Macrolide antibiotics inhibit the degradation of the glucocorticoid-responsive cytochrome P-450p in rat hepatocytes in vivo and in primary monolayer culture

Treatment of rats with macrolide antibiotics such as triacetyloleandomycin (TAO) dramatically increases the hepatic concentration of a cytochrome P-450 indistinguishable from P-450p, the major liver cytochrome induced by glucocorticoids such as dexamethasone (Wrighton, S. A., Maurel, P., Schuetz, E. G., Watkins, P. B., Young, B., and Guzelian, P. S. (1985) Biochemistry 24, 2171-2178). To investigate the mechanism of induction of P-450p, we treated rats for 4 days with these agents and found that dexamethasone and TAO induced the synthesis of P-450p at least 70- and 35-fold over control values, respectively, as estimated from measurements of P-450p mRNA translatable in a cell-free system. However, the accumulation of P-450p holocytochrome (measured as TAO-metabolite spectral complex) or P-450p protein (measured by quantitative immunoblotting) increased at least 150-fold by TAO but only 32-fold by dexamethasone. The possibility that TAO decreases the degradation of P-450p was supported by the observation that administration of TAO to dexamethasone-treated rats labeled with NaH[14C]O3 and [3H]-delta-aminolevulinic acid retarded the decay of radioactive immunoprecipitable P-450p protein (t1/2 = 60 versus 14 h) and heme (t1/2 = 73 versus 10 h). To confirm these results, P-450p protein synthesis was measured as radioactivity incorporated into immunoprecipitable P-450p in primary monolayer cultures of adult rat hepatocytes incubated with [3H]leucine. Dexamethasone treatment of the cultures stimulated P-450p synthesis by at least 30-fold whereas macrolides were without effect. However, macrolide antibiotics but not dexamethasone inhibited the disappearance of radiolabeled P-450p from cultured hepatocytes similar to the results obtained in vivo. We conclude that macrolide antibiotics induce P-450p, the most rapidly turning over cytochrome yet reported, by stimulating its synthesis indirectly and by blocking its degradation, significantly.     

cDNA cloning and functional expression in yeast Saccharomyces cerevisiae of beta-naphthoflavone-induced rabbit liver P-450 LM4 and LM6

A cDNA library was constructed from liver mRNA of a beta-naphthoflavone-induced rabbit. Two clones pLM4-1 and pLM6-1 containing 2.2-kbp inserts that hybridized at low stringincy with a mouse P1 P-450 probe were selected. The clone pLM4-1 was fully sequenced and found to contain a full-length cDNA coding for cytochrome P-450 LM4. Partial sequence and restriction mapping made it possible to identify pLM6-1 as coding for the major part of cytochrome P-450 LM6. Cloned LM4-1 cDNA was reformed by deletion of the 5' and 3' non-coding regions before insertion into yeast expression vectors PYe DP1/10. A similar operation was performed on pLM6-1 cDNA after replacement of the missing N-terminus-coding sequences by homologous sequences form the pLM4-1 clone resulting in a chimeric cytochrome P-450 coding sequence. Expression of cloned rabbit cytochrome P-450 into transformed yeast was optimized by studying the effect of the nature of the DNA sequence just preceding the initiation codon on the level of cytochrome P-450 production. Yeast synthesized cytochromes P-450 were characterized by immunoblotting, spectra and catalytic activity determinations. Cloned cytochrome P-450 LM4 was found by all criteria to be identical to the authentic rabbit one. The chimeric cytochrome P-450 that contains the 143 N-terminal amino acids of cytochrome P-450 LM4 and the remaining 375 amino acids of cytochrome P-450 LM6 was found to exhibit most of the authentic cytochrome P-450 LM6 catalytic properties. Enzymatic and evolutionary implications of these results are discussed.     

