Highly purified cytochrome P-448 and P=450 from rat liver microsomes

Recombinant Technologies Facilitate Drug Metabolism, Pharmacokinetics, and General Biomedical Research

The development of safe and effective medications requires a profound understanding of their pharmacokinetic (PK) and pharmacodynamic properties. PK studies have been built through investigation of enzymes and transporters that drive drug absorption, distribution, metabolism, and excretion (ADME). Like many other disciplines, the study of ADME gene products and their functions has been revolutionized through the invention and widespread adoption of recombinant DNA technologies. Recombinant DNA technologies use expression vectors such as plasmids to achieve heterologous expression of a desired transgene in a specified host organism. This has enabled the purification of recombinant ADME gene products for functional and structural characterization, allowing investigators to elucidate their roles in drug metabolism and disposition. This strategy has also been used to offer recombinant or bioengineered RNA (BioRNA) agents to investigate the posttranscriptional regulation of ADME genes. Conventional research with small noncoding RNAs such as microRNAs (miRNAs) and small interfering RNAs has been dependent on synthetic RNA analogs that are known to carry a range of chemical modifications expected to improve stability and PK properties. Indeed, a novel transfer RNA fused pre-miRNA carrier-based bioengineering platform technology has been established to offer consistent and high-yield production of unparalleled BioRNA molecules from Escherichia coli fermentation. These BioRNAs are produced and processed inside living cells to better recapitulate the properties of natural RNAs, representing superior research tools to investigate regulatory mechanisms behind ADME. SIGNIFICANCE STATEMENT: This review article summarizes recombinant DNA technologies that have been an incredible boon in the study of drug metabolism and PK, providing investigators with powerful tools to express nearly any ADME gene products for functional and structural studies. It further overviews novel recombinant RNA technologies and discusses the utilities of bioengineered RNA agents for the investigation of ADME gene regulation and general biomedical research.

A history of the roles of cytochrome P450 enzymes in the toxicity of drugs

The history of drug metabolism began in the 19th Century and developed slowly. In the mid-20th Century the relationship between drug metabolism and toxicity became appreciated, and the roles of cytochrome P450 (P450) enzymes began to be defined in the 1960s. Today we understand much about the metabolism of drugs and many aspects of safety assessment in the context of a relatively small number of human P450s. P450s affect drug toxicity mainly by either reducing exposure to the parent molecule or, in some cases, by converting the drug into a toxic entity. Some of the factors involved are enzyme induction, enzyme inhibition (both reversible and irreversible), and pharmacogenetics. Issues related to drug toxicity include drug-drug interactions, drug-food interactions, and the roles of chemical moieties of drug candidates in drug discovery and development. The maturation of the field of P450 and drug toxicity has been facilitated by advances in analytical chemistry, computational capability, biochemistry and enzymology, and molecular and cell biology. Problems still arise with P450s and drug toxicity in drug discovery and development, and in the pharmaceutical industry the interaction of scientists in medicinal chemistry, drug metabolism, and safety assessment is critical for success.

Recollection of the early years of the research on cytochrome P450

Since the publication of the first paper on "cytochrome P450" in 1962, the biochemical research on this novel hemoprotein expanded rapidly in the 1960s and the 1970s as its principal roles in various important metabolic processes including steroid hormone biosynthesis in the steroidogenic organs and drug metabolism in the liver were elucidated. Establishment of the purification procedures of microsomal and mitochondrial P450s in the middle of the 1970s together with the introduction of molecular biological techniques accelerated the remarkable expansion of the research on P450 in the following years. This review paper summarizes the important developments in the research on P450 in the early years, for about two decades from the beginning, together with my personal recollections.

