Purification and characterization of the rat liver microsomal cytochrome P-450 involved in the 4-hydroxylation of debrisoquine, a prototype for genetic variation in oxidative drug metabolism

Selective 3-hydroxylation deficiency of lidocaine and its metabolite in Dark Agouti rats

Selective and marked 3-hydroxylase deficiency of lidocaine and its N-deethylated metabolite, MEGX, were observed in male and female DA rats. These findings suggest that cytochrome P450 isozymes metabolizing debrisoquine may be involved in the 3-hydroxylations of lidocaine and MEGX in rats and humans.

Sex and age-related differences in aminopyrine N-demethylase activity in DA- and Wistar-strain rat liver microsomes. Effect of ovariectomy

1. Liver microsomal mixed-function oxidase components were studied in Wistar and Dark Agouti (DA) rats (4-45 weeks) with regard to sex- and age-related differences. Total cytochrome P-450 ranged from 0.29 to 1 nmol/mg in Wistar rats and from 0.21 to 1.27 nmol/mg in DA rats, males had higher levels than females (P less than 0.0025). Cytochrome b5 ranged between 0.42-1.37 nmol/mg and 0.42-1.56 nmol/mg in Wistar and DA strains, respectively, and NADPH-reductase activity ranged between 14-43 and 11-46 nmol/min per mg (Wistar and DA respectively). 2. Significant age-related differences were found in DA rats with four- to six-fold increase in N-demethylase activity from young to adult rats. Sex-related differences were found in both Wistar- and DA-strain rats, with males having higher (about twice) metabolic activity than females. In contrast, no significant sex- or age-related differences in cytochrome b5 content, or NADPH-reductase activity, were found. 3. Ovariectomy of 10-13-week-old females did not affect N-demethylase activity, cytochrome P-450, cytochrome b5 or NADPH-reductase activity in Wistar or DA rats. 4. Cytochrome P-450 content did not correlate (r = 0.35) with aminopyrine N-demethylase activity. 5. Results indicate that sex- and age-related differences are due to changes in the isozymic composition of cytochrome P-450, and that these changes are not subject to oestrogen regulation.

A constitutive form of guinea pig liver cytochrome P450 closely related to phenobarbital inducible P450b(e)

In order to provide evidence that a cytochrome P450 belonging to the IIB subfamily is expressed as a constitutive form in the guinea pig, we tried to purify an isozyme from liver microsomes of untreated guinea pigs by assessing its reactivity with anti-P450b antibody in the present study. One form of cytochrome P450, named P450GP-1, was obtained. The minimum molecular weight of this isozyme was estimated to be 52,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The amino terminal sequence up to the 33rd amino acid of P450GP-1 was determined. As expected, comparison of the amino acid sequence with those of cytochrome P450 isozymes from other species reported so far indicated that P450GP-1 was highly homologous to P450s categorized in the IIB subfamily; that is, 67% similarity to rat P450b, 82% to rabbit LM2, 76% to dog PBD-2, 70% to mouse pf 3/46, and 73% to human IIB1. On the other hand, P450GP-1 showed only low similarity, less than 41%, to other cytochrome P450s of the II subfamily and those of the I, III, and IV families. Affinity of P450GP-1 to anti-P450b immunoglobulin G was confirmed to be comparable with that of a principal antigen, P450b. Immunoblot analysis revealed that P450GP-1 in the guinea pig liver microsomes was induced by phenobarbital treatment, but the increase was not as large as in the rat. P450GP-1 efficiently catalyzed benzphetamine N-demethylation, strychnine 2-hydroxylation, and testosterone 16 beta-hydroxylation, all of which are also catalyzed by P450b. Based on these results, it was strongly suggested that the IIB-type of cytochrome P450 in guinea pigs, at least one of them, is a constitutive form which is moderately induced by phenobarbital.

Cimetidine: an inhibitor and an inducer of rat liver microsomal cytochrome P-450

1. Cimetidine pretreatment of male Sprague-Dawley rats caused a significant increase in the specific content of total hepatic cytochrome P-450, supporting the hypothesis that this H2-receptor antagonist has monooxygenase induction effects. 2. Quantitative ultrastructural studies of liver of cimetidine-pretreated animals also supported this hypothesis in showing a significant proliferation of smooth endoplasmic reticulum. These ultrastructural changes were qualitatively similar to those produced by treatment of rats with phenobarbital, a well-characterized monooxygenase-inducing agent whose effects were studied for comparative purposes. 3. Competitive inhibition of metoprolol alpha-hydroxylation by cimetidine in liver microsomes prepared from untreated animals (Ki = 18.8 microM) was also demonstrated. 4. These results allowed testing of the hypothesis (Burnet et al. 1986) that inhibition of a defined monooxygenase should lead to induction of the synthesis of the relevant cytochrome P-450 isozyme. 5. The finding that metoprolol alpha-hydroxylase activity of liver microsomes was lowered, not elevated, by pretreatment of animals with cimetidine argues against the concept of a causal link between monooxygenase inhibition and induction.

SKF 525-A and cytochrome P-450 ligands inhibit with high affinity the binding of [3H]dextromethorphan and sigma ligands to guinea pig brain

The DM1/sigma 1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [3H]dextromethorphan, [3H]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[3H]1,3-Di-o-tolyl-guanidine ([3H]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM Ki values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma 1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed Ki values of 9-13 and 3-4 microM respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma 1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.

Flumecinol, a novel inducer of testosterone 16 alpha-hydroxylation in male rats

1. Flumecinol, a new inducer of the cytochrome P-450 monooxygenases, was studied in rats as a possible effector of liver microsomal testosterone oxidases. The drug enhanced the total content of liver cytochrome P-450 in immature and adult rats. 2. When total testosterone oxidation activity was compared in liver microsomes of treated and untreated rats, no differences in activities were observed in 60-day-old-rats, but a slight decrease was found in 35-day-old treated rats. 3. Several regio- and stereo-specific hydroxylases were modified by flumecinol administration; in 35-day-old rats only 16 alpha-hydroxylation was induced, whereas in 60-day-old rats a slight increase in 2 alpha-hydroxylation was also observed.

