Purification and NH2-terminal amino acid sequences of human and rat kidney fatty acid omega-hydroxylases

Biochemical and molecular properties of the cytochrome P450 arachidonic acid monooxygenases

The cytochrome P450 (P450) arachidonic acid (AA) monooxygenase metabolizes the fatty acid to a series of epoxy- and hydroxy-acid derivatives. Catalytic turnover requires NADPH, and requires the redox-coupled activation and cleavage of diatomic oxygen, and the delivery of an active form of atomic oxygen to ground state carbon atoms. Past and present advances in P450 biochemistry and molecular biology are beginning to provide a description of the P450 isoform specificity of AA bioactivation, and the mechanisms of action and physiological relevance of the P450 metabolites. The demonstration of the endogenous biosynthesis of many of these metabolites has established the P450 pathway as an important route for AA bioactivation, and has begun to uncovered new and important functional roles for this enzyme system in cell and organ physiology.

In vitro metabolism of chlorpromazine by cytochromes P450 4F4 and 4F5 and the inhibitory effect of imipramine

The metabolism of chlorpromazine by expressed recombinant cytochromes P450 4F4 and 4F5 cloned from rat brain was analyzed to characterize the individual activities of the isoforms. Both isoforms metabolized chlorpromazine to both the N-demethylated and the S-oxide products. When isoforms were incubated with chlorpromazine in the presence of increasing concentrations of imipramine, imipramine significantly inhibited both N-demethylation and S-oxidation of chlorpromazine. A dilution of the serum fraction of anti-4F antibody was also found to significantly inhibit both S-oxidation and N-demethylation of chlorpromazine by both 4F4 and 4F5.

Human fatty acid omega-hydroxylase, CYP4A11: determination of complete genomic sequence and characterization of purified recombinant protein

The gene of the human fatty acid omega-hydroxylase, CYP4A11, has been isolated from a human BAC library, and its complete genomic sequence has been determined. The CYP4A11 gene spanned 12,568 bp and contained 12 exons. The known PPAR recognition elements (PPRE), which were reported to be involved in the induction of CYP4A6 by clofibric acid, were not observed within the 5'-flanking region of the CYP4A11 gene. The recombinant CYP4A11 protein expressed in Escherichia coli using the pCWOri expression vector was purified to an almost electrophoretically homogeneous state with a specific content of 6.4 nmol of P450/mg of protein. This P450 exhibited omega-hydroxylation activity toward laurate, with a turnover number of 14.7 nmol/min/nmol of P450. The apparent K(m) and V(max) values were 56.7 microM and 15.2 nmol/min/nmol of P450, respectively. It also showed omega-hydroxylation activity toward palmitate, with a turnover number of 0.78 nmol/min/nmol of P450. Although several reports from other groups described that CYP4A11 preparations catalyzed omega-hydroxylation of arachidonic acid, our purified recombinant protein exhibited no activity toward arachidonic acid nor prostaglandin A(1).

Hydrogen peroxide-induced endothelium-dependent relaxation of rat aorta involvement of Ca2+ and other cellular metabolites

