Purification of a human liver cytochrome P-450 immunochemically related to several cytochromes P-450 purified from untreated rats

Expression of hepatic drug-metabolizing cytochrome p450 enzymes and their intercorrelations: a meta-analysis

Cytochrome P450 is a family of enzymes that catalyze reactions involved in the metabolism of drugs and other xenobiotics. These enzymes are therefore important in pharmacologic and toxicologic studies, and information on their abundances is of value in the process of scaling in vitro data to in vivo metabolic parameters. A meta-analysis was applied to data on the abundance of human hepatic cytochrome P450 enzymes in Caucasian adult livers (50 studies). Despite variations in the methods used to measure the abundance of enzymes, agreement between the studies in 26 different laboratories was generally good. Nonetheless, some heterogeneity was detected (Higgins and Thompson heterogeneity test). More importantly, large interindividual variability was observed in the collated data. Positive correlations between the expression levels of some cytochrome P450 enzymes were found in the abundance data, including the following pairs: CYP3A4/CYP3A5*1/*3 (Rs = 0.70, P < 0.0001, n = 52), CYP3A4/CYP2C8 (Rs = 0.68, P < 0.0001, n = 134), CYP3A4/CYP2C9 (Rs = 0.55, P < 0.0001, n = 71), and CYP2C8/CYP2C9 (Rs = 0.55, P < 0.0001, n = 99). These correlations can be used to demonstrate common genetic transcriptional mechanisms.

Disparity in holoprotein/apoprotein ratios of different standards used for immunoquantification of hepatic cytochrome P450 enzymes

An analysis of reported hepatic abundances of CYP3A4 and 3A5 indicated that values determined by immunoquantification using commercially available, unpurified recombinant enzymes as standards are significantly lower than those determined using purified enzymes or human liver microsomes characterized with lysosomal peptides (CYP3A4: mean 45 versus 121 pmol/mg protein, p < 0.01; CYP3A5: mean 28 versus 83 pmol/mg protein, p < 0.05). When immunoquantifying cytochromes P450 (P450s), it is assumed that the holoprotein (holo)/apoprotein ratio is the same in the samples and the standard. Estimates of holo/apoprotein ratios from data reported for a range of P450s purified from human liver and non-commercial recombinant systems indicated less than complete and variable heme coupling dependent on enzyme and system.

Identification of cytochrome P450 and arylamine N-acetyltransferase isoforms involved in sulfadiazine metabolism

Sulfadiazine hydroxylamine has been postulated to be the mediator of the greatly increased rates of adverse reactions to sulfadiazine experienced by people with human immunodeficiency virus infection. Therefore, we investigated the in vitro human cytochrome P450 (P450) and N-arylamine acetyltransferase (detoxification) metabolism of sulfadiazine. Formation of both the hydroxylamine and 4-hydroxy sulfadiazine was NADPH-dependent in human liver microsomes (HLM). The average K(m) (+/-S.D.) and V(max) in HLM (n = 3) for hydroxylamine formation was 5.7 +/- 2.2 mM and 185 +/- 142 pmol/min/mg, respectively. Significant (p < 0.05) inhibition by selective P450 isoform inhibitor sulfaphenazole (2.1 microM; CYP2C9) indicated a role for CYP2C9 in the formation of the hydroxylamine. Hydroxylamine formation correlated strongly with tolbutamide 4-hydroxylation (CYP2C8/9) in HLM (r = 0.76, p < or = 0.004, n = 12). Fluconazole (CYP2C9/19 and CYP3A4 inhibitor at clinical concentrations) inhibited hydroxylamine formation, with one-enzyme model K(i) estimates ranging from 9 to 40 microM. Acetylation of sulfadiazine in human liver cytosol (HLC) correlated strongly with NAT2 activity as measured by sulfamethazine N-acetylation (r = 0.92, p < 0.001, n = 12). The average K(m) (+/-S.D.) and V(max) in HLC (n = 3) was 3.1 +/- 1.7 mM and 221.8 +/- 132.3 pmol/min/mg, respectively. The polymorphic acetylation of sulfadiazine may predispose slow acetylator patients to adverse reactions to sulfadiazine. On the basis of our K(i) estimates, clinical fluconazole concentrations of 25 microM would produce decreases of 40 to 70% in hepatic-mediated hydroxylamine production. Therefore, we predict that fluconazole may prove useful in the clinic as an in vivo inhibitor of sulfadiazine hydroxylamine formation to suppress adverse reactions to this drug.