Presence of functional cytochrome P-450 on isolated rat hepatocyte plasma membrane

Antibodies against cytochrome P-450 are found in some children with autoimmune hepatitis (antiliver/kidney microsome 1) and in patients with ticrynafen hepatitis (antiliver/kidney microsome 2). For an immune reaction against cytochrome P-450 to possibly destroy the hepatocytes, one must assume that cytochrome P-450 is present on the plasma membrane surface of hepatocytes. In a first series of experiments, plasma membranes were prepared with a technique based on the electrostatic attachment of isolated hepatocytes to polyethyleneimine-coated beads. After vortexing, beads were coated with a very pure plasma membrane fraction. Microsomal contamination, judged from the specific activities of glucose-6-phosphatase or NADH-cytochrome c reductase, was less than 1%. Nevertheless, the specific content (per milligram of protein) of CO-binding cytochrome P-450 was 20% of that in microsomes; the specific benzo(a)pyrene hydroxylase activity was 25%, and ethoxycoumarin deethylase 11%. Immunoblots showed the presence of cytochromes P-450 UT-A, UT-H, PB-B, ISF-G and PCN-E, the last three isoenzymes being inducible by, respectively, phenobarbital, 3-methylcholanthrene and dexamethasone. In a second series of experiments, nonpermeabilized isolated hepatocytes from untreated rats were incubated with anticytochrome P-450 antibodies. Immunofluorescence and immunoperoxidase staining confirmed the presence of cytochromes P-450 UT-A, PB-B and ISF-G on the membrane. In a last series of experiments, human antiliver-kidney microsomal 1 antibodies were found to react specifically with rat liver plasma membrane cytochrome P-450 UT-H (IID subfamily). We conclude that several cytochrome P-450 isoenzymes are present, active and inducible on the plasma membrane surface of hepatocytes. It is therefore conceivable that immunization against plasma membrane cytochrome P-450 might lead to the immunological destruction of hepatocytes in some patients.

Inhibition of metoprolol metabolism by chloroquine and other antimalarial drugs

The ability of a series of antimalarial drugs to impair the metabolism of metoprolol in rat and man has been examined. Chloroquine was a potent inhibitor in rat liver microsomes (Ki value for metoprolol alpha-hydroxylation = 0.18 microM and for O-demethylation = 0.36 microM). The other antimalarial drugs also inhibited metoprolol oxidation. Quinine was similar to chloroquine in potency, while quinidine, primaquine and mefloquine were slightly less potent. Chloroquine also inhibited metoprolol oxidation in human liver microsomes, although it was about two orders of magnitude less potent than in the rat and the extent of impairment varied greatly between individual livers. Intraperitoneal administration of chloroquine to anaesthetized rats decreased the clearance of metoprolol (40 mg tartrate salt kg-1 i.p.) to 54, 34, 20 and 26% of the control value at doses of 2.5, 4.0, 25 and 40 mg kg-1, respectively. We conclude that antimalarial treatment might have contributed to a previously reported difference in the metabolic pattern of metoprolol between Caucasians and Nigerians.

The rat P450 IID subfamily: complete sequences of four closely linked genes and evidence that gene conversions maintained sequence homogeneity at the heme-binding region of the cytochrome P450 active site

Four genes in the P450 IID gene subfamily were isolated from Sprague-Dawley rat lambda EMBL 3 and Charon 4A genomic libraries and completely sequenced. Their transcription start sites were determined by primer extension analysis. The four genes designated IID2, IID3, IID4, and IID5 span 4036, 4371, 4678, and 4567 bp, respectively, and are closely linked head to tail on a 60-kb segment of DNA. All IID genes contained nine exons, and interestingly, the IID2, IID3, and IID4 genes possessed an atypical GC5' splice junction in intron 2. All four genes are transcribed, however, IID4 mRNA is produced at a level of less than one-tenth of those of IID2, IID3, and IID5. The exonic regions of these genes displayed from 79 to 84% sequence similarties. Several regions of extremely high nucleotide similarity were found within the introns, exons, and in the flanking regions of the four genes. These localized areas of high nucleotide similarities are the result of former gene conversion events. Of interest was the finding that the most highly similar region of all IID genes that was maintained by gene conversion covers portions of the eighth and ninth exons and the eighth intron. The ninth exon codes for a region of the P450 protein that is well conserved among all P450 gene families and in all species and that is associated with the noncovalently bound heme iron at the enzyme's active site. These data indicate that gene conversions have maintained sequence homogeneity within a critical region of the four P450 IID proteins.

Kinetic properties of the metabolism of imipramine and desipramine in isolated rat hepatocytes

The metabolism of imipramine and desipramine was examined by using isolated rat hepatocytes. The enzyme systems having high-affinity-and-low-capacity and low-affinity-and-high-capacity kinetic properties were found to catalyze aromatic 2-hydroxylations of imipramine and desipramine, and aliphatic N-demethylation of imipramine, respectively. The Km and Vmax values for N-demethylation of imipramine (which formed desipramine) were about 5-10 and 5 times larger than those of both 2-hydroxylations respectively. A competitive inhibition between the 2-hydroxylations of imipramine and desipramine ("parallel pathway interaction") (Chiba M, Fujita S and Suzuki T, J Pharm Sci 77: 944-947, 1988), observed using liver microsomes, was found also in isolated hepatocytes. It was concluded that the characteristics of imipramine metabolism observed in liver microsomes were well reproduced in isolated rat hepatocytes.

Kinetics of in vitro metabolism of methoxyphenamine in rats

1. Methoxyphenamine (MP) was metabolized in vitro by rat liver preparations to O-desmethylmethoxyphenamine (O-desmethyl-MP), N-desmethylmethoxyphenamine (N-desmethyl-MP) and 5-hydroxymethoxyphenamine (5-hydroxy-MP). These metabolic pathways were inhibited by SKF 525-A and carbon monoxide, which indicates that these reactions were mediated at least partly by an NADPH-dependent cytochrome P-450 system. 2. Strain differences in the metabolism of this drug in vitro were observed in female Lewis and Dark Agouti (DA) rats, which are proposed models for human debrisoquine phenotypes. Methoxyphenamine O-demethylase and 5-hydroxylase activity in DA rats were lower than those in Lewis rats. 3. The metabolic transformation of methoxyphenamine in vitro to O-desmethyl-MP was inhibited competitively by debrisoquine and sparteine. This indicates that the cytochrome P-450 isoenzyme mediating the metabolism of MP to O-desmethyl-MP is similar to that mediating metabolism of debrisoquine and sparteine. However, no inhibition was observed with methenytoin.

Purification and characterization of cytochrome P-450 isozymes from phenobarbital-induced adult hen liver

1. Two cytochrome P-450 isozymes (P-450 PB-A, PB-B) and cytochrome b5 were purified from livers of phenobarbital-treated adult hens. 2. Both the enzymes exhibited the same apparent molecular weight (54,000). 3. They could be distinguished on the basis of immunochemical properties, spectral properties, peptide pattern after partial proteolysis, tryptic peptide pattern, and N-terminal sequence. 4. The antibodies raised against P-450 PB-A and PB-B did not cross-react with microsomal P-450s of rat, mice, cat, or catfish species by immunoblotting.

