The mechanism of induction of the microsomal drug hydroxylating system in rat liver by phenobarbital

Popliteal lymph node enlargement induced by procainamide

The ability of procainamide (PA) to induce primary local popliteal lymph node (PLN) reactions has been investigated. We employed an in vitro drug-metabolizing system, based on the hypothesis that the negative result for PA in a PLN assay (PLNA) was due to insufficient metabolizing activity at the reaction site. PA was incubated previously in vitro with the S-9 mixture derived from rat liver. The reactants were ultrafiltered in order to eliminate high molecular-weight molecules, and then the low molecular-weight fraction was subcutaneously (s.c.) injected into the hind footpad of mice. PLN reactions were assessed by weighing the popliteal lymph node of the injected side. The reactants of more than 5 mg of PA and S-9 mixture induced PLN enlargement in C3H/He mice 8 days after injection. BALB/c and C57BL/6 mice were also susceptible to PLN reaction in response to the reactants of PA (10 mg) and S-9 mixture. PLN reactions to PA were induced 2 days after the injection and sustained until 18 days. Contact of PA with the S-9 mixture for 30 min at 37 degrees C was sufficient to induce PLN enlargement. However, contact for 24 h reduced the peak reaction. On the other hand, PA which had not been incubated with the S-9 mixture and acetylprocainamide (acetylPA) gave little or no reaction. Local PNA has not been considered to be suitable for the detection of drugs with the potential to induce immune disorders in cases where a metabolite contributes to the adverse reaction. However, the employment of an in vitro drug-metabolizing system may overcome that defect.

Cytochrome P450 induction by phenobarbital (PB) is inhibited by 12-O-tetradecanoylphorbol-13-acetate (TPA): evidence that protein kinase C regulates induction

The hepatic microsomal monooxygenase system was studied in hypophysectomized male rats exposed for 24 or 48 h to PB and/or TPA, an activator of kinase C. TPA attenuated basal and PB-induced levels of P450, aniline hydroxylase (ANH), ethylmorphine demethylase (EDM) and cytochrome c reductase. Hence, PB may effect induction via the inhibition of kinase C. Supporting this is spectral evidence that PB and TPA do not bind and the fact that TPA did not decrease P450 when co-incubated with O2 and NADPH. Hemin failed to increase P450 levels previously depressed by TPA indicating that TPA acts by lowering apocytochrome levels. This is consistent with its attenuation of PB-effected increases in hepatic RNA. TPA effects were associated with increased hepatic RNA and were blocked by puromycin.

A comparative study of the effects of benzene, toluene, and xylenes on their in vitro metabolism and drug-metabolizing enzymes in rat liver

Male Sprague-Dawley rats were injected ip with benzene, toluene, or a mixture of xylene isomers at 20 mmol hydrocarbon/kg daily for 3 days. The effects of administration of these hydrocarbons upon their own in vitro metabolism, as well as upon cytochrome P-450, NADPH-cytochrome c reductase, aminopyrine N-demethylase, aniline hydroxylase, glutathione, glutathione S-transferase, and UDPglucuronyltransferase in liver were studied. Each hydrocarbon studied increased its own in vitro metabolism. Benzene had no effect on the metabolism of toluene or xylenes. Toluene and xylenes increased the metabolism of benzene, toluene, and xylenes. Cytochrome P-450 was elevated by toluene and xylenes, but was not affected by benzene. NADPH-cytochrome c reductase was induced by all three hydrocarbons. Aminopyrine N-demethylase and aniline hydroxylase were induced by toluene and xylenes and were not affected by benzene. Glutathione was elevated by benzene, decreased by xylenes, and not affected by toluene. Glutathione S-transferase was induced differentially by these hydrocarbons toward various substrates: toward 1-chloro-2,4-dinitrobenzene by benzene and toluene, toward 1,2-dichloro-4-nitrobenzene by benzene and xylenes, and no effect toward 1,2-epoxy-3-(p-nitrophenoxy)propane by any hydrocarbons. UDPglucuronyltransferase was induced by benzene and toluene when o-aminophenol and phenol were used as the substrate. Xylenes had no effect. Benzene was more effective at inducing conjugation enzymes. Xylenes were more effective at inducing cytochrome P-450 dependent enzymes. Toluene was equipotent at inducing both types of enzymes. The results indicate that the addition of methyl groups to the aromatic ring affects the inductive pattern of these monocyclic aromatic hydrocarbons.