Alcohol exposure and paracetamol-induced hepatotoxicity

Most instances of hepatotoxicity due to paracetamol in the United Kingdom and Australia are the result of large overdoses of the drug taken with suicidal or parasuicidal intent. In contrast, serious hepatotoxicity at recommended or near-recommended doses for therapeutic purposes has been reported, mainly from the United States and in association with chronic alcohol use, leading to the widely held belief that chronic alcoholics are predisposed to paracetamol-related toxicity at relatively low doses. Yet the effects of alcohol on paracetamol metabolism are complex. Studies performed in both experimental animals and humans indicate that chronic alcohol use leads to a short-term, two- to threefold increase in hepatic content of cytochrome P4502E1, the major isoform responsible for the generation of the toxic metabolite from paracetamol, although increased oxidative metabolism of paracetamol at recommended doses has not been demonstrated clinically. A reduced hepatic content of glutathione, required to detoxify the reactive metabolite, has been documented in chronic alcoholics, due probably to associated fasting and malnutrition, providing a metabolic basis for any possible predisposition of this group to hepatotoxicity at relatively low paracetamol doses. Simultaneous alcohol and paracetamol ingestion reduces oxidative metabolism of paracetamol in both rodents and humans, predominantly as a consequence of depletion in cytosol of free NADPH. The possibilities that chronic alcohol use may predispose to paracetamol-related hepatotoxicity and that alcohol taken with paracetamol may protect against it, based on these metabolic observations, are examined in this review.

Population distribution and effects on drug metabolism of a genetic variant in the 5' promoter region of CYP3A4

BACKGROUND

There are large interindividual differences in CYP3A4 expression and in the metabolism of drug substrates for this enzyme. We and others have identified a polymorphism in the 5' promotor region of the CYP3A4 gene; however, its functional significance is not currently known. This study was conducted to determine whether this polymorphism plays a clinically important role in determining CYP3A4 phenotype.

METHODS

An adenine (A) to guanine (G) transition was identified in the 5' promotor region of the CYP3A4 gene at position -292 (from the start codon), in a sequence motif known as the nifedipine-specific element. The frequency of this polymorphism was assessed in 802 healthy volunteers from five broadly defined racial groups. The population distribution of the G allele in these groups was as follows: white Americans (3.6%; n = 273), black Americans (54.6%, n = 186), Hispanic Americans (9.3%; n = 188), Japanese Americans (0.0%; n = 77), and Chinese Americans (0.0%; n = 78). In a subsequent study, 90 additional black Americans were genotyped, and a subset of the homozygous subjects (AA, n = 8; GG, n = 23) were given the CYP3A4 probe substrates erythromycin and nifedipine to allow genotype-phenotype comparisons to be made.

RESULTS

There was no difference in the rate of CYP3A4-dependent demethylation of erythromycin (erythromycin breath test) or the pharmacokinetics of nifedipine or its CYP3A4-dependent metabolite dehydronifedipine between the two genotype groups (AA or GG).

CONCLUSIONS

This promotor region polymorphism does not appear to play a major role in determining constitutive CYP3A4 expression.

750 MHz HPLC-NMR spectroscopic identification of rat microsomal metabolites of phenoxypyridines

Directly coupled 750 MHz HPLC-1H NMR spectroscopy has been applied to the characterisation of low level metabolites of 3-amino-2-(2-fluorophenoxy)pyridine (AP) and 3-nitro-2-(2-fluorophenoxy)pyridine (NP) in rat microsomes. In stop-flow HPLC-NMR mode, the direct injection of microsomal extracts enabled the separation and characterisation of minor metabolites. NP is converted into AP to an extent of 93.4% and this is further metabolised to 4- and 6-hydroxy-AP (6 and 0.6% respectively). Unequivocal identification of these metabolites was achieved without the use of a radiolabel or synthetic standards and thus demonstrates the applicability of directly coupled HPLC-NMR to metabolite identification in in vitro systems. The potential exists for HPLC-NMR and HPLC-NMR-MS to provide rapid metabolic information within the timescale of high throughput lead optimisation exercises in drug discovery.

Role of intestinal P-glycoprotein (mdr1) in interpatient variation in the oral bioavailability of cyclosporine

Interpatient differences in the oral clearance of cyclosporine (INN, ciclosporin) have been partially attributed to variation in the activity of a single liver enzyme termed CYP3A4. Recently it has been shown that small bowel also contains CYP3A4, as well as P-glycoprotein, a protein able to transport cyclosporine. To assess the importance of these intestinal proteins, the oral pharmacokinetics of cyclosporine were measured in 25 kidney transplant recipients who each had their liver CYP3A4 activity quantitated by the intravenous [14C-N-methyl]-erythromycin breath test and who underwent small bowel biopsy for measurement of CYP3A4 and P-glycoprotein. Forward multiple regression revealed that 56% (i.e., r2 = 0.56) and 17% of the variability in apparent oral clearance [log (dose/area under the curve)] were accounted for by variation in liver CYP3A4 activity (p < 0.0001) and intestinal P-glycoprotein concentration (p = 0.0059), respectively. For peak blood concentration, liver CYP3A4 activity accounted for 32% (p = 0.0002) and P-glycoprotein accounted for an additional 30% (p = 0.0024) of the variability. Intestinal levels of CYP3A4, which varied tenfold, did not appear to influence any cyclosporine pharmacokinetic parameter examined. We conclude that intestinal P-glycoprotein plays a significant role in the first-pass elimination of cyclosporine, presumably by being a rate-limiting step in absorption. Drug interactions with cyclosporine previously ascribed to intestinal CYP3A4 may instead be mediated by interactions with intestinal P-glycoprotein.

