Comparison of different forms of liver, kidney, and lung microsomal cytochrome P-450 by immunological inhibition of regio- and stereoselective metabolism of warfarin

Effects of CYP2C11 gene knockout on the pharmacokinetics and pharmacodynamics of warfarin in rats

1. CYP2C11 is the most abundant isoform of cytochrome P450s (CYPs) in male rats and is considered the main enzyme for warfarin metabolism. 2. To further access the in vivo function of CYP2C11 in warfarin metabolism and efficacy, a CYP2C11-null rat model was used to study warfarin metabolism with both in vitro and in vivo approaches. Prothrombin time (PT) of warfarin was also determined. 3. The maximum rate of metabolism (V_max) and intrinsic clearance (CL_int) of liver microsomes from CYP2C11-null males were reduced by 37 and 64%, respectively, compared to those in Sprague Dawley (S-D) rats. The K_m of liver microsomes from CYP2C11-null males was increased by 73% compared to that of S-D rats. The time to reach the maximum plasma concentration (T_max) of warfarin in CYP2C11-null males was significantly delayed compared to that in S-D males, and the CL rate was also reduced. The PT of CYP2C11-null rats was moderately longer than that of S-D rats. 4. In conclusion, the clearance rate of warfarin was mildly decreased and its anticoagulant effect was moderately increased in male rats following CYP2C11 gene knockout. CYP2C11 played a certain role in the clearance and efficacy of warfarin, while it did not seem to be essential.

Targeted label-free approach for quantification of epoxide hydrolase and glutathione transferases in microsomes

The aim of this study was to investigate the expression and organ distribution of cytochrome P450 (CYP450) enzymes, microsomal epoxide hydrolase (MEH), and microsomal glutathione-S-transferase (MGST 1, 2, 3) in human liver, lung, intestinal, and kidney microsomes by targeted peptide-based quantification using nano liquid chromatography-tandem multiple reaction monitoring (nano LC-MRM). Applying this method, we analyzed 16 human liver microsomes and pooled lung, kidney, and intestine microsomes. Nine of the CYP450s (CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5) could be quantified in liver. Except for CYP3A4 and 3A5 existing in intestine, other CYP450s had little content (<0.1 pmol/mg protein) in extrahepatic tissues. MEH and MGSTs could be quantified both in hepatic and in extrahepatic tissues. The highest concentrations of MEH and MGST 1, 2 were found in liver; conversely MGST 3 was abundant in human kidney and intestine compared to liver. The targeted proteomics assay described here can be broadly and efficiently utilized as a tool for investigating the targeted proteins. The method also provides novel CYP450s, MEH, and MGSTs expression data in human hepatic and extrahepatic tissues that will benefit rational approaches to evaluate metabolism in drug development.

Vkorc1 variation in house mice during warfarin and difenacoum field trials

BACKGROUND

Field studies guided by genetic monitoring of Vkorc1 need to be done to implicate mutations conclusively with rodent control problems due to the presence of animals resistant to anticoagulant rodenticides. Rodent control success in relation to Vkorc1 genotypes in house mice (Mus musculus domesticus) was studied on two farms (I and II) in Germany. Tests were carried out to determine whether certain resistance profiles and Vkorc1 genotypes displayed dynamics over the course of sequential treatments with warfarin and difenacoum that were consistent with single nucleotide polymorphisms (SNPs) in Vkorc1 as indicators of resistance.

RESULTS

On farms I and II, respectively, three (A to C) and two (A and B) types of control problem with anticoagulants (i.e. proxies for resistance) were encountered in spatially segregated subunits: A = none; B = control problems with warfarin but not with difenacoum; C = control problems with both anticoagulants. Unexpectedly, resistance was encountered in a population where only Vkorc1 wild-type mice were detected. In addition, the Arg58Gly Vkorc1 variant was found not to correlate with observed control failures.

CONCLUSION

Control problems were encountered that cannot be explained by Vkorc1 coding or intronic SNPs, and therefore are likely due to non-coding Vkorc1 SNPs or due to other genetic or non-genetic factors.

Comparison of warfarin sensitivity between rat and bird species

Scattering coumarin derivative rodenticides in broad areas have caused primary- and secondary-poisoning incidents in non-target wild birds. In this study, we compared factors determining warfarin sensitivity between bird species and rats based on vitamin K 2,3-epoxide reductase (VKOR) kinetics, VKOR inhibition by warfarin and warfarin metabolism assays. In VKOR characterization, chickens and ostriches showed significantly lower enzymatic efficiencies than rats (one-sixth and one-third, respectively), suggesting bird species depend more on a non-VKOR vitamin K source. On the other hand, the inhibition constants (K(i)) of VKOR for warfarin were significantly different between chickens and ostriches (11.3+/-2.5 microM and 0.64+/-0.39 microM, respectively). Interestingly, the ostrich K(i) was similar to the values for rats (0.28+/-0.09 microM). The K(i) results reveal a surprising possibility that VKOR in some bird species are easily inhibited by warfarin. Warfarin metabolism assays also showed a large inter-species difference in bird species. Chickens and ostriches showed higher metabolic activity than that of rats, while mallards and owls showed only a slight ability to metabolize warfarin. In this study, we clarified the wide inter-species difference that exists among birds in xenobiotic metabolism and sensitivity to a rodenticide.