Metabolic activation of the antidepressant tianeptine. I. Cytochrome P-450-mediated in vitro covalent binding

Incubation under air of [14C]tianeptine (0.5 mM) with a NADPH-generating system and hamster, mouse or rat liver microsomes resulted in the in vitro covalent binding of [14C]tianeptine metabolites to microsomal proteins. Covalent binding to hamster liver microsomes required NADPH and oxygen; it was decreased in the presence of the cytochrome P-450 inhibitors, carbon monoxide, piperonyl butoxide (4 mM), and SKF 525-A (4 mM) or in the presence of the nucleophile, glutathione (1 or 4 mM). In vitro covalent binding to hamster liver microsomes was not decreased in the presence of quinidine (1 microM), and was similar with microsomes from either female Dark Agouti, or female Sprague-Dawley rats. In contrast, in vitro covalent binding to hamster liver microsomes was decreased in the presence of troleandomycin (0.25 mM), while covalent binding was increased with microsomes from either hamsters, mice or rats pretreated with dexamethasone. Preincubation with IgG antibodies directed against rabbit liver glucocorticoid-inducible cytochrome P-450 3c(P-450 IIIA4) decreased in vitro covalent binding by 53 and 89%, respectively, with microsomes from control hamsters and dexamethasone-pretreated hamsters, and by 60 and 81%, respectively, with microsomes from control and dexamethasone-pretreated rats. We conclude that tianeptine is activated by hamster, mouse and rat liver cytochrome P-450 into a reactive metabolite. Metabolic activation is mediated in part by glucocorticoid-inducible isoenzymes but not by the isoenzyme metabolizing debrisoquine. In vivo studies are reported in the accompanying paper.

Effect of liver disease on dextromethorphan oxidation capacity and phenotype: a study in 107 patients

1. The O-demethylation of dextromethorphan to dextrorphan exhibits a genetically-controlled polymorphism, co-segregating with that of debrisoquine hydroxylation. Dextromethorphan has been proposed as a test compound to assess drug oxidation polymorphism. 2. We studied the effects of liver disease of varying severity on dextromethorphan oxidation capacity. Phenotyping was performed using the urinary dextromethorphan/dextrorphan metabolic ratio after oral administration of 40 mg dextromethorphan hydrobromide in 56 patients with cirrhosis and in 51 patients with moderately severe liver disease. 3. Dextromethorphan oxidation capacity was impaired in cirrhotic patients and, to lesser extent, in non cirrhotic patients, as compared with 103 control subjects. 4. The impairment in dextromethorphan oxidation induced by liver disease, was however, much less than that caused by the genetic deficiency. As a result, the prevalence of the poor metabolizer phenotype remained in the same range in patients with cirrhosis (1.8%) and with moderately severe disease (2.0%) as in controls (3.9%). 5. This observation shows that, although liver disease causes some impairment of dextromethorphan O-demethylation, this impairment is not sufficient to modify the assignment of phenotypes.

Extensive oxidative metabolism of dextromethorphan in patients with almitrine neuropathy

Almitrine bismesylate can induce a stereotypical sensory peripheral neuropathy probably through a toxic mechanism. High plasma concentrations of almitrine have been reported in a patient with neuropathy. Since large inter-individual variations in plasma drug concentrations are found it is possible that the development of toxicity may be linked to genetically determined polymorphic oxidation of the drug. Oxidation phenotyping was performed in fifteen patients with almitrine neuropathy using dextromethorphan, a test compound subject to oxidative metabolism similar to that of debrisoquine. All patients were of the extensive metaboliser phenotype. This result shows that, in contrast to perhexiline neuropathy, almitrine neuropathy is not related to slow oxidation of the compound with regard to the particular P-450 iso-enzyme involved in dextromethorphan and debrisoquine metabolism.

Genetically determined oxidation polymorphism and drug hepatotoxicity. Study of 51 patients

The influence of genetically determined oxidation polymorphism on drug hepatotoxicity has been poorly investigated and results are controversial. We studied drug oxidation capacity in 51 patients with hepatitis caused mainly by drugs undergoing oxidative metabolism, using dextromethorphan, a test compound recently proposed as a substitute for debrisoquine. Phenotyping was performed using the metabolic ratio (MR) calculated as MR = 0-10 h urinary output of dextromethorphan/0-10 h urinary output of dextrorphan (the main oxidative metabolite), after oral administration of 40 mg dextromethorphan hydrobromide. Dextromethorphan oxidation capacity was similar in patients and in 103 control subjects as judged by: (a) the prevalence of each phenotype (5.9% versus 3.9% for the poor metabolizer phenotype and 94.1% versus 96.1% for the extensive metabolizer phenotype; (b) the frequency distribution histograms of log metabolic ratio; (c) the mean values of dextromethorphan and dextrorphan urinary outputs and of log metabolic ratio for each phenotype. These results show that hepatotoxicity of several drugs, including amineptine, amodiaquine and Plethoryl, is related neither to an impairment in dextromethorphan oxidation capacity nor to an unusually high capacity to oxidize this drug.

Antibodies against human cytochrome P-450db1 in autoimmune hepatitis type II

In a subgroup of children with chronic active hepatitis, circulating autoantibodies occur that bind to liver and kidney endoplasmic reticulum (anti-liver/kidney microsome antibody type I or anti-LKM1). Anti-LKM1 titers follow the severity of the disease and the presence of these antibodies serves as a diagnostic marker for this autoimmune hepatitis type II. We demonstrate that anti-LKM1 IgGs specifically inhibit the hydroxylation of bufuralol in human liver microsomes. Using two assay systems with different selectivity for the two cytochrome P-450 isozymes catalyzing bufuralol metabolism in human liver, we show that anti-LKM1 exclusively recognizes cytochrome P-450db1. Immunopurification of the LKM1 antigen from solubilized human liver microsomes resulted in an electrophoretically homogenous protein that had the same molecular mass (50 kDa) as purified P-450db1 and an identical N-terminal amino acid sequence. Recognition of both purified P-450db1 and the immunoisolated protein on western blots by several monoclonal antibodies confirmed the identity of the LKM1 antigen with cytochrome P-450db1. Cytochrome P-450db1 has been identified as the target of a common genetic polymorphism of drug oxidation. However, the relationship between the polymorphic cytochrome P-450db1 and the appearance of anti-LKM1 autoantibodies as well as their role in the pathogenesis of chronic active hepatitis remains speculative.