Targeted antipeptide antibodies to cytochrome P450 2C18 based on epitope mapping of an inhibitory monoclonal antibody to P450 2C51

The epitope recognized by the inhibitory monoclonal antibody designated 2F5, which was raised against P450 2C5, was mapped to amino acids 237-260 by immunoblotting using a combination of recombinant antigens and chimeric and partial fusion proteins constructed from rabbit P450s 2C2, 2C4, 2C5, and 2C16, which are recognized by 2F5, and from 2C1 and 2C3, which are not. When the sequence of the epitope for 2F5 (amino acids 237-260) was compared with those of other rabbit 2C P450s, a single lysine residue at position 253 appeared to be a likely determinant of 2F5 immunoreactivity. Substitution of lysine for glutamic acid 253 in P450 2C3 (2C3E253K) conferred immunoreactivity and the ability of 2F5 to inhibit progesterone metabolism catalyzed by P450 2C3E253K. Sequence alignment revealed that this epitope lies in close proximity to the epitope identified for LKM-1 autoantibodies to P450 2D6. Based on these results, an antipeptide antibody was raised to the corresponding region (amino acids 252-263) of human P450 2C18. The resulting antipeptide antiserum recognizes P450 2C18 but not P450 2C8, 2C9, or 2C19. However, the antipeptide 2C18 antiserum did not inhibit 2C18-catalyzed diazepam N-demethylation. Human 2C P450s were also quantitated by immunoblot analysis in a panel of six human liver microsomes using Escherichia coli expressed P450s as standards. Analysis of immunoblots indicated that, if present, P450 2C18 was expressed at very low levels (<2.5 pmol/mg), whereas P450s 2C8, 2C9, and 2C19 were easily detected.

Pharmacogenetics in pediatrics. Implications for practice

Cumulative experience with pharmacotherapy in children indicates that it is difficult to prescribe medications rationally solely on the basis of patient age. Furthermore, the apparent drug biotransformation phenotype may be influenced by disease (e.g., infection), environmental factors (e.g., diet and environmental contaminants), and concurrent medications. Therefore, characterization of drug biotransformation pathways during development and, at a given developmental stage, the effects of known modulators of drug biotransformation are essential for optimum treatment. This is particularly true when one considers that altered drug biotransformation may contribute significantly to therapeutic failure (e.g., graft rejection with inadequate serum and tissue concentrations of cyclosporin and myelotoxicity consequent to a relative inability to metabolize normal doses of certain antineoplastic agents). Accordingly, the goals of coordinated clinical and basic investigations should be to characterize important drug biotransformation pathways for compounds under development and intended for use in pediatrics and to identify the population extremes or "outliers" to aid in selection of an appropriate dosage range for efficacy studies. Acquired knowledge should then be incorporated into the drug-design process to further maximize the efficacy-toxicity ratio. The development of acceptable, preferably noninvasive, phenotyping procedures for all age ranges including neonates, infants, and older children is a major challenge for investigators but, if met, will be rewarded with improved pediatric pharmacotherapy.

Catalytic and immunologic characterization of hepatic and lung cytochromes P450 in the polar bear

The Arctic Ocean is subject to considerable influx of anthropogenic pollutants including halogenated organic compounds. The polar bear (Ursus maritimus) is at the top of the arctic marine food web and is an ideal species for monitoring the level and distribution of contaminants in the arctic ecosystem. As the first step in the development of a biological method for assessing the functional exposure of polar bears to xenobiotics, biochemical studies were undertaken to characterize polar bear cytochromes P450. Liver and lung samples were obtained in the field from four, freshly killed, adult, male polar bears and immediately frozen at -196 degrees. Microsomes were subsequently prepared and used for the measurement of total cytochrome P450 content and aminopyrine N-demethylase, benzphetamine N-demethylase, ethylmorphine N-demethylase, p-nitrophenol hydroxylase and testosterone hydroxylase activities. Immunoblots containing hepatic and lung microsomal samples from the polar bears were probed using antibodies generated against several purified rat cytochrome P450 isozymes. Monoclonal antibody to rat cytochrome P450 1A1 and polyclonal antibodies to rat cytochromes P450 1A1, 2B1 and 3A1, as well as antibody to epoxide hydrolase, cross-reacted to varying degrees with polar bear hepatic microsomes. In addition, polyspecific antibody to the rat cytochrome P450 2C subfamily gave several immunostained protein bands, but antibodies specific to rat cytochrome P450 2C7 and 2C13 did not react, while antibody specific to cytochrome P450 2C11 yielded an ambiguous result. Except for anticytochrome P450 2B1 and polyspecific antibody to the cytochrome P450 2C subfamily, the antibodies listed above did not cross-react with polar bear lung microsomes at the protein concentrations used. The results demonstrate that polar bear liver contains multiple forms of cytochrome P450 that are catalytically active toward diverse substrates and that several of these forms are immunochemically related to rat cytochrome P450 isozymes. Immunochemical homologues of rat cytochrome P450 1A, 2B, 2C and 3A subfamilies, and of rat epoxide hydrolase are present in polar bear liver. In addition, the polar bears all had high levels of immunoreactive cytochrome P450 1A and 2B proteins, probably as a consequence of induction by environmental contaminants.