Cytochrome P-450 isozyme LM3b from rabbit liver microsomes. Induction by triacetyloleandomycin purification and characterization

Oral administration of triacetyloleandomycin (TAO), 1 mmol/kg/day for 7 days to mature male New Zealand White rabbit results in a significant increase in the content of liver microsomal cytochrome P-450. This increase is accompanied by the occurrence on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the microsomes of a strong band in the zone of electrophoretic mobility associated with the LM3 isozymes and the stimulation of a number of monooxygenase activities of these microsomes including aminopyrine, chlorcyclizine, TAO, and erythromycin demethylation as well as 2-OH-estradiol and 6 beta OH-testosterone hydroxylation. Cytochrome P-450 LM3 (TAO) from these liver microsomes, purified to electrophoretic homogeneity, had Mr = 52,000 as determined by calibrated sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Comparison with isozymes LM3a, LM3b, and LM3c isolated from control animals, by a number of criteria including spectral data, amino acid content, NH2-terminal sequence analysis, peptide mapping, immunological properties, and monooxygenase activities of reconstituted system, indicated that isozymes LM3 (TAO) and LM3b are very similar, if not identical, proteins. We conclude that TAO must be considered as a new type of inducer of microsomal cytochrome P-450 from rabbit liver.     

Differential induction and tissue-specific expression of closely related members of the phenobarbital-inducible rabbit cytochrome P-450 gene family

We have examined the tissue-specific expression of three rabbit genes that are closely related members of a subfamily of the phenobarbital-inducible cytochrome P-450 gene family. Analysis of the levels of mRNA in liver revealed that (a) cytochrome P-450PBc1 mRNA was not detectable in livers from control animals but was present in livers from animals treated with phenobarbital, (b) cytochrome P-450PBc2 was present in control tissue and was increased by about 3-fold 24 h after phenobarbital treatment, and (c) the levels of cytochrome P-450PBc3 mRNA was the same in livers from control and treated animals. In the kidney, only P-450PBc2 mRNA was detected at a level 15% of that in the liver, and the levels increased about 3-fold after phenobarbital treatment. None of the mRNAs was detected in lung tissue. Multiple species of RNA were observed that hybridized to probes for cytochrome P-450PBc1 and P-450PBc2 cDNAs by Northern blot analysis ranging in size from 2300 to 4000 nucleotides. Differential sites for polyadenylation probably cause the heterogeneity in size. A single species of RNA of 2200 nucleotides that hybridized to cytochrome P-450PBc3 cDNA probes was observed. These data demonstrate that three closely related cytochrome P-450 genes are differentially responsive to phenobarbital treatment and that they exhibit different tissue-specific patterns of expression.     

Identification of the cytochrome P-450 induced by macrolide antibiotics in rat liver as the glucocorticoid responsive cytochrome P-450p

We administered triacetyloleandomycin (TAO) to rats and found that this macrolide antibiotic is the most efficacious inducer of liver microsomal cytochrome P-450 (P-450) examined to date. Liver microsomes prepared from TAO-treated rats contained greater than 5.0 nmol of P-450/mg of protein and a single induced protein as judged by analysis on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein comigrated with P-450p, the major form of P-450 induced in liver microsomes of rats treated with pregnenolone-16 alpha-carbonitrile (PCN) or dexamethasone (DEX). On immunoblots of such gels developed with antibodies to P-450p, the TAO-induced protein reacted strongly as a single band. There was strict parallelism between the amount of immunoreactive P-450p in liver microsomes prepared from untreated rats or from rats treated with phenobarbital, TAO, DEX, or PCN, the ability of these microsomes to catalyze conversion of TAO to a metabolite which forms a spectral complex, and the ethylmorphine and erythromycin demethylase activities. Antibodies to P-450p specifically blocked microsomal TAO metabolite complex formation and ethylmorphine and erythromycin demethylase activities. Moreover, anti-P-450p antibodies completely immunoprecipitated solubilized TAO metabolite complexes prepared by detergent treatment of liver microsomes obtained from TAO-treated rats. Finally, we found that the major form of P-450 isolated from liver microsomes of TAO-treated rats and purified to homogeneity was indistinguishable from purified P-450p as judged by molecular weights, spectral characteristics, enzymatic activities, ability to bind TAO, peptide maps, and amino-terminal amino acid sequences. We concluded that, in addition to glucocorticoids, macrolide antibiotics are specific inducers of P-450p.     

Characterization of ethanol-inducible human liver N-nitrosodimethylamine demethylase

Through the use of monospecific antibodies directed against hepatic cytochrome P-450j, an enzyme induced in rats treated with ethanol or isoniazid, we have purified from human liver the related cytochrome P-450 termed HLj. HLj resembles rat P-450j and P-450 LM3a, the homologous cytochrome in rabbit liver, in its NH2-terminal amino acid sequence, in being in highest concentration in liver microsome samples prepared from two patients intoxicated by ethanol and one patient given isoniazid, and in catalyzing the metabolic activation of the procarcinogen N-nitrosodimethylamine. Furthermore, each of nine human liver RNA samples contained a species of mRNA hybridizable to a cloned HLj cDNA. We conclude that HLj is related by structure, function, and some regulatory characteristics to rat P-450j and rabbit P-450 LM3a, cytochromes critical for metabolism of several clinically relevant cytotoxic and carcinogenic agents.     