Further characterization of RLM2 and comparison with a related form of cytochrome P-450, RLM2b

We have extended the characterization of RLM2, a constitutive form of rat liver cytochrome P-450, using immunochemical means to quantitate its presence in microsomes, to follow its development in maturing male and female rats, and to determine its response to prototypical P-450 inducers. In addition, RLM2 is compared to RLM2b, a form of P-450 with similar migration on SDS-PAGE and NH2-terminal amino acid sequence. RLM2b is expressed in both sexes at a level of 0.08 nmol/mg microsomal protein at 2 weeks of age. In female rats, this level is unchanged with maturation. However, in the male, the level declined with maturation to reach 0.02 nmol/mg protein by 12 weeks of age. RLM2 is a male-specific form of cytochrome P-450. Originally absent in the 2-week-old rat, it reached a level of 0.03 nmol/mg protein in the adult male, its appearance and increase coinciding with the onset of puberty. Both phenobarbital and 3-methylcholanthrene induced microsomal levels of RLM2b in the adult male and female rat. RLM2, however, was suppressed in the male rat, 58 and 42%, respectively, by the same treatments. RLM2b and RLM2 each catalyze a unique spectrum of hydroxytestosterone metabolites. RLM2b is highly site specific. In contrast, RLM2 produces several isomeric products in the same region of the testosterone molecule. Substitution of the acetyl group of progesterone for the 17-hydroxy group of testosterone did not alter the site specificity of RLM2b, but did alter it for RLM2, indicating, further, a difference in the active site conformation of the two enzymes. Although RLM2b and RLM2 responded differently to inducers and to a changing physiology during maturation, and were functionally quite distinct, the proteins showed a high degree of immunologic relatedness which is suggestive of significant structural similarities. Structural differences do exist, however, as alpha-chymotryptic digestion formed a number of peptide fragments that differed between the two proteins.

Effects of growth hormone on cytochrome P-450j

Hypophysectomy of male and female rats resulted in a 5 and 10-fold increase respectively, in cytochrome P-450j as determined by immunoblotting. Treatment of hypophysectomized rats with growth hormone resulted in a decrease in P-450j. Microsomal N-nitrosodimethylamine demethylase activity was increased approximately 23 and 75 per cent in male and female rats respectively, over the activity in microsomes of intact animals. These studies suggest that hypophysectomy results in an increase in cytochrome P-450j and that growth hormone acts as a repressive factor for constitutive P-450j.

In-vivo and in-vitro dextromethorphan metabolism in SD and DA rat. An animal model of the debrisoquine-type polymorphic oxidation in man

The female dark Agouti (DA) rat is well established as an animal model for the debrisoquine poor metabolizer phenotype (PM), whereas the SD rat represents the extensive metabolizer (EM). It is not known, however, if the DA rat also is representative for the dextromethorphan (DEM) PM, a compound recently demonstrated to be subjected to the debrisoquine phenotype in man. Studies were performed, therefore, to evaluate in-vivo and in-vitro metabolism of DEM in DA and SD rats. After oral administration of 50 mg/kg of DEM, the DA rat excreted 25 +/- 6% of the dose in 72-hr urine as O-demethylated product (dextrorphan), whereas the SD excreted 40 +/- 9% (P less than 0.002). Metabolic ratio of O-demethylation was 0.46 +/- 0.11 in DA and 0.02 +/- 0.01 in SD (P less than 0.001). As a compensatory mechanism, N-demethylation was ninefold increased in DA compared to SD (8.0 +/- 3% of the dose excreted in urine of DA as methoxymorphinan vs 0.9 +/- 0.7% in SD) (P less than 0.001). Total plasma clearance of DEM was 95 +/- 20 ml/min/kg in SD and 45 +/- 13 ml/min/kg in DA (P less than 0.001). In vitro, microsomal affinity for DEM O-demethylation was greater than 50 times higher in SD than in DA rats (P less than 0.004), whereas Vmax did not differ statistically. Vmax for N-demethylation was 80% increased in DA (P less than 0.01), whereas corresponding Km values did not differ. It appears that the differences in DEM metabolism between DA and SD rats are qualitatively similar to human EM and PM phenotypes, respectively. Whether this is also true for the underlying mechanism(s) however, remains to be resolved.

Acute and chronic drug-induced hepatitis

Adverse drug reactions may mimic almost any kind of liver disease. Acute hepatitis is often due to the formation of reactive metabolites in the liver. Despite several protective mechanisms (epoxide hydrolases, conjugation with glutathione), this formation may lead to predictable toxic hepatitis after hugh overdoses (e.g. paracetamol), or to idiosyncratic toxic hepatitis after therapeutic doses (e.g. isoniazid). Both genetic factors (e.g. constitutive levels of cytochrome P-450 isoenzymes, or defects in protective mechanisms) and acquired factors (e.g. malnutrition, or chronic intake of alcohol or other microsomal enzyme inducers) may explain the unique susceptibility of some patients. Formation of chemically reactive metabolites may also lead to allergic hepatitis, probably through immunization against plasma membrane protein epitopes modified by the covalent binding of the reactive metabolites. This may be the mechanism for acute hepatitis produced by many drugs (e.g. amineptine, erythromycin derivatives, halothane, imipramine, isaxonine, alpha-methyldopa, tienilic acid, etc.). Genetic defects in several protective mechanisms (e.g. epoxide hydrolase, acetylation) may explain the unique susceptibility of some patients, possibly by increasing exposure to allergenic, metabolite-altered plasma membrane protein epitopes. Like toxic idiosyncratic hepatitis, allergic hepatitis occurs in a few patients only. Unlike toxic hepatitis, allergic hepatitis is frequently associated with fever, rash or other hypersensitivity manifestations; it may be hepatocellular, mixed or cholestatic; it promptly recurs after inadvertent drug rechallenge. Lysosomal phospholipidosis occurs frequently with three antianginal drugs (diethylaminoethoxyhexestrol, amiodarone and perhexiline). These cationic, amphiphilic drugs may form phospholipid-drug complexes within lysosomes. Such complexes resist phospholipases and accumulate within enlarged lysosomes, forming myeloid figures. This phospholipidosis has little clinical importance. In a few patients, however, it is associated with alcoholic-like liver lesions leading to overt liver disease and, at times, cirrhosis. Subjects with a deficiency in a particular isoenzyme of cytochrome P-450 poorly metabolize perhexiline and are at higher risk of developing liver lesions. Prolonged, drug-induced liver-cell necrosis may also lead to subacute hepatitis, chronic hepatitis or even cirrhosis. This usually occurs when the drug administration is continued, either because the liver disease remains undetected or because its drug aetiology is overlooked. Several autoantibodies may be present.(ABSTRACT TRUNCATED AT 400 WORDS)

Phenacetin O-deethylation by human liver microsomes: kinetics and propranolol inhibition

1. Phenacetin O-deethylation catalysed by human liver microcomes has been examined over a substrate concentration range of 2.5 to 700 microM using preparations of eight human liver samples. Michaelis-Menten kinetics described phenacetin oxidation satisfactorily in five of these samples; apparent Km values were in the range of 17.7 to 38.4 microM. 2. In the three other livers a single rectangular hyperbolic relationship did not describe the substrate saturation data adequately; analyses in these three cases requiring two classes of catalytic site. The apparent Km value for the higher affinity class of site in these three samples was within the range quoted above, but limitations imposed by assay sensitivity and phenacetin solubility obviated accurate characterization of the lower affinity class. 3. Estimates of Vmax for the high affinity class of site in the eight livers varied eleven-fold and there was no correlation between either Km or Vmax and microsomal cytochrome P-450 specific content, NADPH cytochrome c (P-450) reductase specific activity or ethylmorphine demethylase activity. 4. Propranolol was a potent competitive inhibitor of phenacetin deethylation with apparent Ki values of 2 to 7 microM describing its effect on the higher affinity class of activity. Propranolol was also an inhibitor of the lower affinity phenacetin deethylase identified in three of the livers, however the mechanism of inhibition could not be characterized. 5. These data suggest the possibility that propranolol oxidation may be mediated in part, by one or more human liver cytochrome P-450 species catalyzing phenacetin oxidation.