Cytochrome P450 2C9 is responsible for hydroxylation of the naphthoquinone antimalarial drug 58C80 in human liver

2-(4-t-Butylcyclohexyl)-3-hydroxy-1,4-naphthoquinone (58C80) is an experimental naphthoquinone antimalarial drug which undergoes extensive alky hydroxylation in man. By means of purification, N-terminal amino acid sequencing and inhibition by antibodies and sulfaphenazole, we have identified the form of cytochrome P450 primarily responsible for 58C80 hydroxylation in human liver, P450hB20-27, to be a member of the P450 2C9 subfamily. P450hB20-27 is a low-spin haemoprotein with molecular mass 54 kDa. 58C80 hydroxylation in human liver microsomes was dependent on either NADPH or NADH, with the activity supported by NADH being 35% of that supported by NADPH. With purified P450hB20-27 cytochrome b5 stimulated the NADH-dependent activity 8-fold but inhibited the NADPH-dependent activity by 30%. 58C80 is a novel substrate structure for human P450 2C and these results significantly broaden the range of drugs which have been directly shown (i.e. using a purified enzyme as opposed to expressed cDNA) to be metabolized by human P450 2C forms that are incontrovertibly expressed in human liver in vivo.

Identification of the fetal liver cytochrome CYP3A7 in human endometrium and placenta

Placenta and endometrium carry out steroidogenic biotransformation reactions such as 6-beta-hydroxylation of cortisol, a reaction characteristic of the dominant family of cytochromes P450 in human liver, CYP3A. To investigate the possible role in these extrahepatic tissues of the CYP3A microsomal hemoproteins, we analyzed placental and endometrial microsomes on Western blots developed with an anti-human CYP3A antibody. We found an immunoreactive 51,500 D protein that migrated between CYP3A3 (HLp) and CYP3A5 (HLp2) identical with CYP3A7 (HFLa). CYP3A7, a form found prominently in human fetal liver microsomes, was first isolated as a liver 16-alpha-dehydroepiandrosterone-sulfate hydroxylase. Northern blot analysis of total RNA isolated from placenta or from endometrium demonstrated a single band that cross-hybridized with a CYP3A7 cDNA. Amplification of the same RNA samples with the use of primers specific for CYP3A7, produced a 552-bp segment that had the predicted size and the same DNA sequence as does liver CYP3A7 cDNA. Hybridizable endometrial CYP3A7 mRNA was detected more frequently (six of seven samples) and in higher amounts (approximately 12-fold higher) in pregnant compared with nonpregnant women (4 of 12 samples). In addition, during the secretory phase of the menstrual cycle CYP3A7 expression was sixfold higher than in the one sample from the proliferative phase that had detectable CYP3A7 mRNA. Moreover, the amounts of placental and endometrial CYP3A7 mRNA and protein increased substantially from the first to the second trimester of pregnancy. We conclude that placenta and endometrium express the same P450 as is found in fetal liver. These tissues represent a previously unrecognized and quantitatively important site for 6-beta-hydroxylation and 16-alpha-hydroxylation of specific steroid precursors, possibly for protection of the fetus from the toxic effects of endogenous steroids and foreign substrates.

Cytochrome P450 isoforms in human fetal tissues related to phenobarbital-inducible forms in the mouse

Four polyclonal antibodies raised against purified mouse liver cytochrome P450s representing Cyp1a, Cyp2a, Cyp2b and Cyp2c subfamilies were used to detect their related forms in human adult and fetal tissues. In immunoblot analysis, anti-Cyp2c antibody detected two to three proteins in adult livers and one to three proteins in 70% of the 18 fetal livers studied. Anti-Cyp2a-5 antibody recognized a 50-kDa protein in 50% of the fetal adrenals. Anti-Cyp1a-2 antibody reacted with a single protein (55 kDa) in adult liver. The anti-Cyp2b-10 antibody did not detect proteins in any of the tissues. No proteins were detected in fetal kidneys. There was no coumarin 7-hydroxylase activity (COH) in fetal liver or adrenals. The 7-ethoxycoumarin O-deethylase (ECOD) activities were slightly higher in fetal adrenals (mean 6.1 pmol/mg protein/min) vs livers. The fetal adrenal ECOD activity was not inhibited by the anti-Cyp2a-5 antibody. Aryl hydrocarbon hydroxylase (AHH) activities in fetal livers were about 5% of those in adult livers. AHH activity in fetal liver was not inhibited by the anti-Cyp2c antibody. Testosterone 6 beta-hydroxylase activity was much lower in fetal liver than in adult liver (about 20 and 1700 pmol/mg protein/min, respectively). No immunoinhibition occurred in fetal adrenal progesterone hydroxylation, hepatic benzphetamine N-demethylation and hepatic ethylmorphine N-demethylation. These data suggest that members of the P450 subfamilies 1A, 2A and 2B are expressed at a very low level in fetal liver, and that fetal liver may contain members of the 2C subfamily.

