suppressed the activity of liver microsomal UDP-glucuronosyl transferase toward 1-naphthol. However, no such enzyme suppression was observed in thyroidectomized rats. This suggests that the suppressive effect of vitamin A on UDP-glucuronosyl transferase activity may be dependent on T 3. Neither vitamin A nor thyroid status had any major effect on the inducibility of UDP- glucuronosyl transferase and cytochrome P-450-dependent enzyme activities by TCDD. However, vitamin A and TCDD had a nearly additive effect on suppression of serum T 4. It is concluded that liver microsomal enzyme induction is not associated with the modulatory effect of vitamin A and thyroid hormones on the toxicity of TCDD.

Studies on the fate of thiabendazole(TBZ) in mice. II. Distribution of TBZ, 5-hydroxythiabendazole including its conjugates and the changes of liver microsomal drug-metabolizing enzyme activities in pregnant mice fed TBZ orally.

Thiabendazole (TBZ) suspended in olive oil or 3% gum arabic was given orally to pregnant ICR mice on d 15 or during d 6-15 of gestation at a dose of 1300 mg/kg/d, and TBZ including its metabolites, 5-hydroxythiabendazoles (5-OH TBZ and its conjugates) and the activities of the liver microsomal drug metabolizing enzymes were analyzed. The concentration of 5-OH TBZ was higher than that of its conjugates (glucuronide and sulfate) in the liver and placenta, whereas the levels of the conjugates were greater than that of the free form in the kidney 1 or 25 h after a single or repetitive administration of TBZ. Little sulfate of 5-OH TBZ was detected in the fetus. Aniline hydroxylase activity increased significantly after repetitive and a single administration (25 h after) of TBZ. The amount of cytochrome P-450 also increased significantly after repetitive administration of TBZ. Two vehicles (olive oil and 3% gum arabic) showed no significant difference in the data obtained above.

Early, selective and reversible suppression of cytochrome P-450-dependent monooxygenase of liver microsomes following the administration of low doses of carbon disulfide in mice

The effects of carbon disulfide (CS 2) on the liver microsomal drug-metabolizing enzyme system and other enzyme activities were studied 1 hr after the oral administration of 3–300 mg/kg of CS 2 in mice. Considerable decreases in drug-metabolizing enzyme activities (such as hydroxylation of aniline, O-dealkylation of p-nitroanisole, 7-ethoxycoumarin and 7-ethoxyresorufin, and N-demethylation of N, N-dimethylaniline), NADPH-cytochrome P-450 reductase (but not NADPH-cytochrome c reductase), and P-450-associated peroxidase activities were already observed at 3 and 30 mg/kg of CS 2, dose dependency. At the same dosage levels, the magnitudes of microsomal spectral changes induced by aniline and nicotinamide (type 2 substrates), but not those induced by hexobarbital and SKF-525A (type 1 substrates), were also reduced to a considerable extent. The degrees of these alterations were all greater than that of the measurable loss of P-450 content, i.e. the loss of functional activity of P-450 was much greater than simply expected from the apparent decrease in the hemoprotein content. Cytochrome b 5 content and NADH-ferricyanide reductase activity were unchanged at 30 and 300 mg/kg of CS 2, although NADH-cytochrome c reductase activity was increased at the latter dose. The following enzyme activities did not change significantly at up to 300 mg/kg of CS 2: flavin-containing monooxygenase, UDP-glucuronyl transferase, glucose-6-phosphatase and heme oxygenase in microsomes, and glutathione S-transferases in the soluble fraction. Microsomal conjugated diene levels and liver glutathione content were also unchanged. These observations support the theory that P-450 is a sensitive and selective site for CS 2 action, where CS 2 itself is bioactivated. It was also shown that the loss of P-450 was reversible after a single, or repeated, administration of CS 2.

Effects of gomisin A, a lignan component of Schizandra fruits, on experimental liver injuries and liver microsomal drug-metabolizing enzymes

