Endogenous substrates of monooxygenases: fatty acids and prostaglandins

Characterization of the CYP4A11 gene, a second CYP4A gene in humans

Comparison between the cDNA sequence of CYP4A11 and that deduced from a published genomic clone suggested the presence of an additional CYP4A gene in humans, CYP4A22. PCR amplification of genomic DNA yielded overlapping clones covering 13kb of genomic DNA and extending from 1003bp upstream from CYP4A11 translation initiation to 135bp upstream of the mRNA polyadenylation signal. Sequence and Southern blot analysis showed the presence in humans of two highly homologous CYP4A genes, CYP4A11 and CYP4A22. These two genes share 96% sequence identity and have similar intron/exon sizes and distribution. Short nucleotide insertions (< or =10bp) in introns 1, 3, 9, and 11, and deletions (< or =18bp) in introns 4, 6, and 11 differentiate the two genes. RT-PCR amplification of human kidney RNA followed by restriction fragment analysis showed that CYP4A11 is the predominant isoform expressed in kidney.

Structural determination of the substrate specificities and regioselectivities of the rat and human fatty acid omega-hydroxylases

The substrate and regiospecificities of the known CYP4A enzymes from rat (CYP4A1, -4A2, -4A3, and -4A8) and human (CYP4A11) have been determined using lauric (C12), myristic (C14), palmitic (C16), oleic (C18:1), and arachidonic (C20:4) acids. The CYP4A2 and CYP4A8 cDNAs required to complete the enzyme set were cloned from a rat kidney library. All five proteins were expressed in Escherichia coli and were purified with the help of a six-histidine tag at the carboxyl terminus. Two complementary CYP4A2-CYP4A3 chimeras fused at residue 119 (CYP4A2) and 122 (CYP4A3) were constructed to explore the roles of the 18 amino acid differences between the parent proteins in determining their catalytic profiles. The chimera in which the first 119 amino acids are from CYP4A2 indicates that the first 120 amino acids control the substrate specificity. The chimera in which the first 122 amino acids are from CYP4A3 is inactive due to a defect in electron transfer to the heme group. The highest activity for lauric acid was obtained with CYP4A1 and CYP4A8, but for all the proteins the activity decreased with increasing fatty acid chain length. The fact that none of the rat and human CYP4A enzymes exhibits a high activity with arachidonic acid appears to limit their role as catalysts for the physiologically important conversion of arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-HETE).

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.

Metabolism of several 14C-nonylphenol isomers by rainbow trout (Oncorhynchus mykiss): in vivo and in vitro microsomal metabolites

1. Gas chromatographic/mass spectroscopic analysis of a mixture of 14C-nonylphenols produced by alkylation of 14C-RUL-phenol with n-1-nonene indicated that the radio-synthesis produced three major isomers, 2-(4-hydroxyphenyl)-nonane, 3-(4-hydroxyphenyl)-nonane and 4-(4-hydroxyphenyl)-nonane. 2. Bile from rainbow trout exposed to a mixture of these isomers of 14C-nonylphenol was found to contain the glucuronic acid conjugates of three radiolabelled metabolites, which were more polar than their parent compounds. 3. Incubation of trout hepatic microsomes with NADPH and the 14C-nonylphenol isomers resulted in the production of three radiolabelled metabolites whose mobility on silica thin layer chromatography were similar to the deglucuronidated metabolites recovered from trout bile. 4. Metabolism of the 14C-nonylphenol isomers by trout hepatic microsomes was inhibited by omission of NADPH from the incubations as well as by addition of a P450 inhibitor, piperonyl butoxide to the incubations. 5. Analysis of the metabolites extracted from the microsomal incubations by gas chromatography/mass spectroscopy indicated that the parent isomers had been hydroxylated in the C-8 position on the nonane chain to give 2-(4-hydroxyphenyl)-8-hydroxynonane, 3-(4-hydroxyphenyl)-8-hydroxynonane and 4-(4-hydroxyphenyl)-8-hydroxynonane.

Immunostimulating lipopeptide, LtriP (RP 56142): comparison of the effect on hepatic cytochrome P 450 modulation and radioprotection in male and female of three mouse strains

The sex-dependent effect of lauroyl-L-Ala-D-gamma-Glu-L,L-A2pmNH2 (LtriP, RP 56142) on hepatic microsomal cytochromes P 450 (cyt P 450) was studied in three mouse strains NMRI, C3H/OuJ and C3H/HeJ. In NMRI and C3H/OuJ, strains which are responsive to bacterial lipopolysaccharides (LPS-responsive), regardless of the sex of the mouse, significant decrease in the amount of cyt P 450 was observed after LtriP treatment, with a concomitant reduction in ethoxyresorufin-O-deethylase (cyt P 450 1A-dependent) and 7-ethoxycoumarin-O-deethylase activities. This was not seen in C3H/HeJ (LPS-hyporesponsive) mice. These effects may be related to LtriP-dependent cytokine induction, since neither LtriP nor LPS stimulated interleukin-1 (IL-1) secretion by C3H/HeJ macrophages. 11- and 12-hydroxylations (11- and 12-OH) of lauric acid were compared in C3H/OuJ and C3H/HeJ mice. LtriP depressed the total enzymatic conversion of lauric acid in the two strains without modification of the 11/12-OH ratio for C3H/OuJ or male C3H/HeJ mice. However, in females C3H/HeJ mice this decrease was particularly significant and concerned especially the 12-OH activity (a marker of cyt P450 4A family). Although males of the three strains were more sensitive to irradiation than females, LtriP exerted a sex-independent radioprotection on NMRI and C3H/OuJ mice. Its radioprotective effect was illustrated by the preservation of all the enzymatic activities studied in treated NMRI mice, contrary to irradiated control animals. In contrast, for the C3H/HeJ strain, males were not protected by LtriP treatment and, furthermore, females showed a marked sensitization to irradiation. The effects in CH3/HeJ strain implicate LtriP in the control of cyt P 450 induction and of sensitivity to irradiation independently of IL-1 induction.