Purification and characterization of liver microsomal cytochrome P-450 from untreated male rats

Different forms of cytochrome P-450 from untreated male rats were simultaneously purified to homogeneity using the HPLC technique. The absorption maximum, molecular weight, NH2-terminal sequence and catalytic activity of them were determined. The NH2-terminal sequences of six forms of cytochrome P-450 (designated P450 UT-1, UT-2, UT-4, UT-5, UT-7 and UT-8) indicate that these cytochrome P-450 isozymes are of different molecular species. The hydrophobicity values of the NH2-terminal sequences of P450 UT-1 and P450 UT-8 were lower than that of other forms. P450 UT-8 has the highest molecular weight, 54,000, of the six forms of P-450. P450 UT-2 was active in demethylation of benzphetamine, P450 UT-4 was active in the metabolism of 7-ethoxycoumarin and p-nitroanisole. P450 UT-1 and P450 UT-2 were active in the 2 alpha- and 16 alpha-hydroxylation of testosterone, whereas P450 UT-4 was active in the 6 beta-, 7 alpha- and 15 alpha-hydroxylation of the same steroid. We believe that P450 UT-1, P450 UT-7 and P450 UT-8 are as yet unrecognized forms of cytochrome P-450.

Studies on the pregnenolone-16 alpha-carbonitrile-inducible form of rat liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase

Treatment of rats with pregnenolone-16 alpha-carbonitrile (PCN) markedly induces rat liver microsomal cytochrome P-450p and UDP-GT-dt1, a glucuronosyltransferase active towards the digitoxin metabolite, digitoxigenin monodigitoxoside. The present study characterizes the regulation of these two enzymes in rats treated with different xenobiotics. Like PCN, treatment of rats with dexamethasone, spironolactone, troleandomycin or erythromycin estolate markedly induced both UDP-GT-dt1 and cytochrome P-450p (measured as erythromycin demethylase and testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activities). However, compared to PCN and dexamethasone, both troleandomycin and erythromycin estolate preferentially induced cytochrome P-450p, whereas spironolactone preferentially induced UDP-GT-dt1. Treatment of rats with the polychlorinated biphenyl mixture, Aroclor 1254, increased both cytochrome P-450p and UDP-GT-dt1 activity to about 40% of that in liver microsomes from rats induced with PCN or dexamethasone. Treatment of rats with phenobarbital or chlordane caused a relatively small increase in cytochrome P-450p and UDP-GT-dt1 activity. Neither enzyme was induced by treatment of rats with 3-methylcholanthrene, rifampin or digitoxin. The induction of cytochrome P-450p and UDP-GT-dt1 by PCN followed similar dose-response curves. Although cytochrome P-450p and UDP-GT-dt1 are differentially affected by the age and the sex of rats, the enzymes responded similarly, but not identically, to xenobiotic treatment. This suggests that cytochrome P-450p and UDP-GT-dt1 are co-inducible but not coordinately regulated.

Polymorphism of dextromethorphan oxidation in a French population

Genetically-controlled drug oxidation capacity was studied using dextromethorphan, an anti-tussive drug, as the test compound in 103 healthy white French subjects (61 males and 42 females). Phenotyping was performed using the metabolic ratio (MR) calculated as MR = 0-10 h urinary output of dextromethorphan/0-10 h urinary output of dextrorphan, after oral administration of 40 mg (113.6 mumol) of dextromethorphan hydrobromide. The log MR was bimodally distributed: 99 subjects (96.1%) were phenotyped as extensive metabolizers; they had a log MR between -3.1 and -1.1, a urinary output of dextromethorphan below 5 mumol 10 h-1 and a urinary output of dextrorphan above 20 mumol 10 h-1. Four subjects (3.9%) were phenotyped as poor metabolizers; they had a log MR between -0.5 and +0.7, a urinary output of dextromethorphan above 5 mumol 10 h-1 and a urinary out of dextrorphan below 20 mumol 10 h-1.

Isozyme specificity of testosterone 7 alpha-hydroxylation in rat hepatic microsomes: is cytochrome P-450a the sole catalyst?

In the present study we show that monospecific antibody against cytochrome P-450a completely inhibits testosterone 7 alpha-hydroxylation in hepatic microsomes of untreated male or female rats or rats of either sex treated with dexamethasone. These data are in contrast with those of K. Nagata et al. (1987, J. Biol. Chem. 262, 2787-2793) who recently reported that an antibody prepared against cytochrome P-450a completely inhibited testosterone 7 alpha-hydroxylase activity in microsomes from untreated or 3-methylcholanthrene-treated rats but only inhibited 50% of the activity in microsomes from dexamethasone-treated rats. They proposed that dexamethasone treatment of rats induced another testosterone 7 alpha-hydroxylase in rat liver. The discrepancy in the two sets of data was due, at least in part, to the use of a chromatography system by Nagata et al. that is incapable of resolving a number of testosterone metabolites. Dexamethasone treatment of rats leads to a marked increase in the production of several testosterone metabolites, including 15 beta-hydroxytestosterone which is cochromatographic with 7 alpha-hydroxytestosterone in their chromatography system. Our results indicate that cytochrome P-450a accounts for all of the testosterone 7 alpha-hydroxylase activity in microsomes from dexamethasone-treated rats, and that testosterone 7 alpha-hydroxylation continues to be a useful marker for monitoring cytochrome P-450a in rat hepatic microsomes.

Target inactivation analysis applied to determination of molecular weights of rat liver proteins in the purified state and in microsomal membranes

In principle, target inactivation analysis provides a means of determining the molecular weights (Mr) and states of aggregation of proteins in native environments where they are functionally active. We applied this irradiation technique to the rat liver microsomal membrane proteins: cytochrome b5, epoxide hydrolase, flavin-containing monooxygenase, NADH-ferricyanide reductase, NADPH-cytochrome P-450 reductase, and seven different forms of cytochrome P-450. Catalytic activities, spectral analysis of prosthetic groups, and sodium dodecyl sulfate-polyacrylamide electrophoresis/peroxidase-coupled immunoblotting were used to estimate apparent Mr values in rat liver microsomal membranes. Except in one case (cytochrome P-450PCN-E), the estimated Mr corresponded most closely to that of a monomer. Purified cytochrome P-450PB-B, NADPH-cytochrome P-450 reductase and epoxide hydrolase were also subjected to target inactivation analysis, and the results also suggested monomeric structures for all three proteins under these conditions. However, previous hydrodynamic and gel-exclusion results clearly indicate that all three of these proteins are oligomeric under these conditions. The discrepancy between target inactivation Mr estimates and hydrodynamic results is attributed to a lack of energy transfer between monomeric units. Thus, while P-450PCN-E may be oligomeric in microsomal membranes, target inactivation analysis does not appear to give conclusive results regarding the states of aggregation of these microsomal proteins.