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature

We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1-11, 1987; Nebert et al., DNA 8, 1-13, 1989; Nebert et al., DNA Cell Biol. 10, 1-14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen should precede the final number in mouse genes. "P" ("p" in mouse) after the gene number denotes a pseudogene. If a gene is the sole member of a family, the subfamily letter and gene number need not be included. We suggest that the human nomenclature system be used for all species other than mouse. The mRNA and enzyme in all species (including mouse) should include all capital letters, without italics or hyphens. This nomenclature system is identical to that proposed in our 1991 update. Also included in this update is a listing of available data base accession numbers for P450 DNA and protein sequences. We also discuss the likelihood that this ancient gene superfamily has existed for more than 3.5 billion years, and that the rate of P450 gene evolution appears to be quite nonlinear. Finally, we describe P450 genes that have been detected by expressed sequence tags (ESTs), as well as the relationship between the P450 and the nitric oxide synthase gene superfamilies, as a likely example of convergent evolution.

The role of cytochrome P450 3A (CYP3A) isoform(s) in oxidative metabolism of testosterone and benzphetamine in human adult and fetal liver

Testosterone metabolism was studied in human adult and fetal liver microsomes. In fetal livers 6 beta-hydroxylase (6 beta OH) activity (1-2% of adult activity) and 2 alpha-hydroxylase (2 alpha OH) activity (about 40% of adult activity) were present. Also some fetal livers produced two unknown metabolites. Androstenedione was formed in all fetal livers studied (10-20% of adult activity). Testosterone hydroxylations at 6 beta-, 2 beta-, 15 alpha- and 15 beta-positions were associated with CYP3A isoform(s) in adult liver, because they were strongly inhibited by midazolam, a known substrate for CYP3A4 and by anti-CYP3A4 antibody. Fetal liver activities were consistently inhibited less than the activities in adult livers. The formation of androstenedione was not affected by these inhibitors in fetal or adult liver microsomes. Benzphetamine N-demethylase activity in the fetal livers was about 40% of adult activity. Anti-CYP3A4 antibody had no effect on that activity in fetal or in adult liver microsomes, whereas a monoclonal antibody 1-68-11 (generated against rat CYP2C11) slightly inhibited benzphetamine N-demethylase activity in adult liver. This study indicates that human fetal and adult liver are dissimilar in their testosterone metabolism pattern. The formation of androstenedione from testosterone in fetal liver may have a physiological role. Testosterone hydroxylases are less inhibited by anti-CYP3A4 antibody, midazolam and progesterone in fetal than in adult liver.

Human cytochrome P-450 enzymes

Cytochrome P-450 (P-450) enzymes have been studied extensively in experimental animal models and much is known regarding their structures, regulation, and mechanisms of catalysis. In recent years investigations have been extended to the human P-450s. There are more than 30 different characterized human P-450s in the superfamily, and collectively they are probably the most significant enzymes involved in the metabolism of drugs, carcinogens, and steroids. The levels of many of the P-450s and their catalytic activities can vary considerably because of polymorphism, induction, and inhibition. The catalytic specificity of the P-450s can range from being very non-discriminatory to very exacting, and clinical consequences of drugs and steroids can be related to variations in P-450 levels. Defects in the rate-limiting P-450 reactions in steroidogenesis (due to genetic deficiencies) have been shown to be debilitating and even fatal.

The human hepatic cytochromes P450 involved in drug metabolism

The cytochromes P450 are a superfamily of hemoproteins that catalyze the metabolism of a large number of xenobiotics and endobiotics. The type and amount (i.e., the animal's phenotype) of the P450s expressed by the animal, primarily in the liver, thus determine the metabolic response of the animal to a chemical challenge. A majority of the characterized P450s involved in hepatic drug metabolism have been identified in experimental animals. However, recently at least 12 human drug-metabolizing P450s have been characterized at the molecular and/or enzyme level. The characterization of these P450s has made it possible to "phenotype" microsomal samples with respect to their relative levels of the various P450s and their metabolic capabilities. The purpose of this review is to compare and contrast the human P450s involved in drug metabolism with their related forms in the rat and other experimental species.

Cloning and expression of complementary DNAs for multiple members of the human cytochrome P450IIC subfamily

The present study characterizes the profile of cDNAs from the human P450IIC subfamily in a library from one individual, and it describes three new members of this subfamily (IIC17, IIC18, and IIC19) isolated from two human cDNA libraries. cDNA libraries were constructed from two human livers which differed phenotypically in the hepatic content of P450 HLx (IIC8). The library from the phenotypically low HLx individual was screened by using a cDNA for rat liver P450IIC13 and an oligonucleotide probe for human IIC8. One clone, 245c, was isolated which clearly represents a new member of the human P450IIC subfamily (IIC17). This clone lacked the first 358 nucleotides at the N-terminus but was only 91% homologous in its nucleic acid sequence to IIC9 and 79% homologous to IIC8. Near-full-length clones for IIC9 were also isolated from this library, but no clones for IIC8 were found. Northern blots indicated that the mRNA for IIC8 was low or absent in this individual. A second cDNA library (from a liver phenotypically high in HLx) was then screened. Eighty-three essentially full-length (greater than 1.8 kb) clones belonging to the IIC subfamily were isolated from this library. These include full-length clones for two additional new members of the IIC subfamily. Clones 29c and 6b appear to be allelic variants (IIC18), differing by one nucleotide (one amino acid change) in the coding region. Clone 11a represents a full-length clone for a third new P450 (IIC19).(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of cytochrome P-450lpr is developmentally regulated and limited to house fly