Common and uncommon cytochrome P450 reactions related to metabolism and chemical toxicity

Cytochrome P450 (P450) enzymes catalyze a variety of reactions and convert chemicals to potentially reactive products as well as make compounds less toxic. Most of the P450 reactions are oxidations. The majority of these can be rationalized in the context of an FeO(3+) intermediate and odd electron abstraction/rebound mechanisms; however, other iron-oxygen complexes are possible and alternate chemistries can be considered. Another issue regarding P450-catalyzed reactions is the delineation of rate-limiting steps in the catalytic cycle and the contribution to reaction selectivity. In addition to the rather classical oxidations, P450s also catalyze less generally discussed reactions including reduction, desaturation, ester cleavage, ring expansion, ring formation, aldehyde scission, dehydration, ipso attack, one-electron oxidation, coupling reactions, rearrangement of fatty acid and prostaglandin hydroperoxides, and phospholipase activity. Most of these reactions are rationalized in the context of high-valent iron-oxygen intermediates and Fe(2+) reductions, but others are not and may involve acid-base catalysis. Some of these transformations are involved in the bioactivation and detoxication of xenobiotic chemicals.

Kinetics of ferric cytochrome P450 reduction by NADPH-cytochrome P450 reductase: rapid reduction in the absence of substrate and variations among cytochrome P450 systems

The reduction of ferric cytochrome P450 (P450) to ferrous is the first chemical step in almost all P450 reactions, and many characteristics of this step have been reported. Reduction kinetics of rabbit and human P450s were measured in a variety of systems. As reported earlier, P450 reduction is biphasic in microsomes and some purified P450 systems. However, this is not an inherent property of P450s, and some low- and high-spin iron P450s were reduced with single-exponential kinetics. Contrary to a generalized view, the presence of substrate is not necessary for rapid reduction of all P450s. Also, low-spin heme can be reduced as rapidly as high-spin in several P450s. P450s varied considerably in their reduction behavior, and even a single P450 showed remarkably different reduction kinetics when placed in various environments. P450 3A4 reduction was examined in liver microsomes, a reconstituted system, a fusion protein in which it was linked to NADPH-P450 reductase, and baculovirus and bacterial membranes in which P450 3A4 and NADPH-P450 reductase were coexpressed; the systems differed considerably in terms of the need for the substrate testosterone and cytochrome b5 (b5) for reduction and as to whether reduction was rate-limiting in the overall catalytic cycle. When b5 was included in reconstituted systems, it reduction kinetics were linked with those of some P450s. This behavior could be simulated in kinetic models in which electrons flowed from the ferrous P450.CO complex to oxidized b5. Overall, the kinetics of ferric P450 reduction cannot be generalized among different P450s in various systems, and concepts regarding influence of substrate, reaction sequence, and a rate-limiting step are not very universal.

Drug metabolism in rats with cancer induced by N-nitrosodiethylamine and phenobarbital

The metabolism of R- and S-warfarin in vivo and in vitro, bufuralol in vitro, and antipyrine and debrisoquine in vivo were studied in rats with cancer induced by N-nitrosodiethylamine and phenobarbital treatment. Microsomal cytochrome P-450 content was greatly reduced in both healthy and cancerous parts of the livers of tumour-bearing animals. The specific activities of R-warfarin and bufuralol 1'-hydroxylases were significantly elevated in rats with cancer. The activities of S-warfarin hydroxylases expressed per mg microsomal protein were reduced in animals with cancer, whereas those of R-warfarin and bufuralol 1'-hydroxylases were not. The urinary excretion of R-7-hydroxywarfarin was increased and those of S-6- and S-4'-hydroxywarfarin decreased in rats with cancer. The correlations between microsomal formation and urinary excretion of all warfarin metabolites were poor, except for R-7-hydroxywarfarin. Antipyrine oxidation was increased in the cancerous state but the urinary metabolic profiles were similar in rats with cancer and in controls. The metabolism of debrisoquine was decreased in tumour-bearing animals. Antipyrine metabolism did not show any correlation with either warfarin or debrisoquine metabolism, whereas several relationships were observed between warfarin and debrisoquine metabolism and between warfarin and bufuralol metabolism.