Rat liver cytochrome P-450 isozymes as catalysts of aldrin epoxidation in reconstituted monooxygenase systems and microsomes

To explore which rat liver cytochrome P-450 species are involved in aldrin epoxidation, we have studied the catalytic activities of a series of cytochrome P-450 isozymes purified from untreated and inducer-treated Sprague-Dawley rats. Of ten cytochrome P-450 forms analyzed, seven isozymes, listed in order of decreasing activity, catalyzed aldrin epoxidation: P-450UT-A, P-450PB-C, P-450UT-H, P-450PB-B, P-450PCN-E, P-450UT-F, and P-450PB-D. P-450UT-I, P-450BNF-B, and P-450ISF-G were not very active at all. A novel aldrin metabolite, endo-dieldrin, was formed by cytochrome P-450UT-F in a 6-fold excess over dieldrin, which is the exo-isomer. The activity of aldrin epoxidase furthermore was assayed in liver microsomes from Sprague-Dawley rats of diverse physiological status and after pretreatment with various inducers resulting in a peculiar pattern of cytochrome P-450 isozymes. Untreated animals, at an age of 3 weeks, showed similar enzyme activities in both genders. During maturation, the activity of males increased by 3-fold, while the activity in females did not significantly change during this period. Pretreatment with pregnenolone-16-alpha-carbonitrile or dexamethasone strongly increased the activity in females. Pretreatment with dexamethasone did not increase the activity of males. A 50% depression of epoxidase activity was noted for males pretreated with 5,6-benzoflavone. Phenobarbital pretreatment increased the activity of females by 12-fold and of males by 2-fold. Males responded to pretreatment with polychlorinated biphenyls in a strain dependent fashion: enzyme activity was increased 2-fold in Sprague-Dawley rats but was not altered in Wistar rats. "Theoretical" values of microsomal epoxidase activity were calculated for weanling and adult Sprague-Dawley rats from turnover numbers and published data on the relative abundance of aldrin epoxidizing P-450 isozymes (Waxmann et al., Biochemistry 24, 4409, 1985). These values agreed with the activities determined. A similar statement can be made for male rats of both strains pretreated with inducers, when the ratio of enzyme activity of pretreated to control animals was used as a basis of comparison. The activity ratio of females pretreated with pregnenolone-16-alpha-carbonitrile, dexamethasone and phenobarbital, however, was much higher than the ratio calculated. Our results reveal that aldrin epoxidation is a reaction indicative of male specific and of phenobarbital-inducible cytochrome P-450 isozymes in rat liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Irreversible binding and metabolism of propranolol by human liver microsomes--relationship to polymorphic oxidation

Studies were performed to investigate the irreversible binding and oxidative metabolism of propranolol in human liver microsomes and the relationship of binding and metabolism to the polymorphic oxidation of debrisoquine. Incubation of microsomes with 14C-labelled propranolol in the presence of a NADPH-generating system gave rise to irreversible binding which increased linearly with time and became saturated at high substrate concentrations. The extent of binding was decreased by the exclusion of cofactors, boiling, anaerobic conditions, and the addition of reduced glutathione and SKF-525A. Trichloropropene oxide had a negligible effect on cofactor-dependent binding. However, debrisoquine, antipyrine and phenacetin decreased binding to a considerable extent. The latter compound abolished cofactor-dependent binding completely at the concentration used (1 mM). Electrophoresis of microsomes which had been incubated with tritiated propranolol revealed that binding was probably occurring to a large number of proteins particularly in the 40,000-90,000 molecular weight range. Glutathione, debrisoquine and antipyrine did not inhibit the 4'-hydroxylation and N-deisopropylation of propranolol. In contrast, phenacetin exerted a very potent inhibitory action on both routes of metabolism. It is concluded that a product or products of propranolol oxidation bind irreversibly but non-selectively to human liver microsomal protein, the enzyme system responsible for the activation of propranolol appears to be related more closely to the cytochrome P-450 system which metabolizes phenacetin than to that metabolising debrisoquine, and radiolabelled propranolol is not a sufficiently specific probe for studying these cytochrome P-450 systems.

Immunochemical evidence for multiple steroid-inducible hepatic cytochromes P-450 in the rat

It has been established that there are glucocorticoid-inducible hepatic cytochromes P-450 in the rat (P-450p), the rabbit (LM3c) and man (HLp) which share extensive structural, functional and regulatory features. We prepared immunochemical probes to P-450p and identified a unique monoclonal antibody, 1G8, that recognizes purified P-450p, but neither purified LM3c nor HLp, on immunoblot analysis. The N-terminal amino acid sequence of purified samples of P-450p was identical with that previously reported for P-450PCN1 [Gonzalez, Nebert, Hardwick & Kasper (1985) J. Biol. Chem. 260, 7435-7441]. Immunoblot analyses of liver microsomes from untreated male rats revealed two 1G8-reactive proteins, whereas liver microsomes from untreated females contained none. Another monoclonal antibody, 13-7-10, reacted specifically with LM3c and HLp, but not with P-450p. A single 13-7-10-reactive microsomal protein was detected in untreated male and female rats, the latter protein exhibiting a greater apparent Mr. 1G8-reactive proteins were induced to the greatest extent by triacetyloleandomycin, followed by dexamethasone, chlordane, pregnenolone-16 alpha-carbonitrile and 2,4,2',4'-tetrachlorobiphenyl. In contrast, 13-7-10-reactive proteins were most strongly induced by dexamethasone, only moderately by triacetyloleandomycin and pregnenolone-16 alpha-carbonitrile, weakly by chlordane and not at all by 2,4,2',4'-tetrachlorobiphenyl. We conclude that the P-450p family in rat liver consists of three or more proteins that are structurally related and yet appear to be under distinct regulatory control.

Human liver debrisoquine 4-hydroxylase: test for specificity toward various monooxygenase substrates and model of the active site

Polyclonal antibodies raised toward a debrisoquine 4-hydroxylating cytochrome P-450 species purified from rat liver (P-450UT-A) were used to determine which monooxygenase reactions are linked to debrisoquine hydroxylation in human liver. Anti P-450UT-A did not inhibit the oxidation of dimethylnitrosamine, morphine, diazepam, vinylidene chloride, trichloroethylene, benzo(a)pyrene and its 7.8-dihydrodiol, but was inhibitory for the hydroxylation of debrisoquine, (+/-)-bufuralol, lasiocarpine and monocrotaline. A model interpreting the substrate specificity of the human liver enzyme is presented.