In phenylephrine-precontracted rings, H2O2 produced an endothelium-dependent relaxation at concentrations of 4.4 x 10(-7) to approximately 4.4 x 10(-5) M. Removal of extracellular Ca2+ ([Ca2+]0) markedly attenuated the relaxant effects of H2O2. Complete inhibition of the H2O2 relaxant action was obtained after buffering intracellular Ca2+ ([Ca2+]i) in endothelial cells, with 10 microM acetyl methyl ester of bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). These relaxant effects of H2O2 were nearly abolished by 15 x 10(-5)M N(G)-monomethyl-arginine (L-NMMA) or 5 x 10(-5) M N(G)-nitro-L-arginine (L-NAME) and were attenuated markedly by the presence of either 10(-6) M Fe2+, 10(-6) M Fe3+, or 5 x 10(-6) M methylene blue. These inhibitory effects of L-NMMA or L-NAME could be reversed partly by 5 x 10(-5) M L-arginine. These Fe(2+)- and Fe(3+)-induced inhibitions of H2O2-stimulated relaxation were reduced significantly by either 1.0 mM deferoxamine (a Fe2+ chelator) or 100 microM dimethyl sulfoxide (DMSO). In addition, 17-octadecynoic acid (2.5 microM) or proadifen (10 microM) (both antagonists of cytochrome P450 metabolism of fatty acids) markedly decreased the H2O2 relaxant effects. Proadifen (10 microM) produced concentration-dependent impairment of vasorelaxation to acetylcholine. A variety of amine antagonists and a cyclo-oxygenase inhibitor all fail to interfere with or attenuate the H2O2-induced relaxations. Our observations suggest that, at suitable pathophysiologic concentrations, H2O2 could induce release of an endothelium-derived relaxing factor, probably nitric oxide, from endothelial cells. The H2O2 relaxant effects are clearly Ca(2+)-dependent and require formation of cyclic guanosine monophosphate (cGMP). These vasorelaxing effects of H2O2 appear to be induced by H2O2 itself. Hydrogen peroxide may stimulate production of some unknown metabolites metabolized by cytochrome P450-dependent enzymes.

Molecular modelling of CYP4A subfamily members based on sequence homology with CYP102

1. Homology modelling of various members of the CYP4A subfamily based on the CYP102 template structure is reported. 2. The binding interactions of specific substrates to the CYP4A forms from rat (CYP4A1), human (CYP4A11) and rabbit (CYP4A4) are shown to be consistent with experimental evidence regarding regioselectivity of metabolism. 3. The differences in substrate specificity between CYP4A1, CYP4A11 and CYP4A4 towards fatty acids and prostaglandins respectively are rationalized in terms of variations in active site amino residues.

The effect of immunosuppressive agents (FK-506, rapamycin) on renal P450 systems in rat models

It is well known that cyclosporin, rapamycin and FK-506 (tacrolimus) are metabolized by the liver microsomal cytochrome P450 enzyme system. Although there have been reports of interaction between these drugs and the renal P450 enzyme system, differences among these immunosuppressants has not been comprehensively demonstrated. We have studied the individual capacities of these immunosuppressants to induce renal microsomal P450 enzymes similar to CYP2B4 and CYP4A2 by examining renal function in treated rats, and have correlated the results by means of biochemical, immunological and immunohistochemical assays of renal P450 enzymes. Cyclosporin caused impairment of renal function with an increase in renal-specific P450 content, but FK-506 and rapamycin did not. Laurate omega- and (omega-1)-hydroxylase activity increased in rats treated with rapamycin but decreased in those treated with FK-506. Prostaglandin A1 (PGA1) omega-hydroxylase activity increased in rats treated with FK-506 but was reduced by treatment with cyclosporin. Aminopyrine N-demethylase activity increased in rats treated with cyclosporin or FK-506, but not in those treated with rapamycin. Western-blot analysis revealed significant induction of P450, (similar to CYP2B4 of the rabbit P450 isozyme) in kidneys from rats treated with cyclosporin but not in those from rats receiving FK-506 or rapamycin. Histochemical studies clearly demonstrated a form of P450 such as CYP4A2 in the proximal tubules of rats treated with cyclosporin, but not in those of rats treated with FK-506 or rapamycin. These results show that although cyclosporin has a strong effect on renal P450 systems and induces such a system in kidney cortex (microsomal P450), FK-506 and rapamycin have no substantial effect on the induction of renal P450. These findings might clarify the nephrotoxicity induced by these immunosuppressive drugs.