Effects of oral administration of gomisin A, one of the components isolated from Schizandra fruits, on liver injuries induced by CCl4, d-galactosamine and dl-ethionine and on liver microsomal drug-metabolizing enzyme activities were investigated. Gomisin A suppressed the increase of serum transaminase activities and the appearances of histological changes such as degeneration and necrosis of hepatocyte, inflammatory cell infiltration and fatty deposition in each type of liver injury. The repeated administration of gomisin A (30 or 100 mg/kg, p.o., daily for 4 days) induced an apparent increase of liver weight in liver-injured and normal rats. Gomisin A decreased serum triglyceride and lipid contents of the liver in biochemical studies. Increases of microsomal cytochrome b5 and P-450, elevations of NADPH cytochrome C reductase, aminopyrine N-demethylase and 7-ethoxycoumarin O-deethylase activities and decrease of 3,4-benzo(a)pyrene hydroxylase activity per cytochrome P-450 were observed after the administration of gomisin A. In addition, gomisin A was found to enhance the incorporation of 14C-phenylalanine into liver protein and to shorten the hexobarbital-induced sleeping time. These changes caused by gomisin A were similar to those by phenobarbital. However, gomisin A is distinctly different from phenobarbital in the finding that phenobarbital lessened the survival ratio of CCl4-intoxicated mice, but gomisin A did not. Our observation suggest that gomisin A shows an antihepatotoxic action by oral application and also has hypolipidemic (mainly triglyceridemic) and liver protein synthesis-facilitating actions and that the enlargement of the liver seen with gomisin A is the adaptive hypertrophy which is due to the induction of drug-metabolizing enzymes.

Induction of rat liver microsomal drug-metabolizing enzymes by a new sleep inducer 450191-S and plasma levels of 450191-S-metabolites.

Liver microsomal 7-alkoxycoumarin O-dealkylase activities in rats were stimulated by the administration of large doses of 5-[(2-aminoacetamide)methyl]-1-[4-chloro-2-(o-chlorobenzoyl)phenyl ]-N, N-dimethyl-1 H-s-triazole-3-carboxamide hydrochloride dihydrate (450191-S), a new sleep inducer which is a 1H-1,2,4-triazolyl benzophenone derivative. To obtain the correlation between the stimulation or induction of hepatic enzymes and the plasma level of the metabolites of 450191-S, various amounts of 450191-S were administered orally to rats and the metabolites in plasma were determined by high performance liquid chromatography. Plasma concentration-time profiles for metabolites in rats showed the appearance of metabolites in plasma followed by their rapid disappearance from blood when the animals received non-inducing amounts of 450191-S. On the other hand, the profiles of metabolites in rats administered higher amounts of the drug showed very high plasma concentrations of metabolites, especially 8-chloro-6-(2-chlorophenyl)-N-methyl-4H-1,2,4-triazolo [1,5-a] [1,4]benzodiazepine-2-carboxamide (M-2) and 8-chloro-6-(2- chlorophenyl)-N-hydroxymethyl-4H-1,2,4-triazolo [1,5-a] [1,4]benzodiazepine-2-carboxamide (M-A), which were maintained for a long time with slow elimination. These results led to the conclusion that the induction of hepatic drug-metabolizing enzymes is closely correlated with the high plasma concentrations of metabolites and their prolonged existence in plasma.

Induction of drug metabolizing enzymes in the liver of diabetic mice

The effects of two classical inducers, phenobarbital and 3-methylcholanthrene, have been tested on some liver microsomal drug-metabolizing enzymes (monooxygenases and phase II enzymes) and on benzo(a)pyrene metabolism in genetically (ob/ob) and chemically (streptozotocin) diabetic mice. 1) In ob/ob mice, the basal activities and the inducibility of phase I and phase II enzymes, as well as the electrophoretic pattern of microsomal proteins, were not notably different from those of similarly treated lean mice. 2) A possibly common form of cytochrome P 450 present both in microsomes from steptozotocin-diabetic non-induced mice and in those from phenobarbital-treated non-diabetic mice could explain the increased "phenobarbital-like" enzyme activities in chemically diabetic animals. 3) The increase of monooxygenase activities produced by streptozotocin treatment is partially depressed by 3-methylcholanthrene, probably as a result of the dilution of "phenobarbital-like" cytochrome P 450 forms by 3-methylcholanthrene-induced cytochrome P 448. 4) The increased formation of the most carcinogenic metabolites of benzo(a)pyrene, and the slight decrease of phase II conjugation enzyme activities, may add their deleterious effects in 3-methylcholanthrene-induced streptozotocin-diabetic animals.

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

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

Effects of aging and phenobarbital on the rat liver microsomal drug-metabolizing system

Significant declines in the non-induced activities of liver microsomal drug-metabolizing enzymes and in the amount of cytochrome P-450 occur between maturity (16 months) and senescence (27 months) in male Fischer 344 rats, whereas there are essentially no differences between very young (1 month) and mature animals. Several hepatic responses to chronic phenobarbital administration also demonstrate marked age-dependent changes. The livers of young and mature animals exhibit: (1) greater hepatomegaly; (2) faster rates of induction and post-induction recovery of microsomal mixed function oxidase enzyme activities and hemoprotein concentration; and (3) higher maximally induced levels of these components in comparison to senescent rats. When considered with information from previous studies, the present data suggest that the age-related decline in liver drug metabolism may be due to qualitative and/or quantitative changes in the structural and/or functional components of the hepatic microsomal mixed function oxidase system.