Expression of house fly cytochrome P-450lpr was examined using immunoblotting in male and female adult LPR house flies, mixed sex adult house flies at 12 different ages, larvae, and pupae. P-450lpr was expressed in both male and female adult house flies. P-4501pr was clearly present in all adult stages examined, was barely detectable in pupae, and could not be detected in larvae. Thus, cytochrome P-450lpr is developmentally regulated and present in both sexes of house fly. Expression of cytochrome P-450, immunologically homologous to house fly cytochrome P-4501pr, was examined in other species using immunoblot analysis. Eleven animal species were tested in the orders Diptera, Hymenoptera, Lepidoptera, Orthoptera, Acari, and Rodentia, using microsomes in some species from both induced and noninduced animals or insecticide-resistant and susceptible strains. P-450lpr appears to be restricted to house flies, as none of these species contained cytochrome P-450 that reacted with antiserum to cytochrome P-450lpr.

Differential regulation of liver P-450III cytochromes in choline-deficient rats: implications for the erythromycin breath test as a parameter of liver function

Progressive liver fibrosis in rats develops when they are fed a diet deficient in choline. This diet also results in a pronounced and selective decrease in the liver microsomal content of a phase I drug-metabolizing enzyme belonging to the cytochrome P-450III gene family. Because P-450III cytochromes characteristically catalyze the N-demethylation of erythromycin, we believed that the production of breath CO2 from erythromycin would be dramatically reduced in choline-deficient rats. However, when 12 choline-deficient rats were compared with 9 control rats, the reduction in CO2 production from erythromycin (mean decrease 71%) was essentially identical to that from aminopyrine (mean decrease 69%), a substrate believed to be metabolized normally by the hepatocyte in fibrotic liver disease. Furthermore, we found that the relative erythromycin and aminopyrine demethylase activities were comparable when measured in vitro in liver microsomes prepared from the choline-deficient rats. To determine the molecular basis for the erythromycin demethylase activity in the choline-deficient rats, the liver microsomes were subjected to immunoblot analysis using a variety of polyclonal and monoclonal antibodies capable of distinguishing individual P-450III-related proteins. Our studies confirm that a major erythromycin demethylase belonging to the P-450III family, termed P-450p, was greatly reduced in the choline-deficient rat liver. However, the specific concentration of a second P-450p-related protein was essentially normal and that of a third P-450p-related protein was actually increased in the choline-deficient rat liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic polymorphisms of drug metabolism

The molecular mechanisms of 3 genetic polymorphisms of drug metabolism have been studied at the level of enzyme activity, enzyme protein and RNA/DNA. As regards debrisoquine/sparteine polymorphism, cytochrome P-450IID6 was absent in livers of poor metabolizers; aberrant splicing of premRNA of P-450IID6 may be responsible for this. Moreover, 3 mutant alleles of the P-450IID6 locus on chromosome 22 associated with the poor metabolizer phenotype were identified by Southern analysis of leucocyte DNA. The presence of 2 identified mutant alleles allowed the prediction of the phenotype in approximately 25% of poor metabolizers. The additional gene-inactivating mutations which are operative in the remainder of poor metabolizers are now being studied. Regarding mephenytoin polymorphism, although the deficient reaction, S-mephenytoin 4'-hydroxylation, has been well defined in human liver microsomes, the mechanism of this polymorphism remains unclear. All antibodies prepared to date against cytochrome P-450 fractions with this activity recognize several structurally similar enzymes and several cDNAs related to these enzymes have been isolated and expressed in heterologous systems. However, which isozyme is affected by this polymorphism is not known. As regards N-acetylation polymorphism, N-acetyltransferases have been purified from human liver, specific antibodies prepared; it was observed that immunoreactive N-acetyltransferase is decreased or undetectable in liver of "slow acetylators". Two genes that encode functional N-acetyltransferase were characterized. The product of one of these genes has identical activity and characteristics as the polymorphic liver enzyme. Cloned DNA from rapid and slow acetylator individuals has been analyzed to identify the structural or regulatory defect that causes deficient N-acetyltransferase.