Fatty acid discrimination and omega-hydroxylation by cytochrome P450 4A1 and a cytochrome P4504A1/NADPH-P450 reductase fusion protein

The omega-hydroxylation of fatty acids by certain cytochrome P450 enzymes shows a degree of chain-length and regionspecificity which is remarkable in view of the conformational flexibility of these substrates, the strong similarity in properties among homologs, and the lack of polar groups (other than the carboxy terminus) with which to guide and strength enzyme-substrate interactions. To investigate the chemical basis for these features of omega-hydroxylation we designed and synthesized a series of lauric acid analogs and evaluated them as substrates and inhibitors of omega-hydroxylation catalyzed by cytochrome P4504A1 and a cytochrome P450 4A1/NADPH-P450 reductase fusion protein. Among n-alkanoic acids, lauric acid was found to have the optimum chain length for the fusion protein, as it does for native cytochrome P450 4A1. With both enzymes, chain shortening caused a precipitous drop in turnover while chain lengthening caused a gradual drop in turnover. The fusion protein omega-hydroxylated methyl laurate and lauryl alcohol about 1/10th as efficiently as lauric acid, but it did not hydroxylate lauramide. 10-Methoxydecanoic acid underwent O-demethylation (via omega-hydroxylation). The branched substrate 11-methyllauric acid was hydroxylated efficiently and selectively at the omega-position. In contrast, the cyclopropyl analog 11,12-methanolauric acid was not detectably hydroxylated, although it induced Type I binding spectrum and inhibited lauric acid omega-hydroxylation by 43% at equimolar concentrations. omega-(Imidazolyl)-decanoic acid induced a Type II heme-binding spectrum and was an especially potent inhibitor of lauric acid hydroxylation. Collectively these data suggest that the active site of cytochrome P450 4A1 has an elongated tubular shape of definite length (ca. 14 A) with a recognition site for polar groups (including but not limited to carboxyl) at its entrance and the (oxo)heme group at its terminus.

Altered hepatic eicosanoid concentrations in rats treated with the peroxisome proliferators ciprofibrate and perfluorodecanoic acid

Several hypolipidemic drugs, plasticizers, and other chemicals induce hepatic peroxisome proliferation and hepatocellular carcinomas in rodents. These agents induce and promote hepatocarcinogenesis by unknown mechanisms, since most studies have not found them to be genotoxic. Peroxisome proliferators increase the expression of several genes, including those for the enzymes of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids and their precursor fatty acids. The peroxisome proliferators ciprofibrate and perfluorodecanoic acid (PFDA) were therefore examined for their ability to alter hepatic eicosanoid concentrations. Rats received injections of 3 or 10 mg PFDA/kg body weight every 14 days or were fed 0.01% ciprofibrate for 10 days, 24 days, 6 weeks, 26 weeks, or 54 weeks. The activity of the peroxisomal enzyme fatty acyl CoA oxidase was significantly increased by both ciprofibrate and PFDA at all times. Hepatic concentrations of prostaglandins E2 and F2a (PGE2, PGF2a), thromboxane B2 (TXB2), and leukotriene C4 (LTC4) were measured by immunoassay. Concentrations of PGE2, PGF2a, and TXB2 were decreased in livers of rats receiving ciprofibrate or PFDA compared to livers of control rats, with ciprofibrate exerting a greater effect than PFDA at the doses used. Hepatic LTC4 concentrations were significantly increased by ciprofibrate at 10 days and PFDA at 54 weeks, and significantly decreased by PFDA at 26 weeks. These alterations in eicosanoid concentrations may be important in the natural history of peroxisome proliferator-induced hepatocarcinogenesis.

omega-Oxidation of lipoxin B4 by rat liver. Identification of an omega-carboxy metabolite of lipoxin B4

Lipoxin B4 (LXB4) is metabolized to 20-hydroxy-LXB4 by rat liver microsomes. The omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, indicating involvement of a cytochrome P-450 (P-450). This is supported by inhibition of the reaction by antibodies raised against NADPH-P-450 reductase. The P-450 appears to be the one responsible for leukotriene B4 omega-hydroxylation, because leukotriene B4 inhibits the formation of 20-hydroxy-LXB4 and LXB4 blocks the leukotriene B4 omega-hydroxylase activity in microsomes. Incubation of 20-hydroxy-LXB4 with both rat liver cytosol and NAD+ leads to formation of a more polar metabolite on high-performance liquid chromatography. The metabolite is identified as 20-carboxy-LXB4, a novel metabolite of LXB4, based on analyses by ultraviolet spectrometry and by gas chromatography/mass spectrometry. The 20-carboxy-LXB4-forming activity is localized in cytosol, with an optimal pH of 8.5. The activity is dependent on NAD+, but NADP+ can not replace NAD+. The reaction is inhibited by pyrazole and 4-methylpyrazole, inhibitors of alcohol dehydrogenase, and by substrates of the enzyme such as ethanol and 20-hydroxy-leukotriene B4. Disulfiram, an inhibitor of aldehyde dehydrogenase, also blocks the 20-carboxy-LXB4 formation. These observations suggest that both alcohol dehydrogenase and aldehyde dehydrogenase participate in the oxidation of 20-hydroxy-LXB4 to 20-carboxy-LXB4.

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

Cytochrome P450-mediated prostaglandin omega/omega-1 hydroxylase activities in porcine ciliary body epithelial cells

The ocular hypotensive effect of topically applied prostaglandins (PGs) is well documented. Although PGs introduced in the posterior chamber accumulate in the anterior tissues (e.g. iris/ciliary complex), little is known about the metabolism of PGs by these tissues. We have recently found that non-pigmented epithelial (NPE) cells and pigmented epithelial (PE) cells are readily separated from porcine ciliary body and cytochrome P450-dependent xenobiotic metabolism is considerably higher in NPE cells than in PE cells. We have therefore investigated in this study the cytochrome P450-mediated PG omega/omega-1 hydroxylase activities of porcine ciliary epithelial cells. The NPE cells show about three times higher activities than do PE cells; the NPE cells, in fact, demonstrate the highest PG omega/omega-1 hydroxylase activities among different ocular tissues. Both omega and omega-1 hydroxylases show broad substrate specificities and hydroxylate PGA1, A2, E1, E2, and lauric acid. The omega/omega-1 hydroxylase activities of NPE and PE, as determined with PGA2 and lauric acid as substrates, are enhanced or induced by treatment of primary cultures of the individual epithelial cells with clofibrate, both activities reaching maximum levels within 48 hr of induction. The induced activities are inhibited almost completely by cycloheximide and actinomycin D. The omega/omega-1 hydroxylase activities of both NPE and PE cells require NADPH and molecular oxygen, are associated with the microsomal fraction, respond to inducers such as clofibrate, and are inhibited by metyrapone and SKF525 A (inhibitors of P450 enzymes). These results support the suggestion that PG omega/omega-1 hydroxylations by NPE and PE are cytochrome P450-mediated reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential tissue-specific expression and induction of cytochrome P450IVA1 and acyl-CoA oxidase