Purification and immunological characterization of a novel cytochrome P450 from C3H/10T1/2 cells

The major polycyclic aromatic hydrocarbon-metabolizing cytochrome P450 in the mouse embryo fibroblast-derived C3H/10T1/2 CL8 cell line (P450-EF) has been partially purified from benz[a]anthracene (BA)-induced 10T1/2 cells (40 pmol P450/mg). The purification of P450-EF was carried out by sequential chromatography of solubilized microsomes over hydrophobic aminohexyl-Sepharose 4B, anion exchange DE-52 cellulose, and cation exchange carboxymethyl trisacryl columns. The final preparation (1700 pmol/mg) appeared as a single major 55-kDa band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Reconstitution of detergent-free partially purified P450-EF yielded a relatively high turnover for 7,12-dimethylbenz[a]anthracene (DMBA) metabolism (5.4 nmol/nmol/min). Polyclonal antibodies to purified P450-EF (anti-P450-EF), raised in, respectively, rabbit and chicken, detected a single 55-kDa band in 10T1/2 cell microsomes that was highly inducible by BA (approximately 20-fold) and TCDD (approximately 5-fold). Rabbit anti-P450-EF was much more effective than the corresponding chicken antibody at binding denatured P450-EF protein on Western blots. Conversely, only the chicken antibody was effective at inhibiting DMBA metabolism catalyzed by microsomal P450-EF. This antibody did not inhibit P450IA1-mediated DMBA metabolism. Rabbit anti-P450-EF recognized very weakly (less than 1% of homologous protein response) pure P450IA1, IIB1, IIC7, IIE1, and IIIA1 proteins on Western blots but exhibited substantial cross-reactivity (approximately 10%) with pure P450IIA1 and very strong cross-reactivity (approximately 75%) with a hormonally regulated rat adrenal P450. Polyclonal antibodies to several major P450 subfamilies either did not recognize P450-EF (anti-P450IA, IIB, and IIC) or recognized it very weakly (anti-P450IIA1). P450-EF is probably distantly related to the P450IIA subfamily and may belong to a new P450 subfamily.

Metabolic enantiomeric interactions: the inhibition of human (S)-warfarin-7-hydroxylase by (R)-warfarin

Inhibition of the metabolism of (S)-warfarin, the more pharmacologically active enantiomer of the racemic drug, by (R)-warfarin was investigated in microsomes obtained from three human livers. In each case the production of both (S)-6- and (S)-7-hydroxywarfarin was found to be competitively inhibited by (R)-warfarin. The KiS for inhibition of (S)-6- and (S)-7-hydroxylation by (R)-warfarin ranged from 7.0 to 8.4 microM and from 6.0 to 6.9 microM, respectively, while the KmS for the 6- and 7-hydroxylation of (S)-warfarin ranged from 3.6 to 3.8 microM and from 3.3 to 3.9 microM, respectively. In contrast, except for the 4'-hydroxylation pathway (S)-warfarin was found to be a weak inhibitor of the metabolism of (R)-warfarin. Possible implications of these findings include the following: (1) the kinetic parameters defining the interactions of two enantiomers of a racemic drug with the cytochrome P-450s or other macromolecular systems in the living organism can only be properly defined from experiments with the pure enantiomers, (2) an enantiomer of a racemic drug may contribute significantly to biological effect not by its inherent activity but by altering the pharmacokinetics of the eutomer, and (3) enantiomeric interactions are not easily detected unless directly sought and may be relatively common.

Hepatic microsomal warfarin metabolism in warfarin-resistant and susceptible mouse strains: influence of pretreatment with cytochrome P-450 inducers

In the present paper, the heterogeneity of hepatic cytochrome P-450 isoenzymes in the mouse has been probed, using warfarin as the substrate. Both sex and strain differences in the in vitro microsomal metabolism of warfarin have been investigated in male and female warfarin-resistant HC and warfarin-susceptible LAC-grey mouse strains. Animals were either untreated or treated with the cytochrome P-450 inducers phenobarbitone, beta-napthoflavone or clofibrate. In both sexes and strains of mice, metabolism of warfarin was stereoselective in favour of the R(+) enantiomer. However, regioselectively was different in both strains and sexes of untreated animals. After pretreatment with phenobarbitone, increases in the rate of formation of 4' and 7-hydroxy R(+) and S(-) warfarin metabolites in HC mice were observed, compared with untreated animals. In LAC-grey mice increases in 4'-, 6-, 7- and 8-hydroxy R(+) and S(-) warfarin metabolites were noted, compared with untreated animals. This data indicated that different amounts or forms of cytochrome P-450s were responsible for warfarin metabolism after phenobarbitone treatment in the two strains. Pretreatment of animals with beta-napthoflavone resulted in significant decreases in the rat of R(+) warfarin metabolism in both strains and sexes of mice indicating that the beta-naphthoflavone-inducible cytochrome P-450 isoenzymes were less active in the metabolism of warfarin, as compared to the uninduced isoenzymes. In addition, the cytochrome P-450 isoenzyme composition in the two mouse strains was different after clofibrate pretreatment, as reflected in reduced levels of some warfarin metabolites and a reduced total metabolism of warfarin, consistent with the narrow substrate specificity of clofibrate-induced cytochrome P450IVA1 for fatty acid hydroxylation. Accordingly, it is clear that both the basal and xenobiotic inducible hepatic cytochrome P-450 isoenzymes in warfarin-resistant and susceptible mice are different and therefore have implications for the in vivo disposition of warfarin.

