Purification of a new cytochrome P-450 from human liver microsomes

Using a classical methodology of purification consisting of three chromatographic steps (Octyl-Sepharose, DEAE-cellulose, CM-cellulose) we have purified a new cytochrome P-450 from human liver microsomes. It was called cytochrome P-450(9). It has been proven to be different from all precedingly purified human liver microsomal cytochrome P-450 isozymes by its immunological and electrophoretical properties. It does not cross-react with any rat liver cytochrome P-450 and anti-cytochrome P-450(9) does not recognize rat liver microsomes; thus this cytochrome P-450(9) is specific to humans. This cytochrome P-450 isozyme exists in low amounts in human liver microsomes and exhibits an important quantitative polymorphism. In reconstituted system, cytochrome P-450(9) is able to hydroxylate all substrates tested but is not specific of any; its exact role in xenobiotic metabolism in man remains to be elucidated.

The Salmonella/mammalian microsome mutagenicity test: comparison of human and rat livers as activating systems

The mutagenicity of several test compounds was verified by the Salmonella/microsome mutagenicity test (Ames test), using both human liver and rat liver (untreated or pretreated with Aroclor 1254) S9 under identical experimental conditions. Aflatoxin B1, 3-methylcholanthrene, and cigarette-smoke condensate were less mutagenic in the presence of human-liver S9 than in the presence of rat-liver S9 (particularly after treatment with Aroclor 1254). The opposite was observed with 2-aminonanthracene and to a lesser degree with 2-aminofluorene; correlation studies indicate that the two compounds were activated by the same or by very similar enzymes, probably cytochrome P-450s. These results clearly indicate that human-liver S9, as an activating system, behaves differently than rat-liver S9; therefore, it may constitute a useful, additional tool for the study of mutagenicity and probably, carcinogenicity in man.

Attempts to phenotype human liver samples in vitro for debrisoquine 4-hydroxylase activity

Twenty-eight samples of human liver have been characterised for cytochrome P-450 content, aldrin epoxidase, debrisoquine 4-hydroxylase and bufuralol 1'-hydroxylase activities. Evidence is presented here and elsewhere that bufuralol 1'-hydroxylase and debrisoquine 4-hydroxylase are activities catalysed by the same form of cytochrome P-450 in man, and that this form is different from that catalysing the epoxidation of aldrin. Attempts to phenotype liver samples in vitro, in the absence of any metabolic data in vivo for debrisoquine 4-hydroxylation status, met with limited success. A combination of enzyme assays will most probably be required in any such phenotyping of human liver samples.

Some erythromycin derivatives are strong inducers in rats of a cytochrome P-450 very similar to that induced by 16 alpha-pregnenolone carbonitrile

Erythromycin derivatives having lost the cladinose moiety, erythralosamine and its mono- and diacetate, are strong inducers of liver cytochrome P-450, better than troleandomycin, in rats. The major cytochrome P-450 form induced by all these macrolides is electrophoretically and immunologically indistinguishable from the major form induced in rats by pregnenolone carbonitrile. This form is particularly able to metabolize the macrolides and to lead to the corresponding 456 nm absorbing cytochrome P-450 metabolite complexes in vivo and in vitro.

The effect of different hyperglucagonemic states on monooxygenase activities and isozymic pattern of cytochrome P-450 in mouse

The continuous infusion of a low dose of glucagon (35 micrograms/kg/d, for 5 d) constitutes, in view of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities, a reliable experimental model of hyperglucagonemia. By conjunction of monooxygenase assays and immunoquantitation of specific isozymes of cytochrome P-450, the actual inducing ability of glucagon has been shown and it might explain some of the modifications of the drug metabolizing system in diabetic mice. The isozymic pattern of cytochrome P-450 of liver microsomes from diabetic mice appears very different from that produced by classical inducers.

Quantification of two cytochrome P-450 isoenzymes by an enzyme-linked immunosorbent assay (ELISA)

An enzyme linked immunosorbent assay (ELISA) using monoclonal and polyclonal antibodies has been developed to quantify individual cytochrome P-450 isoenzymes in microsomal preparations, namely UT-A and PB-B. This very sensitive method can be used for the rapid processing of large quantities of determinations and requires only limited amounts of antibodies.