We have examined the tissue-specific expression and inducibility of acyl-CoA oxidase and cytochrome P450IVA1 (P450IVA1) RNA in rats. Groups of three rats were dosed daily by gavage with methylclofenapate at 25 mg/kg in 5 ml/kg corn oil for nine weeks, or were administered a vehicle control. P450IVA1 and acyl-CoA oxidase RNA were detected using an RNase protection assay. Similar levels of acyl-CoA oxidase RNA were present in control liver and kidney, but the level of this RNA in lung, muscle and testis was 6-11%, and in pancreas was 0.13%, of that in liver. Treatment of rats with methylclofenapate led to an 11-fold induction of acyl-CoA oxidase RNA in liver and also produced a significant induction of this RNA in kidney, lung, muscle and testis of 1.7-fold, 1.3-fold, 2-fold and 1.7-fold, respectively. Acyl-CoA oxidase RNA was not induced in pancreas. P450IVA1 RNA was present in control liver and also in kidney of control rats at 28% of the level in liver. In contrast to acyl-CoA oxidase RNA, P450IVA1 RNA was not detected in lung, pancreas or testis. Methylclofenapate treatment of rats led to an 18-fold induction of P450IVA1 RNA in liver, and a sevenfold induction in kidney. Induction of P450IVA1 was not detected in any of the other tissues examined. Quantification of the relative amounts of acyl-CoA oxidase and P450IVA1 RNA in control liver revealed that acyl-CoA oxidase RNA was present in a 17.5-fold molar excess over P450IVA1 RNA. Western blotting with an anti-P450IVA IgG revealed two bands of similar apparent molecular mass in liver and kidney microsomes, but not in microsomes from the testis of control rats. Methylclofenapate treatment of rats caused an increase in the intensity of these bands in microsomes from liver, but no induction was obvious in kidney. Immunocytochemical staining for both the microsomal P450IVA and peroxisomal acyl-CoA oxidase proteins was restricted to the proximal convoluted tubule in the kidney cortex, with staining being most intense in the S3 region.

Fatty acid monooxygenation by P450BM-3: product identification and proposed mechanisms for the sequential hydroxylation reactions

The soluble P450 isolated from Bacillus megaterium (the product of the CYP 102 gene) (P450BM-3) is a catalytically self-sufficient fatty acid hydroxylase which converts lauric, myristic, and palmitic acids to omega-1, omega-2, and omega-3 hydroxy analogs. The percentage distribution of the regioisomers depends on the substrate chain length. Lauric and myristic acids were preferentially metabolized to their omega-1 hydroxy counterparts while no hydroxylation occurred when capric acid was used as the substrate. Palmitic acid, when present at concentrations greater than the concentration of oxygen in the reaction medium (greater than 250 microM), was hydroxylated to its omega-1, omega-2, and omega-3 hydroxy analogs, with the percentage distribution of the regioisomers being 21:44:35, respectively. No omega hydroxylation of any of the fatty acids was detected. When the concentration of palmitic acid was less than the concentration of oxygen in the reaction mixture, it was noted that a number of additional products were formed. Under these conditions, unlike lauric and myristic acids, it was observed that palmitic acid was first converted to its monohydroxy isomers which were subsequently metabolized to a mixture of 14-ketohexadecanoic, 15-ketohexadecanoic, 13-hydroxy-14-ketohexadecanoic, 14-hydroxy-15-ketohexadecanoic, and 13,14-dihydroxyhexadecanoic acids with a relative distribution of 8:2:40:30:20, respectively. Thus, P450BM-3 is able not only to monohydroxylate a variety of fatty acids but also to further metabolize some of these primary metabolites to secondary and tertiary products. The present paper characterizes the products formed during the sequential hydroxylation of palmitic acid and proposes reaction pathways to explain these results.

Induction of omega-oxidation of monocarboxylic acids in rats by acetylsalicylic acid

The accumulation of dicarboxylic acids, particularly long chain, is a prominent feature of Reye's syndrome and diseases of peroxisomal metabolism. We assessed the omega-oxidation of a spectrum of fatty acids in rats and asked whether pretreatment of rats with aspirin, which is known to predispose children to Reye's syndrome, would affect omega-oxidation of long chain fatty acids. We found that aspirin increased liver free fatty acids and increased the capacity for omega-oxidation three- to sevenfold. Omega-oxidation of long chain substrate was stimulated to a greater degree than medium chain substrate and was apparent within one day of treatment, at serum aspirin concentrations below the therapeutic range in humans. The apparent Km for lauric acid was 0.9 microM and 12 microM for palmitate. We also found a difference in the storage stability of activity toward medium and long chain substrate. Saturating concentrations of palmitate had no effect on the formation of dodecanedioic acid, whereas laurate decreased but never eliminated the omega-oxidation of palmitate. 97% of the total laurate omega-oxidative activity recovered was found in the microsomes, but 32% of palmitate omega-oxidative activity was present in the cytosol. These results demonstrate that aspirin is a potent stimulator of omega-oxidation and suggest that there may be multiple enzymes for omega-oxidation with overlapping substrate specificity.