Chemical synthesis, absolute configuration, and stereochemistry of formation of 10-hydroxywarfarin: a major oxidative metabolite of (+)-(R)-warfarin from hepatic microsomal preparations

The synthesis of a diastereomerically pure 10-hydroxywarfarin [4-hydroxy-3-(2-hydroxy-3-oxo-1-phenylbutyl)-2H-1 benzopyran-2-one] was accomplished in three steps from racemic warfarin. The relative configuration of the synthetic product was established by conversion to a cyclic derivative followed by NMR and X-ray diffraction analysis. Absolute stereochemistry was determined by enzymatic conversion of either of the pure enantiomers of warfarin to a 10-hydroxy metabolite of known relative configuration. Metabolic formation of 10-hydroxywarfarin was studied using hepatic microsomal preparations from female rats and man. The formation of 10-hydroxywarfarin catalyzed by hepatic microsomes from both dexamethasone-treated rats and man was highly stereoselective [(R)/(S): 3.4-9.0] for (R)-warfarin. In contrast, little stereoselectivity was observed in reactions catalyzed by untreated rat liver microsomes. The resultant stereochemistry at the site of oxidation was also found to be highly dependent on substrate stereochemistry. (R)-Warfarin gave (9R;10S)-10-hydroxywarfarin with only a trace of the (9R;10R) isomer irrespective of which enzyme preparation was used for catalysis, while (S)-warfarin gave (9S;10R)-10-hydroxywarfarin with only a trace of the (9S;10S) isomer, again irrespective of which enzyme preparation was used for catalysis.

Characterization of a phenobarbital-inducible dog liver cytochrome P450 structurally related to rat and human enzymes of the P450IIIA (steroid-inducible) gene subfamily

A cytochrome P450 called PBD-1 isolated from liver microsomes of an adult male Beagle dog treated with phenobarbital (PB) is structurally and functionally similar to members of the P450IIIA gene subfamily in rat and human liver microsomes. The sequence of the first 28 amino-terminal residues of PBD-1 is identical in 15 and 20 positions, respectively, to the P450IIIA forms P450p from rat and P450NF (and HLp) from human. Upon immunoblot analysis, anti-PBD-1 IgG recognizes PCNa (P450p) and PCNb (PB/PCN-E) from rat, P450NF from human, and two proteins in liver microsomes from both untreated and PB-treated dogs. Similarly, anti-PCNb IgG cross-reacts with PBD-1 and with at least one protein in microsomes from untreated dogs and two proteins in microsomes from PB-treated dogs. P450IIIA-form marker steroid 6 beta-hydroxylase activities increase 2.5-fold upon PB-treatment of dogs and are selectively inhibited by anti-PBD-1 IgG. NADPH-dependent triacetyloleandomycin (TAO) complex formation and erythromycin demethylase, also marker activities for P450IIIA forms from rats and humans, increase 4- and 5-fold in dog liver microsomes upon PB treatment, whereas immunochemically reactive PBD-1 is induced 3-fold. In microsomes from PB-treated dogs, 5 mg anti-PBD-1 IgG/nmol P450 inhibits greater than 75 and 50% of TAO complex formation and erythromycin demethylase activity, respectively. TAO complex formation is not inhibited by chloramphenicol, a selective inhibitor of the major PB-inducible dog liver cytochrome P450, PBD-2. These data suggest that PBD-1 or another immunochemically related form is responsible for a major portion of macrolide antibiotic metabolism by microsomes from PB-treated dogs and for steroid 6 beta-hydroxylation by microsomes from both untreated and PB-treated dogs. Major species differences were noted, however, in the apparent Km for 6 beta-hydroxylation of androstenedione by liver microsomes from untreated rats (24 microM), humans (380 microM), and untreated dogs (4700 microM).

