The P450 superfamily: updated listing of all genes and recommended nomenclature for the chromosomal loci

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.

Characterization of a cDNA encoding a new member of the glucocorticoid-responsive cytochromes P450 in human liver

Adult human liver contains a form of cytochrome P450, termed HLp, that resembles the glucocorticoid-inducible cytochrome P450p in rat liver in its structure, function, and regulation and catalyzes the oxidation of such clinically important substrates as cyclosporin, nifedipine, erythromycin, and midazolam. Recent evidence, however, suggests that HLp may represent two or more closely related forms of cytochromes P450, one of which is termed P450nf. To search for additional members of the Class III human subfamily of HLp related genes, we screened a human liver cDNA library cloned in phage vector lambda gt11 with oligonucleotides and with a cDNA fragment related to HLp. We isolated a full-length cDNA (1709 nucleotides) encoding a new form of human cytochrome P450 termed HLp2. Analysis of HLp2 cDNA predicted a protein of 502 amino acids, weighing 57,294 Da 83% similar to HLp. HLp2 appears to represent a distinct gene as judged by partial sequence analysis of a cloned human gene and by hybridizations of Southern blots, under conditions of varying stringency, with a 3'-portion of HLp cDNA and with an oligonucleotide specific for HLp2. Northern blot analysis revealed that HLp/P450nf was present in all samples of liver mRNA from adult patients not treated with inducers of HLp, whereas HLp2 mRNA was undetectable in more than two-thirds. Human fetal liver RNA contained mRNA species 2.1 and 1.9 kb which hybridized with an HLp2 oligonucleotide. We conclude that HLp2 represents a third member of the Class III glucocorticoid-responsive gene family that is expressed in both fetal and adult human liver and may account for polymorphism in metabolism of clinically important drugs.

Hybrid cytochromes P-450 identify a substrate binding domain in P-450IIC5 and P-450IIC4

The cytochrome P-450 superfamily of enzymes catalyzes the oxidative metabolism of innumerable lipophilic compounds (e.g., drugs, carcinogens, steroids). Although the three-dimensional structure of a soluble bacterial P-450 (P-450cam) has been solved, little is known about the structures of the membrane-bound mammalian P-450s. Thus, the structural features of these enzymes that determine their multisubstrate specificity are unknown. In this report, we identify a segment of the primary structure of the structurally similar but functionally distinct cytochromes P-450IIC5 and P-450IIC4, which determines the apparent affinity of these cytochromes for the conversion of progesterone into the mineralocorticoid deoxycorticosterone. P-450IIC5 exhibits a greater than 10-fold lower apparent Km than P-450IIC4 for progesterone 21-hydroxylation. Chimeric cDNAs were constructed and expressed in COS-1 cells, which encode hybrids between these enzymes. The hybrid enzymes were assayed for catalytic activity and compared to the parental proteins. A segment of P-450IIC5 was identified that conferred the lower Km of P-450IIC5 to P-450IIC4. Sequential reduction of the length of the exchanged segments led to a hybrid enzyme with a high affinity derived largely from P-450IIC4, which contains three amino acid residues derived from P-450IIC5 clustered between positions 113 and 118. This suggests that this region is part of a substrate binding domain. This region maps by alignment of amino acid sequences to a residue of P-450cam, which has been implicated in substrate binding, suggesting that these segments of the primary structure serve a similar functional role in these two distantly related proteins.

Complexity of steroid hydroxylase gent expression in the adrenal cortex A microcosm of regulated transcription

Regulation of the expression of steroid hydroxylase genes in the adrenal cortex involves mechanisms required for maintenance of optimal steroidogenesis, tissue specificity, and ontogeny of the steroidogenic pathway. Evaluation of the molecular basis of this complexity promises to unfold new aspects of regulated eukaryotic gene expression.

Novel exogenous heme-dependent expression of mammalian cytochrome P450 using baculovirus

Rat P450 IIA1 is a membrane-bound hemoprotein that catalyzes the 7 alpha- and 6 alpha-hydroxylation of testosterone. A recombinant baculovirus, containing the cDNA encoding IIA1, was constructed and used to infect Spodoptera frugiperda cells. Infected cells contained up to 10% IIA1 protein as quantified by Western blot analysis; however, only a small percentage of this protein contained heme and was catalytically active. Addition of hemin to the culture medium during the course of viral infection resulted in a marked sixfold increase in both testosterone hydroxylase activity and heme-containing IIA1 protein. When the exogenously added protoheme was substituted with modified heme molecules containing hydrogen or ethyl groups at the 2 and 4 positions of the protoporphyrin IX, enzymes with only marginal changes in catalytic properties were produced. These data indicate that baculovirus can be used as a system to produce high levels of mammalian cytochrome P450s containing custom modified heme moieties.