In vitro effect of fluoroquinolones on theophylline metabolism in human liver microsomes

Some quinolone antibiotics cause increases in levels of theophylline in plasma that lead to serious adverse effects. We investigated the mechanism of this interaction by developing an in vitro system of human liver microsomes. Theophylline (1,3-dimethylxanthine) was incubated with human liver microsomes in the presence of enoxacin, ciprofloxacin, norfloxacin, or ofloxacin. Theophylline, its demethylated metabolites (3-methylxanthine and 1-methylxanthine), and its hydroxylated metabolite (1,3-dimethyluric acid) were measured by high-pressure liquid chromatography, and Km and Vmax values were estimated. Enoxacin and ciprofloxacin selectively blocked the two N demethylations; they significantly inhibited the hydroxylation only at high concentrations. Norfloxacin and ofloxacin caused little or no inhibition of the three metabolites at comparable concentrations. The extent of inhibition was reproducible in five different human livers. Inhibition enzyme kinetics revealed that enoxacin caused competitive and mixed competitive types of inhibition. The oxo metabolite of enoxacin caused little inhibition of theophylline metabolism and was much less potent than the parent compound. Nonspecific inhibition of cytochrome P-450 was ruled out since erythromycin N demethylation (cytochrome P-450 mediated) was unaffected in the presence of enoxacin. These in vitro data correlate with the clinical interaction described for these quinolones and theophylline. We conclude that some quinolones are potent and selective inhibitors of specific isozymes of human cytochrome P-450 that are responsible for theophylline metabolism. This in vitro system may be useful as a model to screen similar compounds for early identification of potential drug interactions.

Distribution of cytochromes P-450, cytochrome b5, and NADPH-cytochrome P-450 reductase in an entire human liver

In rat liver there appear to be significant differences between lobes in the concentration of individual cytochrome P-450 isozymes (Sumner and Lodola, Biochem Pharmacol 36: 391-393, 1987). Because studies in patients often rely on small pieces of liver obtained from diverse anatomical locations, it seemed important to determine if the cytochromes P-450 were also heterogeneously distributed in human liver. Accordingly, tissue was obtained from ten different locations in a single human liver including those most commonly biopsied by percutaneous needles, and by surgeons during laparotomy. The differences observed between locations in the microsomal concentrations of carbon monoxide-binding protein (total cytochrome P-450), cytochrome b5, and NADPH-cytochrome P-450 reductase appeared to be small and were not statistically significant. Likewise, no significant differences were observed between locations in the specific content of HLp, HLp3, HLj, HLx or P450MP. However, the specific concentrations of HLd varied almost 2-fold between the microsomes and this was statistically significant in some cases (P less than 0.05). Our results suggest that, in human livers, regional differences in the content of cytochromes P-450 are generally small but may be significant for some isozymes. With the exception of HLd, tissue obtained by percutaneous or surgical liver biopsies is probably representative of the entire organ with regard to the enzymes assayed.

Modulation of cytochrome P450 isozymes in human liver, by ethanol and drug intake

Cytochromes P450 (P450) are a family of isozymes which play an important role in xenobiotic metabolism. The concentration of three P450 isozymes, namely P450-IIE1(A1c),-IIIA(NF) and -IIC8-10(MP) has been measured in human liver biopsies of patients with different alcohol and drug intake status. All these three P450s were expressed in all subjects. Ethanol intake increased P450-IIE1(A1c) content with no effect on the content of the two other P450s. Drug intake (barbiturates) increased both P450-IIIA(NF) and -IIC8-10(MP) content without any effect on P450-IIE1(A1c). This paper brought, at protein level, further evidence of the importance of environmental conditions on P450 isozyme pattern, and therefore, on drug metabolizing capacity of human liver.

Isolation of a new human fetal liver cytochrome P450 cDNA clone: evidence for expression of a limited number of forms of cytochrome P450 in human fetal livers

From a human fetal liver cDNA library, a new cDNA clone (lambda HFL10) was isolated using an antiserum to P450 HFLa, which has been isolated from livers of human fetuses. Cytochrome P450 cDNAs, namely lambda hPA6, lamda hP2-1, and lambda hPD4 which were highly homologous to cDNA clones, pHY13, Hp1-1, and phP450j, respectively, were also isolated from the cDNA library of human adult livers. Using these cDNA clones as probes together with Lambda HFL10, Northern blot analysis was conducted to determine whether all of these cytochromes were expressed in human fetal livers. The results clearly showed that only P450 HFL10 mRNA was detected in human fetal livers. This result supports the allegation that there is a much more limited number of forms of cytochrome P450 in human fetal livers than in adult livers.

Metabolism of retinol and retinoic acid by human liver cytochrome P450IIC8

Liver microsomes obtained from nine subjects were found to metabolize retinol to polar metabolites, including 4-hydroxyretinol. In a reconstituted monooxygenase system containing human liver P450IIC8, retinol was converted to 4-hydroxyretinol and other polar metabolites, with a Km of 0.071 mM and a Vmax of 1.73 nmol/min/nmol P450. Neither P450IIC9 nor P450IIE1, two other purified human P450s, displayed significant retinol hydroxylase activity. Immunoblots performed with a monospecific antibody directed against human P450IIC8 revealed that appreciable amounts of this enzyme were present in human liver microsomes. The same antibody significantly inhibited retinol metabolism in liver microsomes and in the system reconstituted with P450IIC8. The system reconstituted with P450IIC8 also converted retinoic acid to polar metabolites. Thus, this study shows, for the first time, metabolism of two physiologic substrates by a human liver cytochrome P450 related to a group of "constitutive" rodent P450s believed to participate in the metabolism of endogenous compounds. Through its involvement in vitamin A metabolism, P450IIC8 may participate in maintaining the balance between those vitamin A concentrations that promote cellular integrity (and oppose the development of cancer) and those concentrations that cause cellular toxicity.