Effect of phenobarbital on the activity of RNA polymerase II during diethylnitrosamine-induced hepatocarcinogenesis in the rat

The effects of dietary phenobarbital (PB) on RNA polymerase II (nucleoside triphosphate: RNA nucleotidyltransferase, EC 2. 7. 7. 6) in the liver of male Sprague-Dawley rats previously administered diethylnitrosamine (DENA) were studied. When a diet containing 0.05% phenobarbital was administered, the activity of DNA-dependent RNA polymerase II in the liver nuclei of rats was temporarily increased 1 and 2 weeks after the onset of feeding and expressed to be relatively higher afterwards, compared with the activity in rats fed the control diets.

Serum lipids and hepatic microsomal enzymes with special reference to serum cholinesterase in Wistar rats

Serum pseudocholinesterase in accordance with indicators revealing the lipid metabolism and liver function was studied in rats exposed to phenobarbital, bis-p-nitrophenylphosphate and disulfiram in vivo. Phenobarbital markedly induced the hepatic microsomal drug-metabolizing enzymes and tended to decrease the triglyceride content in serum. Serum lipids remained almost unaffected after the treatment of bis-p-nitrophenylphosphate. However, serum gamma-glutamyltransferase activity showed a marked decrease to this organophosphorus compound. Disulfiram activated the microsomal UDP-glucuronosyltransferase (p-nitrophenol) activity, and caused an enhancement of serum total cholesterol, which suggests that this drug might be atherogenic.

Age-and sex-related differences in rat liver microsomal enzymes and their inducibility by toluene

In the liver microsomes of toluene-treated and control Sprague-Dawley rats (n = 148, males and females aged 13-93 days), the contents of cytochrome P-450 and b5 and the activities of NADPH-cytochrome c reductase and four monooxygenases were studied. In male control rats, cytochrome contents and NADPH-cytochrome c reductase, aminopyrine N-demethylase and aniline hydroxylase activities increased to the age of one month, and after a slight decrease in cytochrome concentrations, the average adult level was reached by the age of two months. Aniline hydroxylase and 7-ethoxycoumarine O-deethylase activities decreased to about half at the same age period. In control female rats, the activities of aminopyrine N-demethylase and aniline hydroxylase decreased after the age of one month, and they remained at a considerably lower level in adult females than in males. The sex-dependence of other enzymes was negligible. Toluene induction was already well developed in the youngest age group of both sexes; in most cases the induced enzyme levels in young rats were as high or higher than in adults. In adult female rats, toluene induction of all enzymes was weaker than in males. In male rats, the toluene-induced level of aniline hydroxylase and 7-ethoxycoumarine O-deethylase showed deep minima at the age of 43-53 days, at the puberty of rats.

Polychlorinated biphenyls as phenobarbitone-type inducers of microsomal enzymes. Structure-activity relationships for a series of 2,4-dichloro-substituted congeners

Several polychlorinated biphenyl (PCB) isomers and congeners resemble phenobarbitone (PB) in their mode of induction of the hepatic drug-metabolizing enzymes; however, unlike PCBs which induce aryl hydrocarbon hydroxylase, no apparent structure-activity correlations have been reported. This study examines the effects of structure on the activity of a series of 2,4-dichloro-substituted biphenyls as inducers of several microsomal enzyme activities including dimethylaminoantipyrine N-demethylase, benzo[a]pyrene hydroxylase, aldrin epoxidase, and ethoxyresorufin O-deethylase. The results clearly illustrate a marked effect of structure on activity: all of the 2,4-dichloro-substituted PCBs resembled PB in their mode of induction. However, the potency of the induction response was dependent on the substitution pattern of the second phenyl ring (i.e. 2,3,4,5-tetrachloro greater than or equal to 2,3,4,5,6-pentachloro greater than 2,3,4,6-tetrachloro greater than 2,3,5,6-tetrachloro greater than 2,4,6-trichloro); the structure of the lower chlorinated ring also determined induction potency since the 2,4-dichloro-substituted PCBs were generally more active than their 4-chloro-substituted analogs, whereas the 2-substituted PCB homologs were inactive. The structural factors which typify the most active PB-type inducer, 2,2',3,4,4',5-hexachlorobiphenyl, include the presence of two para-, at least two meta- and two ortho-chloro substituents. In addition to the structure-activity correlations noted for PCBs, the 2,2',3,4,4',5-hexachlorobiphenyl congener also elicited a dose-response induction of two PB-inducible enzymes, aldrin epoxidase and dimethylaminoantipyrine N-demethylase.