Fingerprinting rat liver microsomal cytochromes P-450 as a means of delineating sexually distinctive forms

The cytochrome P-450 fraction of microsomes separated on lauric acid AH-Sepharose 4B columns contains about 75% of the microsomal P-450. This was fingerprinted by means of two dimensional isoelectric focusing/SDS-PAGE. Separation of the fraction by highly reproducible, standard procedures on carboxymethyl Sepharose CL6B into four fractions allowed ready isolation and purification of seven forms of P-450, RLM2, 2b, 3, f4, 5, 5a and f5a. Comparison of the four fractions CMI, CMII, CMIII and CMIV revealed qualitative differences in the proteins contained in CMI and CMII of male and female rats. Identification of these proteins revealed RLM2, present in the CMI fraction of adult male rats, is not present in detectable levels in the comparable fraction from females. Similarly, RLM3 and 5 were present in the CMII fraction of male rats but could not be detected in the corresponding fraction of females. Instead, another protein, fRLM4, was found in the females. RLM5a, found in the CMII fraction of males, was also present in females. Examination of the physical properties of these P-450 proteins revealed those isolated in the CMI and CMII fractions to have fairly neutral isoelectric points (7.1-7.6). Based upon the NH2-terminal amino acid sequence, three classes of constitutive forms of P-450 can be recognized. All of the constitutive forms have methionine in position one and leucine in position seven. By comparing sequence homologies, RLM2 and 2b form one sub-class, RLM3, f4 and 5 form a second sub-class, and P-450f and RLM5a form a third sub-class.

Polymorphism of propafenone metabolism and disposition in man: clinical and pharmacokinetic consequences

The relationship between debrisoquine metabolic phenotype and the pharmacokinetics and pharmacodynamics of propafenone was studied in 28 patients with chronic ventricular arrhythmias (22 extensive metabolizers [EMs] and six poor metabolizers [PMs] of debrisoquine). EMs were characterized by a shorter propafenone elimination half-life (5.5 +/- 2.1 vs 17.2 +/- 8.0, p less than .001), lower average plasma concentration (Cp) (1.1 +/- 0.6 vs 2.5 +/- 0.5 ng/ml/mg daily dosage, p less than .001), and higher oral clearance (1115 +/- 1238 vs 264 +/- 48 ml/min, p less than .001). The active metabolite 5-hydroxypropafenone, assayed in 12 patients, was identified in nine of 10 EMs but in neither of the PMs. A lower incidence of central nervous system side effects was noted in EMs (14% vs 67%, p less than .01). The magnitude of QRS widening at any given propafenone Cp was greater in EMs than PMs. There was no significant difference between EMs and PMs in effective propafenone dose or frequency of antiarrhythmic response. Inhibition of debrisoquine 4-hydroxylation by propafenone was demonstrated both in vivo and in a human liver microsomal system in vitro. We conclude that propafenone is metabolized via the same cytochrome P-450 responsible for debrisoquine's 4-hydroxylation, and that its pharmacokinetics and concentration-response relationships and the incidence of central nervous system side effects are different in patients of different debrisoquine metabolic phenotype.

Debrisoquine 4-hydroxylase: characterization of a new P450 gene subfamily, regulation, chromosomal mapping, and molecular analysis of the DA rat polymorphism

Debrisoquine 4-hydroxylase (P450db1) was purified from rat liver microsomes. Polyclonal antibody was produced and, in conjunction with immunoblots, was used to identify and purify a second immunorelated P450 (P450db2) that does not have debrisoquine hydroxylating activity. The cDNA clones to db1 and db2 were isolated from a lambda gt11 expression library, sequenced, and found to share 78% nucleotide and 73% deduced amino acid similarities. These similarities are evenly dispersed along the sequence except for a region of 190 nucleotides with 99% similarity near the carboxyl terminus of the protein-coding region; this similarity is probably the remnant of a gene conversion event. Both proteins share between 38% and 43% amino acid similarity with P450a, P450b, P450e, P450f, P450PB1, and P450j; these data indicate that P450db1 and P450db2 are members of a separate subfamily within the P450II gene family. Southern blot analysis and preliminary genomic cloning suggest that at least four genes exist in the subfamily, although the present evidence suggests that only db1 and db2 are expressed in rat liver. With the use of 19 mouse X hamster somatic cell hybrids, the db1 and db2 genes were localized to mouse chromosome 15 (P450-2D locus). A polymorphism has been described for debrisoquine metabolism in the DA rat strain, adult females having markedly decreased debrisoquine 4-hydroxylase activity. Our immunoblot analysis and mRNA analysis suggest that debrisoquine 4-hydroxylase deficiency in the female DA rat is not due to a decrease in db1 protein or mRNA. The db1 and db2 proteins are differentially regulated: during development db2 is present at birth while db1 is absent, and db1 increases by 1 week of age; in addition, db1 is slightly induced by phenobarbital, 3-methyl-cholanthrene, and dexamethasone whereas db2 is marginally increased by these latter two agents. These results demonstrate that debrisoquine 4-hydroxylase is a member of a new constitutively expressed P450II sub-family containing two or more genes in the rat and establish that the debrisoquine polymorphism in the DA rat is probably due to a structurally altered db1 protein.

Polymorphism of human cytochrome P-450

The cytochrome P-450 forms involved in debrisoquine 4-hydroxylation (P-450DB), phenacetin O-deethylation (P-450PA), S-mephenytoin 4-hydroxylation (P-450MP), and nifedipine 1,4-oxidation (P-450NF) have been purified to electrophoretic homogeneity from human liver microsomes. All of these reactions show in vivo polymorphism in humans. Evidence for the roles of the purified proteins in these processes comes from in vitro reconstitution and immunoinhibition studies. The rat orthologs of these enzymes are as follows--P-450DB: P-450UT-H; P-450PA: P-450ISF-G; P-450MP: P-450UT-I; P-450NF: P-450PCN-E. Only in the case of P-450UT-H is the primary rat ortholog the same cytochrome P-450 which catalyses the catalytic reaction under consideration. Reconstitution and immunochemical studies establish that the following reactions are catalysed by the individual P-450s--P-450DB: debrisoquine 4-hydroxylation, sparteine delta 5-oxidation, bufuralol 1'-hydroxylation, encainide O-demethylation, and propanolol 4-hydroxylation; P-450PA: phenacetin O-deethylation; P-450MP: S-mephenytoin 4-hydroxylation and tolbutamide methyl hydroxylation; P-450NF: oxidation of nifedipine and 16 other substituted dihydropyridines, estradiol 2- and 4-hydroxylation, aldrin epoxidation, benzphetamine N-demethylation and 6 beta-hydroxylation of testosterone, androstenedione and cortisol. A cDNA clone has been isolated that corresponds to rat P-450UT-H, as shown by a number of criteria. Studies with this probe establish that the sex and strain variation in debrisoquine 4-hydroxylase and related activities is related to differences in the levels of a 2.0 kb length mRNA present.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease in the levels of a constitutive cytochrome P-450 (RLM5) in hepatic microsomes of diabetic rats

Cytochrome P-450 dependent hydroxylation of testosterone has been measured in hepatic microsomes of control, diabetic and insulin-treated diabetic rats. The observed decrease in testosterone 16 alpha-hydroxylase activity in diabetes, an activity previously shown to be largely due to RLM5, was accompanied by a dramatic decrease in immunodetectable RLM5. Diabetic rats which received insulin had elevated testosterone 16 alpha-hydroxylase activity relative to the diabetic animals, which was accompanied by a corresponding increase in the levels of RLM5. These results provide evidence that specific constitutive cytochrome P-450 enzymes are altered in the diabetic state and that these changes are not permanent since they can be overcome, at least partially, by insulin replacement therapy.

