The molecular biology of the polycyclic aromatic hydrocarbon inducible cytochrome P-450; the past is prologue

The heme-containing cytochromes P-450 are a ubiquitous family of monooxygenase isozymes responsible for the oxidative metabolism of a wide variety of endogenous as well as exogenous compounds. Many of the compounds metabolized by this enzyme system are effectively detoxified and converted to derivatives more easily eliminated from the organism. However, some compounds can be activated to reactive species capable of eliciting a cascade of toxic lesions, including cancer. Since its discovery nearly 30 years ago, the cytochrome P-450 enzyme system has received a great deal of attention, particularly in the areas of their mechanisms of metabolism, range of substrate specificity, the purification and characterization of the multiple isozymes and, more recently, the regulation of expression of specific forms. This review will discuss current notions concerning the expression of at least one cytochrome P-450 isozyme and future directions that should lead to a more complete understanding of cytochrome P-450 gene expression in general, particularly as it impacts upon biochemical pharmacology.     

Studies on the rate-determining factor in testosterone hydroxylation by rat liver microsomal cytochrome P-450: evidence against cytochrome P-450 isozyme:isozyme interactions

The aim of the present study was to examine a recent proposal that inhibitory isozyme:isozyme interactions explain why membrane-bound isozymes of rat liver microsomal cytochrome P-450 exert only a fraction of the catalytic activity they express when purified and reconstituted with saturating amounts of NADPH-cytochrome P-450 reductase and optimal amounts of dilauroylphosphatidylcholine. The different pathways of testosterone hydroxylation catalyzed by cytochromes P-450a (7 alpha-hydroxylation), P-450b (16 beta-hydroxylation), and P-450c (6 beta-hydroxylation) enabled possible inhibitory interactions between these isozymes to be investigated simultaneously with a single substrate. No loss of catalytic activity was observed when purified cytochromes P-450a, P-450b, or P-450c were reconstituted in binary or ternary mixtures under a variety of incubation conditions. When purified cytochromes P-450a, P-450b, and P-450c were reconstituted under conditions that mimicked a microsomal system (with respect to the absolute concentration of both the individual cytochrome P-450 isozyme and NADPH-cytochrome P-450 reductase), their catalytic activity was actually less (69-81%) than that of the microsomal isozymes. These results established that cytochromes P-450a, P-450b, and P-450c were not inhibited by each other, nor by any of the other isozymes in the liver microsomal preparation. Incorporation of purified NADPH-cytochrome P-450 reductase into liver microsomes from Aroclor 1254-induced rats stimulated the catalytic activity of cytochromes P-450a, P-450b, and P-450c. Similarly, purified cytochromes P-450a, P-450b, and P-450c expressed increased catalytic activity in a reconstituted system only when the ratio of NADPH-cytochrome P-450 reductase to cytochrome P-450 exceeded that normally found in liver microsomes. These results indicate that the inhibitory cytochrome P-450 isozyme:isozyme interactions described for warfarin hydroxylation were not observed when testosterone was the substrate. In addition to establishing that inhibitory interactions between different cytochrome P-450 isozymes is not a general phenomenon, the results of the present study support a simple mass action model for the interaction between membrane-bound or purified cytochrome P-450 and NADPH-cytochrome P-450 reductase during the hydroxylation of testosterone.     

Regioselective progesterone hydroxylation catalyzed by eleven rat hepatic cytochrome P-450 isozymes