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.

Induction by phenobarbital of the mRNA for a specific variant of rat liver microsomal cytochrome P-450

The treatment of rats for 4 days with phenobarbital causes an apparent 3-fold increase in the amount of total liver cytochrome P-450. By sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, metyrapone binding and immunoprecipitation, this increase was found to be due to a much larger increase in a restricted number of specific cytochrome P-450 variants. A radioimmunoassay technique demonstrated that the major phenobarbital-inducible variant, of molecular weight 52 000, is induced 24-fold by phenobarbital. Immunoprecipitation analysis of products of translation in vitro with an antibody specific to the 52 000-mol.wt. cytochrome P-450 showed that phenobarbital induces the mRNA in polyribosomes for this variant 20-fold. Evidence is presented for the action of phenobarbital at the transcriptional and translational levels.

The TCDD receptor in rat intestinal mucosa and its possible dietary ligands

Induction of aryl hydrocarbon hydroxylase (AHH) by polycyclic aromatic hydrocarbons and other inducers such as 2,3,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to occur following binding of the inducer to a soluble receptor protein similar to steroid hormone receptors. This receptor is usually called the TCDD receptor, since TCDD has the highest affinity of all known ligands for the receptor. In the present paper a receptor for TCDD in cytosol from rat intestinal mucosa has been studied, using isoelectric focusing in polyacrylamide gel. This receptor's biochemical properties were found to be similar to those of the TCDD-receptor in rat liver cytosol. The dissociation constant (Kd) of the 3H-TCDD-receptor complex in rat intestinal mucosa was 0.7-3.1 nM, and it was present at a concentration of 70-80 fmol/mg protein. Starvation did not significantly increase the receptor level. The affinities of some potential dietary ligands for the TCDD receptor in rat intestinal mucosa were also studied. Indole-3-carbinol had 1/2,600 of the affinity of TCDD for the receptor protein. Butylated hydroxyanisole (BHA), transstilbene oxide and quercetinpentamethylether competed even more weakly with 3H-TCDD for binding to the receptor. The biological significance of the occurrence of low-affinity ligands of dietary origin for the TCDD receptor is uncertain at the present time.

Asymmetric distribution of cytochrome P-450 and NADPH--cytochrome P-450 (cytochrome c) reductase in vesicles from smooth endoplasmic reticulum of rat liver

1. The topography of cytochrome P-450 in vesicles from smooth endoplasmic reticulum of rat liver has been examined. Approx. 50% of the cytochrome is directly accessible to the action of trypsin in intact vesicles whereas the remainder is inaccessible and partitioned between luminal-facing or phospholipid-embedded loci. Analysis by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis reveals three major species of the cytochrome. Of these, the variant with a mol.wt. of 52000 is induced by phenobarbitone and this species is susceptible to trypsin. 2. After trypsin treatment of smooth membrane, some NADPH-cytochrome P-450 (cytochrome c) reductase activity remains and this remaining activity is enhanced by treatment with 0.05% deoxycholate, which renders the membranes permeable to macromolecules. In non-trypsin-treated control membranes the reductase activity is increased to a similar extent. These observations suggest an asymmetric distribution of NADPH-cytochrome P-450 (cytochrome c) reductase in the membrane. 3. As compared with dithionite, NADPH reduces only 44% of the cytochrome P-450 present in intact membranes. After tryptic digestion, none of the remaining cytochrome P-450 is reducible by NADPH. 4. In the presence of both a superoxide-generating system (xanthine plus xanthine oxidase) and NADPH, all the cytochrome P-450 in intact membrane (as judged by dithionite reducibility) is reduced. The cytochrome P-450 remaining after trypsin treatment of smooth vesicles cannot be reduced by this method. 5. The superoxide-dependent reduction of cytochrome P-450 is prevented by treatment of the membranes with mersalyl, which inhibits NADPH-cytochrome P-450 (cytochrome c) reductase. Thus the effect of superoxide may involve NADPH-cytochrome P-450 reductase and cytosolically orientated membrane factor(s).