Selective mouse breeding for short ethanol sleep time has led to high levels of hepatic aromatic hydrocarbon (Ah) receptor

Following a selective breeding program of heterogeneous mice for more than 30 generations, SS ("short sleep") and LS ("long sleep") lines have been developed on the basis of their sleep times when challenged with a single intraperitoneal dose of ethanol. The aromatic hydrocarbon responsiveness (Ah) locus encodes the Ah receptor, which regulates the induction of certain drug-metabolizing enzymes by polycyclic aromatic compounds such as 3-methylcholanthrene and tetrachlorodibenzo-p-dioxin. The C57BL/6 inbred mouse strain (B6; Ahb/Ahb) has a high-affinity Ah receptor, while the DBA/2 inbred mouse strain (D2; Ahd/Ahd) has a low-affinity Ah receptor. We show here that the SS inbred mouse line exhibits markedly elevated hepatic levels of the high-affinity Ah receptor, while the LS outbred mouse line contains the low-affinity Ah receptor. Among progeny of (B6D2)F1 X D2 backcross, the b/d heterozygote (having the high-affinity Ah receptor) was found to be several times more resistant than the d/d homozygote to a single dose of intraperitoneal ethanol. The D2.B6-Ahb congenic line is also several times more resistant to intraperitoneal ethanol than the B6.D2-Ahb congenic line is also several times more resistant to intraperitoneal ethanol than B6.D2-Ahd congenic line. We found that the waking blood ethanol levels are the same in b/d and d/d mice, suggesting that the relative ethanol resistance in b/d mice cannot be explained on the basis of a difference in central nervous system sensitivity. There are no differences between SS and LS mice or between b/d and d/d mice with regard to (i) blood acetaldehyde levels after a single intraperitoneal dose of ethanol, or (ii) hepatic alcohol dehydrogenase activities. There is a difference in the rate of ethanol elimination: SS more rapid than LS; b/d more rapid than d/d. Although SS mice have lower hepatic aldehyde dehydrogenase activities (cytosolic, mitochondrial low-Km: and mitochondrial high-Km forms) than LS mice, b/d and d/d do not show this difference. These data suggest that a selected mouse breeding program, based on resistance to a single intraperitoneal dose of ethanol, selects concurrently for the hepatic high-affinity Ah receptor. This selective advantage cannot be explained on the basis of changes in alcohol dehydrogenase or aldehyde dehydrogenase activities and might provide insight into the nature of the endogenous ligand for the Ah receptor.

Heterogeneity within the alkane-inducible cytochrome P450 gene family of the yeast Candida tropicalis

The reexamination of a genomic lambda gt11 Candida tropicalis expression library for the presence of genes related to the previously reported alkane-inducible cytochrome P450alk gene (P450alk), which is the first member of the P450LII gene family, was undertaken. A positive clone with a DNA fragment having 69% similarity with a portion of P450alk was isolated. As in the case of P450alk, this new putative P450 gene was also induced by tetradecane when C. tropicalis was grown on this carbon source and was therefore named P450alk2, P450alk1 corresponding to the first isolated P450 gene. In addition to P450alk2, the existence of other P450alk-related genes is suggested by the hybridization pattern of P450alk1 and P450alk2 probes with the C. tropicalis genomic DNA. The P450LII gene family in C. tropicalis appears therefore to include several different members. This heterogeneity is presently a unique feature within yeast P450 gene families and resembles the situation existing in P450 gene families of higher eukaryotes.

Identification of a new P450 expressed in human lung: complete cDNA sequence, cDNA-directed expression, and chromosome mapping

A cDNA coding for a P450 expressed in human lung was isolated from a lambda gt11 library constructed from human lung mRNA using a cDNA probe to rat P450 IVA1. The cDNA-deduced amino acid sequence of this P450, designated IVB1, consisted of 511 amino acids and had a calculated molecular weight of 59,558. The IVB1 amino acid sequence bore 51%, 53%, and 52% similarities to rat IVA1, IVA2, and rabbit P450p-2, respectively. Comparison of the primary amino acid sequence of human IVB1 with rat IVA and rabbit p-2 P450 sequences revealed a region of absolute sequence identity of 17 amino acids between residues 304 and 320. However, the functional significance of this conserved sequence is unknown. Human IVB1 also appears to be related to P450 isozyme 5 that has been extensively characterized in rabbits. The IVB1 cDNA was inserted into a vaccinia virus expression vector and the enzyme expressed in human cell lines. The expressed enzyme had an absorption spectrum with a lambda max at 450 nm when reduced and complexed with carbon monoxide, typical of other cytochrome P450s. Unlike rabbit P450 isozyme 5, however, human IVB1 was unable to activate the promutagen 2-aminofluorene. Human lung microsomal P450s were also unable to metabolize this compound despite the presence of IVB1 mRNA in three out of four human lungs analyzed. In contrast to its expression in lung, IVB1 mRNA was undetectable in livers from 14 individuals, including those from which the lungs were derived. IVB1-related mRNA was also expressed in rat lung and was undetectable in untreated rat liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochrome P450 forms in fish: catalytic, immunological and sequence similarities

1. Hepatic microsomal enzymes from teleost and elasmobranch fishes catalyse a diversity of monooxygenase reactions, consistent with the presence of multiple, distinct P450 forms. Protein purification and immunological studies have confirmed that multiple microsomal P450s occur in teleosts. 2. A member of the aromatic hydrocarbon-inducible P450 IA family is present in all fish species examined to date. This protein appears to be most closely related to P450 IA1. Certain of the immunological probes for a teleost P450 IA1 (scup P450E) appear to be reagent antibodies, recognizing the homologous protein in members of all vertebrate groups examined. The nature of the epitope recognized by such antibodies is not known. 3. Based on immunological and amino acid sequence comparisons, teleost P450 IA1 appears to be orthologous to both P450 IA1 and P450 IA2 in mammals. Multiple P450 IA genes may appear in teleosts, but divergence on separate lines from that involving mammalian P450 IA2 could include additional, new members (P450 IA3?) of the P450 IA family. 4. There are greater similarities in the N-terminal amino acid sequences of different teleost (scup and trout) P450 IA1 forms, than seen in the N-terminal sequence relationships found in P450 IA1 of mammalian species. Whether this similarity extends to the rest of these teleost proteins is unknown. 5. The induction of P450 IA1 in teleosts involves transcriptional and translational events. However, the temporal patterns involved in induction of mRNA or protein are different from those in mammalian species, indicating additional aspects of the regulation in teleosts. 6. Relationships between other teleost and mammalian P450 forms, or between other P450 forms isolated from different teleosts, remain to be conclusively established. However, certain relationships are suggested, based on catalytic and other comparisons.