Human prostate CYP3A5: identification of a unique 5'-untranslated sequence and characterization of purified recombinant protein

We have isolated a cDNA clone coding for CYP3A5 from a human prostate cDNA library. The human prostate CYP3A5 cDNA had a unique 5'-untranslated sequence, suggesting that the prostate specific regulation of CYP3A5 is different from liver. Hybridization screening using a human genomic BAC library yielded four positive clones, two of which were shown to contain the unique 5'-untranslated sequence by Southern blot analysis. The CYP3A5 recombinant protein expressed in Escherichia coli using the pCWOri expression vector was purified to an almost electrophoretically homogeneous state with a specific content of 4.4 nmol of P450/mg of protein. This P450 exhibited 6beta-hydroxylation activity toward both testosterone and progesterone. No polar metabolite of 5alpha-dihydrotestosterone (DHT) was detected. The apparent K(m) values for testosterone and progesterone 6beta-hydroxylation were 143 and 114 microM, respectively, with V(max) values of 0.48 and 0. 21 nmol/min/nmol of P450, respectively. This is the first report that a particular form of P450, CYP3A5, has been isolated from human prostate and that the purified recombinant protein of CYP3A5 has been shown to be active in the metabolism of sex hormones.

Role of human CYP4F2 in hepatic catabolism of the proinflammatory agent leukotriene B4

Leukotriene B4 (LTB4), an arachidonic acid derivative, is a potent proinflammatory agent whose actions are terminated by catabolism via a microsomal omega-hydroxylation pathway. Although the liver serves as the principal site for LTB4 clearance from the systemic circulation, the attributes of hepatic LTB4 metabolism are ill defined in humans. Thus, we examined metabolism of LTB4 to its omega-hydroxylated metabolite 20-hydroxyleukotriene B4 (20-OH LTB4) by human liver microsomes and also purified the hepatic P450 enzyme underlying this reaction. Liver microsomes from 10 different subjects converted LTB4 to 20-OH LTB4 at similar rates (1.06 +/- 0.3 nmol/min/nmol P450; 0.25 +/- 0.1 nmol/min/mg protein). Analysis of the microsomal LTB4 20-hydroxylation reaction revealed kinetic parameters (apparent Km of 74.8 microM with a VMAX of 2.42 nmol/min/nmol P450) consistent with catalysis by a single P450 enzyme. Conventional chromatography combined with immunochemical screening with rat CYP4A1 antibodies was then used to isolate a P450 enzyme from human liver microsomes with a molecular weight of 57,000 and an NH2-terminal amino acid sequence 94% homologous (12Trp --> 12Gly) over the first 17 residues with the human CYP4F2 cDNA-derived sequence. Upon reconstitution with P450 reductase and phospholipid, CYP4F2 converted LTB4 to 20-OH LTB4 at a turnover rate of 392 pmol/min/nmol P450, whereas the other human liver P450s tested, including CYP4A11, exhibited neglible LTB4 omega-hydroxylase activity. Polyclonal antibodies to CYP4F2 were found to markedly inhibit (91.9 +/- 5%; n = 5) LTB4 20-hydroxylation by human liver microsomes. Microsomal 20-OH LTB4 formation was also inhibited 30% by arachidonic acid, a known CYP4F2 substrate, and 50% by prostaglandin A1 but was unaffected by lauric acid, palmitic acid, and PGF2alpha. Finally, a strong correlation (r = 0.86; P < 0.002; n = 10) was observed between CYP4F2 content and LTB4 20-hydroxylase activity in the human liver samples. Our results indicate that CYP4F2 is the principle LTB4 omega-hydroxylating enzyme expressed in human liver and, as such, may play an important role in regulating circulating as well as hepatic levels of this powerful proinflammatory eicosanoid.

Protein expression, characterization, and regulation of CYP4F4 and CYP4F5 cloned from rat brain

We previously reported the cDNA cloning of three new forms of P450, CYP4F4, CYP4F5, and CYP4F6, from a rat brain cDNA library. In the present study, we expressed CYP4F4 and CYP4F5 in Escherichia coli using the pCWOri expression vector with a modification of their N-terminal amino acid sequences and the incorporation of a C-terminal [His]4 tag to aid in purification. CYP4F5 recombinant protein was purified to a specific content of 7.7 nmol/mg protein from the membrane fraction of E. coli and showed omega-hydroxylation activity toward leukotriene B4 (LTB4), a chemical mediator of inflammation. On the other hand, the solubilized membrane fraction of CYP4F4-expressed recombinant protein catalyzed the omega-hydroxylation of prostaglandin A1, prostaglandin E1, and 6-trans-LTB4 as well as LTB4. The effects of the peroxisome proliferator, clofibrate, on mRNA expression of CYP4F4, 4F5, and 4F6 were studied by Northern blot analysis. The expression levels of the mRNA of these CYP4Fs were shown to be reduced by clofibrate in liver.

