Interaction of isosafrole in vivo with rat hepatic microsomal cytochrome P-450 following treatment with phenobarbitone or 20-methylcholanthrene

HUMAN CYTOCHROME P450 INHIBITION AND METABOLIC-INTERMEDIATE COMPLEX FORMATION BY GOLDENSEAL EXTRACT AND ITS METHYLENEDIOXYPHENYL COMPONENTS

The concurrent use of herbal medicinals with prescription and over-the-counter drugs carries a risk for unanticipated adverse drug-botanical pharmacokinetic interactions, particularly as a result of cytochrome P450 (P450) inhibition. Extracts of goldenseal (Hydrastis canadensis) containing approximately equal concentrations ( approximately 17 mM) of two methylenedioxyphenyl alkaloids, berberine and hydrastine, inhibited with increasing potency (CYP2C9) diclofenac 4'-hydroxylation, (CYP2D6) bufuralol 1'-hydroxylation, and (CYP3A4) testosterone 6beta-hydroxylation activities in human hepatic microsomes. The inhibition of testosterone 6beta-hydroxylation activity was noncompetitive with an apparent Ki of 0.11% extract. Of the methylenedioxyphenyl alkaloids, berberine (IC50 = 45 microM) was the more inhibitory toward bufuralol 1'-hydroxylation and hydrastine (IC50 approximately 350 microM for both isomers), toward diclofenac 4'-hydroxylation. For testosterone 6beta-hydroxylation, berberine was the least inhibitory component (IC50 approximately 400 microM). Hydrastine inhibited testosterone 6beta-hydroxylation with IC50 values for the (+)- and (-)-isomers of 25 and 30 microM, respectively. For (-)-hydrastine, an apparent Ki value of 18 microM without preincubation and an NADPH-dependent mechanism-based inhibition with a kinactivation of 0.23 min(-1) and a KI of approximately 110 microM were determined. Cytochrome P450 metabolic-intermediate (MI) complex formation could be demonstrated for both hydrastine isomers. With expressed P450 isoforms, hydrastine formed a P450 MI complex with CYP2C9, CYP2D6, and CYP3A4. Coexpression of cytochrome b5 with the P450 isoforms enhanced the rate but not the extent of P450 MI complex formation.

Selective induction of cytochrome P450 isozymes in rat liver by 4-n-alkyl-methylenedioxybenzenes

To examine the structural requirements of cytochrome P450 induction by 4-n-alkyl-substituted methylenedioxybenzenes (MDBs), rats were treated in vivo with a series of MDBs that differed in alkyl carbon side-chain length (0, 1, 2, 3, 4, 5, 6, or 8). Expression patterns of specific P450 isozymes were evaluated with Western and Northern blotting, enzymatic assays, and solution hybridization assays. As determined by carbon monoxide difference spectroscopy, maximal hepatic induction of total P450 content occurred when rats were treated with MDB derivatives with alkyl chain lengths of five or six carbons. However, maximum induction of the specific P450s--P450IA1, P450IIB1, and P450IIB2--occurred with n-hexyl-MDB. In contrast to effects observed with phenobarbital, treatment with MDBs resulted in higher levels of P450IIB2 than of P450IIB1 in rat hepatic microsomes. Western blot quantitation of MDB-induced hepatic P450IIB1 and P450IIB2 apoenzymes did not correlate to measured levels of the corresponding P450 mRNAs. In fact, P450IIB1 and P450IIB2 apoenzyme levels were consistently lower than expected based on Northern blot and solution hybridization measures of the respective mRNAs. These data suggest that the n-alkyl-MDBs effect increases in levels of hepatic P450 in a complex manner, producing accumulation of P450 mRNAs concomitant with alterations in processes regulating steady-state levels of P450 apoenzyme.

Inducers of cytochrome P-450d: influence on microsomal catalytic activities and differential regulation by enzyme stabilization