Human hepatic cytochrome P-450 composition as probed by in vitro microsomal metabolism of warfarin

Human liver microsomal fractions from 27 renal donors (tissue obtained post mortem) and from six cancer patients (tissue obtained during surgery) were used to investigate human hepatic cytochrome P-450 isozyme compositions. In vitro microsomal metabolism of the R and S enantiomers of warfarin to dehydrowarfarin and 4'-, 6-, 7-, 8-, and 10-hydroxywarfarin is catalyzed by cytochrome P-450 isozymes and was used as the basis for evaluating similarities and differences between human cytochrome P-450 isozyme compositions. The mean hepatic cytochrome P-450 concentration from postmortem samples was not significantly different from that of surgical patients (0.51 +/- 0.16 vs. 0.35 +/- 0.14 nmol/mg protein), but the NADPH-cytochrome P-450 reductase activity of the former was significantly higher than that of the latter (141 +/- 56 vs. 29 +/- 6 nmol cytochrome c reduced/min/mg protein). In general, the microsomal preparations were overall stereoselective for R warfarin metabolism. The stereoselectivities for formation of the individual metabolites of the R enantiomer were 6-, 8-, and 10-hydroxywarfarin and the S enantiomer were 4'- and 7-hydroxywarfarin. Of the 33 microsomal preparations, 21 exhibited qualitatively similar warfarin metabolite profiles with 6R- and 7S-hydroxywarfarin having the highest formation rates. Some of the preparations exhibited markedly different metabolite profiles, the most notable having 10R-hydroxywarfarin as the major metabolite. Based on the known warfarin metabolite profiles of five purified cytochrome P-450 isozymes, the isozyme composition of the microsomes can be estimated. The majority of the microsomal preparations apparently had similar isozyme compositions but some preparations were markedly different.

Preparation and characterization of monoclonal antibodies against phenobarbital-inducible cytochrome P-450

Monoclonal antibodies against cytochrome P-450 were prepared from phenobarbital-induced rat liver microsomes. The immunoglobulin classes and subclasses, as well as the binding capacity to cytochrome P-450, of the different antibodies were characterized. Their specificity was verified by various techniques and seemed to correspond to a single form of cytochrome P-450, the major phenobarbital-inducible form. However, the antibodies were unable to inhibit completely the monooxygenase activities investigated. These antibodies may constitute very specific and powerful analytical tools for characterizing and quantifying cytochrome P-450 isoenzymes.

Induction of drug-metabolizing enzymes by tricyclic antidepressants in human liver: characterization and partial resolution of cytochromes P-450

Drug-metabolizing enzyme activities were determined in liver microsomes from six kidney-transplant donors, one tricyclic antidepressant-treated and five untreated donors. The tricyclic antidepressant treatment modifies neither the overall cytochrome P-450 content of the liver, nor enzymatic activities of 4-nitroanisole demethylase, aniline hydroxylase, epoxide hydrolase and glutathione S-transferase. Only benzphetamine and ketotifen demethylation and conjugation of bilirubin with UDP-glucuronic acid are markedly augmented (more than two-fold). Separation of the different cytochrome P-450 fractions on a DEAE cellulose column indicates a modification of the elution pattern: the fraction increased by tricyclic antidepressants is responsible for the enhanced monooxygenase activity towards benzopyrene and benzphetamine.

Purification and characterization of six cytochrome P-450 isozymes from human liver microsomes

Six cytochrome P-450 (P-450) isozymes were purified to electrophoretic homogeneity from the livers of four human organ donors, with three of these isozymes purified from a single individual. Differences were noted between all six P-450s for some or all of the parameters determined by the techniques of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, peptide mapping, spectral analysis of ferrous-carbon monoxide complexes, double-diffusion immunoprecipitin analysis or crossed immunoelectrophoresis (sodium dodecyl sulfate-polyacrylamide gel electrophoresis/peroxidase-coupled staining) with rabbit antisera raised to five of the P-450s, or catalytic activity toward d-benzphetamine, benzo[a]pyrene, acetanilide, debrisoquine, (R)- and (S)-warfarin, and 1-naphthylamine. While NADPH-fortified human liver microsomal preparations showed catalytic activity toward trichloroethylene, 7-ethoxycoumarin, 2-naphthylamine, and 2-aminofluorene in addition to the other substrates mentioned, none of the P-450s which we purified from these microsomes catalyzed the oxidation of these compounds in reconstituted enzyme systems containing purified rat liver NADPH-P-450 reductase. Antibodies raised against one of the purified P-450s inhibited d-benzphetamine N-demethylase activity in microsomal incubations but did not inhibit the metabolism of 7-ethoxycoumarin, acetanilide, benzo[a]pyrene, or debrisoquine. The data provide a strong biochemical basis for the view that distinct isozymes of P-450 exist in humans and that these isozymes differ in catalytic activity toward drugs and carcinogens.