Cytochrome P450 4A and 2E1 expression in human kidney microsomes

Laurate and arachidonate omega and (omega-1)-hydroxylase activities, cytochrome P450 2E1 (CYP2E1), and CYP4A content were measured in 18 human kidney microsomal samples. The rates of laurate and arachidonate were found to be very different from those measured in human liver samples, with a laurate omega/omega-1 ratio of approximately 22 in human kidney vs 0.75 in human liver. Immunoblot analysis of the 18 human kidney microsomal samples identified 1 CYP4A electrophoretic band, but CYP2E1 was not detectable in human kidney, contrary to liver. Laurate and arachidonate omega-hydroxylase activities were significantly correlated with CYP4A content (r = 0.86 and 0.75, respectively). Polyclonal antirat CYP2E1 antibody did not affect omega-hydroxylase activity, whereas the polyclonal antirat CYP4A1 antibody inhibited it by 60%. These results suggest that, in contrast to other species, human kidney microsomes do not contain significant amounts of CYP2E1, but possess CYP4A and fatty acid omega-hydroxylase activity.

The cytochrome P450 4 (CYP4) family

1. The CYP4 family consists of 11 subfamilies (CYP4A-CYP4M), which encode constitutive and inducible isozymes expressed in both mammals and insects. 2. The CYP4A subfamily encodes several cytochrome P450 enzymes that are capable of hydroxylating the terminal omega-carbon and, to a lesser extent, the (omega-1) position of saturated and unsaturated fatty acids, as well as enzymes active in the omega-hydroxylation of various prostaglandins. 3. The CYP4A1, A2 and A3 genes, the most extensively studied members of the CYP4 family, are expressed constitutively in rat liver and kidney and their expression is induced by a class of chemicals known as peroxisome proliferators, which includes the hypolipidemic drug, clofibrate. 4. Induction of CYP4A expression by clofibrate is due to transcriptional activation, mediated possibly via a peroxisome proliferator activated receptor (PPAR). 5. CYP4A gene expression is hormonally regulated. 6. The CYP4A1-3 genes are expressed constitutively and following induction in pregnant and lactating rats. 7. Translactational and transplacental induction of the CYP4A1-3mRNAs and proteins has been demonstrated. 8. There is a close association between microsomal CYP4A1 induction, peroxisome proliferation and induction of the peroxisomal fatty acid metabolizing system. 9. The CYP4A subfamily may be involved in the metabolism of arachidonic acid leading to the formation of physiologically important metabolites involved in such processes as blood flow in the kidney, cornea and brain.

Induction of cytochrome P450 isoenzymes in cultured precision-cut rat and human liver slices

1. The effect of some xenobiotics on levels of selected cytochrome P450 (CYP) isoenzymes determined by Western immunoblotting and associated enzyme activities has been studied in 72-h cultured rat and human precision-cut liver slices. 2. In cultured rat liver slices, 0.5 mM sodium phenobarbitone (PB), 25 microM beta-naphthoflavone (BNF), and 20 micrograms/ml Aroclor 1254 (ARO) induced mixed-function oxidase enzyme activities. Western immunoblotting of liver slice microsomes was performed with antibodies to rat CYP1A2, 2B1/2 and 3A. Compared with 72-h control (dimethyl sulphoxide only treated) rat liver slice microsomes, PB induced CYP2B1/2 and 3A, BNF induced CYP1A2, and ARO induced CYP1A2, 2B1/2, and 3A. 3. The peroxisome proliferators methylclofenapate (MCP), ciprofibrate (CIP) and Wy-14,643 (WY) induced palmitoyl-CoA oxidation in 72-h cultured rat liver slices. Compared with 72-h control rat liver slice microsomes, MCP, CIP, and WY all induced levels of CYP4A. 4. In cultured human liver slices, 20 micrograms/ml ARO, but not 0.5 mM MCP, induced 7-ethoxyresorufin O-deethylase activity. Neither ARO nor MCP had any effect on homogenate palmitoyl-CoA oxidation and microsomal lauric acid 11- and 12-hydroxylase activities. Compared with 72-h control human liver slice microsomes, ARO induced CYP1A2, and MCP appeared to induce CYP4A. Further studies would be required to confirm that CYP4A isoenzymes could be induced by xenobiotics in human liver slices. 5. These results demonstrate that cultured liver slices may be used in evaluating the effect of xenobiotics on both rat and human CYP isoenzymes.