Quantitative high-pressure liquid chromatographic assays were developed that separate progesterone and 17 authentic monohydroxylated derivatives. The assays were utilized to investigate the hydroxylation of progesterone by 11 purified rat hepatic cytochrome P-450 isozymes and 8 different rat hepatic microsomal preparations. In a reconstituted system, progesterone was most efficiently metabolized by cytochrome P-450h followed by P-450g and P-450b. Seven different monohydroxylated progesterone metabolites were identified. 16 alpha-Hydroxyprogesterone, formed by 8 of the 11 isozymes, was the only detectable metabolite formed by cytochromes P-450b and P-450e. 2 alpha-Hydroxyprogesterone was formed almost exclusively by cytochrome P-450h, and 6 alpha-hydroxyprogesterone and 7 alpha-hydroxyprogesterone were only formed by P-450a. 6 beta-hydroxylation of progesterone was catalyzed by four isozymes with cytochrome P-450g being the most efficient, and 15 alpha-hydroxyprogesterone was formed as a minor metabolite by cytochromes P-450g, P-450h, and P-450i. None of the isozymes catalyzed 17 alpha-hydroxylation of progesterone, and only cytochrome P-450k had detectable 21-hydroxylase activity. 16 alpha-Hydroxylation catalyzed by cytochrome P-450b was inhibited in the presence of dilauroylphosphatidylcholine (1.6-80 microM), while this phospholipid either stimulated (up to 3-fold) or had no effect on the metabolism of progesterone by the other purified isozymes. Results of microsomal metabolism in conjunction with antibody inhibition experiments indicated that cytochromes P-450a and P-450h were the sole 7 alpha- and 2 alpha-hydroxylases, respectively, and that P-450k or an immunochemically related isozyme contributed greater than 80% of the 21-hydroxylase activity observed in microsomes from phenobarbital-induced rats.     

Use of monoclonal antibody probes against rat hepatic cytochromes P-450c and P-450d to detect immunochemically related isozymes in liver microsomes from different species

Nine distinct monoclonal antibodies raised against purified rat liver cytochrome P-450c react with six different epitopes on the antigen, and one of these epitopes is shared by cytochrome P-450d. None of these monoclonal antibodies recognize seven other purified rat liver isozymes (cytochromes P-450a, b, and e-i) or other proteins in the cytochrome P-450 region of "Western blots" of liver microsomes. Each of the monoclonal antibodies was used to probe "Western blots" of liver microsomes from untreated, or 3-methylcholanthrene-, or isosafrole-treated animals to determine if laboratory animals other than rats possess isozymes immunochemically related to cytochromes P-450c and P-450d. Two protein-staining bands immunorelated to cytochromes P-450c and P-450d were observed in all animals treated with 3-methylcholanthrene (rabbit, hamster, guinea pig, and C57BL/6J mouse) except the DBA/2J mouse, where no polypeptide immunorelated to cytochrome P-450c was detected. The conservation of the number of rat cytochrome P-450c epitopes among these species varied from as few as two (guinea pig) to as many as five epitopes (C57BL/6J mouse and rabbit). The relative mobility in sodium dodecyl sulfate-gels of polypeptides immunorelated to cytochromes P-450c and P-450d was similar in all species examined except the guinea pig, where the polypeptide related to cytochrome P-450c had a smaller Mr than cytochrome P-450d. With the use of both monoclonal and polyclonal antibodies, we were able to establish that purified rabbit cytochromes P-450 LM4 and P-450 LM6 are immunorelated to rat cytochromes P-450d and P-450c, respectively.     

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.     

NH2-terminal sequence analyses of four rat hepatic microsomal cytochromes P-450

Cytochromes P-450f, P-450g, P-450h, and P-450i are four hepatic microsomal hemoproteins that have been purified from adult rats. Whereas cytochromes P-450g and P-450h appear to be male-specific hemoproteins, cytochrome P-450i is apparently a female-specific enzyme purified from untreated adult female rats. Cytochrome P-450f has been purified from adult male and female rats with equivalent recoveries. Amino-terminal sequence analyses of the first 15-20 amino acid residues of each of these cytochromes P-450 has been accomplished in the current investigation. Each protein possesses a hydrophobic leader sequence consisting of 65-87% hydrophobic amino acids, and only one charged amino acid (Asp) in the amino-terminal region. Although differences in the amino-terminal sequences of cytochromes P-450f, P-450g, P-450h, and P-450i are identified, these hemoproteins all begin with Met-Asp, and marked structural homology is observed among certain of these enzymes. Cytochromes P-450g and P-450h, two male-specific proteins, have 11-12/15 identical residues with cytochrome P-450i, a female-specific isozyme. Cytochromes P-450f and P-450h have 16/20 identical amino-terminal residues. Only limited sequence homology is observed between the amino-terminal sequences of cytochromes P-450f-i compared to rat liver cytochromes P-450a-e. The results demonstrate that cytochromes P-450f, P-450g, P-450h, and P-450i are isozymic to each other and five additional rat hepatic microsomal cytochrome P-450 isozymes (P-450a-e).     