In vivo and in vitro effects of a new macrolide antibiotic roxithromycin on rat liver cytochrome P-450: comparison with troleandomycin and erythromycin

The effects of a new macrolide antibiotic (Roxithromycin) and one of its major metabolite (RU 39001) on rat hepatic drug metabolizing enzymes were compared to those of erythromycin, erythralosamine and troleandomycin (TAO) both in vitro and in vivo. In contrast to erythromycin, erythralosamine and TAO, roxithromycin and its metabolite RU 39001 exhibit: (i) a very poor affinity for rat liver cytochrome P-450, (ii) an unability to be metabolized into a stable inhibitory metabolite-cytochrome P-450 complex and (iii) a decreased ability to induce liver cytochrome P-450 PCNE, an isozyme implicated in drug associations involving some macrolide antibiotics.     

Specificity in the activation and inhibition by flavonoids of benzo[a]pyrene hydroxylation by cytochrome P-450 isozymes from rabbit liver microsomes

The effect of flavone and 7,8-benzoflavone on the metabolism of benzo[a]pyrene to fluorescent phenols by five cytochrome P-450 isozymes obtained from rabbit liver microsomes was determined. Benzo[a]pyrene metabolism was stimulated more than 5-fold by the addition of 600 microM flavone to a reconstituted monooxygenase system consisting of NADPH, cytochrome P-450 reductase, dilauroylphosphatidylcholine, and cytochrome P-450LM3c or cytochrome P-450LM4. In contrast, an inhibitory effect of flavone on benzo[a]pyrene metabolism was observed when cytochrome P-450LM2, cytochrome P-450LM3b, or cytochrome P-450LM6 was used in the reconstituted system. 7,8-Benzoflavone (50-100 microM) stimulated benzo[a]pyrene metabolism by the reconstituted monooxygenase system about 10-fold when cytochrome P-450LM3c was used, but benzo[a]pyrene hydroxylation was strongly inhibited when 7,8-benzoflavone was added to the cytochrome P-450LM6-dependent system. Smaller effects of 7,8-benzoflavone were observed on the metabolism of benzo[a]pyrene by the cytochrome P-450LM2-, cytochrome P-450LM3b-, and cytochrome P-450LM4-dependent monooxygenase systems. These results demonstrate that the activating and inhibiting effects of flavone and 7,8-benzoflavone on benzo[a]pyrene metabolism depend on the type of cytochrome P-450 used in the reconstituted monooxygenase system.     

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.     

A form of rabbit liver cytochrome P-450 that catalyzes the 7 alpha-hydroxylation of cholesterol

A form of cytochrome P-450 which comigrates with cytochrome P-450LM4 (molecular weight, 55 000) on SDS-polyacrylamide gel was purified from liver microsomes of cholestyramine-treated rabbits. This form of cytochrome P-450 catalyzed the 7 alpha-hydroxylation of cholesterol with an activity of 37.5 pmol/min per nmol cytochrome P-450 in the reconstituted enzyme system containing cytochrome P-450 and NADPH-cytochrome P-450 reductase. The substrate specificity of this form of cytochrome P-450 was compared with cytochrome P-450LM4 isolated from phenobarbital- and beta-naphthoflavone-treated rabbit liver microsomes. The latter two isoenzymes do not catalyze 7 alpha-hydroxylation of cholesterol, but are more active in O-deethylation of 7-ethoxycoumarin and p-nitrophenetole. Ouchterlony double diffusion revealed cross-reactivity between anti-P-450LM4 (phenobarbital) IgG and cytochrome P-450 isolated from cholestyramine- or beta-naphthoflavone-treated rabbit liver microsomes. A two-dimensional iodinated tryptic peptide fingerprint indicated only minor structural differences among these three cytochrome P-450LM4 preparations.     

cDNA cloning of cytochrome P-450 related to P-450p-2 from the cDNA library of human placenta. Gene structure and expression