Complete cDNA sequence and cDNA-directed expression of CYP4A11, a fatty acid omega-hydroxylase expressed in human kidney

A cDNA was isolated from a human kidney lambda gt10 library using the rat CYP4A3 cDNA as a probe. The cDNA-deduced amino acid sequence encoded a protein of 519 amino acids that was designated CYP4A11 (Nelson et al., 1993) and exhibited 76%, 72%, 80%, and 53% similarities to rat CYP4A1, rat CYP4A3, rabbit CYP4A6, and human CYP4B1, respectively. The deduced amino-terminal amino acid sequence of this cDNA agreed with the amino-terminal amino acid sequence of a major P450 protein purified from human renal microsomes. A second variant form of CYP4A11 cDNA, designated CYP4A11v, was isolated from the same library and had a deletion of a single adenine residue, thereby extending the reading frame and resulting in a protein of 591 amino acids. CYP4A11v is probably encoded by a rare allelic variant of CYP4A11, since no mutant alleles were uncovered in 15 normal individuals, as determined by a polymerase chain reaction (PCR) diagnostic test. Baculovirus-mediated cDNA expression of CYP4A11 yielded a P450 protein having a lambda max of 452 nm when reduced and complexed with carbon monoxide. The expressed enzyme efficiently catalyzed omega-hydroxylation of lauric acid. No detectable activity was uncovered toward arachidonic acid and prostaglandin E1. The cDNA-expressed variant, CYP4A11v, was found to be unstable and not to efficiently metabolize lauric acid, as assessed by both baculovirus and monkey kidney COS cell cDNA expression systems. These studies indicate that CYP4A11 is a major fatty acid-metabolizing P450 that is expressed in human kidney.

Characterization of a cDNA encoding a human kidney, cytochrome P-450 4A fatty acid omega-hydroxylase and the cognate enzyme expressed in Escherichia coli

A cDNA encoding a cytochrome P-450 4A (CYP4AII) was cloned from a human kidney cDNA library. Northern blot analysis and RNase protection assays indicate that related mRNAs occur in kidney and liver with the highest abundance found in kidney. The enzyme was expressed from its cDNA in Escherichia coli. A solubilized preparation of the enzyme reconstituted with cytochrome P-450 reductase catalyzed the omega-hydroxylation of lauric acid, palmitic acid, and arachidonic acid with turnover numbers of 9.8, 2.2 and 0.55 min-1, respectively. Little or no activity was detected toward prostaglandins A1 and E1.

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature

We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1-11, 1987; Nebert et al., DNA 8, 1-13, 1989; Nebert et al., DNA Cell Biol. 10, 1-14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen should precede the final number in mouse genes. "P" ("p" in mouse) after the gene number denotes a pseudogene. If a gene is the sole member of a family, the subfamily letter and gene number need not be included. We suggest that the human nomenclature system be used for all species other than mouse. The mRNA and enzyme in all species (including mouse) should include all capital letters, without italics or hyphens. This nomenclature system is identical to that proposed in our 1991 update. Also included in this update is a listing of available data base accession numbers for P450 DNA and protein sequences. We also discuss the likelihood that this ancient gene superfamily has existed for more than 3.5 billion years, and that the rate of P450 gene evolution appears to be quite nonlinear. Finally, we describe P450 genes that have been detected by expressed sequence tags (ESTs), as well as the relationship between the P450 and the nitric oxide synthase gene superfamilies, as a likely example of convergent evolution.