UDP glucuronosyltransferase and mixed function oxidase activity in microsomes prepared by differential centrifugation and calcium aggregation

Microsomes were prepared from the liver, kidney and lung of phenobarbital or 20-methylcholanthrene treated and control rats with the conventional ultracentrifugation and calcium aggregation methods. The two methods were compared as to the yield of microsomal protein, amount of cytochrome P-450/448 and activity of UDP-GLUCURONOSYLTRANSFERASE, BENZOPYRENE HYDROXYLASE AND P-NITROANISOLE O-demethylase. The absolute amount of cytochrome P-450/448 (nmol/g wet weight), as well as the enzymatic activities dependent on it (nmol produced/g wet weight) did not differ significantly in any tissue of either treated or control animals nor did that of UDPglucuronosyltransferase. However, the ultracentrifugation method resulted in a slightly smaller yield of the hepatic microsomal protein and a correspondingly higher yield of cytochrome P-450/448 per mg protein as well as higher specific enzymatic activities of both the consecutive drug biotransformation reactions studied. The specific activity of UDPglucuronosyltransferase in digitonin treated microsomes was twice as high in the conventional microsomes as in the calcium aggregated microsomes; no differences was found in the trypsin treated microsomes. The specific activity of the hepatic benzpyrene hydroxylase of the benzpyrene treated animals in the calcium harvested microsomes was 55 per cent of that in the ultracentrifugated microsomes.

Forging the links between metabolism and carcinogenesis

Metabolism plays important roles in chemical carcinogenesis, both good and bad. The process of carcinogen metabolism was first recognized in the first half of the twentieth century and developed extensively in the latter half. The activation of chemicals to reactive electrophiles that become covalently bound to DNA and protein was demonstrated by Miller and Miller [Cancer 47 (1981) 2327]. Today many of the DNA adducts formed by chemical carcinogens are known, and extensive information is available about pathways leading to the electrophilic intermediates. Some concepts about the stability and reactivity of electrophiles derived from carcinogens have changed over the years. Early work in the field demonstrated the ability of chemicals to modulate the metabolism of carcinogens, a phenomenon now described as enzyme induction. The cytochrome P450 enzymes play a prominent role in the metabolism of carcinogens, both in bioactivation and detoxication. The conjugating enzymes can also play both beneficial and detrimental roles. As an example of a case in which several enzymes affect the metabolism and carcinogenicity of a chemical, aflatoxin B1 (AFB1) research has revealed insight into the myriad of reaction chemistry that can occur even with a 1s half-life for a reactive electrophile. Further areas of investigation involve the consequences of enzyme variability in humans and include areas such as genomics, epidemiology, and chemoprevention.

Comparisons of catalytic selectivity of cytochrome P450 subfamily enzymes from different species

Historically there has been considerable interest in comparing patterns of biotransformation of xenobiotic chemicals in experimental animal models and humans, e.g. in areas such as drug metabolism and chemical carcinogenesis. With the availability of more basic knowledge it has become possible to attribute the oxidation of selected chemicals to individual cytochrome P450 (P450) enzymes in animals and humans. Further, these P450s can be characterized by their classification into distinct subfamilies, which are defined as having > 59% amino acid sequence identity. Questions arise about how similar these enzymes are with regard to structure and function. More practically, how much can be predicted about reaction specificity and catalysis? In order to address these issues, we need to consider not only the relatedness of P450s from different species but also (i) functional similarity within P450 subfamilies and (ii) the effects of small changes imposed by site-directed mutagenesis. Relationships in the P450 1A, 2A, 2B, 2C, 2D, 2E, 3A, and 17A subfamilies are briefly reviewed. Overall functional similarity is generally seen in subfamily enzymes but many examples exist of important changes in catalysis due to very small differences, even a single conservative amino acid substitution. Some general conclusions are presented about predictability within various P450 subfamilies.