Assay of fatty acid omega-hydroxylation using high-performance liquid chromatography with fluorescence labeling reagent, 3-bromomethyl-7-methoxy-1,4-benzoxazin-2-one (BrMB)

A sensitive nonradioisotopic method is reported for measuring microsomal lauric acid omega-hydroxylation activity. The assay is based upon separation and detection of 12-hydroxylauric acid formed by means of high-performance liquid chromatography following fluorescence labeling of the carboxyl group with 3-bromomethyl-7-methoxy-1,4-benzoxazin-2-one (BrMB). The use of 10-hydroxycapric acid as an internal standard affords the accurate and reproducible assay. The differential effect of dehydroepiandrosterone, a peroxisome proliferator, on the omega-hydroxylation activity in the liver and kidney of rats is also reported.

Cloning and expression of three rabbit kidney cDNAs encoding lauric acid omega-hydroxylases

cDNAs encoding three cytochrome P-450 enzymes were cloned from a rabbit kidney cDNA library. These three cDNAs exhibit greater than 90% nucleotide sequence identity across the coding region. This degree of sequence identity is also seen with P450IVA4, an enzyme that catalyzes the omega-hydroxylation of prostaglandins and that is elevated during pregnancy and induced by progesterone in rabbit lung. The 3' untranslated regions of the three cDNAs display very little sequence identity, suggesting that they are the products of distinct genes. The predicted amino acid sequences derived from each cDNA and for P450IVA4 exhibit about 85% identity. Each cDNA was inserted into an expression vector for transient transfection of COS-1 cells. The transfected cells each expressed a protein recognized by antibodies to P450IVA4. Microsomes isolated from the cells transfected with each cDNA efficiently catalyzed the omega-hydroxylation of lauric acid with rates that greatly exceed that catalyzed by microsomes isolated from the host cell line. One of the cDNAs encodes an enzyme that omega-hydroxylates prostaglandin A1; however, the specific activity was 2 orders of magnitude lower than that for lauric acid. Our results indicate that the substrate selectivity of the kidney P-450s encoded by these cDNAs is distinct from that of the lung P450IVA4 and that multiple enzymes comprise P-450 class IVA in the rabbit.

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.

Role of hepatic and renal cytochrome P-450 IVA1 in the metabolism of lipid substrates

The role of clofibrate-inducible cytochrome P-450 IVA1 in the metabolism of endogenous lipids in both rat liver and kidney microsomal fractions has been investigated. 20(omega)-hydroxyarachidonic acid has been identified as a major metabolite after incubation with both tissue fractions and the structure confirmed by mass spectrometry. The arachidonic acid 20-hydroxylase activity is inducible by clofibrate in both liver and kidney, indicating that cytochrome P-450 IVA1 is probably the enzyme responsible for this activity. In addition, the kidney exhibited higher rates of arachidonate 20-hydroxylase activity than the liver (in both control and induced states). Although leukotriene B4 was also hydroxylated in the 20-position in both liver and kidney, clofibrate induction resulted in a decrease (approximately 50%) in hydroxylase activity. In addition, the absolute level of leukotriene B4 20-hydroxylase activity in both tissue homogenates and by purified cytochrome P-450 IVA1 in a reconstituted system, was 2-3 orders of magnitude lower than the corresponding activity for lauric acid and arachidonic acid as substrates, indicating that the leukotriene was not the preferred substrate for this enzyme. Computer modelling of the conformational geometries of the above three potential cytochrome P-450 IVA1 substrates have shown that both lauric and arachidonic acids adopt a compact, 'hairpin' structure that are almost superimposed on each other, thereby rationalizing why they are relatively good substrates for this isoenzyme. By contrast, leukotriene B4 adopts a more bulky geometry than the two fatty acids, thereby providing a coherent structural reason why it is a poorer substrate for the cytochrome P-450 IVA1 isoenzyme.

Effects of 2-arylbenzimidazoles on rat hepatic microsomal monooxygenase system

1. The effects of eight newly synthesized 2-aryl substituted benzimidazole derivatives on control and phenobarbital (PB) treated rat liver microsomal aniline 4-hydroxylase and ethylmorphine N-demethylase activities, and their binding to control and PB-treated rat liver microsomal oxidized cytochrome P-450 are presented. 2. All compounds inhibited ethylmorphine N-demethylase activity with I50 values ranging from 8.50 x 10(-4) M to 27.83 x 10(-4) M in control and ranging from 2.80 x 10(-4) M to 15.79 x 10(-4) M in PB-treated rats. 3. Aniline 4-hydroxylase activity was inhibited by all of the compounds tested having I50 values in the range of 7.04 x 10(-4) M-31.37 x 10(-4) M in PB-treated rats, but only five of the compounds showed inhibitory activity in control rats. 4. Only a few significant regression coefficients could be found between the parameters of the chemicals studied and their inhibitory patterns. 5. No correlation has been observed between the binding of the derivatives and their inhibitory pattern.

Determination of microsomal lauric acid hydroxylase activity by HPLC with flow-through radiochemical quantitation

An assay for the microsomal hydroxylation of lauric acid (LA), based on HPLC with flow-through radiochemical detection, has been developed. Conditions were optimized for resolution and quantitation of three microsomal metabolites of LA, one of which has not been reported previously as a metabolite of LA in mammalian microsomal incubations. These products, 12-(omega)-hydroxy-LA, 11-(omega-1)-hydroxy-LA, and a novel metabolite, 10-(omega-2)-hydroxy-LA, were isolated by HPLC and identified by gas chromatography/mass spectrometry. In the presence of NADPH, the formation of all three metabolites was linear with time and microsomal protein concentration. Hydrogen peroxide also supported the microsomal metabolism of LA, although the ratio of metabolites was substantially different than that produced by NADPH-supported microsomes. Several biochemical probes (metyrapone, alpha-naphthoflavone, 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride, and 10-undecynoic acid) were used to dissociate the three LA hydroxylase activities. These experiments suggest that the site-specific hydroxylation [omega-, (omega-1)-, (omega-2)-] of LA may be catalyzed by different isozymes of cytochrome P-450.