The increase in urinary excretion of 6 beta-hydroxycortisol as a marker of human hepatic cytochrome P450IIIA induction

1. Urinary excretion of 6 beta-hydroxycortisol, hepatic microsomal cortisol 6 beta-hydroxylase and the specific content of several forms of cytochrome P450 were measured in 8 to 14 patients before and after treatment with rifampicin (600 mg orally per day for 4 days). 2. Rifampicin treatment produced an average five fold increase in daily excretion of urinary 6 beta-hydroxycortisol. 3. Cortisol 6 beta-hydroxylase activity increased from 15 +/- 6 pmol min-1 mg-1 in organ donors (considered as 'control subjects') to 87 +/- 31 pmol min-1 mg-1 in rifampicin treated patients. 4. Among three forms of human P450 (P450IA, IIC and IIIA), (1), (2), measured by Western blots, only P450IIIA was significantly induced by the antibiotic. 5. Only antibodies against P450IIIA selectively inhibited cortisol 6 beta-hydroxylase in human liver microsomes. 6. Cortisol 6 beta-hydroxylase was correlated with P450IIIA specific content. 7. The urinary level of 6 beta-hydroxycortisol correlated with liver microsomal cortisol 6 beta-hydroxylase and P450IIIA specific content. 8. We conclude that P450IIIA is predominantly responsible for cortisol 6 beta-hydroxylase activity in human liver microsomes and that urinary 6 beta-hydroxycortisol is a marker of the induction of this cytochrome P450.

Isolation of a human cytochrome P-450 reductase cDNA clone and localization of the corresponding gene to chromosome 7q11.2

We have isolated and sequenced cDNA clones that code for rat and human NADPH-dependent cytochrome P-450 reductase. The cDNA coding for the human protein was used to analyse, by Southern blot hybridization, DNA isolated from a panel of 8 independent human-rodent somatic cell hybrids. The results indicate that cytochrome P-450 reductase is encoded by a single gene (POR) located on human chromosome 7(pter-q22). Analysis of human metaphase chromosomes by hybridization in situ confirmed the results and refined the localization to 7q11.2. Northern blot hybridization revealed that in human liver the expression of the gene varies by less than 3-fold between different individuals.

Evolution of the cytochrome P450 genes

1. The P450 gene superfamily is presently known to contain more than 78 members, divided into 14 families. 2. The superfamily has undergone divergent evolution, and the ancestral gene is probably more than 2 billion years old. 3. The recent 'burst' in new P450 genes, particularly in the II family during the past 800 million years, appears to be the result of 'animal-plant warfare'. 4. Due to the presence or absence of a particular P450 gene in one species but not the other, it may not be correct to extrapolate toxicity or cancer data from rodent to human. 5. Increases in the P450 gene product (enzyme induction) almost always reflect an elevated rate in gene transcription, although there are several exceptions. 6. The mechanisms of P450 gene regulation (induction) by classes of inducers might become better understood through the comparison of different phyla that differ in response to a particular class of inducers. 7. Amongst several carefully selected phyla, delineation between which electron donor (presence of Fe2S2 protein or NADPH-P450 oxidoreductase, or both) interacts with P450 may provide valuable information about the evolution of eukaryotes from prokaryotes.

Regulation of mouse CYP1A1 gene expression by dioxin: requirement of two cis-acting elements during induction

The mouse cytochrome P1450 (CYP1A1) gene is responsible for the metabolism of numerous carcinogens and toxic chemicals. Induction by the environmental contaminant tetrachlorodibenzo-p-dioxin (TCDD) requires a functional aromatic hydrocarbon (Ah) receptor. We examined the 5'-flanking region of the CYP1A1 gene in mouse hepatoma Hepa-1 wild-type cells and a mutant line having a defect in chromatin binding of the TCDD-receptor complex. We identified two cis-acting elements (distal, -1071 to -901 region; proximal, -245 to -50 region) required for constitutive and TCDD-inducible CYP1A1 gene expression. Three classes of DNA-protein complexes binding to the distal element were identified: class I, found only in the presence of TCDD and a functional Ah receptor, that was heat labile and not competed against by simian virus 40 (SV40) early promoter DNA; class II, consisting of at least three constitutive complexes that were heat stable and bound to SV40 DNA; and class III, composed of at least three constitutive complexes that were thermolabile and were not competed against by SV40 DNA. Essential contacts for these proteins were centered at -993 to -990 for the class I complex, -987, -986, or both for the class II complexes, and -938 to -927 for the class III complexes. The proximal element was absolutely essential for both constitutive and TCDD-inducible CYP1A1 gene expression, and at least two constitutive complexes bound to this region. These data are consistent with the proximal element that binds proteins being necessary but not sufficient for inducible gene expression; interaction of these proteins with those at the distal element was found to be required for full CYP1A1 induction by TCDD.