Phenacetin O-deethylase activity of the rat: strain differences and the effects of enzyme-inducing compounds

Phenacetin O-deethylase activity in microsomal fractions from liver of DA and Fischer rats has been determined. No major sex or strain differences were found. Kinetic analysis revealed two major components of O-deethylase activity in the liver of both strains of rats. Michaelis-Menten analysis revealed no major difference between the strains. Phenacetin O-deethylase activity is inducible by both 3-methylcholanthrene and phenobarbitone in DA and Fischer rats. 3-Methylcholanthrene selectively increases the high-affinity component of activity, by 20- to 25-fold, whereas phenobarbitone selectively increases the low-affinity component, by two- to three-fold. It is concluded that there is no major difference between the DA and Fischer strains in their ability to O-deethylate phenacetin. Thus, unlike poor metabolizers of debrisoquine in the human population, who appear also to have impaired phenacetin O-deethylase activity, the DA rat is deficient in only the former activity.

Digitoxin metabolism by liver microsomal cytochrome P-450 and UDP-glucuronosyltransferase and its role in the protection of rats from digitoxin toxicity by pregnenolone-16 alpha-carbonitrile

The aim of the present study was to investigate whether the mechanism by which pregnenolone-16 alpha-carbonitrile (PCN) protects rats from digitoxin toxicity was dependent on the induction of liver microsomal cytochrome P-450p and/or the UDP-glucuronosyltransferase active toward digitoxigenin monodigitoxoside (UDP-GT-dt1). Evidence is presented that suggests troleandomycin is a selective inhibitor of cytochrome P-450p in vivo, based on the pattern of inhibition observed when zoxazolamine paralysis time and hexobarbital sleeping time were measured in rats treated with different cytochrome P-450 inducers. A single dose of troleandomycin completely reversed the ability of PCN to protect rats from digitoxin toxicity, establishing the importance of cytochrome P-450p induction in the protective effect of PCN. The postpubertal decline in constitutive cytochrome P-450p levels in female but not male rats was paralleled by a female-specific, age-dependent decline in the rate of digitoxin sugar cleavage (i.e., digitoxosyl oxidation of digitoxin to 15'-dehydrodigitoxin and digitoxosyl cleavage to digitoxigenin bisdigitoxoside). This resulted in a marked sex difference in the rate of digitoxin sugar cleavage catalyzed by liver microsomes from mature rats (male/female approximately 6). However, no sex difference in digitoxin toxicity was observed in either immature or mature rats. In contrast to cytochrome P-450p, liver microsomal UDP-GT-dt1 activity increased dramatically with age in both male and female rats (mature/immature approximately 10). However, no age differences in digitoxin toxicity were observed in rats of either sex. The results indicate that cytochrome P-450p and UDP-GT-dt1 can be independently regulated in rat liver and that large changes in the constitutive levels of these microsomal enzymes have no effect on digitoxin toxicity. This suggests that the induction of cytochrome P-450p and UDP-GT-dt1 does not fully account for the mechanism by which PCN protects rats from digitoxin toxicity.

Bufuralol 1'-hydroxylase activity of the rat. Strain differences and the effects of inhibitors

The kinetics of bufuralol 1'-hydroxylase activity of hepatic microsomal fractions have been determined in female DA and Fischer 344 rats, strains between which there is a large difference in debrisoquine 4-hydroxylase activity. Two components of bufuralol 1'-hydroxylase activity could be observed in both strains. Although there were differences between the strains in Vmax and Km of both components of activity, these were much less marked than the differences previously reported for debrisoquine 4-hydroxylase (Kahn et al., Drug Metab. Dispos. 13, 510 (1985)). The kinetics of bufuralol 1'-hydroxylase activity were such that the difference in activity between the strains varied with the concentration of bufuralol, 4-5-fold at 2.5 microM, no difference at 100 microM Competitive inhibitors of debrisoquine 4-hydroxylase activity in man were competitive inhibitors of bufuralol 1'-hydroxylase activity in the Fischer 344 rat, but not in the DA rat. The Ki for inhibition of bufuralol 1'-hydroxylase activity by debrisoquine in the Fischer 344 rat was 184 microM, compared with a Km for the 4-hydroxylation of this compound of 10.5 microM. It is concluded that the major isozyme of cytochrome P-450 catalysing the 1'-hydroxylation of bufuralol in the rat is different from that catalysing debrisoquine 4-hydroxylation (P-450UT-H).

Partial metabolic clearances as determinants of the oral bioavailability of propranolol

The objective of this study was to determine the relationships between the total oral clearance of propranolol and the partial clearances through its primary metabolic pathways, i.e. glucuronidation, side-chain oxidation and ring oxidation. Seven young, white males were given single 80 mg oral doses of the drug together with tritium-labelled propranolol. Plasma propranolol was measured by GC/MS and fourteen metabolites were measured in urine by h.p.l.c. with radiometric detection. The total oral clearance of propranolol in these subjects varied about three-fold, from 27.5 to 71.4 ml min-1 kg-1. The clearance through glucuronidation was very similar in all subjects, ranging from 6.8 to 9.9 ml min-1 kg-1. The clearance through side-chain oxidation varied 2.4-fold, from 10.9 to 25.8 ml min-1 kg-1. Increased clearance through this pathway correlated with increased total oral clearance (r = 0.84; P less than 0.02). The clearance through ring oxidation varied as much as 5.6-fold, from 7.5 to 41.8 ml min-1 kg-1. Increased clearance through this pathway correlated highly with increased total oral clearance (r = 0.94; P less than 0.002). These observations indicate that the intersubject variability in the oral clearance of propranolol in young, white males is due mainly to differences in the activity of the ring oxidation pathway, to some extent in the side-chain oxidation pathway, but not to differences in the glucuronidation pathway. The partial metabolic clearance approach adopted in this study may be useful in the investigation of factors influencing the oral bioavailability of propranolol.