Regulation of three forms of cytochrome P-450 and epoxide hydrolase in rat liver microsomes. Effects of age, sex, and induction

A procedure for the preparation of monospecific antibody directed against rat liver microsomal cytochrome P-45-a is described. This antibody, together with monospecific antibodies to cytochromes P-450b and P-450c, has been used to show that these three forms of cytochrome P-450 are distinct and share no common antigenic determinants. These antibodies (a) give single immunoprecipitin bands with detergent-solubilized microsomes; (b) do not cross-react with the purified heterologous antigens in Ouchterlony double diffusion analyses; (c) have no effect on catalytic activity of the heterologous antigens but completely inhibit the enzymatic activity of the homologous antigens; and (d) remove only the homologous antigen from detergent-solubilized microsomes when covalently bound to a solid support. With radial immunodiffusion assay, we have quantitated these three forms of cytochrome P-450 in liver microsomes after treatment of rats with seven different inducers of cytochrome P-450. The levels of these cytochrome P-450 isozymes vary independently and are also regulated by the age and sex of the animal. The antibodies have also been used to assess the contribution of cytochromes P-450a, P-450b, and P-450c in the metabolism of xenobiotics by rat liver microsomes. A large proportion of benzo(a)pyrene metabolism and testosterone 16 alpha-hydroxylation in microsomes from untreated rats is not catalyzed by cytochromes P-450a, P-450b, and P-450c. Epoxide hydrolase, another microsomal enzyme involved in the metabolism of xenobiotics, was also quantitated by radial immunodiffusion after prior treatment of rats with microsomal enzyme inducers. The inductions of epoxide hydrolase varies independently of the induction of cytochromes P-450a, P-450b, and P-450c.     

Interactive binding at cytochrome P-450 of cell growth regulatory bioamines, steroid hormones, antihormones, and drugs

The virtually universal family of P-450 isozymes contribute to the regulation of cell growth by modulating the levels of steroids and other lipid messengers for cytoplasmic and nuclear processes, including gene expression. In microsomes from rat liver cells, the concentration ( approximately 1 nmole/mg protein) of cytochromes P-450 approximates that of intracellular binding sites (K(d) 1.0-50 microM) for histamine. The potencies of certain therapeutic drugs to inhibit catalytic activity of, and histamine binding to, cytochromes P-450 in vitro were previously shown by us to be predictive of relative propensities to modulate tumor growth in rodents. Also, we demonstrated that growth-regulating polyamines potently interact with histamine at P-450. We now show that several classes of steroid hormones, antiestrogens, and antiandrogens, as well as various arylalkylamine drugs, all potently inhibit (3)H-histamine binding to cytochrome P-450 (K(i) values: testosterone 0.28 microM, progesterone 0.56 microM, flutamide 1.7 microM, tamoxifen 9.0 microM). Furthermore, all the various hormone and drug ligands are mutually inhibitory in their binding to cytochrome P-450; e.g., K(i) values of androstenedione and progesterone, to inhibit imipramine binding to P-450 (determined by spectral analysis), are 11 nM and 26 nM, respectively. The K(i) value of imiprimine to inhibit binding of androstenedione to P-450 is 3.5 microM. We estimate the total P-450 content in microsomes to be greater in male than in female rats and correlated with the number of binding sites for histamine, but not for steroids and drugs that appear to be more selective for P-450 isozymes. Thus, for at least some isozymes, the homeostatic role of the monooxygenases may be governed by histamine, modulated by endogenous ligands, and perturbed by many foreign molecules.     

Specific drug binding to rat liver cytochrome P-450 isozymes induced by pregnenolone-16 alpha-carbonitrile and macrolide antibiotics. Implications for drug interactions

Clinical interactions of macrolides with various drugs lead to elimination impairment, increase of plasma concentration and overdose-like effects, resulting from modifications of their metabolism. Previous studies have shown that treatment of rats by the macrolide antibiotics of the oleandomycin and erythromycin series lead to the induction of an hepatic cytochrome P-450 which is implicated into their own metabolism. We have characterized PCN or macrolides induced cytochromes P-450 by their specific ability to interact with macrolide derivatives and, using the cytochrome P-450 spectral binding assays, we have shown that some compounds, implicated in drug interaction with macrolides, interact preferentially with the same cytochromes. This strongly suggests that specific blockage of cytochrome P-450 IIIA1 family by macrolides, is responsible for these drug interactions and that these interactions can be predicted easily by simple in vitro tests such as those described herein.     