The rat clofibrate-inducible CYP4A gene subfamily. I. Complete intron and exon sequence of the CYP4A1 and CYP4A2 genes, unique exon organization, and identification of a conserved 19-bp upstream element

The P450 CYP4A1 and CYP4A2 genes were isolated from a rat genomic library constructed in the vector lambda EMBL3 and their complete sequences were determined. The CYP4A1 and CYP4A2 genes spanned 14,144 and 10,576 bp and contained 13 and 12 exons, respectively. The CYP4A1 gene contained an additional intron that splits the exon corresponding to exon 12 of the CYP4A2 gene, resulting in a noncoding 13th exon in CYP4A1. The exon numbers of these genes were distinct among known P450 genes, and yet several intron-exon junctions along the P450 amino acid coding region were conserved with P450 genes in the CYP2, CYP11, and CYP21 gene families. On the basis of these data, the number of exons in the putative ancestral P450 gene was estimated. The evolutionary implications of this finding are discussed. No consensus TATA sequence was found upstream of either gene's transcription start site. Comparison of the CYP4A1 and CYP4A2 promoters with other genes that lack TATA boxes did not reveal any strong consensus sequence in their immediate upstream regions. However, a conserved 19-bp sequence was located at the positions of 42 and 48 bp upstream from the CYP4A1 and CYP4A2 genes' start sites, respectively. The CYP4A2 gene also contained two 378-bp direct repeats upstream from the start site; these repeats are derived from portions of the long interspersed middle repetitive element present in high copy numbers in the rat genome.

Transcriptional start site in the mouse Cyp1a1 (cytochrome P1450) gene

Two transcriptional initiation sites have been described by different laboratories for the mouse Cyp1a1 (cytochrome P1450) gene: start site TI, at the beginning of the nontranslated 87-bp first exon, and start site TII, 635 bp downstream from site TI within the 2,380-bp first intron. Site TI was characterized in normal C57BL/6 mouse liver, whereas site TII was described in a high activity variant line (HAV cells) derived from the mouse hepatoma Hepa-1c1c7 wild type (wt) established cell line. It is conceivable that a tumor-specific promoter might be used in the Cyp1a1 gene in malignant tissue, as compared with an alternative transcriptional start site in normal tissue. To test this hypothesis, we constructed fusion plasmids containing the two putative transcription initiation sites and performed transcriptional mapping on the Cyp1a1 transcripts from control and tetrachlorobenzo-p-dioxin (TCDD)-treated wt and HAV cells; Cyp1a1 mRNA sequencing was also carried out in the TCDD-treated wt and HAV cell lines. We conclude that the Cyp1a1 transcription initiation site in both of these hepatoma-derived cell lines is the same (start site TI) as that used in normal mouse liver.

Genetic contributions to the variation among rabbits of liver microsomal deoxycorticosterone synthesis

Outbred New Zealand white rabbits exhibit two phenotypes, 21H and 21L, corresponding to rates greater than or less than 1 nmol/min/mg, respectively, for liver microsomal progesterone 21-hydroxylase activity. In contrast, the inbred strain III/J exhibits only the 21L phenotype. Two 21H male New Zealand white rabbits were mated with several female III/J rabbits to produce a total 46 progeny. Both the 21H and 21L phenotypes were evident among male and female offspring in roughly equal numbers. Backcrosses between 21L progeny and III/J rabbits exhibit only the 21L phenotype, whereas 21H offspring yield both 21H and 21L progeny when backcrossed to the 21L inbred strain III/J. These results are consistent with autosomal dominant inheritance of the 21H phenotype. Analysis of Southern blots of genomic DNA digested with the restriction endonuclease KpnI reveals 20-, 13-, and 9-kb fragments that hybridize with a probe derived from the 3'-untranslated region of the 21-hydroxylase cDNA. The 13-kb band is not observed for strain III/J or 21L progeny of strain III/J crossed with 21H rabbits, but it is detected for both 21H fathers and 21H progeny indicating that the genetically determined difference of 21-hydroxylase expression is inherited cis to the gene for P450IIC5, the hepatic progesterone 21-hydroxylase. Electrophoretic analysis of P450IIC5 synthesized in vitro from mRNA isolated from 21L and 21H rabbits reveals that little or no P450IIC5 is synthesized from 21L mRNAs. A second immunoreactive, electrophoretically distinct protein is synthesized from both 21L and 21H mRNAs to a similar extent but in lesser amounts than P450IIC5. The second protein could represent either an allozymic form of the enzyme or the product of a distinct locus. Thus, it is likely that distinct structural genes for P450IIC5 contribute to the differences in P450-mediated metabolism in 21L as compared to 21H rabbits.

Transcript encoded on the opposite strand of the human steroid 21-hydroxylase/complement component C4 gene locus

The gene encoding human adrenal steroid 21-hydroxylase (P450c21) and its highly similar pseudogene are duplicated in tandem with the two genes encoding the fourth component of human serum hemolytic complement (C4). This 60-kilobase gene complex, which lies within the major histocompatibility complex on the short arm of human chromosome 6, has been studied in considerable detail because genetic disorders in steroid 21-hydroxylation and in C4 are common. We have cloned a cDNA encoded by a previously unidentified gene in this region. This gene lies on the strand of DNA opposite from the strand containing the P450c21 and C4 genes, and it overlaps the last exon of P450c21. The newly identified gene encodes mRNAs of 3.5 and 1.8 kilobases that are expressed in the adrenal and in a Leydig cell tumor but are not expressed in nonsteroidogenic tissues. The sequence of the longest cDNA (2.7 kilobases) shows no similarity to known sequences available in two computerized data bases. The 5' end of this sequence is characterized by three repeats, each encoding about 100 amino acids flanked by potential sites for proteolytic cleavage. Although numerous studies have shown that gene deletions causing congenital adrenal hyperplasia occur in this region, none of these gene deletions extends into this newly identified gene, suggesting that it encodes an essential function.