Microbial models of mammalian metabolism: conversion of warfarin to 4'-hydroxywarfarin using Cunninghamella bainieri

Warfarin, an anticoagulant and "metabolic probe" for cytochrome P-450 isozyme multiplicity, is metabolized to 4'-hydroxywarfarin, a principle mammalian metabolite, using the fungus Cunninghamella bainieri (UI-3065). The metabolite was isolated from cell suspension cultures and characterized by analytical (TLC, HPLC, GC-MS) and spectral (HRMS, EI-MS, PMR) comparisons with authentic 4'-hydroxywarfarin. The mechanism of aromatic hydroxylation was examined in C. bainieri using 4'-deuterowarfarin. The absence of a primary isotope effect (KH/KD = 1.13), migration and retention of deuterium in the phenolic product [80% migration and retention (M&R)], and inhibition of the hydroxylation by carbon monoxide (93% inhibition in a 50:50 CO:O2 atmosphere) are consistent with a cytochrome P-450-mediated hydroxylation involving the classic NIH shift (arene oxide) pathway.

Microbial models of mammalian metabolism: production of novel alpha-diketone metabolites of warfarin and phenprocoumon using Aspergillus niger

1. The coumarin anticoagulants warfarin and phenprocoumon were metabolized by Aspergillus niger via oxidative ring cleavage to yield the corresponding alpha-diketone metabolites. 2. Structural identification was based upon physical, spectral, and chromatographic comparisons of isolated metabolites and synthetic standards generated by the oxidative cleavage of warfarin or phenprocoumon with pyridinium chlorochromate. 3. This pathway of metabolism has been previously observed for coumarin anticoagulants in mammalian systems.

Expression and function of three cytochrome P-450 isozymes in rat extrahepatic tissues

Western blots using a polyclonal and a monoclonal antibody raised against rat liver cytochrome P-450b indicate tissue-specific expression of low levels of cytochrome P-450's b and e. P-450b and P-450e were expressed very selectively in, respectively, lung and adrenal microsomes of untreated rats but neither isozyme was detected in the corresponding kidney or small intestine microsomes. The regioselectivity of microsomal metabolism of 7,12-dimethylbenz[a]anthracene (DMBA) as well as the sensitivity to inhibition by anti P-450b/e IgG established that low levels of "b-like" P-450's are functional in lung and adrenal microsomes from uninduced rats, but not in microsomes from the kidney or small intestine. Functional P-450c was also detected at low levels in liver, lung, kidney, and adrenals of untreated rats. Among the extrahepatic tissues examined, DMBA metabolism was the highest in rat adrenal microsomes. However, only 30% of this activity was due to P-450's b, e, or c. Phenobarbital (PB) treatment of rats increased microsomal DMBA metabolism in all extrahepatic tissues examined. The selectivity of this increase for 12-methyl hydroxylation of DMBA and the near complete inhibition by anti-P-450b/e are consistent with induction of P-450e even though P-450b was preferentially induced in each of the extrahepatic tissues examined. The levels of expression of P-450b were increased by PB in all sets of adrenal, lung, and intestinal microsomes and in three out of six sets of kidney microsomes. The levels of P-450e were also increased by PB in all sets of adrenal microsomes. Following PB treatment, P-450e became immunoquantifiable (greater than 2 pmol/mg protein) in three of six sets of lung and kidney microsomes but remained below detection in all sets of intestinal microsomes. Based on the activity of purified P-450e, undetectable levels (less than 1 pmol/mg protein) could account for increased DMBA metabolism in this tissue. The high constitutive level of P-450b in the lung (approximately 40 pmol/mg), was remarkably inactive in DMBA metabolism and was only slightly increased by PB treatment (50%). In contrast, PB treatment caused a 2.5- to 10-fold increase in 12-methyl hydroxylation of DMBA that was highly sensitive to anti-P-450b/e. A protein comigrating with P-450e was well above detection (6-7 pmol/mg) in two of six preparations of lung microsomes that showed highest induction of this activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspects of anticoagulant action: a review of the pharmacology, metabolism and toxicology of warfarin and congeners

Warfarin is the most widely used anticoagulant in the treatment of thromboembolism in man. It has also been used extensively as a rodenticidal agent. Insofar as its clinical use is concerned, it is now clear that many of the drug interactions observed in patients are mediated via metabolic or pharmacokinetic factors. An understanding of the disposition of warfarin is therefore essential if one is to predict the likely response in patients undergoing anticoagulant therapy with this compound. Warfarin-resistance has been reported in both man and rodents. Understanding resistance in both man and rodents is important for effective anticoagulant therapy, and in control of resistant strains of rodents. Warfarin resistance in rat strains does not appear to have a metabolic or pharmacokinetic basis; in this species, resistance is thought to be due to differences in permeability to, or affinity for a receptor. Apart from its clinical and rodenticidal uses, warfarin is an excellent substrate for probing the heterogeneity of cytochrome P.450, since its metabolic oxidation is mediated by this mixed function oxidase. This review draws together much of the current published literature on the pharmacology, metabolism and toxicology of warfarin and related congeners.