Monoclonal antibodies distinguish among isozymes of the cytochrome P-450b subfamily

Polyclonal antibody has been shown previously to react identically with cytochromes P-450b and P-450e purified from Long Evans rats and a strain variant of cytochrome P-450b purified from Holtzman rats (P-450bH). In the present study, an array of 12 different monoclonal antibodies produced against cytochrome P-450b has been used to distinguish among these closely related phenobarbital-inducible rat hepatic cytochromes P-450. In immunoblots and enzyme-linked immunosorbent assays, 10 monoclonal antibodies bind to cytochromes P-450b, P-450e, and P-450bH; one monoclonal antibody (B50) recognizes cytochromes P-450b and P-450bH but not cytochrome P-450e; and one monoclonal antibody (B51) is specific for cytochrome P-450b. In addition, one monoclonal antibody (BEF29) reacts strongly with cytochrome P-450f, and another antibody (BEA33) reacts weakly with cytochrome P-450a. No cross-reactions with cytochromes P-450c, P-450d, and P-450g-P-450j were detected with any of the monoclonal antibodies in these assays. Six spatially distinct epitopes on cytochrome P-450b were identified, and differences in antibody reactivity provided evidence for three additional overlapping epitopes. Several monoclonal antibodies are potent inhibitors of testosterone and benzphetamine metabolism supported by cytochrome P-450b in a reconstituted system. B50 and BE52 do not inhibit metabolism of the two substrates by microsomes from untreated rats, but inhibit benzphetamine N-demethylation and testosterone metabolism to 16 alpha- and 16 beta-hydroxytestosterone as well as androstenedione formation 67-94% by microsomes from phenobarbital-treated rats. No other pathways of testosterone metabolism are inhibited by these monoclonal antibodies. The differential inhibition of microsomal metabolism of benzphetamine and testosterone by these monoclonal antibodies is a reflection of the content and inducibility of cytochromes P-450b and P-450e as well as other cytochrome P-450 isozymes.     

Expression and catalysis of sex-specific cytochrome P450 isozymes in rat liver

Research interest in the study of cytochromes P450 has recently been shifting to the characterization of "constitutively" expressed isozymes from that of the inducible forms. Several "constitutive" cytochrome P450 isozymes have been purified from rat liver including five immunochemically related proteins designated cytochromes P450f, P450g, P450h, P450i, and P450k. These hemoproteins have been identified as distinct isozymes on the basis of spectral, electrophoretic, and catalytic properties and NH2-terminal sequence analysis. Purification and immunoquantitation studies have indicated that these isozymes are expressed in a developmental as well as sex-related manner, and are relatively refractory to induction by xenobiotics. Cytochromes P450h and P450g are male-specific proteins, cytochrome P450i is a female-specific isozyme, while cytochromes P450f and P450k are present in both male and female adult rats. In addition, the expression of cytochrome P450g was shown to segregate into two phenotypes in outbred rats. Genetic studies utilizing inbred strains have indicated that the gene responsible for inheritance of high levels of cytochrome P450g is autosomal. Although considerable progress has been made in understanding the role of gonadal hormones and growth hormone in the hepatic regulation of cytochromes P450g, P450h, and P450i in particular, the physiological significance of the "constitutive" isozymes in the liver remains largely unresolved.     

Human cytochrome P-450 PB-1: A multigene family involved in mephenytoin and steroid oxidations that maps to chromosome 10

The cytochrome P-450 monooxygenase system possesses catalytic activity toward many exogenous compounds (e.g., drugs, insecticides, and polycyclic aromatic hydrocarbons) and endogenous compounds (e.g., steroids, fatty acids, and prostaglandins). Multiple forms of cytochrome P-450 with different substrate specificities have been isolated. In the present paper we report the isolation and sequence of a cDNA clone for the human hepatic cytochrome P-450 responsible for mephenytoin (an anticonvulsant) oxidation. The mephenytoin cytochrome P-450 is analogous to the rat cytochrome P-450 form termed PB-1 (family P450C2C). We also report that human PB-1 is encoded by one of a small family of related genes all of which map to human chromosome 10q24.1-10q24.3. The endogenous role of this enzyme appears to be in steroid oxidations. This cytochrome P-450 family does not correspond to any of the hepatic cytochrome P-450 gene families previously mapped in humans.     