Comparison of the short-term hepatic effects of orally administered citral in Long Evans hooded and Wistar albino rats

The short-term effects of citral on the liver have been studied in two strains of rat. Hepatomegaly was accompanied in citral-treated rats by an altered distribution of lipid and glycogen in the liver and peroxisome proliferation occurred in a manner reminiscent of that associated with some hypolipidaemic compounds. Specific biochemical markers supported the morphological changes in the peroxisomes. Cyanide-insensitive palmitoyl CoA oxidation showed, at the maximum, fourfold and threefold inductions in Wistar albino and Long Evans hooded rats, respectively. In addition, induction of cytochrome P-450 levels was greater in the Long Evans than in the Wistar rats, the maximal increases recorded being 81 and 27% respectively. A peroxisome-associated polypeptide of molecular weight 80,000 daltons (PPA-80) was induced, especially in Long Evans rats. No alterations in plasma triglycerides or total cholesterol were detected. The differential induction of the mixed-function oxidase system and the differential proliferation of peroxisomes in these two strains of rat suggest that citral may be metabolized differently in the two strains. The study indicates that peroxisomal and possibly also mitochondrial changes are involved in the action of citral on lipid metabolism.

Mechanistic biotransformations of some amphetamine drugs

Amphetamine, fenfluramine and benzphetamine were the drugs investigated for the isolation of toxic metabolites using the biochemical mechanism of cytochrome P-450 monooxygenase mediated reaction. NH3 derived from amphetamine should be innocuous unless the in vivo ammonia detoxifying mechanism is overwhelmed thus culminating in ammonia intoxication in cerebral tissues with consequent concomitant convulsion. +CF3 electrophile derived from fenfluramine is potentially reactive with nucleophiles of proteins, carbohydrates, lipids, DNA and RNA. The derivation of .CF3 was discussed. Methylbenzylamine was derived from benzphetamine. This, in the nitrosating environment of the gastrointestinal tract, could yield the carcinogenic methylbenzylnitrosamine.

In vitro activation of isophosphamide and trophosphamide to metabolites mutagenic for bacteria

The ability of S9 liver fractions from uninduced rats to activate isophosphamide (IP) and trophosphamide (TP) to metabolites mutagenic for bacteria was compared to that of S9 fractions prepared from rats pretreated in vivo with three inducers of hepatic monooxygenase. Pretreatment of rats with phenobarbital (PB) and Aroclor 1254 increased IP and TP mutagenic activation by S9 fractions as compared to control and 3-methylcholanthrene (3-MC)-induced rat liver S9. Furthermore, the effect of mixed-function oxidase inhibitors, such as alpha-naphthoflavone, metyrapone and SKF 525-A on S9-mediated mutagenic activation of IP and TP was investigated. The data obtained suggest the involvement of a PB-inducible form of cytochrome P-450 in the activation of IP and TP to mutagenic species.

Absolute configuration of trans-7,8-dihydroxy-7,8-dihydro-7-methylbenzo[a]pyrene enantiomer and the unusual quasidiequatorial conformation of the diacetate and dimenthoxyacetate derivatives

The enantiomers of trans-7,8-dihydroxy-7,8-dihydro-7-methylbenzo[a]pyrene (7-MBaP 7,8-dihydrodiol) and of trans-7,8-dihydroxy-7,8,9,10-tetrahydro-7-methylbenzo[a]pyrene (7-MBaP 7,8-tetrahydrodiol) were directly resolved by high-performance liquid chromatography (HPLC) using a commercially available column packed with an (R)-N-(3,5-dinitrobenzoyl)-phenylglycine derivative of gamma-aminopropylsilanized silica. The absolute configurations of the resolved enantiomers were determined by the exciton chirality method. Circular dichroism (CD) spectral analysis of the quasidiequatorial benzo[a]pyrene 7R,8R-dihydrodiol enantiomer and its diacetate and dimenthoxyacetate derivatives indicated conformational changes were induced upon derivatization. However, the characteristic CD Cotton effects of the quasidiequatorial 7-MBaP 7,8-dihydrodiol and its diacetate and dimenthoxyacetate derivatives were similar indicating that the conformation of 7-MBaP trans-7,8-dihydrodiol was not altered upon derivatization. Proton nuclear magnetic resonance (NMR) spectral analyses confirmed that 7-MBaP 7,8-dihydrodiol, its diacetate and dimenthoxyacetate derivatives all have quasidiequatorial conformations. The results indicate that the methyl substituent of 7-MBaP 7,8-dihydrodiol maintains a quasiaxial position regardless of the size of the acyl derivatives linked to the hydroxyl groups.