The rat clofibrate-inducible CYP4A subfamily. II. cDNA sequence of IVA3, mapping of the Cyp4a locus to mouse chromosome 4, and coordinate and tissue-specific regulation of the CYP4A genes

A novel P450 cDNA, designated IVA3, was isolated from a lambda gt11 cDNA library from clofibrate-treated rat liver by screening with the lauric acid omega-hydroxylase, IVA1, cDNA probe. This cDNA encoded a protein with 507 amino acids and a calculated Mr of 58,239. The IVA3 cDNA shared 65% and 97% nucleotide and 72% and 96% DNA-deduced amino acid sequence similarities with IVA1 and IVA2, respectively. The CYP4A gene family, designated the Cyp4a locus, was mapped to mouse chromosome 4 using a panel of mouse-hamster somatic cell hybrids. Levels of the IVA mRNAs were analyzed in rat tissues and cell cultures after treatment with the hypolipidemic drug clofibrate. The IVA1, IVA2, and IVA3 mRNAs were present at very low levels in the livers of untreated rats and markedly induced in rats treated with clofibrate. Dose-response and time-course studies revealed that all three genes were regulated coordinately in liver. In contrast to the coordinate induction in liver, only the CYP4A3 gene was induced in the rat hepatoma cell line McA-RH7777. In the kidney, IVA1 and IVA3 mRNAs were present at low levels and were induced by clofibrate in a manner similar to that in liver. In contrast, the IVA2 mRNA was expressed in the kidney of untreated rats at a level similar to that of the maximally induced IVA2 mRNA in liver. These data indicate that the CYP4A1, CYP4A2, and CYP4A3 genes are regulated coordinately in liver while only CYP4A2 is activated constitutively in kidney.

A rabbit liver constitutive form of cytochrome P450 responsible for amphetamine deamination

A cytochrome P450 isozyme responsible for amphetamine deamination was purified from hepatic microsomes of untreated rabbits. The purification procedures consisted of a set of column chromatographies with omega-aminooctyl-Sepharose 4B, DEAE-cellulose, CM-Sephadex C-50, and hydroxyapatite. The deamination activity was determined by measuring the formation of phenylacetone after derivatization to the p-nitrobenzyloxim by HPLC. This isozyme, which was designated P450APD, showed a monomeric molecular weight of 51,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and exhibited an absorption maximum of reduced CO complex at 451 nm. On the basis of the specificity toward testosterone metabolism and the N-terminal amino acid sequence, P450APD was attributed to a member of P450 class IIC subfamily, which is identical or closely related to LM3b (D. R. Koop and M. J. Coon (1979) Biochem, Biophys, Res. Commun. 91, 1075-1081), form 3b (E. F. Johnson (1980) J. Biol. Chem. 255, 304-309), and other similar preparations. Antibody against the P450APD inhibited about 80% of the amphetamine deamination activity in rabbit hepatic microsomes as well as in the reconstitution system of this P450. The present results support that P450APD is the major P450 isozyme responsible for amphetamine deamination in rabbit liver.

Mouse liver phenobarbital-inducible P450 system: purification, characterization, and differential inducibility of four cytochrome P450 isozymes from D2 mouse

Three novel cytochrome P450 isozymes were purified from phenobarbital (PB)-treated D2 mouse liver microsomes and compared to the previously characterized coumarin 7-hydroxylase, P450Coh. The molecular masses were 56.5, 55, 51, and 49.5 kDa, and the peaks of the reduced CO complexes were at 450, 447.5, 451.5, and 449 nm for P450PBI, P450PBII, P450PBIII, and P450Coh, respectively. The NH2-terminal sequences suggest that these isozymes belong to the P450 gene subfamilies 2B, 1A, 2C, and 2A, respectively. On the basis of reconstituted activities and microsomal immunoinhibition studies, P450Coh was the sole catalyst of coumarin 7-hydroxylation. P450PBI was the major isozyme catalyzing the high Km 7-pentoxyresorufin O-dealkylation. This reaction was also mediated at a slower rate by the low Km isozyme, P450PBII. P450PBIII contributed significantly to the microsomal O-deethylation of 7-ethoxyresorufin and N-demethylation of benzphetamine. Western blotting and dot immunobinding analyse of microsomes showed that the induction patterns of the isozymes were different. PB and TCPO-BOP induced all isozymes variably: P450PBI (19- and 31-fold), P450PBII (2- and 3-fold), P450PBIII (9- and 4-fold), and P450Coh (about 2-fold). Pyrazole induced only P450Coh, while all other isozymes were decreased by 30 to 60%. The changes in the microsomal amounts of these isozymes correlated generally well with the variation in the immunoinhibitable enzyme activities. On the basis of the structural and catalytic properties, immunochemical characteristics, and induction profiles, all three isozymes were different from each other and from the previously characterized P450Coh. This mouse PB-inducible P450 model may be valuable in further studies on the induction mechanisms of PB and TCPOBOP.