Genes for cytochrome P-450 and their regulation

The capacity of the liver microsomal mixed-function oxidase system to metabolize a wide variety of exogenous as well as endogenous compounds reflects the participation of multiple forms of the terminal oxidase, cytochrome P-450, which have different broad, but overlapping, substrate specificities. Several of these isozymes accumulate in the liver after exposure of animals to specific inducing agents. Recent studies employing recombinant DNA techniques to investigate the genetic and evolutionary relatedness of various cytochrome P-450 isozymes as well as the molecular basis for the induction phenomenon are described. The conclusions from these investigations are presented in the context of the substantial body of data obtained from the characterization of specific cytochrome P-450 isozymes and from studies on the induction of specific isozymes or enzymatic activities during development or after treatment of animals with various inducing agents.     

Purification of hepatic microsomal cytochromes P-450 from beta-naphthoflavone-treated Atlantic cod (Gadus morhua), a marine teleost fish

Four isozymes of cytochrome P-450 were purified to varying degrees of homogeneity from liver microsomes of cod, a marine teleost fish. The cod were treated with beta-naphthoflavone by intraperitoneal injection, and liver microsomes were prepared by calcium aggregation. After solubilization of cytochromes P-450 with the zwitterionic detergent 3-[(3-cholamidopropyl) dimethylammonio]-1-propansulfonate, chromatography on Phenyl-Sepharose CL-4B, and subsequently on DEAE-Sepharose, resulted in two cytochrome P-450 fractions. These were further resolved on hydroxyapatite into a total of four fractions containing different isozymes of cytochromes P-450. One fraction, designated cod cytochrome P-450c, was electrophoretically homogeneous, was recovered in the highest yield and constituted the major form of the isozymes. The relative molecular mass of this form (58 000) corresponds well with a protein band appearing in cod liver microsomes after treatment with beta-naphthoflavone. Both cytochrome P-450c and a minor form called cytochrome P-450d (56000) showed activity towards 7-ethoxyresorufin in a reconstituted system containing rat liver NADPH-cytochrome P-450 reductase and phospholipid. Differences between these two forms were observed in the rate and optimal pH for conversion of this substrate, and in optical properties. Rabbit antiserum to cod cytochrome P-450c did not show any cross-reactions with cod cytochrome P-450a (Mr 55000) or cytochrome P-450d in Ouchterlony immunodiffusion, but gave a precipitin line of partial identity with cod cytochrome P-450b (Mr 54000), possibly as a result of contaminating cytochrome P-450c in this fraction.     

On the glycosylation state of five rat hepatic microsomal cytochrome P-450 isozymes

The glycosylation states of five rat hepatic microsomal cytochrome P-450 isozymes (cytochromes P-450a, P-450b, P-450c, P-450d, and P-450e) were examined by quantitative carbohydrate analysis. Carbohydrate content of the purified enzymes as determined by acid hydrolysis, reduction, and gas chromatography of the alditol acetates revealed only trace amounts of neutral and amino hexoses in each of the five isozymes. Levels of mannose ranged from 0.3 to 1.7 mol/mol of cytochrome P-450 whereas levels of galactose were less than or equal to 0.2 mol/mol of cytochrome P-450 for the five hemoproteins. The amino sugars glucosamine and galactosamine were usually present at levels less than or equal to 0.2 mol/mol of cytochrome P-450, although one preparation of cytochrome P-450b had as much as 0.5 mol of glucosamine/mol of cytochrome P-450. Other carbohydrate residues (xylose and arabinose) were not detected in significant quantities. Since N- and O-glycosylation of proteins occurs primarily through N-acetylglucosaminyl and N-acetylgalactosaminyl residues, respectively, the lack of significant amounts of these amino sugars indicates that these five cytochrome P-450 isozymes are not normally glycosylated in the native state. Purified NADPH-cytochrome c reductase, which functions as an electron donor for microsomal cytochrome P-450, contained no detectable quantities of hexose sugars.     