Cytochrome P-448 and the activation of toxic chemicals and carcinogens

The metabolic activation of carcinogens and some toxic chemicals appears to involve oxygenation in conformationally hindered positions in the chemical molecules. Oxygenation of xenobiotics in hindered positions is effected by cytochrome P-448 (LM4) but not by cytochrome P-450 (LM2). Substrate-interaction spectra show that cytochrome P-448 has an active site with a conformation different from that of cytochrome P-450. Induction of cytochrome P-448, as specifically measured by ethoxyresorufin O-deethylase activity, occurs in rat liver, kidney and lung after administration of the carcinogens, 3-methylcholanthrene, Aroclor 1254, 2-anthramine, safrole, 7,12-dimethylbenz[a]anthracene, MNNG and 2-acetamidofluorene. The doubtful carcinogens, saccharin, DDT and aldrin, resulted in no significant induction. The drugs paracetamol, antipyrine, imipramine and rifampicin resulted in diminished enzyme activity, indicating the absence of any induction of cytochrome P-448. In studies with the matched pairs of carcinogens and non-carcinogens, benzo[a]pyrene and benzo[e]pyrene, and 1,2,5,6-dibenzanthracene and anthracene, only the carcinogenic analogue resulted in induction of cytochrome P-448. With alpha- and beta-naphthylamine, both resulted in marked induction of cytochrome P-448 in liver, kidney and lung, indicating that both isomers might be carcinogenic.

Aryl hydrocarbon hydroxylase activity and psoriasis

Aryl hydrocarbon hydroxylase (AHH) has been measured in the skin, jejunum and liver of normal and psoriatic individuals. We have been unable to confirm previous reports of an abnormality in AHH activity in patients with psoriasis. Re-examination of the laboratory records on which the original reports were based leads us to doubt their veracity and validity.

Stereoselective metabolism of benzo[a]pyrene and 7-methylbenzo[a]pyrene by liver microsomes from sprague-Dawley rats pretreated with polychlorinated biphenyls

The dihydrodiols formed from the metabolism of benzo[a]pyrene (BaP) and 7-methylbenzo[a]pyrene (7-MBaP) by liver microsomes from male Sprague-Dawley rats treated with polychlorinated biphenyls (PCBs, Aroclor 1254) have been examined by circular dichroism (CD) spectropolarimetry. Comparisons with optically pure enantiomers obtained via high performance liquid chromatography (HPLC) resolution of diastereomeric di-(-)-menthoxyacetates indicated that the trans-4,5-, 7,8- and 9,10-dihydrodiols formed from BaP metabolism are predominantly R,R-enantiomers with optical purities greater than 98%. The major enantiomers of the metabolically formed 7-MBaP-trans-4,5- and 9,10-dihydrodiols and 7-hydroxymethyl-BaP-trans-9,10-dihydrodiol have Cotton effects very similar to those of BaP-4R,5R- and 9R,10R-dihydrodiols, respectively. These 7-MBaP-trans-4,5- and 9,10-dihydrodiol metabolites therefore contain predominantly the R,R-enantiomers. The optical purity of metabolically formed 7-MBaP-trans-4,5-dihydrodiol was determined to be 30.8% enriched in (-)-enantiomer. The optical purity of the 9,10-dihydrodiol was not determined due to the lack of synthetic standards. The major trans-7,8-dihydrodiol enantiomer formed from 7-MBaP metabolism is a (+)-enantiomer (optical purity 60.4%) which has Cotton effects opposite in sign to that of the (-)-7R,8R-dihydrodiol formed from BaP metabolism. The results indicate that a methyl substituent on a polycyclic aromatic hydrocarbon may alter the stereoselective properties of the microsomal drug-metabolizing enzyme systems toward the substrate molecule.