Expression of a full-length cDNA encoding bovine adrenal cytochrome P450C21

Two full-length cDNA clones encoding bovine adrenocortical P450C21 have been constructed in a eukaryotic expression vector using partial-length cDNAs whose structures have been previously reported. Following expression of these cDNAs in COS 1 cells, the substrate specificity of P450C21 was determined. Of the 18 steroids tested, progesterone, 17 alpha-hydroxyprogesterone, and 11 beta,17 alpha-dihydroxyprogesterone were found to be the only steroids to serve as substrates for this adrenal enzyme, a much higher degree of substrate specificity than has been reported for a hepatic 21-hydroxylase. The Vmax for 17 alpha-hydroxyprogesterone was 2.5 times greater than that for progesterone, whereas delta 5-steroids were unable to serve as substrate for this enzyme. A difference between the two cDNAs is located at amino acid 401 where one resultant enzyme contains tyrosine while the other contains histidine. This amino acid difference appears to have no effect on the kinetic properties of adrenal P450C21.

Complete cDNA sequence of a major 3-methylcholanthrene-inducible cytochrome P-450 isozyme (P-450AFB) of Syrian hamsters with high activity toward aflatoxin B1

Cytochrome P-450AFB is major isozyme inducible by 3-methylcholanthrene in Syrian golden hamsters and shows high potency toward aflatoxin B1 activation. We have isolated and sequenced cDNA clones to P-450AFB by immunoscreening a hamster liver cDNA library in lambda gt11. The longest clone contains an open reading frame of 1482 nucleotides and encodes a protein of 494 amino acids with a molecular weight of 57,420. The sequence of P-450AFB shares a 73% and 65% homology with that of mouse P-450 15 alpha (IIA3) and rat P-450a (IIA1), respectively, indicating that P-450AFB is a unique gene of the P-450IIA subfamily. The apparent concentration of a mRNA species hybridizable to the clone as well as the concentration of a protein immunoreactive to P-450AFB was increased significantly by the treatment with 3-methyl-cholanthrene, which indicates that the increase in P-450AFB protein is due mainly to an elevation of the mRNA.

Molecular cloning and characterization of the primary structure of the alkane hydroxylating cytochrome P-450 from the yeast Candida maltosa

A cDNA library was established starting from poly(A) RNA of n-alkane-grown Candida maltosa cells and cDNA clones were isolated containing the entire coding sequence for the alkane hydroxylating cytochrome P-450. The deduced protein consists of 521 amino acids, contains two putative transmembrane segments in the N-terminal region and has a characteristic heme-binding sequence in the C-terminal part. Sequence alignments with members of 11 reported cytochrome P-450 families revealed a strong homology to an alkane-inducible cytochrome P-450 from Candida tropicalis.

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

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

Regulation of mouse CYP1A1 gene expression by dioxin: requirement of two cis-acting elements during induction

The mouse cytochrome P1450 (CYP1A1) gene is responsible for the metabolism of numerous carcinogens and toxic chemicals. Induction by the environmental contaminant tetrachlorodibenzo-p-dioxin (TCDD) requires a functional aromatic hydrocarbon (Ah) receptor. We examined the 5'-flanking region of the CYP1A1 gene in mouse hepatoma Hepa-1 wild-type cells and a mutant line having a defect in chromatin binding of the TCDD-receptor complex. We identified two cis-acting elements (distal, -1071 to -901 region; proximal, -245 to -50 region) required for constitutive and TCDD-inducible CYP1A1 gene expression. Three classes of DNA-protein complexes binding to the distal element were identified: class I, found only in the presence of TCDD and a functional Ah receptor, that was heat labile and not competed against by simian virus 40 (SV40) early promoter DNA; class II, consisting of at least three constitutive complexes that were heat stable and bound to SV40 DNA; and class III, composed of at least three constitutive complexes that were thermolabile and were not competed against by SV40 DNA. Essential contacts for these proteins were centered at -993 to -990 for the class I complex, -987, -986, or both for the class II complexes, and -938 to -927 for the class III complexes. The proximal element was absolutely essential for both constitutive and TCDD-inducible CYP1A1 gene expression, and at least two constitutive complexes bound to this region. These data are consistent with the proximal element that binds proteins being necessary but not sufficient for inducible gene expression; interaction of these proteins with those at the distal element was found to be required for full CYP1A1 induction by TCDD.

Identification of a common molecular basis for combined 17 alpha-hydroxylase/17,20-lyase deficiency in two Mennonite families

During the course of studies to characterize mutations of the CYP17 gene that cause the 17 alpha-hydroxylase-deficient form of congenital adrenal hyperplasia we have discovered two ostensibly unrelated Mennonite families in which affected individuals are homozygous for the same mutation. The defect is a four-base duplication in exon 8 of the CYP17 gene, which alters the reading frame encoding the C-terminal 26 amino acids of cytochrome P450(17 alpha).

Effect of ACTH on the stability of mRNAs encoding bovine adrenocortical P-450scc, P-45011 beta, P-45017 alpha, P-450C21 and adrenodoxin

ACTH treatment of bovine adrenocortical cells in primary culture causes increased accumulation of mRNAs encoding cytochromes P-450SCC, P-450(11)beta, P-450(17) alpha, P-450C21 and adrenodoxin as well as increased transcriptional activity of their respective genes. In this study we have shown that ACTH does not greatly affect the half-life of mRNAs encoding P-450(11)beta, P-450(17)alpha, P-450C21 and adrenodoxin. However, in the case of P-450SCC mRNA, ACTH causes a five-fold increase in the half-life leading to a significant stabilization of P-450SCC mRNA. Thus it appears that the levels of mRNAs encoding P-450(11)beta, P-450(17)alpha, P-450C21 and adrenodoxin are regulated by ACTH primarily at the transcriptional level, while that for P-450SCC is regulated at both the transcriptional and post-transcriptional levels.