The interaction of isosafrole, 3,4,5,3',4',5'-hexabromobiphenyl (HBB) and hexachlorobiphenyl (HCB) with cytochrome P-450d was evaluated by characterization of estradiol 2-hydroxylase activity. Displacement of the isosafrole metabolite from microsomal cytochrome P-450d derived from isosafrole-treated rats resulted in a 160% increase in estradiol 2-hydroxylase. The increase was fully reversed by incubation with 1 microM HBB. Although isosafrole is capable of forming a complex with many different cytochrome P-450 isozymes, it appears to bind largely to cytochrome P-450d in vivo as was demonstrated by measuring the enzymatic activity of microsomal cytochromes P-450b, P-450c, and P-450d from isosafrole-treated rats. When estradiol 2-hydroxylase was measured in rats treated with increasing doses of HCB, there was a gradual decrease in microsomal enzyme activity despite a 20-fold increase in cytochrome P-450d. The ability of cytochrome P-450d ligands to stabilize the enzyme was investigated in two ways. First, cytochromes P-450c and P-450d were quantitated immunochemically in microsomes from rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), at a dose which maximally induced total cytochrome P-450, followed by a single dose of a second inducer. The specific content of cytochrome P-450d was significantly increased when isosafrole or HCB was the second inducer but not when 3-methylcholanthrene was the second inducer. Second, the relative turnover of cytochrome P-450d was measured by the dual label technique. Following TCDD treatment, microsomal protein was labeled in vivo with [3H]leucine, the second inducer was given and protein was again labeled 3 days later with [14C]leucine. A higher ratio of 3H/14C in the cytochrome P-450d from isosafrole + TCDD- and HCB + TCDD-treated rats relative to TCDD (control)-treated rats suggested that isosafrole and HCB were able to retard the degradation of cytochrome P-450d, presumably by virtue of being tightly bound to the enzyme.

Inhibition of mono-oxygenase and oxidase activity of rat-hepatic cytochrome P-450 by H2-receptor blockers

Of four H2 blockers, cimetidine, tiotidine, oxmetidine and ranitidine, all except ranitidine showed ligand (type II) interactions with oxidized cytochrome P-450. High- and low-affinity binding sites were observed in hepatic microsomes of control, phenobarbital (PB)-treated and 3-methylcholanthrene (3-MC)-treated rats. All H2 blockers except for ranitidine (up to 400 microM) produced a concentration-dependent inhibitory effect of the metabolic intermediate (MI)-cytochrome P-450 complex formation which is displayed during metabolism of tofenacine in PB hepatic microsomes in vitro. At 400 microM, of all H2 blockers only oxmetidine was able to dissociate in vitro the isosafrole metabolite-cytochrome P-450 complex formed in vivo. Endogenous NADPH-dependent microsomal H2O2 production is inhibited in control, PB and 3-MC microsomes by the H2 blockers to various extents. In liver microsomes of phenobarbital-pretreated rats, substrate-dependent inhibition of H2O2 production correlates with inhibition of MI-cytochrome P-450 complex formation of tofenacine. Moreover, the magnitude of ligand (type II) binding of the H2 blockers correlates with inhibition of H2O2 formation. This indicates that prevention of oxygen activation by ligand binding decreases endogenous H2O2 production. Inhibition of both mono-oxygenase as well as oxidase activity of cytochrome P-450 may lead to adverse drug interactions. On the other hand formation of reactive or deleterious intermediates formed as a consequence of cytochrome P-450 activities can be prevented.

Induction of phencyclidine metabolism by phencyclidine, ketamine, ethanol, phenobarbital and isosafrole

The in vitro metabolism of phencyclidine (PCP) was investigated in 9000 g supernatant fractions of both control and PCP-, ketamine-, ethanol-, phenobarbital- or isosafrole-pretreated rats. Levels of PCP, trans-4-phenyl-4-piperidinocyclohexanol (I), 1-(1-phenylcyclohexyl)-4-hydroxypiperidine (II), N-(5-hydroxypentyl)-1-phenylcyclohexylamine (IX), and 5-(1-phenylcyclohexylamino)-valeric acid (X) were monitored by gas chromatographic analysis in all cases. The inhibition of metabolism by N2, CO, SKF-525A or 2,4-dichloro-6-phenylphenoxyethylamine (DPEA), or deletion of NADPH or protein, implied the involvement of cytochrome P-450 in the reactions. The various inducing agents affected the metabolism of PCP in different ways, implying that at least several isozymes of cytochrome P-450 were involved in the total metabolism. The majority of the consumed PCP was not accounted for by the measured metabolites so that some other metabolic pathways of major quantitative importance must be operative.

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.

Induction of rat hepatic microsomal cytochrome P-450 by 2,3',4,4',5,5'-hexachlorobiphenyl