Characterization of human neutrophil leukotriene B4 omega-hydroxylase as a system involving a unique cytochrome P-450 and NADPH-cytochrome P-450 reductase

Leukotriene B4 (LTB4), a potent chemotactic agent, was catabolized to 20-hydroxyleukotriene B4 (20-OH-LTB4) by the 150,000 x g pellet (microsomal fraction) of human neutrophil sonicate. The reaction required molecular oxygen and NADPH, and was significantly inhibited by carbon monoxide, suggesting that a cytochrome P-450 is involved. The neutrophil microsomal fraction showed a carbon monoxide difference spectrum with a peak at 450 nm in the presence of NADPH or dithionite, indicating the presence of a cytochrome P-450. The addition of LTB4 to the microsomal fraction gave a type-I spectral change with a peak at around 390 nm and a trough at 422 nm, indicating a direct interaction of LTB4 with the cytochrome P-450. The dissociation constant of LTB4, determined from the difference spectra, is 0.40 microM, in agreement with the kinetically determined apparent Km value for LTB4 (0.30 microM). Such a spectral change was not observed with prostaglandins A1, E1 and F2 alpha or lauric acid, none of which inhibited the LTB4 omega-hydroxylation. The inhibition of the LTB4 omega-hydroxylation by carbon monoxide was effectively reversed by irradiation with monochromatic light of 450 nm wavelength. The photochemical action spectrum of the light reversal of the inhibition corresponded remarkably well with the carbon monoxide difference spectrum. These observations provide direct evidence that the oxygen-activating component of the LTB4 omega-hydroxylase system is a cytochrome P-450. Ferricytochrome c inhibited the hydroxylation of LTB4 and the inhibition was fortified by cytochrome oxidase. An antibody raised against rat liver NADPH-cytochrome-P-450 reductase inhibited both LTB4 omega-hydroxylase activity and the NADPH-cytochrome-c reductase activity of human neutrophil microsomal fraction. These observations indicate that NADPH-cytochrome-P-450 reductase acts as an electron carrier in LTB4 omega-hydroxylase. On the other hand, an antibody raised against rat liver microsomal cytochrome b5 inhibited the NADH-cytochrome-c reductase activity but not the LTB4 omega-hydroxylase activity of human neutrophil microsomal fraction, suggesting that cytochrome b5 does not participate in the LTB4-hydroxylating system. These characteristics indicate that the isoenzyme of cytochrome P-450 in human neutrophils, LTB4 omega-hydroxylase, is different from the ones reported to be involved in omega-hydroxylation reactions of prostaglandins and fatty acids.

Prostaglandin E2 20-hydroxylase in ampulla of vas deferens of sheep

Microsomes were prepared from the ampullary mucosa of ram ductus deferens. The microsomal fraction was supplemented with 1 mM NADPH and was found to metabolize PGE2 to 20-hydroxy-PGE2. The product was identified by gas chromatography-mass spectrometry (GC-MS). The biosynthesis of 20-hydroxy-PGE2 was assessed by reversed-phase high performance liquid chromatography (HPLC) using 19-hydroxy-PGE1 as an internal standard and 0.2 mM PGE2 was found to be metabolized to 20-hydroxy-PGE2 at a rate of 7.1 pmol min-1 mg-1 of protein. Like a prostaglandin 20-hydroxylase of sheep vesicular glands, the 20-hydroxylase of the ampulla was only partly inhibited by metyrapone. 20-Hydroxy E prostaglandins, which occur in ram semen, are likely to be formed by the vesicular glands and the ampulla of vas deferens in the sheep.

Hepatic metabolism of cyclodiene insecticides by constitutive forms of cytochrome P-450 from lower vertebrates

1. Multiple forms of cytochrome P-450 were separated from the hepatic microsomes of untreated male rats, pigeons (Columbia livia), razorbills (Alca torda), puffins (Fratercula arctica), and rainbow trout (Salmo gairdnerii), using anion exchange chromatography and DEAE-cellulose. 2. In some cases cytochrome P-450 forms were further purified on hydroxylapatite and carboxymethyl-sephadex columns. 3. Considerable differences in the distribution of forms between these five species were evident from elution profiles on DEAE cellulose, and on analysis of the cytochrome P-450 containing pools by SDS-PAGE. 4. The metabolism of two organochlorine compounds, aldrin and the dieldrin analogue HCE, were studied in (a) intact microsomes and (b) reconstituted systems containing cytochrome P-450, from each of the five species. 5. In spite of their close structural similarity, significant differences were found between the two substrates in the distribution of catalytic activity between the cytochrome P-450 isozymes of each species.

Studies on the mechanism of omega-hydroxylation of platelet 12-hydroxyeicosatetraenoic acid (12-HETE) by unstimulated neutrophils

Stimulated platelets, in the presence or absence of aspirin, synthesize significant quantities of 12-hydroxyeicosatetraenoic acid (12-HETE), which is chemotactic and chemokinetic, and enhances mononuclear cell procoagulant activity. During a cell-cell interaction between stimulated platelets and unstimulated neutrophils, platelet 12-HETE is metabolized to 12,20-dihydroxyeicosatetraenoic acid (12,20-DiHETE) by neutrophils. Characteristics of the enzyme system in unstimulated neutrophils responsible for this omega-hydroxylation were investigated. A broad range of cytochrome P-450 inhibitors, as well as leukotriene B4, blocked formation of 12,20-DiHETE. Owing largely to released proteases, neutrophil homogenization abolished activity. Pretreatment with diisopropylfluorophosphate preserved activity in neutrophil homogenates. omega-Hydroxylation of 12-HETE was confined solely to the microsomal fraction. Specific activity increased 6.6-fold compared with neutrophil sonicates. The electron donor NADPH was a required cofactor. These results indicate that the enzyme in unstimulated human neutrophils, which metabolizes 12-HETE from stimulated platelets to 12,20-DiHETE in this cell-cell interaction, is a cytochrome P-450 monooxygenase.