Metabolism of ketotifen by human liver microsomes. In vitro characterization of a tertiary amine glucuronidation

Biotransformation of ketotifen was investigated in vitro using human liver microsomes. Three of the four metabolic pathways observed in vivo in man were exhibited under the conditions of incubation, namely demethylation, N-oxidation, and N-glucuronidation, the absent route being the ketoreduction, which probably has a cytosolic localization. The kinetic parameters of the N-glucuronidation (KM for ketotifen and UDPGA and Vmax) were determined with native and detergent-treated microsomes. Treatment by Triton X-100 increased by about 3-fold the conjugation reaction. No sex difference was observed and N-glucuronidation did not seem to be inhibited either by bilirubin or by 4-nitrophenol. Thus, human liver microsomes are a useful and suitable in vitro model for studying metabolic routes, specific for man, as in the case of ketotifen. Obviously, the results obtained can only reflect partially the multiplicity of in vivo events and interpretation has to be complemented by investigations with other models.

Evaluation of isolated human hepatocytes

This preliminary study reports the functional capacities of freshly isolated human hepatocytes in regard to their energetic metabolism and monooxygenase activities. Incubated for 30 or 60 min, isolated cells maintain their membrane integrity, ATP and reduced glutathione content and redox potential estimated by means of lactate to pyruvate and beta-hydroxypyruvate to acetoacetate ratios. Three monooxygenase activities, supported by different isoenzymes of cytochrome P-450 are determined by the accumulation of unconjugated metabolites: their relative magnitudes are similar to those observed in microsomes, indicating a good preservation of hydroxylase activities during cell isolation and incubation. Although incubations did not exceed 60 min, one can conclude that human hepatocytes maintain their viability and metabolic capacities after isolation and might be considered in transplantation process for the treatment of acute hepatic failure. Isolated human hepatocytes might be also used as a tool for studying biochemical and toxicological effects of a drug.

Influence of ethanol and benzene on cytochrome P-450 fractions in rat liver microsomes

The repartition and characteristics of liver microsomal cytochrome P-450 fractions from benzene- or ethanol-treated rats were compared to those observed either in untreated animals, or in rats treated by classic inducers, namely phenobarbital, 3-methylcholanthrene, or beta-naphthoflavone. DEAE-cellulose chromatography allowed the separation of four main cytochrome P-450 fractions called A (nonabsorbed), Ba, Bb, and Bc (successively eluted by a NaCl gradient). In control rats, and in ethanol- and benzene-treated animals, fractions A were predominant; phenobarbital, 3-methylcholanthrene, beta-naphthoflavone, and benzene induced Bb fractions. Enzymatic and immunological methods allowed a characterization of those cytochrome P-450 fractions. Fractions A are similar in all cases, and mainly active towards aniline. This aniline hydroxylase activity is especially increased by ethanol. As a rule, fractions Ba and Bc behave similarly and exhibit rather low monooxygenase activities. On the contrary, fractions Bb differ from each other as a function of the inducer. Phenobarbital-induced Bb fraction is different from all other Bb fractions and especially active towards benzphetamine. 3-Methylcholanthrene- and beta-naphthoflavone-induced Bb fractions are identical, but they are different from all other Bb fractions, and especially active towards 7-ethoxycoumarin. Fraction Bb induced by benzene is different from those induced by classic inducers, but may be identical to the Bb fraction of control animals. As a whole, benzene and ethanol appear to display inducing properties different from those of phenobarbital or polycyclic aromatic hydrocarbon-like inducers.

[Pharmacokinetic elements of sodium valproate. Use in febrile convulsions in children]

A pharmacokinetic study of sodium valproate (DPA) was undertaken in children treated with this drug after a febrile convulsion. In infants, after a single oral dose of 20 mg/kg body weight in the first morning meal, plasma kinetic data were as follows: T max 2.4 hr +/- 1.1; total DPA maximum plasma concentration: 94 micrograms/ml +/- 28; T 1/2: 12.4 hr +/- 4.9; the distribution volume was 0.25 l/kg +2- 0.07 and plasma clearance was 0.014 l/kg/hr +2- 0.004. Control of prolonged treatment showed that the measured plasma concentrations were not correlated with the daily dosage. Furthermore, when febrile convulsions are concerned, one should carefully weight the recently reported hepatic toxicity of DPA, against the usually good prognosis of febrile convulsions.