We have isolated and analyzed cDNA (designated P-450HP cDNA) clones from a human placenta cDNA library, using the cDNA for rabbit pulmonary cytochrome P-450p-2, a prostaglandin omega-hydroxylase, as a hybridization probe. The cDNA obtained encoded a polypeptide comprising 511 amino acids with a calculated molecular mass of 58987 Da, and the amino acid sequence similarity with P-450p-2 and rat liver laurate omega-hydroxylase (P-450LA omega) was only about 50%. RNA blot analysis showed that the mRNA hybridizable with the human P-450HP cDNA was inducibly expressed 3-5-fold in rabbit small intestine and lung by gestation, but the expression remained constant in rabbit liver and kidney. This mode of expression was quite different from that of P-450p-2 and P-450LA omega. Interestingly, the mRNA hybridized with the cDNA of P-450HP was found to be expressed in all the human tumor tissues so far examined, in sharp contrast with the facts that almost all the other species of P-450s are known to disappear in the tumor tissues. Taken together, the deduced hemoprotein termed P-450HP dose not seem to be the human counterpart of rabbit P-450p-2 or rat P-450LA omega, and is presumably a new member of the P-450 family including P-450p-2 and P-450LA omega. Furthermore, the corresponding genomic DNA was also cloned and analyzed. The gene of P-450HP spanned 18.8 kb and was separated into 11 exons by 10 introns whose locations were completely different from those of P-450 genes so far determined.     

Inactivation of sodium dithionite reduced cytochromes P-450 of different origins

The inactivation of five dithionite reduced soluble cytochrome P-450 isoforms has been studied. The inactivation of microsomal rabbit liver isoform LM2 and bacterial linalool cytochrome P-450 is followed by its conversion into cytochrome P-420. Microsomal rabbit liver isoform LM4, bacterial camphor and p-cymene cytochromes P-450 were not inactivated under these conditions. The inactivation of linalool cytochrome P-450 and LM2 isoform is a first order reaction; the rate constants for linalool cytochrome P-450 and LM2 are 0.3 and 0.1 min-1, respectively. In the case of linalool cytochrome P-450 its carboxycomplex (Fe2+-CO) is inactivated, while in the case of LM2 the inactivation affects its oxycomplex (Fe2+-O2). The amino acid residues of linalool cytochrome P-450 are probably modified due to a direct electron transfer in its carboxycomplex. The amino acid residues of LM2 isoform are modified, presumably due to oxidation by oxygen active species which are released during the oxycomplex decay.     

Regulation of testosterone hydroxylation by rat liver microsomal cytochrome P-450

The pathways of testosterone oxidation catalyzed by purified and membrane-bound forms of rat liver microsomal cytochrome P-450 were examined with an HPLC system capable of resolving 14 potential hydroxylated metabolites of testosterone and androstenedione. Seven pathways of testosterone oxidation, namely the 2 alpha-, 2 beta-, 6 beta-, 15 beta-, 16 alpha-, and 18-hydroxylation of testosterone and 17-oxidation to androstenedione, were sexually differentiated in mature rats (male/female = 7-200 fold) but not in immature rats. Developmental changes in two cytochrome P-450 isozymes largely accounted for this sexual differentiation. The selective expression of cytochrome P-450h in mature male rats largely accounted for the male-specific, postpubertal increase in the rate of testosterone 2 alpha-, 16 alpha, and 17-oxidation, whereas the selective repression of cytochrome P-450p in female rats accounted for the female-specific, postpubertal decline in testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activity. A variety of cytochrome P-450p inducers, when administered to mature female rats, markedly increased (up to 130-fold) the rate of testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylation. These four pathways of testosterone hydroxylation were catalyzed by partially purified cytochrome P-450p, and were selectively stimulated when liver microsomes from troleandomycin- or erythromycin estolate-induced rats were treated with potassium ferricyanide, which dissociates the complex between cytochrome P-450p and these macrolide antibiotics. Just as the testosterone 2 beta-, 6 beta-, 15 beta-, and 18-hydroxylase activity reflected the levels of cytochrome P-450p in rat liver microsomes, so testosterone 7 alpha-hydroxylase activity reflected the levels of cytochrome P-450a; 16 beta-hydroxylase activity the levels of cytochrome P-450b; and 2 alpha-hydroxylase activity the levels of cytochrome P-450h. It is concluded that the regio- and stereoselective hydroxylation of testosterone provides a functional basis to study simultaneously the regulation of several distinct isozymes of rat liver microsomal cytochrome P-450.     