Characterization of prostaglandin E2 20-hydroxylase of sheep vesicular glands

The microsomal fraction of homogenates of the sheep vesicular glands, supplemented with 1 mM NADPH, metabolized 0.2 mM prostaglandin E2 to 20-hydroxyprostaglandin E2 at a rate of 76 +/- 9 pmol/min per mg of protein (with a Km of about 0.1 mM and a Vmax of about 0.1 nmol/min per mg of protein). Prostaglandin E1 was metabolized at a rate of only 8.5% of that of prostaglandin E2. The metabolism of prostaglandin E2 was decreased by 66% using 1 mM NADH instead of NADPH. alpha-Naphthoflavone (50 microM) and carbon monoxide inhibited the 20-hydroxylase by more than 60%, while 1 mM beta-diethylaminoethyl-2,2-diphenyl-pentanoate and 1 mM metyrapone inhibited it by less than 50%. The enzyme catalyzed the incorporation of atmospheric oxygen into the substrate. The findings suggest that the 20-hydroxylase could be a cytochrome P-450. The 20-hydroxylase could not be detected in vesicular glands of five rams 3 weeks after castration. The function of the enzyme is presumably to create the high level of 20-hydroxyprostaglandin E compounds in ram semen.

Anthralin (1,8-dihydroxyanthrone) is a potent inhibitor of leukotriene production and LTB4-omega oxidation by human neutrophils

The effect of anthralin and its oxidation products danthrone and anthralin-dimer on the production of 5-lipoxygenase products (5-HETE, leukotriene B4, omega-oxidized LTB4) by Ca-ionophore A 23187-stimulated human neutrophils has been studied in vitro. Anthralin exhibited dose-dependent inhibitory activity showing 50% inhibition at 7 microM with 10(7) neutrophils. Inhibitory effects strongly depended upon cell densities and maximal inhibition occurred at low cell concentrations, whereas inhibitory rates of anthralin were low at high cell densities. Inhibition of leukotriene production persisted after washing of anthralin-treated neutrophils. Also, with increasing amounts of arachidonic acid as substrate only slight changes of inhibitory activity were detected, indicating a noncompetitive way of action. In addition to the inhibition of leukotriene-production, the formation of omega-OH-LTB4 from LTB4 as well as omega-COOH-LTB4 from omega-OH-LTB4 was inhibited with IC50 (half maximum inhibition concentration) near 4.4 microM and 2.2 microM, respectively. In contrast to anthralin, both metabolites--danthrone as well as anthralin-dimer--did not show any effect on leukotriene production and omega-oxidation even at high concentrations (up to 70 microM and 44 microM, respectively).

Mechanism-based in vivo inactivation of lauric acid hydroxylases

The hepatic cytochrome P-450 isozymes that catalyze omega- and (omega - 1)-hydroxylation of lauric acid are specifically inactivated in vitro but not in vivo by 10-undecynoic acid. The lack of in vivo activity may result from rapid degradation of the inhibitor by beta-oxidation. Strategies for the construction of fatty acid analogues that retain the ability to inactivate fatty acid hydroxylases but are resistant to metabolic degradation have therefore been sought. Fatty acid analogues in which the carboxylic acid group is replaced by a sulfate moiety, or in which two methyl groups are placed vicinal to the carboxylic acid group, have been found to inactivate lauric acid hydroxylases in vitro and in vivo without causing time-dependent inhibition of ethoxycoumarin O-deethylation or N-methyl-p-chloroaniline N-demethylation.

Prostaglandin release by the chick embryo heart is increased by 2,3,7,8-tetrachlorodibenzo-p-dioxin and by other cytochrome P-448 inducers

Exposure of chick embryos in ovo to cytochrome P-448 inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3,4,3',4'-tetrachlorobiphenyl, 3,4,5,3',4',5'-hexachlorobiphenyl and beta-napthoflavone, increased cardiac prostaglandins in vitro. The dose response relationships were biphasic with prostaglandin release increasing at the low doses and returning to basal levels at higher doses. Phenobarbital was ineffective. Increased cardiac prostaglandin release was detected at doses that induced hepatic 7-ethoxyresorufin deethylase (7-ER) but which were below the threshold for cardiac induction. The fall in prostaglandin release coincided with induction of cardiac 7-ER and therefore may be attributable to increased prostaglandin metabolism. These studies show that the P-450 system may interact with the arachidonic acid metabolizing system to increase PG release and that this effect may be part of the pleiotypic response to Ah-receptor activation.

Metabolism of prostaglandin E analogs in guinea pig and rat liver microsomes

Tritium-labelled 16,16-dimethyl-PGE2, 9-methylene-PGE2 (9-deoxo-16,16-dimethyl-9-methylene-prostaglandin E2) and tetranor-9-methylene-PGE2 were incubated with guinea pig liver microsomes. All three compounds were converted to omega-oxidized products in yields of a few per cent. In addition, from incubations with 9-methylene-PGE2 and tetranor-9-methylene-PGE2 were also obtained metabolites with the methylene group transformed into a dihydrodiol. In a comparative study with rat liver microsomes, it was found that these converted tetranor-9-methylene-PGE2 in a 50 per cent yield to omega-oxidized products. Finally, 20.000 X G supernatants from guinea pig and rat liver were compared with respect to omega-oxidation. The rat liver 20.000 X G supernatant was found to convert the substrate to the same extent as washed microsomes. By contrast, the guinea pig liver 20.000 X G supernatant was considerably more efficient than washed microsomes.

Immunochemical study on the contribution of hypolipidaemic-induced cytochrome P-452 to the metabolism of lauric acid and arachidonic acid

The influence of four hypolipidaemic drugs (clofibrate, WY-14,643, clobuzarit and bezafibrate) on hepatic cytochrome P-450 and fatty acid metabolism in male rat liver microsomes has been investigated. All of the hypolipidaemic drugs tested significantly induced the hydroxylation of lauric acid and, furthermore, this was accompanied by a concomitant 3-fold induction of a specific isoenzyme of cytochrome P-450 (termed cytochrome P-452) as determined by a single radial immunodiffusion technique. In addition, immunochemical quantitation of cytochrome P-452 in control, uninduced rat liver microsomes revealed that this particular isoenzyme constituted 22% of the total carbon monoxide-discernible cytochrome P-450 population. This has led us to the conclusion that cytochrome P-452 is a constitutive cytochrome P-450 isoenzyme and therefore that hypolipidaemic agents function as inducers of constitutive haemoprotein isoenzymes. Cytochrome P-452 plays a significant role in the hydroxylation of lauric acid as evidenced by inhibition of hydroxylase activity in the presence of an anti-P-452 IgG fraction. In addition, this antibody preferentially inhibits the 12-hydroxylation of lauric acid in rat liver microsomes by comparison to the 11-hydroxylase activity. Our studies have also shown that arachidonic acid serves as an excellent substrate for hypolipidaemic-induced cytochrome P-452, resulting in the formation of several metabolites that have been separated by reverse phase HPLC. Furthermore, a specific metabolite (or group of metabolites) of arachidonic acid is induced by clofibrate pretreatment and that the formation of this metabolite(s) is inhibited by an antibody to cytochrome P-452. By comparison, other metabolites of arachidonic acid remain refractory to induction by clofibrate and are not inhibited by the presence of anti-P-452 IgG. In addition, a reconstituted enzyme system containing highly purified cytochrome P-452 actively catalyses the above specific oxidation of arachidonic acid, a reaction that is significantly stimulated by the presence of cytochrome b5. Taken collectively, our data provide compelling evidence that hypolipidaemic agents induce a specific isoenzyme of hepatic microsomal P-450 that readily oxidizes fatty acids and that arachidonic acid may serve as an excellent endogenous substrate for this novel haemoprotein.