Stereoselective and regioselective hydroxylation of warfarin and selective O-dealkylation of phenoxazone ethers in human placenta

The oxidative metabolism of warfarin and a series of phenoxazone ethers was studied in two groups of human placentas which exhibited high or low levels of aryl hydrocarbon hydroxylase (AHH). Warfarin metabolism was stereoselective (mean R/S = 2.48) for the R-enantiomer and regioselective for the 6- and 8- positions in the high AHH group whereas warfarin metabolism in the low AHH group displayed no significant overall stereoselectivity (mean R/S = 1.24) and was regioselective for the 7- position. The high AHH group metabolized the methyl, ethyl, propyl and butyl ethers of phenoxazone rapidly, while the low AHH group catalyzed their biotransformation at very low or negligible rates. Neither group detectably metabolized phenoxazone or pentyloxyphenoxazone whereas both groups metabolized benzyloxyphenoxazone at low but similar rates. Rates of warfarin R-6 and R-8 hydroxylation were highly correlated with metabolism of benzo(alpha)pyrene (r = 0.99) and the C1-C4 phenoxazone ethers (r greater than 0.87), but poorly correlated with metabolism of benzyloxyphenoxazone (r less than 0.50). These data support the use of warfarin and the phenoxazone ethers as sensitive biochemical probes for P-450 isozymes in human extrahepatic tissues. They indicate the presence of a multiplicity of xenobiotic metabolizing P-450's in placental tissue which has not been exposed to inducing agents that elevate AHH.

Regulation of renal cytochrome P-450. Effects of two-thirds hepatectomy, cholestasis, biliary cirrhosis and post-necrotic cirrhosis on hepatic and renal microsomal enzymes

The possibility of a relationship between hepatic and renal cytochrome P-450 contents was assessed in rats with liver disease. In rats killed 3 days after two-thirds hepatectomy (a model for hepatocellular insufficiency), the total microsomal cytochrome P-450 content of the whole liver was decreased by 60% as compared to that in control rats; renal cytochrome P-450 was increased by 30% while the 7-ethoxycoumarin deethylase activity of kidney microsomes was increased by 80%. In rats killed 7 days after bile duct ligation (a model for cholestasis) or 35 days after bile duct ligation (a model for biliary cirrhosis), hepatic cytochrome P-450 was decreased by 60% and 45%, respectively, while renal cytochrome P-450 content was increased by 50% and 150%, respectively. In contrast, in rats killed 15 days after the last dose of carbon tetrachloride, 1.3 ml/kg twice weekly for 3 months (a model for post-necrotic cirrhosis), both hepatic and renal cytochrome P-450 contents remained unchanged. Phenobarbital (80 mg/kg daily for 3 days) was a poor inducer of renal cytochrome P-450 in sham-operated rats but became a potent inducer of renal cytochrome P-450 in rats with two-thirds hepatectomy. We conclude that renal cytochrome P-450 is increased in three models in which hepatic cytochrome P-450 contents are decreased (two-thirds hepatectomy, cholestasis and biliary cirrhosis), but remains unchanged in a model of severe liver pathology, in which hepatic cytochrome P-450 content is not modified (late, post-necrotic cirrhosis). The hypothetical role of endogenous inducer(s) is discussed.

Chemically induced renal papillary necrosis and upper urothelial carcinoma. Part 1

In the past, renal papillary necrosis (RPN) has been commonly associated with long-term abusive analgesic intake, but over recent years a wide variety of industrially and therapeutically used chemicals have been shown to induce this lesion experimentally or in man. Destruction of the renal papilla may result in: (1) secondary degenerative cortical changes which precede chronic renal failure or (2) a rapidly metastasizing upper urothelial carcinoma, which has a very poor prognosis. This article will briefly review the published data on the morphology, function, and biochemistry of the normal renal medulla and the pathology associated with RPN, together with the secondary changes which give rise to cortical degeneration or epithelial carcinoma. It will then examine in detail those chemicals which have been reported to cause RPN in an attempt to delineate structure-activity relationships. Finally, the many different theories that have been proposed to explain the pathophysiology of RPN will be examined and an hypothesis will be put forward to explain the primary pathogenesis of the lesion and its secondary consequences.