Microheterogeneity in the major phenobarbital-inducible forms of rabbit liver microsomal cytochrome P-450 as revealed by nucleotide sequencing of cloned cDNAs

We have isolated one full-length cDNA clone, termed pHP1, and a number of clones of shorter insert lengths, tentatively called b14, b46, etc., all encoding phenobarbital- (PB-) inducible forms of rabbit liver microsomal cytochrome P-450, and determined their nucleotide sequences. The polypeptides encoded by these cDNAs can be classified into five types, represented by HP1, b14, b46, b52, and b54, the deduced amino acid sequences of which are more than 95% similar to one another. Amino acid differences among them total 24 positions, which are distributed over the entire sequence, in contrast to the microheterogeneity observed in two PB-inducible rat liver microsomal cytochromes P-450 (P-450b and P-450e). The primary structure deduced for the HP1 protein is 97% similar to that determined for rabbit P-450 LM2 (form 2), which has been purified by Coon and co-workers [van der Hoeven, T. A., Haugen, D. A., & Coon, M. J. (1974) Biochem. Biophys. Res. Commun. 60, 569-675; Haugen, D. A., & Coon, M. J. (1976) J. Biol. Chem. 251, 7929-7939] as the major PB-inducible form of rabbit liver microsomal cytochrome P-450. The amino acid sequence of P-450(1), which we have purified as the major PB-inducible rabbit liver cytochrome P-450, was partially determined with the sequence reported for P-450 LM2 as a reference. The two sequences are closely similar to each other, but at least two amino acid differences can be detected between them.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of complementary DNA clones coding for two forms of 3-methylcholanthrene-inducible rat liver cytochrome P-450

Double-stranded DNA complementary to the partially purified mRNA prepared from 3-methylcholanthrene (MC)-treated rat liver was constructed and cloned in Escherichia coli. Twenty clones were verified to carry a complementary DNA (cDNA) insert coding for MC-inducible cytochrome P-450 by positive hybridization translation assay and immunochemical assay with anti-cytochrome P-450 antibody. The identified cDNA clones were divided into at least two groups on the basis of comparison of restriction maps of the cDNA inserts. A clone pAU157 whose cDNA insert was approximately 2.7 kb in length contained nearly full-length mRNA information for cytochrome P-450MC or P-450c, which is the major form of MC-inducible cytochrome P-450. Other cDNA clones pTZ286-pTZ330 contained the 1.2 kb sequence complementary to cytochrome P-450d mRNA. RNA blot analysis revealed that pAU157 and pTZ286-pTZ330 cDNA clones were derived from 22S and 18S mRNAs, respectively, both of which were induced in rat liver by MC treatment. Sequence analysis revealed that there were closely homologous sequence regions in pAU157 and pTZ286-pTZ330 cDNA inserts and most of the homologous sequences were localized in two limited coding regions of the two cytochrome P-450 species. pAU157 encoded the total amino acid sequence of cytochrome P-450MC or P-450c and pTZ286-pTZ330 coded for the C-terminal 368 amino acid residues of cytochrome P-450d. Two highly homologous regions were found in the amino acid sequences of these cytochrome P-450 species.     

Expression of human liver cytochrome P450 IIIA4 in yeast. A functional model for the hepatic enzyme