Leukotriene B4 omega-hydroxylase in human polymorphonuclear leukocytes. Partial purification and identification as a cytochrome P-450

Human polymorphonuclear leukocytes (PMN) not only synthesize and respond to leukotriene B4 (LTB4), but also catabolize this mediator of inflammation rapidly and specifically by omega-oxidation. To characterize the enzyme(s) responsible for omega-oxidation of LTB4, human PMN were disrupted by sonication and subjected to differential centrifugation to yield membrane, granule, and cytosol fractions (identified by biochemical markers). LTB4 omega-hydroxylase activity was concentrated (together with NADPH cytochrome c reductase activity) only in the membrane fraction (specific activity increased 10-fold as compared to whole sonicates, 41% recovery). Negligible activity was detected in granule or cytosol fractions. LTB4 omega-hydroxylase activity in isolated PMN membranes was linear with respect to duration of incubation and protein concentration, was maximal at pH 7.4, had a Km for LTB4 of 0.6 microM, and was dependent on oxygen and on reduced pyridine nucleotides (apparent Km for NADPH = 0.5 microM; apparent Km for NADH = 223 microM). The LTB4 omega-hydroxylase was inhibited significantly by carbon monoxide, ferricytochrome c, SKF-525A, and Triton X-100, but was not affected by alpha-naphthoflavone, azide, cyanide, catalase, and superoxide dismutase. Finally, isolated PMN membranes exhibited a carbon monoxide difference spectrum with a peak at 452 nm. Thus, we have partially purified the LTB4 omega-hydroxylase in human PMN and identified the enzyme as a membrane-associated, NADPH-dependent cytochrome P-450.

16,16-Dimethyl PGE2 and fatty acids protect hepatocytes against CCl4-induced damage

16,16-Dimethyl PGE2 (dmPGE2) has previously been shown to protect the in vivo rat liver against CCl4-induced damage. These studies were undertaken to determine if this protection could be demonstrated in vitro where factors of absorption, secretion, and blood flow are not present. Primary hepatocyte cultures were established by perfusing rat liver with collagenase. Hepatocytes were plated at a density of 2 X 10(4) cells/cm, allowed 90 min to attach, then stabilized in L15 medium for 18 h. Hepatocytes were then challenged with CCl4 with concomitant exposure to 10(-9) to 10(-5) M dmPGE2, stearic acid, oleic acid, or ethanol vehicle (0.00001 to 0.1%). After 1 h, challenge was aspirated and cells were stained with 0.04% trypan blue to determine viability. Hepatocytes in the vehicle groups took up more trypan when exposed to CCl4 than those treated with dmPGE2, stearic acid, or oleic acid at concentrations of 10(-9) to 10(-7) M. At 0.1% ethanol vehicle protected as well as all other treatments. Protection against CCl4 by dmPGE2, stearic, and oleic acids as well as high concentrations of ethanol may occur by altering the metabolism of CCl4.

Spectral properties of substrate-cytochrome P-450 interaction and catalytic activity of xenobiotic metabolizing enzymes in isolated rainbow trout liver cells

A method for the isolation of intact viable rainbow trout liver cells in high numbers is described. The technique involves perfusion of collagenase through the liver. A major part of the cytochrome P-450 in isolated liver cells was present in the oxidized non-substrate bound form. It was observed that 7-ethoxycoumarin was rapidly taken up by the liver cells and bound to cellular cytochrome P-450. The substrate binding spectrum for isolated trout liver cells was slightly modified compared with that obtained with trout liver microsomes. The microsomal affinity of 7-ethoxycoumarin, calculated as the apparent spectral dissociation constant (ks), was elevated 11-fold after fish were treated with beta-naphthoflavone, indicating a qualitative alteration in the nature of the constitutive cytochrome P-450. The metabolism of 7-ethoxycoumarin in isolated liver cells was found to be of a comparable rate to that obtained in liver microsomes. Pretreatment of fish with Clophen A50 or beta-naphthoflavone significantly increased the content of cytochrome P-450 and elevated the rate of 7-ethoxycoumarin deethylation in isolated liver cells. Furthermore, the rate of conjugation of 7-hydroxycoumarin was significantly elevated in liver cells isolated from beta-naphthoflavone treated fish when compared with the control rate. In isolated liver cells, 90% of the 7-hydroxycoumarin formed from deethylation of 7-ethoxycoumarin was further metabolized to conjugated products. However, in beta-naphthoflavone of Clophen A50 treated fish the fraction of conjugated metabolites was markedly decreased, indicating a changed balance between cytochrome P-450 dependent reactions and conjugation reactions in the cell.

Metabolism of leukotriene B4 in hepatic microsomes

Leukotriene B4 was metabolized in rat hepatic microsomes to two products. Mass spectral analysis of these two metabolites indicated that the major metabolite was the 20-hydroxy metabolite while the minor metabolite was the 19-hydroxy metabolite. The formation of these metabolites required NADPH and was linear with time (20 min) and protein (1.6 mg/ml). The Km apparent and Vmax for omega hydroxylation of LTB4 was 14 uM and 0.138 nmol/min/mg protein. In contrast, the km and Vmax for omega minus one hydroxylation was 54 uM and 0.093 nmol/min/mg protein. These results suggest that omega and omega minus one hydroxylations of LTB4 may be mediated by different isozymes of hepatic P-450.