Chemically induced renal papillary necrosis and upper urothelial carcinoma. Part 2

In the past, renal papillary necrosis (RPN) has been commonly associated with long-term abusive analgesic intake, but over recent years a wide variety of industrially and therapeutically used chemicals have been shown to induce this lesion experimentally or in man. Destruction of the renal papilla may result in: (1) secondary degenerative cortical changes which precede chronic renal failure or (2) a rapidly metastasizing upper urothelial carcinoma, which has a very poor prognosis. This article will briefly review the published data on the morphology, function, and biochemistry of the normal renal medulla and the pathology associated with RPN, together with the secondary changes which give rise to cortical degeneration or epithelial carcinoma. It will then examine in detail those chemicals which have been reported to cause RPN in an attempt to delineate structure-activity relationships. Finally, the many different theories that have been proposed to explain the pathophysiology of RPN will be examined and an hypothesis will be put forward to explain the primary pathogenesis of the lesion and its secondary consequences.

Enzymatic activation of chemicals to toxic metabolites

A variety of enzymes function in the oxygenation, oxidation-reduction, conjugation, and hydrolysis of drugs and other foreign chemicals. Often these enzymes detoxicate chemicals to prevent detrimental effects. In this review we will, however, concentrate on cases in which metabolism activates chemicals to reactive species which cause cellular damage. Particular attention will be given to mixed-function oxidases, which carry out a variety of oxygenations, as well as other reactions. (We will focus on cellular toxicity as opposed to initiation of tumorigenesis in this review.) In many cases, considerable circumstantial evidence exists linking these enzymes to enhanced toxicity of chemicals, although causal relationships have seldom been demonstrated. Further, in very few cases is the explicit cause of toxicity known. Modification of critical protein residues is suspected, although oxidative stress may also be involved in some cases. We discuss general aspects of mechanisms of toxic action, briefly list all cases in which metabolism is suspected to play a role in enhancing toxicity, and review a few examples in detail where substantial chemical and enzymatic information is available. The latter instances would involve knowledge of the enzymes involved, chemical evidence on the structures of the reactive metabolites, identification of adducts, and some inference into the biological processes which are effected to elicit toxicity. We consider, in this regard, vinyl halides (which have been a focus in our own laboratory), acetaminophen, pyrrolizidine alkaloids, and fluoroxene.

Expression of cytochromes P-450 in rat hepatoma cells. Analysis by monoclonal antibodies specific for cytochromes P-450 from rat liver induced by 3-methylcholanthrene or phenobarbital

We have studied the expression of aldrin eposidase (AE), 7-ethoxycoumarin-O-deethylase (ECDE), and aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) in nine differentiated or dedifferentiated cell lines derived from H4IIEC3 rat hepatoma cells. The nature of the cytochromes P-450 mediating AE, ECDE and AHH activities was analysed using monoclonal antibodies (MAb) made to the major 3-methylcholanthrene-induced cytochrome P-450 (MAb-MC) or phenobarbital-induced cytochrome P-450 (MAb-PB) from rat liver. The cells were treated with 5 microM dexamethasone for 30 h to increase the levels of the monoxygenase activities. (a) The six differentiated cell lines examined (Faza967, Fao, HF1-4, 2sFou, C2Rev7, and H4IIEC3/G-) contained MAb-PB-sensitive AE comprising 30-75% of the total AE activity. In most of these cell lines MAb-PB also markedly inhibited ECDE; however, the antibody had a considerably weaker effect on AHH. (b) MAb-PB-sensitive AHH, ECDE and AE activities were also observed in untreated and phenobarbital-treated cells. (c) MAb-MC inhibited AHH and ECDE in the two dedifferentiated lines HF1 and H5 by 50-80%. The antibody also inhibited AHH activities in the poorly differentiated line H4IIEC3/T and in the majority of the differentiated lines by 40-65%. MAb-MC-sensitive AHH was found in Fao cells after treatment with benz[a]anthracene but induced AHH in H4IIEC3/T, H4IIEC3/G-, and 2sFou cells 20-30-fold and in Faza967 and Fao cells 3-5-fold. Benz[a]anthracene remained without effect on AHH activity in C2Rev7 cells. The results show that the hepatoma cells examined express to various degrees phenobarbital-inducible cytochrome P-450 and/or 3-methylcholanthrene-inducible cytochrome P-450. These cell lines are versatile tools for studying the regulation of monooxygenase activities and analysing their role in the activation and inactivation of xenobiotics such as carcinogens, drugs and pesticides.