Disruption of the Saccharomyces cerevisiae gene for NADPH-cytochrome P450 reductase causes increased sensitivity to ketoconazole

Strains of Saccharomyces cerevisiae deleted in the NADPH-cytochrome P450 reductase gene by transplacement are 200-fold more sensitive to ketoconazole, an inhibitor of the cytochrome P450 lanosterol 14 alpha-demethylase. Resistance is restored through complementation by the plasmid-borne wild type gene from either S. cerevisiae or Candida tropicalis. Neither Southern hybridization nor Western immunoblot techniques provided evidence for a second NADPH-cytochrome P450 reductase gene, suggesting that an alternate pathway may provide for the functions of this reductase in S. cerevisiae.

Close linkage of the human cytochrome P450IIA and P450IIB gene subfamilies: implications for the assignment of substrate specificity

We have isolated from human liver libraries two cytochrome P450 cDNA clones (lambda MP14 and lambda MP3) which are highly similar (83% over the coding region) to mouse testosterone 15 alpha hydroxylase and are therefore part of the cytochrome P450IIA gene subfamily. The P450IIA (CYP2A) gene subfamily was found to be closely linked to the P450IIB (CYP2B) subfamily and their chromosomal location could not be distinguished using somatic cell hybrids containing fragments of chromosome 19 between 19q12 and 19q13.2. Pulsed field gel electrophoresis indicates that both gene subfamilies are contained within 350-kb genomic DNA fragments, but were separated using various restriction enzymes. Northern blot analysis identified three P450IIA mRNAs each showing a wide inter-individual variation in their levels in the liver. High levels of P450IIA transcript were associated with high levels of P450IIB transcript suggesting that common factors may influence the expression of genes within these subfamilies. Genetic analysis has suggested previously that a member of the P450IIB subfamily is responsible for coumarin hydroxylase activity in the mouse. We discuss the possibility, based on our findings of tight linkage of the human P450IIA and IIB subfamilies, that a member of the IIA subfamily is a better candidate for this enzyme activity.

Olfactory-specific cytochrome P-450. cDNA cloning of a novel neuroepithelial enzyme possibly involved in chemoreception

We isolated cDNA clones for cytochrome P-450 genes expressed in the olfactory neuroepithelium by screening a corresponding rat cDNA library. Sequence analysis and RNA blot hybridization revealed a new cytochrome P-450, designated cytochrome P-450olf1, which is the first reported cytochrome P-450 mRNA uniquely expressed in the chemosensory organ. Cytochrome P-450olf1 shows intermediate level of sequence similarity (38-53% identity) to several liver cytochrome P-450 enzymes, suggesting that it belongs to the cytochrome P-450II family, but defines a new subfamily (cytochrome P-450IIG) within it. Cytochrome P-450II enzymes are known to process diverse organic compounds, including odorants. This, together with the specificity of cytochrome P-450olf1 to the sensory neuroepithelium, may indicate a role for this protein in olfactory reception.

Posttranslational modification of hepatic cytochrome P-450. Phosphorylation of phenobarbital-inducible P-450 forms PB-4 (IIB1) and PB-5 (IIB2) in isolated rat hepatocytes and in vivo

Phosphorylation of hepatic cytochrome P-450 was studied in isolated hepatocytes incubated in the presence of agents known to stimulate protein kinase activity. Incubation of hepatocytes isolated from phenobarbital-induced adult male rats with [32P]orthophosphate in the presence of N6,O2'-dibutyryl-cAMP (diBtcAMP) or glucagon resulted in the phosphorylation of microsomal proteins that are immunoprecipitable by polyclonal antibodies raised to the phenobarbital-inducible P-450 form PB-4 (P-450 gene IIB1). Little or no phosphorylation of these proteins was observed in the absence of diBtcAMP or glucagon or in the presence of activators of Ca2+-dependent protein kinases. Two-dimensional gel electrophoresis revealed that these 32P-labeled microsomal proteins consist of a mixture of P-450 PB-4 and the closely related P-450 PB-5 (gene IIB2), both of which exhibited heterogeneity in the isoelectric focusing dimension. Phosphorylation of both P-450 forms was markedly enhanced by diBtcAMP at concentrations as low as 5 microM. In contrast, little or no phosphorylation of P-450 forms reactive with antibodies to P-450 PB-1 (gene IIC6), P-450 2c (gene IIC11), or P-450 PB-2a (gene IIIA1) was detected in the isolated hepatocytes under these incubation conditions. Phosphoamino acid analysis of the 32P-labeled P-450 PB-4 + PB-5 immunoprecipitate revealed that these P-450s are phosphorylated on serine in the isolated hepatocytes. Peptide mapping indicated that the site of phosphorylation in hepatocytes is indistinguishable from the site utilized by cAMP-dependent protein kinase in vitro, which was previously identified as serine-128 for the related rabbit protein P-450 LM2.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence of a chicken phenobarbital-inducible cytochrome P450 cDNA: regulation of two P450 mRNAs transcribed from different genes