Preparation of homogeneous NADPH-cytochrome P-450 reductase from rat hepatoma

NADPH-cytochrome P-450 reductase was purified to homogeneity from hepatoma 5123t.c.(H) microsomes from phenobarbital and hydrocortisone-treated rats by detergent solubilization and affinity chromatography with an overall 8% recovery. The purified enzyme has a minimum subunit molecular weight of 79 000 and contains one molecule each of FMN and FAD per 79 000 molecular weight. The purified hepatoma cytochrome P-450 reductase catalyzes electron transfer to artificial electron acceptors with Km values similar to those of purified liver reductase. The Km value of the hepatoma reductase for NADPH, 13 microM, is also similar to that of purified liver reductase. The tumor reductase appears immunochemically identical to liver reductase by Ouchterlony double-diffusion analysis and inhibition of activity. Peptide maps of the hepatoma and hepatic enzymes after proteolysis demonstrate the identity of the two proteins.

Comparisons of highly purified hepatic microsomal cytochromes P-450 from Holtzman and Long-Evans rats

The present study describes the purification and characterization of strain variant forms of a major phenobarbital-inducible microsomal hemoprotein, cytochrome P-450b, from Holtzman and Long-Evans rats. The strain variant hemoproteins cannot be resolved by sodium dodecyl sulfate gel electrophoresis, but can be partially separated in two-dimensional isoelectric focusing SDS gels. If, however, sodium tetradecyl sulfate is incorporated into the one-dimensional gel system, separation of the cytochromes P-450b is achieved. Minor structural differences are detected in the peptides of the cytochromes P-450b following limited proteolysis by Staphylococcus aureus V8 protease, cleavage by cyanogen bromide, or reverse-phase high-pressure liquid chromatography of tryptic peptides. The strain variant cytochromes P-450b are immunochemically and spectrally indistinguishable. The optical spectra of the ferric and ferrous hemoproteins are identical, as are the CO- and ethylisocyanide-reduced difference spectra. Ferrous cytochromes P-450b from both rat strains effectively bind metyrapone with equivalent affinities. In addition, the cytochromes P-450b do not differ in their catalytic activities toward benzphetamine, hexobarbital, benzo [a]pyrene, zoxazolamine, 7-ethoxycoumarin, estradiol-17 beta and testosterone. Cytochrome P-450c, the predominant isozyme inducible in rat liver by 3-methylcholanthrene, was purified from Holtzman and Long-Evans rats. Cytochromes P-450c from both rat strains are indistinguishable based on electrophoretic, immunological, spectral and catalytic properties. Minor structural differences in the cytochromes P-450c were revealed in the reverse-phase high-pressure liquid chromatographic profiles of the tryptic peptides of these hemoproteins, but not in the peptides generated by limited proteolysis or cleavage with cyanogen bromide.

Purification and partial characterization of squalene epoxidase from rat liver microsomes

Squalene epoxidase (EC 1.14.99.7, squalene 2,3-monooxygenase (epoxidizing) was purified to an apparent homogeneity from rat liver microsomes. The purification was carried out by solubilization of microsomes by Triton X-100, fractionation with ion exchangers, hydroxyapatite, Cibacron Blue Sepharose 4B, and chromatofocusing column chromatography. A total purification of 143-fold over the first DEAE-cellulose fraction was achieved. The purified enzyme gave a single major band on SDS-polyacrylamide gel electrophoresis and the Mr was estimated to be 51 000 as a single polypeptide chain. The enzyme showed no distinct absorption spectrum in the visible regions. The squalene epoxidase activity was reconstituted with the purified enzyme, NADPH-cytochrome P-450 reductase (EC 1.6.2.4), FAD, NADPH and molecular oxygen in the presence of Triton X-100. The apparent Michaelis constants for squalene and FAD were 13 microM and 5 microM, respectively. The Vmax was about 186 nmol per mg protein per 30 min for 2,3-oxidosqualene. The enzyme activity was not inhibited by potent inhibitors of cytochrome P-450. It is suggested that squalene epoxidase is distinct from cytochrome P-450 isozymes.