Regioselectivity of hydroxylation of prostaglandins by liver microsomes supported by NADPH versus H2O2 in methylcholanthrene-treated and control rats: formation of novel prostaglandin metabolites

The effects of methylcholanthrene (MC) treatment of male rats on the regioselectivity of hydroxylation of prostaglandins E1 and E2 (PGE1 and PGE2) by liver microsomes, supplemented with NADPH or H2O2, was examined. In the presence of NADPH, control microsomes catalyzed the hydroxylation at omega-1 (C19) and at omega-(C20) sites with minimal formation of novel monohydroxy metabolites of PGE1 and PGE2, referred to as compounds X1 and X2, respectively. Similarly, H2O2 supported the 19-hydroxylation and the formation of compounds X1 and X2, but yielded only minimal amounts of 20-hydroxy products. With NADPH, MC-treated microsomal incubations demonstrated only minor quantitative change in the 19- and 20-hydroxylation as compared with controls, but showed a 7- to 11-fold increase in formation of compound X1 and a 10-fold increase in formation of X2. By contrast with H2O2, MC-treatment increased by about 3-fold the 19- and 20-hydroxylation of PGE1 and by 35- to 46-fold the formation of X1; similarly, there was an approximate 2-fold increase in 19- and 20-hydroxylation of PGE2 and a 10-fold increase in formation of X2. These findings suggest that several monooxygenases are involved in catalyzing the hydroxylation at the various sites of the PGE molecule. Inhibitors of monooxygenases (SKF 525A, alpha-naphthoflavone, and imidazole derivatives) provided further evidence that the hydroxylation at the three sites of PGEs is catalyzed by different P-450 monooxygenases. It is striking that the inhibitors had a much lesser effect on the 20-hydroxylation of PGE1 as compared with other sites of hydroxylation. Structural identification of compounds X1 and X2 was elucidated as follows. Resistance of the PGB derivative of X1 to periodate oxidation and mass fragmentation analysis of the t-butyldimethylsilyl ether methyl ester, placed the hydroxylation at C17 or C18. Finally, mass fragmentation of trimethylsilyl ether methyl ester PGB derivatives of X1 and X2 provided conclusive evidence that X1 and X2 are 18-hydroxy-PGE1 and 18-hydroxy-PGE2, respectively. The above findings indicate that the high regioselectivity of hydroxylation of PGE1 and PGE2, resulting in the formation of 18-hydroxy-PGE1 and 18-hydroxy-PGE2, respectively, is catalyzed by P-450 isozyme(s) which are induced by MC, possibly by P-450c.

Regulation of renal cytochrome P-450. Effects of two-thirds hepatectomy, cholestasis, biliary cirrhosis and post-necrotic cirrhosis on hepatic and renal microsomal enzymes

The possibility of a relationship between hepatic and renal cytochrome P-450 contents was assessed in rats with liver disease. In rats killed 3 days after two-thirds hepatectomy (a model for hepatocellular insufficiency), the total microsomal cytochrome P-450 content of the whole liver was decreased by 60% as compared to that in control rats; renal cytochrome P-450 was increased by 30% while the 7-ethoxycoumarin deethylase activity of kidney microsomes was increased by 80%. In rats killed 7 days after bile duct ligation (a model for cholestasis) or 35 days after bile duct ligation (a model for biliary cirrhosis), hepatic cytochrome P-450 was decreased by 60% and 45%, respectively, while renal cytochrome P-450 content was increased by 50% and 150%, respectively. In contrast, in rats killed 15 days after the last dose of carbon tetrachloride, 1.3 ml/kg twice weekly for 3 months (a model for post-necrotic cirrhosis), both hepatic and renal cytochrome P-450 contents remained unchanged. Phenobarbital (80 mg/kg daily for 3 days) was a poor inducer of renal cytochrome P-450 in sham-operated rats but became a potent inducer of renal cytochrome P-450 in rats with two-thirds hepatectomy. We conclude that renal cytochrome P-450 is increased in three models in which hepatic cytochrome P-450 contents are decreased (two-thirds hepatectomy, cholestasis and biliary cirrhosis), but remains unchanged in a model of severe liver pathology, in which hepatic cytochrome P-450 content is not modified (late, post-necrotic cirrhosis). The hypothetical role of endogenous inducer(s) is discussed.

Metabolism of prostaglandin E analogs in the guinea pig liver mitochondrial fraction

Tritium-labeled 16,16-dimethyl-PGE2 and 9-methylene-PGE2 were incubated with the guinea pig liver mitochondrial fraction. The tetranor and dinor metabolites were obtained, a large portion of the latter contained a saturated carboxyl side chain. Also the beta-hydroxydinor metabolites, analogous to the beta oxidation of fatty acids, were identified. Such metabolites have not been reported earlier in connection with beta oxidation of prostaglandins or prostaglandin analogs. Furthermore, omega-hydroxylated analogs were incubated with the guinea pig liver mitochondrial fraction and were found to pass through beta oxidation to a certain extent.

Carbon monoxide inhibits omega-oxidation of leukotriene B4 by human polymorphonuclear leukocytes: evidence that catabolism of leukotriene B4 is mediated by a cytochrome P-450 enzyme

Carbon monoxide significantly inhibits omega-oxidation of exogenous leukotriene B4 to 20-OH-leukotriene B4 and 20-COOH-leukotriene B4 by unstimulated polymorphonuclear leukocytes as well as omega-oxidation of leukotriene B4 that is generated when cells are stimulated with the calcium ionophore, A23187. Inhibition of omega-oxidation by carbon monoxide is concentration-dependent, completely reversible, and specific. Carbon monoxide does not affect synthesis of leukotriene B4 by stimulated polymorphonuclear leukocytes or other cell functions (i.e., degranulation, superoxide anion generation). These findings suggest that a cytochrome P-450 enzyme in human polymorphonuclear leukocytes is responsible for catabolizing leukotriene B4 by omega-oxidation.