The nucleic acid and derived amino acid sequences of a chicken phenobarbital-inducible cytochrome P450 cDNA clone, pCHP7, are presented. The amino acid sequence shares 92% identity with that of a previously characterized chicken cytochrome P450 cDNA clone, pCHP3. The two clones, pCHP7 and pCHP3, represent two distinct mRNAs of 2.2 and 3.5 kb and appear to be transcribed from separate cytochrome P450 genes. Sequence comparisons with P450s from mammalian species indicate that the chicken P450s are most closely related to the IIC subfamily. According to the cytochrome P450 nomenclature (Nebert et al., 1989), the chicken P450 gene encoding the 3.5-kb mRNA is termed P450IIF1 and the P450 gene encoding the 2.2-kb mRNA is designated P450IIF2. When chick embryos were treated with 2-allyl-2-isopropylacetamide, the levels of the 3.5- and 2.2-kb P450 mRNAs in liver were elevated maximally by about 100-fold, while phenobarbital treatment resulted in a maximal increase of about 50-fold. These increases in mRNA levels were accompanied by a less than sixfold increase in the corresponding gene transcription rates. The results indicate that the increase in the amount of these two mRNAs is due to both transcriptional activation of the P450 genes and to a marked post-transcriptional mechanism. By contrast, the drug-induced hepatic mRNA levels for 5-aminolevulinate synthase, the rate-controlling enzyme of the heme biosynthetic pathway, could be accounted for predominantly by activation of gene transcription. A close correlation was observed between the time courses of hepatic mRNA accumulation for cytochrome P450 and 5-aminolevulilnate synthase following drug treatment of chick embryos. In adult hens it was demonstrated that a tissue-specific drug induction of cytochrome P450 mRNAs occurred with levels being substantially elevated in the liver, kidney, and small intestine. The mRNA for 5-aminolevulinate synthase was also drug-induced in the same tissue-specific fashion. The results are compatible with the gene for 5-aminolevulinate synthase being activated in response to an increased cytochrome P450 heme requirement.

Developmental aspects of P450IIIA: prenatal activity and inducibility

We have used antiserum of defined specificity as well as a specific inducers and inhibitors of P450IIIA1(2) to determine the fetal occurrence and inducibility of this enzyme in rats. Apparently absent from uninduced fetal rat liver (or present in extremely low amounts) cytochrome P450IIIA1(2) becomes increasingly inducible as a function of gestational age. In adult rats, it is now apparent that there are at least two inducible members and one male-specific constitutive member of the IIIA subfamily. The ontogenesis of these enzymes from 2 weeks post partum to puberty has also been determined. The male-specific occurrence of P450IIIA2 subject to testosterone imprinting and maintenance has been proposed. Inconsistencies persist, however. Waxman et al. have proposed the perinatal occurrence in male and female rats with subsequent suppression in females, whereas others have not detected P450IIIA1(2) in uninduced perinatal rat liver. These differences remain unresolved and reflect the difficulties in defining the individual enzyme specificities for various substrates and of antiserum reactivity. Approaches recently applied to investigations of the IIB subfamily of cytochromes P-450 should contribute greatly to the elucidation of factors governing the ontogenesis of IIIA in rats and humans. Recently, cDNA probes capable of discriminating P450IIB1 and P450IIB2 (commonly referred to as P450s b and e, respectively) were utilized to discriminate the developmental regulation of these immune cross-reactive enzymes. cDNA probes specific for the constitutive and inducible P450IIIA enzymes should clarify the P450IIIA ontogeny in rats. However, in light of regulatory differences among the human and rat members of P450IIIA, it is apparent that the extrapolation of human biotransforming potential from results of animal models must be approached with great caution.

Mutagen activation by cDNA-expressed P(1)450, P(3)450, and P450a

cDNAs for rodent P(1)450, P(3)450, and P450a were expressed in the modified vaccinia virus-T7 RNA polymerase system. Each P450 exhibited its appropriate molecular weight and characteristic enzyme activity. Aryl hydrocarbon hydroxylase activity was catalyzed by P(1)450, acetanilide hydroxylase by P(3)450, and testosterone 7 alpha-hydroxylase by P450a. Ethoxycoumarin deethylase was exhibited by both P(1)450 and P(3)450. Each expressed P450 was also analyzed for its ability to activate 19 carcinogens of diverse classes to their mutagenic forms. Most notable was the activation of several polycyclic aromatic hydrocarbons by P1 and the activation of acetylaminofluorene, 4-aminobiphenyl, and several heterocyclic amine food pyrolysate products by P(3)450. P450a, in contrast, showed slight mutagen activation only toward N-hydroxy-2-acetyl aminofluorene. The vaccinia virus-T7 RNA polymerase system described here can express cDNAs for diverse forms of P450, each of which can then be characterized for substrate and product specificity and for mutagen activation.

Human cDNA-expressed cytochrome P450 IA2: mutagen activation and substrate specificity

The vaccinia virus cDNA expression system was used to produce human cytochrome P450 IA2 in a hepatoma cell line that is devoid of significant basal levels of P450. The expressed enzyme yielded a reduced carbon monoxide-bound difference spectrum with a lambda max of 449 nm. Catalytic activities and mutagen activation ability of the human enzyme were assessed and directly compared with results obtained with the orthologous mouse IA2, which was also expressed using vaccinia virus. Both the human and mouse enzymes were able to catalyze efficiently the p-hydroxylation of aniline. Mouse IA2 also catalyzed ethoxyresorufin O-deethylation, and its activity was sevenfold greater than expressed human IA2. The mouse and human enzymes also activated several promutagens and procarcinogens. Mouse IA2 was five- to sevenfold more active than the human enzyme for activation of the procarcinogens 2-acetylaminofluorene and benzo[a]pyrene-trans-7,8-dihydrodiol and the promutagens Glu-P-2 and Trp-P-1. Comparable activities were observed with 2-aminoanthracene, 2-aminofluorene, and Glu-P-1. These data demonstrate the utility of cDNA expression for examining the activities of human P450s and further suggest potentially important differences in catalytic activities of orthologous P450s found in different species.