Erythromycin breath test as an assay of glucocorticoid-inducible liver cytochromes P-450. Studies in rats and patients

The major P-450IIIA gene family member present in human liver is HLp which, like its rat liver orthologue P-450p, is inducible by glucocorticoids and catalyzes erythromycin N-demethylation. To develop a practical method to estimate the amounts of HLp in patients [14C]N-methyl erythromycin was injected into rats that had been pretreated with dexamethasone or with inducers of other forms of cytochrome P-450. The rate of demethylation of this substrate, measured simply as 14CO2 in the breath, correlated well with the concentrations of immunoreactive P-450p protein (r = 0.70), holocytochrome P-450p (r = 0.70), or with erythromycin N-demethylase activity (r = 0.90) determined in the liver microsomes prepared from each rat. Next, [14C]N-methyl erythromycin was administered to 30 patients and there was a sixfold interindividual variation in breath 14CO2 production seemingly unrelated to medications, smoking status or age. However, the average breath test values were twofold greater in female as compared to male patients (P less than 0.01). Breath 14CO2 production rose in patients retested after treatment with the P-450IIIA inducers dexamethasone (P less than 0.05) or rifampicin (P less than 0.05) and was decreased after treatment with the HLp inhibitor triacetyloleandomycin (P less than 0.05). We conclude that the erythromycin breath test provides a convenient assay of P-450IIIA cytochromes in rats and in some patients.

Isolation and characterization of human fetal liver cytochrome P450HLp2: a third member of the P450III gene family

We purified from human fetal livers a form of cytochrome P450, termed HLp2, related to adult human liver cytochrome P450HLp by monitoring the purification procedures by immunoblots developed with a monoclonal antibody to HLp. The preparation of HLp obtained was judged to be homogeneous by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and found to have an apparent monomeric molecular weight slightly greater than that of HLp, 51,500 versus 51,000. Immunoblot and Ouchterlony double-diffusion analyses indicated that HLp and HLp2 are immunochemically related. However, ferrous-CO versus ferrous difference spectra yielded different Soret maxima for HLp and HLp2 (448.5 and 449.5 nm, respectively). In addition, structural comparisons between HLp and HLp2 indicated that they were distinct isozymes. Specifically, the 28 amino acid amino-terminal sequence determined for HLp2 was found to be 79% homologous to that of HLp. In addition, peptide mapping of HLp and HLp2 by limited proteolysis with three proteases yielded similar but different patterns indicating that HLp and HLp2 are structurally distinct. These results demonstrate that HLp2 and HLp are highly related but distinct proteins and that HLp2 is a member of the steroid-inducible P450III gene family.

Sex change in cytochrome P-450 phenotype by growth hormone treatment of adult rat hepatocytes maintained in a culture system on matrigel

Results of studies of hypophysectomized rats suggest that growth hormone serves as a final common mediator through which gonadal steroids and other modifiers of pituitary function alter the expression of gender-specific liver genes such as the sexually dimorphic pair of cytochrome P-450 isozymes, male-specific P-450h and female-specific P-450i. We tested the effects of growth hormone in a system for primary monolayer culture of adult rat hepatocytes on a laminin-rich extracellular matrix (matrigel), which permits sustained expression of both constitutive and inducible liver genes in a chemically defined medium. Cultures of freshly isolated hepatocytes prepared from untreated male rats and samples of the intact donor liver contained readily detectable quantities of immunoreactive P-450h protein (measured on immunoblots of cell microsomes) and P-450h mRNA (measured on Northern blots of cellular RNA). Neither P-450i immunoreactive protein nor P-450i mRNA were present. Addition of physiologic concentrations of human or bovine growth hormone, but not of prolactin, to culture medium lacking insulin or other hormones resulted in prompt induction of P-450i immunoreactive protein and P-450i mRNA. Induction of P-450i mRNA in male hepatocyte cultures was dependent on the concentration of growth hormone, required as little as 24 hr of exposure, and was markedly attenuated in cultures maintained on type I collagen rather than on matrigel. Growth hormone treatment also induced the level of mRNA for insulin-like growth factor I, whereas the amount of mRNA for the male-specific urinary protein alpha 2 mu-globulin was unaffected. Cultures of hepatocytes derived from untreated adult female rats retained high levels of P-450i mRNA but only if the culture medium contained growth hormone. None of the tested treatments with estrogens, androgens, glucocorticoids, or growth hormone induced P-450h mRNA or P-450h immunoreactive protein in cultures of female hepatocytes. We conclude that the somatogenic effects of growth hormone acting alone and directly on the hepatocyte in culture are sufficient to "feminize" the cytochrome P-450 phenotype. The present culture system offers a way to explore the molecular basis for hormonal control of liver gene expression.