Infantile phytanic acid storage disease, a possible variant of Refsum's disease: three cases, including ultrastructural studies of the liver

Three cases of phytanic acid storage disease with symptoms during the first months of life are reported. Hepatomegaly, facial dysmorphia, growth and/or mental retardation and osteopenia were observed in addition to retinitis pigmentosa and neurosensory deafness. The presence of phytanic acid in serum (160-320 mumol/1 (50-100 micrograms/ml)) was accompanied by hypocholesterolaemia. Electron microscopy showed that a storage material had accumulated in mesenchymal and parenchymal liver cells. Lamellar structures were seen in hepatocytes and other storing cells. These inclusions resembled the structures found in plant chloroplasts containing phytol. Some of the clinical and biological data obtained were consistent with Refsum's disease. However, other characteristics such as mental retardation, hepatomegaly, osteopenia, hypocholesterolaemia and hypoalphalipoproteinaemia, as well as the ultrastructural findings in the liver, suggested that our patients' illness was either a phytanic acid storage disease different from the classical form of Refsum's disease, or a more severe early symptomatic form of Refsum's disease. Early diagnosis by phytanic acid assay and electron microscopic liver examination calls for prescription of a low phytanate diet in the hope of improving the child's condition.

Cytochrome P-450 monooxygenase activities in human and rat liver microsomes

Microsomes were prepared from human livers obtained from renal donors of various ages and both sexes. Their drug-metabolizing capacity was measured and compared to that of rat liver microsomes. The following parameters were investigated: cytochrome P-450, cytochrome B5, NADPH-cytochrome c reductase, epoxide hydrolase, aryl hydrocarbon hydroxylase, benzphetamine N-demethylase, p-nitroanisole-O-demethylase, ethoxycoumarin-O-deethylase, steroid-16 alpha-hydroxylase. In addition, the metabolism of benzo(a)pyrene, progesterone, pregnenolone, testosterone, dehydroepiandrosterone and estradiol was studied in detail in vitro. The inhibitory effect of metyrapone and alpha-naphthoflavone on 7-ethoxycoumarin-O-deethylase was measured. The microsomal proteins of both species were separated by polyacrylamide gel electrophoresis in the presence of dodecyl sulfate. The following conclusions were drawn from the results obtained. Human liver microsomes can be stored under optimal conditions for the measurement of a large variety of enzymic activities. Human liver microsomes are able to metabolize the various xenobiotics used as substrates with a rate similar to that of female rat liver microsomes. No sex-linked difference in enzymic activity was observed in human microsomes. Significant differences in benzo(a)pyrene and steroid metabolism were registered when human and rat liver microsomes were compared. The monooxygenase activities, the sensitivity to in vitro alpha-naphthoflavone and metyrapone, the results of steroid metabolism, and slab gel electrophoresis are strong indications for multiplicity of human liver cytochrome P-450.

Ellipticines and human liver microsomes: spectral interaction with cytochrome P-450 and hydroxylation. Inhibition of aryl hydrocarbon metabolism and mutagenicity

Some pharmacological properties of ellipticine (E) and its derivatives linked to their interaction with cytochrome P-450 have been investigated with human liver microsomes. 9-Hydroxyellipticine (9-OHE) interacts with human liver cytochrome P-450 exhibiting a type II spectrum (lambda max: 428 nm, Ks = 1.1 microM). After incubation with human liver microsomes the E was converted to 9-OHE; 7-hydroxyellipticine was not produced. The cytotoxic effect of this biotransformation has been evaluated on leukemic L1210 cells, in vitro, and found to be equal to those elicited by liver microsomes of control or phenobarbital (PB) pretreated rats. Moreover, 9-OHE and 9-fluoroellipticine (9-FE) strongly inhibit the benzo[alpha]pyrene hydroxylase (AHH) activity of human liver microsomes (I50 = 2.6 microM and 1.6 microM, respectively) as well as the mutagenesis induced by the polycyclic aromatic hydrocarbon 2-acetylaminofluorene (AAF); 1 microgram/plate of each of these compounds is able to inhibit by more than 50% the mutagenicity of 5 microgram/plate AAF.