The following evidence suggests that 2,3',4,4',5,5'-hexachlorobiphenyl resembles isosafrole as an inducer of hepatic microsomal cytochrome P-450d in the immature male Wistar rat. First, the major hepatic microsomal polypeptide (Mr = 52,000), intensified after treatment of rats with 2,3',4,4',5,5'-hexachlorobiphenyl, comigrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with cytochrome P-450d (i.e. the major isosafrole-inducible polypeptide) but had an electrophoretic mobility intermediate between cytochrome P-450b (Mr approximately equal to 51,500) and cytochrome P-450c (Mr = 56,000) (i.e. the major phenobarbital- and 3-methylcholanthrene-inducible polypeptides respectively). Second, when pairs of various xenobiotics were coadministered to rats at doses effecting maximal induction of hepatic microsomal cytochrome P-450, the inductive effects of 2,3',4,4',5,5'-hexachlorobiphenyl were additive with those of phenobarbital, 3-methylcholanthrene and pregnenolone-16 alpha-carbonitrile but not with those of isosafrole. The inductive effects of phenobarbital, 3-methylcholanthrene, pregnenolone-16 alpha-carbonitrile and isosafrole were all expressed additively with each other. Third, in contrast to phenobarbital and pregnenolone-16 alpha-carbonitrile treatment, treatment of rats with 2,3',4,4',5,5'-hexachlorobiphenyl, isosafrole or 3-methylcholanthrene failed to increase markedly the proportion of total cytochrome P-450 capable of forming a 446 nm-absorbing complex with metyrapone. Fourth, the in vitro metabolism of isosafrole, catalyzed by hepatic microsomes from rats treated with 2,3',4,4',5,5'-hexachlorobiphenyl, isosafrole or 3-methylcholanthrene, produced complexes between ferrous cytochrome P-450 and a methylenedioxyphenyl metabolite, the spectra of which were between 400 and 500 nm and were similar to each other but which were readily distinguishable from the spectra of the product adducts formed during the metabolism of isosafrole by hepatic microsomes from rats treated with corn oil (control), phenobarbital, or pregnenolone-16 alpha-carbonitrile.

Selective inhibition of the safrole-induced mixed-function oxidase activities by 9-hydroxyellipticine

Intraperitoneal administration of 9-hydroxyellipticine, a specific cytochrome P-448 inhibitor, inhibited 3-methylcholanthrene-induced cytochrome P-448 activity (ethoxyresorufin O-deethylase, biphenyl 2-hydroxylase) and formation of the safrole carbene ligand complex with this cytochrome, but did not inhibit phenobarbital-induced cytochrome P-450 activity (ethyl-morphine N-demethylase) or formation of the safrole carbene ligand complex with this cytochrome. Biphenyl displaced the 9-hydroxyellipticine ligand from cytochrome P-448 leading to increased free cytochrome, but with no corresponding increase in mixed-function oxidase activity when biphenyl was used as substrate. It is concluded that following dissociation of the ligand complex, 9-hydroxyellipticine, which also exhibits type I binding, competes with biphenyl for the substrate binding site. Administration of 9-hydroxyellipticine to safrole-pretreated rats inhibited the cytochrome P-448-catalysed activity, but had no effect on the cytochrome P-450-catalysed activity. These results indicate that safrole induces a mixture of cytochromes P-450 and P-448 rather than a single novel haemo-protein. The type I substrate biphenyl displaced both the safrole carbene and the 9-hydroxyellipticine ligands from cytochrome P-448 resulting in increased free cytochrome. Displacement of the safrole carbene ligand was accompanied by increased mixed-function oxidase activity but, in contrast, displacement of the 9-hydroxyellipticine ligand resulted in no increase in mixed-function oxidase activity.

The induction of hepatic cytochrome P-450 in C57 BL/10 and DBA/2 mice by isosafrole and piperonyl butoxide. A comparative study with other inducing agents

The formation of cytochrome P-450 metabolite complexes with isosafrole and piperonyl butoxide in vivo in genetically 'responsive' C57 BL/10 mice and 'non-responsive' DBA/2 mice is described. Displacement of the isosafrole metabolite complex can be brought about by incubation with certain type I ligands. The capacity of isosafrole and piperonyl butoxide to induced cytochrome P-450 was evaluated by measurement of biphenyl 2- and 4-hydroxylase, ethoxyresofurin O-deethylase and ethylmorphine N-demethylase and by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis and compared with results obtained for phenobarbitone, 3-methylcholanthrene and pregnenolone-16 alpha-carbonitrile. All four monooxygenase activities were elevated by isosafrole and piperonyl butoxide, as were cytochrome P-450 levels in both strains of mice. There was a large increase in biphenyl 2-hydroxylase in microsomes from isosafrole treated mice of both strains on displacement of the metabolite complex. SDS polyacrylamide gel electrophoresis demonstrated that isosafrole and piperonyl butoxide induce protein bands of mol. wt., 54000 in both the responsive and non-responsive strains. In addition, piperonyl butoxide induces a protein band of mol. wt. 49000 in both strains of mice. The changes in metabolic activities on pretreatment with isosafrole and piperonyl butoxide do not correspond to those seen with any single inducing agent. The differences in the inducing capabilities of isosafrole and 3-methylcholanthrene in the 'non-responsive' DBA/2 strain are discussed with reference to possible mechanisms of induction by benzodioxole (methylenedioxyphenyl) compounds.