Cytochrome P-450 (P450) NF, a member of the P450 IIIA subfamily, is the major contributor to the oxidation of the calcium-channel blocker nifedipine in human liver microsomes. A cDNA clone designated NF25 encoding for human P450 NF was isolated from a bacteriophage lambda gt11 expression library [Beaune, P. H., Umbenhauer, D. R., Bork, R. W., Lloyd, R. S. & Guengerich, F. P. (1986) Proc. Natl Acad. Sci. USA 83, 8064-8068]. We have expressed NF25 cDNA in Saccharomyces cerevisiae using an expression vector constructed from pYeDP1/8-2 [Cullin, C. & Pompon, D. (1988) Gene 65, 203-217]. Yeast transformed with the plasmid containing the NF25 sequence (pVNF25) showed a ferrous-CO spectrum typical of cytochrome P-450. Microsomal preparations contained a protein with an apparent molecular mass identical to that of P450-5 (a form isolated from human liver indistinguishable from P450 NF) that was not present in microsomes from control yeast (transformed with pYeDP1/8-2 alone), as revealed by immunoblotting with anti-P450-5 antibodies. On the other hand, antibodies raised in rabbits against human liver P450 IIC8-10 and rat liver P450 IA1 and P450 IIE1 did not recognize yeast-expressed P450 NF25. The P450 NF25 content in microsomes was about 90 pmol/mg protein. Microsomal, yeast-expressed P450 NF25 exhibited a high affinity for different substrates including macrolide antibiotics, dihydroergotamine and miconazole as shown by difference visible spectroscopy. Microsomal suspensions containing P450 NF25 were also able to catalyze several oxidation reactions that were expected from the activities of the protein isolated from human liver, including nifedipine 1,4-oxidation, quinidine 3-hydroxylation and N-oxygenation, and N-demethylation of the macrolide antibiotics erythromycin and troleandomycin. The yeast endogenous NADPH-cytochrome P-450 reductase thus couples efficiently with the heterologous P450 NF25 though its level is far lower than that of its ortholog in human liver. Indeed addition of rabbit liver NADPH-cytochrome P-450 reductase increased the oxidation rates. Rabbit liver cytochrome b5 also caused a marked enhancement of catalytic activities, as had been noted previously for this particular P450 enzyme in a reconstituted system involving the protein purified from human liver. Furthermore, the level of the yeast endogenous cytochrome P-450 (lanosterol 14-demethylase) has been found to be negligible compared to the heterologously expressed cytochrome P-450 (30 times less). Thus, yeast microsomes containing P450 NF25 constitute by themselves a good functional model for studying the binding capacities and catalytic activities of this individual form of human hepatic cytochrome P-450.     

Isolation and sequence analysis of three cloned cDNAs for rabbit liver proteins that are related to rabbit cytochrome P-450 (form 2), the major phenobarbital-inducible form

We have isolated from rabbit liver three cDNA clones of 1400-1800 base pairs that hybridize selectively to RNA from animals treated with phenobarbital. The nucleotide sequences of the cDNAs have been determined. In the protein coding region the nucleotide sequences of two of the cDNAs are 88% homologous, and the third cDNA is about 72-74% homologous to the other two. All three are 55-60% homologous to rat liver cytochrome P-450b cDNA. The amino acid sequences derived from the cDNA sequences are about 50% homologous to those of rat liver cytochrome P-450b and rabbit liver cytochrome P-450 (form 2). The degree of homology differs substantially in different regions of the protein. The hydrophobicity profiles of these five mammalian cytochromes P-450 are very similar and contain up to eight regions of hydrophobicity that are long enough to span a membrane. These results indicate that these three cDNAs code for rabbit liver cytochromes P-450 which are different from any rabbit liver cytochrome P-450 for which amino acid sequence information is published. These cDNAs are part of a family of genes that are related to rabbit liver cytochrome P-450 (form 2) and rat liver cytochrome P-450b which are the major phenobarbital-inducible forms. The divergence of amino acid sequence between the rat and rabbit forms and the divergence of nucleotide sequences of silent sites in the two most closely related rabbit forms suggest that cytochromes P-450 have a relatively high rate of amino acid divergence compared to many other vertebrate proteins.     

Immunochemical studies on the metabolism of nitrosamines by ethanol-inducible cytochrome P-450

The ethanol-induced rabbit liver microsomal cytochrome P-450, P-450LM3a, has been shown previously to efficiently catalyze the demethylation of N-nitrosodimethylamine (NDMA) with a Km of 2.9 mM. Since the predominant Km in hepatic microsomes from ethanol-treated rabbits is 0.07 mM, the role of P-450LM3a in the activation of this carcinogen has been uncertain. In the present study, antibodies to P-450LM3a were shown to almost completely inhibit NDMA demethylation by the purified P-450 in a reconstituted system as well as the low-Km activity of liver microsomes from control or ethanol-treated rabbits. In contrast, the antibody did not inhibit the high-Km NDMA demethylase activity in the microsomes. These results indicate that P-450LM3a is the major P-450 responsible for the low-Km NDMA demethylase activity. In addition, evidence is provided for the existence of a cytochrome immunochemically similar to P-450LM3a in liver microsomes from rats, mice, and guinea pigs that effectively catalyzes the demethylation of NDMA.