The use of formic acid in carrier gas. Rapid method for identification and determination of phytanic acid by gas chromatography-mass spectrometry-chemical ionization

A rapid gas chromatographic method to determine phytanic acid in plasma from Refsum's disease is described. After a brief alkaline hydrolysis of lipids, the biological sample is directly injected into a glass pre-column; an acid carrier gas (formic acid in nitrogen) is used to displace the long-chain fatty acids from their sodium salts and from their binding to proteins. Formic acid introduced through the column may also be used as a reagent gas for chemical ionization in combined gas chromatography--mass spectrometry; fatty acids (C14 to C18:2 and phytanic acid) are easily identified by their M + 1 (base peak) and M - 17 peaks. The described procedure is also suitable for studying normal fatty acids from plasma lipids.

Biotransformation of chloroform by rat and human liver microsomes; in vitro effect on some enzyme activities and mechanism of irreversible binding to macromolecules

The effects of chloroform on some rat microsomal enzyme activities were studied in vitro. Maximum inhibition of oxygen consumption, NADPH oxidase and NADPH-cytochrome c reductase was observed at 0.5 mM chloroform; prior metabolization of CHCl3 by microsomal monooxygenases increased inhibition by about 50% at 0.2-0.5 mM chloroform. Higher concentrations produced a paradoxical reversal of inhibition, whereas p-nitroanisole demethylase was steadily inhibited by about 50% up to 10 mM chloroform. Irreversible binding of 14CHCl3 was confirmed to depend on chloroform metabolization by monooxygenases. The increased irreversible binding due to phenobarbital induction is accompanied by a diminished affinity towards chloroform as shown by increased KM of irreversible binding, and a higher spectral dissociation constant KS. Aminoacids with nucleophilic functions (histidine, cysteine) partially prevented the irreversible binding of chloroform metabolites to microsomes; non-volatile radioactive derivatives were recovered in trichloracetic acid supernatants when microsomes were incubated with cysteine, but not with histidine. Phosgene has been demonstrated as a biological metabolite of chloroform: its possible reactions with nucleophilic groups of macromolecules, water and added aminoacids partly explain these experimental data. Similar results were obtained with human microsomes, showing that chloroform hepatotoxicity in man could involve the same mechanisms.

Formation of complexes between microsomal cytochrome P-450-Fe(II) and nitrosoarenes obtained by oxidation of arylhydroxylamines or reduction of nitroarenes in situ

A cytochrome P-450 complex exhibiting a Soret peak at 454 nm is formed by direct interaction of nitrosobenzene with NADPH-reduced rat liver microsomes in anaerobic conditions, by reaction of phenylhydroxylamine with aerobic microsomes or during nitrobenzene reduction by NADPH-reduced or dithionite-reduced microsomes. In the latter conditions, the complex formation is only transient as it is unstable to dithionite. Analogous reactions with myoglobin lead to the previously described myoglobin-Fe(II)-nitrosobenzene complex which has similar properties to those of the 454-nm-absorbing cytochrome P-450 complex. This analogy, together with the various conditions of its formation, strongly indicates that it is a cytochrome-P-450-Fe(II)-nitrosobenzene complex. The corresponding complex with the 4-chloro-nitrosobenzene ligand is formed in similar conditions. Cytochrome P-450-Fe(II) complexes with nitrosoarenes seem less stable than the previously described complexes with nitrosoalkanes.

[Origin and fixation of pyrophosphates in the parietal bones of young rats]

Calvaria from 6 to 13-day-old rats mineralize and bind pyrophosphates which are transferred from nucleotides, as shown by : 1 -- the increase of pyrophosphates in young rat calvaria incubated with nucleoside triphosphate, especially ATP ; 2 -- the more important increase of pyrophosphate content when ATP is renewed in the incubation medium ; and 3 -- binding of [32P] pyrophosphate [beta 32P] ATP. The effect of preheating of calvaria on 32P binding from [beta32P]ATP led us to assume that two systems might be involved in pyrophosphate fixation : a heat-labile, non-specific, phosphatase system, and a heat-stable pyrophosphotransferase system. cAMP increases the pyrophosphate content of calvaria incubated with ATP : that effect may result from either an inhibition of the phosphatase system, or an activation of the pyrophosphotransferase system.