Comparisons of warfarin metabolism by liver microsomes of rats treated with a series of polybrominated biphenyl congeners and by the component-purified cytochrome P-450 isozymes

R- and S-warfarin metabolite profiles (regio- and stereoselectivity) has been determined with hepatic microsomes from untreated rats and rats treated with nine individual polybrominated biphenyl (PBB) congeners, a commercial mixture of PBBs, and, for comparison with phenobarbital and 3-methylcholanthrene. The metabolic rates have been correlated with cytochrome P-450 (P-450) isozyme concentrations in the microsomes determined by immunochemical quantitation techniques (G.A. Dannan, F.P. Guengerich, L.S. Kaminsky, and S.D. Aust, (1983) J. Biol. Chem. 258, 1282-1288). The warfarin hydroxylase activities of the P-450 isozyme components of the various microsomal preparations (F.P. Guengerich, G.A. Dannan, S.T. Wright, M.V. Martin, and L.S. Kaminsky (1982) Biochemistry 21, 6019-6030) were multiplied by the corresponding isozyme concentrations to obtain an assessment of the potential warfarin hydroxylase capacity of the microsomes, and the results were compared with actual activities. The results of these studies and comparisons indicate that substrate regio- and stereoselectivities of microsomal-bound P-450s are essentially retained on purification of the isozymes to homogeneity and reconstitution, that warfarin metabolism by microsomal preparations can be used to predict microsomal P-450 isozyme compositions, and that microsomal warfarin hydroxylase activity is greater than would be predicted based on the approx 20:1 ratio of P-450 to NADPH-P-450 reductase in the microsomes and on the known activities of constituent isozymes. Two P-450 isozymes which are induced by treatment of rats with phenobarbital appear to be more tightly linked to NADPH-P-450 reductase than does an isozyme induced by beta-naphthoflavone.

A sensitive and specific stable isotope assay for warfarin and its metabolites

A capillary gas chromatographic mass spectrometric method for the quantification of warfarin and its known metabolites from microsomal incubations is described. Deuterium labelled 4', 6-, 7- and 8-hydroxy warfarins are used as internal standards and the method has detection limits of 1 ng ml-1 with 20 ng ml-1 being the lower limit for accurate quantification.

Microbial transformations of warfarin: stereoselective reduction by Nocardia corallina and Arthrobacter species

The microbiological metabolism of warfarin was examined as a model of metabolism in higher organisms, including humans, and to determine the chirality of microbial reductases for application in organic synthesis. Nineteen cultures were examined based on their reported abilities to reduce ketonic substrates, and several were shown to catalyze the desired reaction. Nocardia corallina (ATCC 19070) exhibited complete substrate and product stereoselectivity as it reduced S-warfarin to the corresponding S-alcohol. Arthrobacter species (ATCC 19140) exhibited marked substrate and complete product stereoselectivity since S-warfarin, and to a lesser extent R-warfarin, were reduced to the corresponding S-alcohols. These reductions parallel those reported to occur in mammalian species.

Modification of the mutagenicity and teratogenicity of cyclophosphamide in rats with inducers of the cytochromes P-450

Cyclophosphamide must be enzymatically activated to be either mutagenic or teratogenic. This activation is thought to be catalyzed by the cytochrome P-450 monooxygenase system. To study the relationship between the mutagenic and teratogenic metabolites of cyclophosphamide, the mutagenicity and teratogenicity of this drug were compared after activation by rats pretreated with chemicals (phenobarbital and beta-naphthoflavone) inducing different cytochromes P-450. Activation of cyclophosphamide to mutagenic metabolites by enzyme fractions from rats on day 13 of gestation was measured with the Ames test using Salmonella typhimurium TA1535. Teratogenicity was assessed in vivo by treatment of rats with cyclophosphamide on day 13 of gestation. Cyclophosphamide was activated to mutagenic metabolites to the same extent (on a tissue wet weight basis) by enzyme fractions from maternal liver, kidney and placenta, despite differences in cytochrome P-450 content. Fetal homogenates did not activate cyclophosphamide. Phenobarbital pretreatment increased the activation of cyclophosphamide to mutagenic metabolites by maternal liver microsomes 10-fold and liver cytochrome P-450 content 1.8 fold; however, this drug did not alter the activation of cyclophosphamide by maternal kidney, by placenta or by the fetus. Phenobarbital pretreatment increased the teratogenicity of cyclophosphamide in rats on day 13 of gestation (increased incidence of malformed embryos, decreased fetal weight). Pretreatment with beta-naphthoflavone did not induce liver cytochrome P-450 in the pregnant rat and did not change the activation of cyclophosphamide to mutagenic metabolites by liver, kidney, placenta or the fetus. Pretreatment with this polycyclic aromatic hydrocarbon had no effect or decreased the teratogenicity of cyclophosphamide. Thus, these experiments suggest that the mother, rather than the fetus, is the site of activation of cyclophosphamide; after phenobarbital pretreatment the predominant site of cyclophosphamide activation is the maternal liver. There appears to be a correlation between the teratogenicity and mutagenicity of cyclophosphamide after induction of the cytochromes P-450. We can speculate that the "proximate teratogen" of cyclophosphamide may also be the "proximate mutagen".