Immunofluorescence of phenobarbital inducible cytochrome P-450 in the hepatic lobule of normal and phenobarbital-treated rats

The localization of the form of cytochrome P-450 that is induced by phenobarbital was studied by direct immunofluorescence in the hepatocytes of rats pretreated with phenobarbital in comparison with saline-treated rats. Specific fluorescence was seen in the hepatocyte cytoplasm in saline- and phenobarbital-treated rats; a more concentrated halo of fluorescence was detected surrounding the nuclei in the centrilobular zones after phenobarbital treatment. In the saline-treated rats, fluorescence was barely discernible but slightly more intense in the centrilobular than perilobular zones. In phenobarbital-treated rats, fluorescence was much more intense, with a similar but much greater difference between the centrilobular and perilobular zones. The tissue distribution and induction site of this component of the cytochrome P-450-dependent microsomal system may be relevant to the site of drug toxicity and the development of chemical carcinogenesis.

Difference in liver homogenates from Donryu, Fischer, Sprague-Dawley and Wistar strains of rat in the drug-metabolizing enzyme assay and the Salmonella/hepatic S9 activation test

Comparison studies for detecting differences between liver microsome and S9 preparations from 4 strains (Donryu, Fischer, Sprague-Dawley, Wistar) of young male rats were carried out with pretreatment of the animals by inducers such as PCBs and PB plus 5,6-BF. Each microsome fraction was assayed for the enzymic activity of metabolism of model substrates such as aniline, benzophetamine, BP, DMN and 7-ethoxycoumarin. The hepatic S9 sample was also compared, as regards its metabolizing ability to activate 9 pre-mutagens (2AA, AAF, o-AAT, BP, DAB, DMBA, DMN, m-PDA, quinoline) to directly acting mutagens in the Salmonella/hepatic S9 activation test by using TA98, TA100 and TA1537 strains with or without cytochrome P450 inhibitors (SKF-525A, metyrapone, 7,8-benzo-flavone). In the enzymic assay with PCBs-induced microsomes, BP hydroxylation a strain-specific difference: the microsomes from Fischer and Wistar rats were more effective for metabolizing BP than those from the other strains of rat. The effect of induction by BP plus 5,6-BF for Fischer rats showed relatively higher enzymic activity in the same induction group. Other microsomes prepared from rats with and without induction by PB plus, 5,6-BF did not show a clear-cut strain dependency in the enzymic activities assayed. In the mutation experiments with hepatic S9 samples, the examination of DAB and quinoline revealed a marked strain difference when S9 samples prepared from PCBs-pretreated and PB-plus-5,6-BF-induced rats were used: the S9 sample from Fischer rats was available for activating the two pre-mutagens to directly acting mutagens. No marked difference in the metabolic activation of the remaining 7-pre-mutagens was observed on other S9 preparations. In examinations of mutagenicity activities with the use of three inhibitors, the two S9 preparations made with the two induction methods showed inhibition profiles closely similar to each other. However, there were minor differences in the profiles by these inhibitors. From these findings it was concluded that Fischer rat-liver S9 is useful for detecting mutagens in the metabolic activation test, when induction by PB plus 5,6-BF was used in the Ames Salmonella test.