Protein engineering of cytochromes P-450

The cytochromes P-450 are an immensely important superfamily of heme-containing enzymes. They catalyze the monooxygenation of an enormous range of substrates. In bacteria, cytochromes P-450 are known to catalyze the hydroxylation of environmentally significant substrates such as camphor, phenolic compounds and many herbicides. In eukaryotes, these enzymes perform key roles in the synthesis and interconversion of steroids, while in mammals hepatic cytochromes P-450 are vital for the detoxification of many drugs. As such, the cytochromes P-450 are of considerable interest in medicine and biotechnology and are obvious targets for protein engineering. The purpose of this article is to illustrate the ways in which protein engineering has been used to investigate and modify the properties of cytochromes P-450. Illustrative examples include: the manipulation of substrate selectivity and regiospecificity, the alteration of membrane binding properties, and probing the route of electron transfer.     

Studies on the substrate-binding sites of liver microsomal cytochrome P-448.

The interaction of substrates of the microsomal mixed-function oxidases with cytochromes P-450 and P-448 was investigated by using liver microsomes from rats pretreated with phenobarbital or 3-methylcholanthrene, and with purified forms of the cytochromes isolated from rabbit liver. The two forms of the cytochrome have different substrate specificities; cytochrome P-450 has one type 1 substrate-binding site that can accommodate a large variety of substrates, but in contrast cytochrome P-448 may possess two type 1 substrate-binding sites, one of which is different to that of cytochrome P-450 in that it shows a specificity for substrates such as safrole and 9-hydroxy-ellipticine. These findings explain why the two forms of the cytochrome have different substrate specificities and play contrasting roles in the activation and deactivation of xenobiotics.     

Interactions of safrole and isosafrole and their metabolites with cytochromes P-450

The structural features which determine interaction of safrole and related methylenedioxyphenyl compounds with cytochromes P-450 or P-448, and determine the induction of these two classes of the cytochrome, have been studied. All methylenedioxyphenyl compounds studied interact with both cytochromes P-450 and P-448 eliciting type I spectral changes and it has been found that the allyl 4-substituent is important in these interactions. Methylenedioxyphenyl compounds with an oxidised allyl 4-substituent exhibited higher affinity for cytochrome P-448 while those possessing an intact allyl or methylvinyl group generally showed higher affinity for cytochrome P-450. Compounds possessing intact allyl and methylenedioxyphenyl groups (safrole, isosafrole and myristicine) were the most potent inducers of cytochromes P-450 and P-448; compounds containing an intact allyl group only (estragole, allybenzene and eugenol methyl ether) or an oxidized allyl group and an intact methylenedioxyphenyl group (epoxysafrole) were inducers of P-448 only.     

Decreased expression of cytochrome P-452 in the resistance phenotype characteristic of putative preneoplastic hepatocyte nodules during hepatocarcinogenesis

Hepatocyte nodules, a characteristic early step in the development of liver cancer in rats, has a distinctive resistance phenotype including a large decrease in total cytochromes P-450 and in two isozymes induced by phenobarbital and two by 3-methylcholanthrene. In this study, it has been observed that the nodules show a large decrease in an additional cytochrome P-450, cytochrome P-452, which is very active in the hydroxylation of lauric acid at C-11 and C-12. The decrease in activity of this microsomal cytochrome P-452 is of the same order of magnitude as the decreases in the other cytochrome P-450 components. These observations are consistent with the hypothesis that there is some more basic alteration in the synthesis or availability of heme and that the changes in the activities of the cytochromes P-450 are secondary.     

Biocatalytic Hydroxylations Catalyzed by Cytochromes P-450—Problems and Prospects

Due to their selectivity biocatalytic hydroxylations catalysed by monooxygenases or dioxygenases arouse considerable interest. The most studied and distributed type of monooxygenases are cytochromes P-450, which are a supergene family of proteins that catalyse the oxidation of lipophilic compounds through the insertion of 1 oxygen atom of O₂ into the substrate. For biocatalytic processes those cytochromes deserve attention, are involved in the catabolism of nutrients in microorganisms or in the metabolism of physiological substrates in higher eucaryotes. Mammalian cytochromes P-450, which catalyse the oxidation of xenobiotics (synthetic drugs, pesticides, hazardous waste compounds, are hardly suitable for this purpose. Processes with immobilized cytochromes P-450 can be excluded. The use of wild or genetically transformed strains of microorganisms is a suitable way.