Pharmacological properties of dibenzo[a,c]cyclooctene derivatives isolated from Fructus Schizandrae chinensis. II. Induction of phenobarbital-like hepatic monooxygenases

Schizandrin (Sin) A, B and C, Schizandrol (Sol) A and B and Schizandrer (Ser) A and B were isolated from Fructus Schizandrae chinensis, a traditional Chinese tonic. These components are the derivatives of dibenzo[a,c]cylooctene. Dimethyl-4,4'-dimethoxy-5,6,5',6'-dimethylenedioxy-biphenyl-2,2'-dicarboxylate (DDB) is an intermediate for synthesizing Sin C. The effect of these compounds on rat liver microsomal monooxygenases and epoxide hydrolase has been studied. Among these compounds, Sin B, Sin C, Sol B and DDB significantly increased rat liver cytochrome P-450 concentration, NADPH-cytochrome c reductase, benzphetamine and aminopyrene demethylase activities. The four compounds also markedly stimulated proliferation of smooth endoplasmic reticulum of liver cells. Metyrapone (1 mM) inhibited to a same extent (about 50%) the activity of aminopyrene demethylase of microsomes from rats treated by Sin B, Sin C, Sol B, DDB and phenobarbital (PB). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of liver microsomal preparations showed that Sin B and Sol B induce a major protein band of P-450 similar to that induced by PB. In addition, the effect of Sin B-, Sol B- and DDB-treated rat liver microsomes on [G-3H]-benzo[a]pyrene (BP) metabolism and covalent binding of reactive metabolites to DNA, in vitro, resembles that of PB. Dual induction of rats by Sol B and BP decreased mutagenicity of BP.

O-demethylation of scoparone and studies on the scoparone-induced spectral change of cytochrome P-450 in rat liver microsomes

Scoparone (6,7-dimethoxycoumarin) is demethylated to scopoletin (7-hydroxy-6-methoxycoumarin) and isoscopoletin (6-hydroxy-7-methoxycoumarin) by the cytochrome P-450-dependent monooxygenase system of rat liver microsomes. Under the conditions used, the ratio of scopoletin to isoscopoletin was determined to 1:1.8 +/- 0.1 for microsomes from untreated rats. Based on this reaction, a direct fluorometric method for the microsomal O-demethylation activity for scoparone is described. The fluorescence of the scopoletin formed in the incubation mixture is recorded after the adjustment of the excitation and emission wavelengths to 398 and 460 nm, respectively. The fluorescence of scoparone and isoscopoletin does not interfere with the test. Pretreatment of rats with phenobarbital or polycyclic hydrocarbons (3-metylcholanthrene, benzo[a]pyrene) causes a change in the ratio of the demethylation products scopoletin to isoscopoletin which was determined to be 1:2.5 +/- 0.1 (benzo[a]pyrene or 3-methylccholanthrene) or 1:5.9 +/- 0.01 (phenobarbital) respectively, and a significant increase in the amount of microsomal O-demethylation activity. Thus the ratio of the two products varies significantly with the state of induction. The difference spectra of scoparone with liver microsomes obtained from benzo[a]pyrene- and 3-methylcholanthrene-pretreated rats show an absorption peak at 416 nm and a trough at 393 nm with an isosbestic point at 405 nm. This scoparone-induced modified Type II spectral change seems to indicate the interaction of the 6- or 7-methoxy group of scoparone with the heme ion of the 394-nm form and its conversion into a modified ferrihemochrome with a absorption peak at 416 nm. This is modified ferrihemochrome is not identical with the 418-nm form of the cytochrome P-450.

Arylhydrocarbon hydroxylase activity and cytochrome P-450 in human tissues

The stability and distribution of arylhydrocarbon hydroxylase activity in four human tissues has been examined. Two tissues, liver and lung, were obtained from autopsy samples while lymphocytes and placenta were obtained from cell lines and donors. Marked differences in arylhydrocarbon hydroxylase activity were observed between tissues and individuals, with liver being the richest source. Activity in all tissues was stable at 4 degrees C for 24 h, but freeze-thawing markedly reduced hydroxylase activity in liver. Using gel exclusion chromatography, the molecular weight of a non-dissociated form of arylhydrocarbon hydroxylase was estimated to be about 400000. A heme staining band corresponding to a molecular weight of 50000 was observed after polyacrylamide gel electrophoresis of liver microsomal preparations. This appears to be a cytochrome P-450 subunit based on correlations between staining intensity and hydroxylase activity in tissues and partially purified preparations examined.

Aldrin epoxidation catalyzed by purified rat-liver cytochromes P-450 and P-448. High selectivity for cytochrome P-450

Aldrin epoxidation was studied in monooxygenase systems reconstituted from purified rat liver microsomal cytochrome P-450 or P-448, NADPH-cytochrome c reductase, dilauroylphosphatidylcholine and sodium cholate. Cytochrome P-450, purified from hepatic microsomes of phenobarbital-treated rats, exhibited a high rate of dieldrin formation. The low enzyme activity observed in the absence of the lipid and sodium cholate was increased threefold by addition of dilauroylphosphatidylcholine and was further stimulated twofold by addition of sodium cholate. The apparent Km for aldrin in the complete system was 7 +/- 2 microM. SKF 525-A, at a concentration of 250 microM, inhibited aldrin epoxidation by 65%, whereas 7,8-benzoflavone had no inhibitory effect at concentrations up to 250 microM. Addition of ethanol markedly increased epoxidase activity. The increase was threefold in the presence of 5% ethanol. When cytochrome P-448 purified from hepatic microsomes of 3-methylcholanthrene-treated rats was used, a very low rate of epoxidation was observed which was less than 3% of the activity mediated by cytochrome P-450 under similar assay conditions. Enzyme activity was independent of the lipid factor dilauroylphosphatidylcholine. The apparent Km for aldrin was 27 +/- 7 microM. The modifiers of monooxygenase reactions, 7,8-benzoflavone, SKF 525-A and ethanol, inhibited the activity mediated by cytochrome P-448. The I50 was 0.05, 0.2 and 800 mM, respectively. These results indicate that aldrin is a highly selective substrate for cytochrome P-450 species present in microsomes of phenobarbital-treated animals and is a poor substrate for cytochrome P-448. The two forms of aldrin epoxidase can be characterised by their turnover number, their apparent Km and their sensitivity to modifiers, like 7,8-benzoflavone and ethanol.

Two-dimensional gel electrophoresis of rat liver microsomal membrane proteins

Total rat liver microsomal proteins are not suitable for isoelectric focusing in polyacrylamide gels, even in the presence of sodium dodecyl sulphate and excess non-ionic detergent; considerable quantities of protein form an aggregate in the isoelectric focusing gel. This prevents resolution of microsomal proteins by the increasingly popular two-dimensional electrophoresis technique employing isoelectric focusing followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The problem is caused by the extreme insolubility of some microsomal proteins, especially cytochrome P-450 species, which precipitate during isoelectric focusing. A selective extraction of microsomes with sodium deoxycholate excludes these poorly soluble proteins. The extracted proteins can then be resolved without difficulty by isoelectric focusing, and give excellent two-dimensional gel patterns showing more than 100 proteins, mainly in the pI range 5--7. The technique should be useful in studies on microsome protein topology and on changes in microsome composition.

Purification and structural comparison of pulmonary and hepatic cytochrome P-450 from rabbits

A procedure is described for the purification of a major form of cytochrome P-450 from the livrs of rabbits treated with phenobarbital and a major form of the cytochrome from the lungs of untreated rabbits. Preparations in good yield (13--17%) and of high purity (up to 21 nmol of cytochrome per mg of protein) that were free of lipid and contained minimal amounts of non-ionic detergent were obtained from either tissue. The two cytochromes cannot be distinguished from each other on the basis of absorption spectra, extinction coefficients, apparent molecular weights (52 000), amino acid compositions, or peptide fragments produced by treatment of the proteins with cyanogen bromide. These data are consistent with a major indigenous form of rabbit pulmonary cytochrome P-450 being the same as the major form of hepatic cytochrome induced by phenobarbital.