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

Identification of the cytochrome P450 enzymes responsible for the ω-hydroxylation of phytanic acid

Patients suffering from Refsum disease have a defect in the alpha-oxidation pathway which results in the accumulation of phytanic acid in plasma and tissues. Our previous studies have shown that phytanic acid is also a substrate for the omega-oxidation pathway. With the use of specific inhibitors we now show that members of the cytochrome P450 (CYP450) family 4 class are responsible for phytanic acid omega-hydroxylation. Incubations with microsomes containing human recombinant CYP450s (Supersomes) revealed that multiple CYP450 enzymes of the family 4 class are able to omega-hydroxylate phytanic acid with the following order of efficiency: CYP4F3A>CYP4F3B>CYP4F2>CYP4A11.

Induction of cytochrome CYP4F3A in all-trans-retinoic acid-treated HL60 cells

Cytochrome P-450 CYP4F3A catalyzes the inactivation of leukotriene B(4) by omega-hydroxylation, an activity of which is specifically expressed in human neutrophils. Here, we examined expression of the LTB(4) omega-hydroxylating activity during the differentiation of HL60 cells, an acute promyelocytic leukemia cell line, in the presence of various inducers. Among the inducers used, all-trans-retinoic acid (ATRA) most strongly induces the LTB(4) omega-hydroxylating activity in a dose-dependent manner. The time course of the induction of the omega-hydroxylating activity correlates well with that of the superoxide-generating activity, indicative of cell differentiation. ATRA-treated cell microsomes convert LTB(4) to its 20-hydroxyl derivative under aerobic conditions in the present of NADPH. The reaction is inhibited by carbon monoxide, an inhibitor of cytochrome P-450, and by antibodies raised against NADPH-P-450 reductase. CYP4F3A appears to be responsible for the LTB(4) omega-hydroxylase activity, based on the following observations: expression of the mRNA for CYP4F3A is observed together with the induction of LTB(4) omega-hydroxylating activity in ATRA-treated HL60 cells; and the apparent K(m) values for the omega-hydroxylation of LTB(4) and lipoxin B(4) by ATRA-treated cell microsomes are essentially the same as those of CYP4F3A in human neutrophil microsomes.

Correlation between inflammatory cellular responses and chemotactic activity in bronchoalveolar lavage fluid following intratracheal instillation of nickel sulfate in rats

In a preceding study, we reported that the numbers of macrophages and polymorphonuclear leukocytes (PMN) were increased in bronchoalveolar lavage fluid (BALF) following the intratracheal instillation of nickel sulfate (NiSO4) in rats. In the present study, BALF chemotactic activities for both macrophages and PMN were measured to investigate if the increases of these inflammatory cells in BALF depend on increases in chemotactic activities in epithelial lining fluid (ELF) of the lung. Both the number of PMN and the PMN chemotactic activity peaked at 2 days post-instillation and they were significantly correlated. However, the PMN chemotactic activity was inversely correlated with concentration of leukotriene B4 (LTB4), a well-known chemotaxin. Although PMN were not observed in control BALF, LTB4 concentration in the control ELF (ca. 5 x 10(-7) M) was estimated to have a potential to attract PMN chemotactically through a membrane in in vitro migration assay. These results suggest that the presence of LTB4 in ELF itself does not trigger transpulmonary PMN infiltration. The rat BALF were fractionated by high performance liquid chromatography (HPLC), and PMN chemotactic activity of each fraction was measured. The elution profiles of PMN chemotactic activity showed that there were at least two different chemotaxins in BALF obtained from the NiSO4-exposed rats. Macrophage chemotactic activity in BALF also peaked at 2 days post-instillation. However, the number of macrophages was not significantly correlated with the chemotactic activity for macrophage in BALF.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of effect of phenobarbital on ex vivo leukocyte oxidative metabolism in healthy volunteers

Oxidative metabolism in activated human polymorphonuclears catabolizes leukotriene B4 by a cytochrome P450 omega-hydroxylase and procainamide by a myeloperoxidase. The percentage of leukotriene B4 metabolized by activated human polymorphonuclears and the apparent enzymatic parameters of procainamide metabolism were studied ex vivo in six healthy volunteers before and after phenobarbital intake (100 mg/day) for 10 days and in six healthy control volunteers. No differences were found between groups for the difference in percentage of leukotriene B4 metabolized between day 11 and day 1. The apparent enzymatic parameters, Km and Vm, of procainamide oxidation did not differ significantly between the groups both on day 1 and day 11. These results do not show any evidence of inducibility of leukotriene B4 and procainamide metabolism in human polymorphonuclears. However, a positive correlation between 6 beta OH-cortisol excretion and percentage of leukotriene B4 metabolized was observed on day 11. This study suggests that human polymorphonuclears cytochrome P450 leukotriene B4 omega-hydroxylase and procainamide metabolism is not a useful method to study cytochrome P450 induction in clinical pharmacology.

Leukotrienes: their possible role in pulpal and periapical diseases

Leukotrienes play an important role in inflammation and its sequelae such as pain, swelling, and bone resorption. This review covers, their history, structure, synthesis, metabolism, biological effects, inhibitors, antagonists, and their possible role in pathogenesis of pulpal and periapical disease.

Characterization of lipoxins by combined gas chromatography and electron-capture negative ion chemical ionization mass spectrometry: formation of lipoxin A4 by stimulated human whole blood

The lipoxins are a recent addition to the family of bioactive products derived from arachidonic acid. Here, we have prepared pentafluorobenzyl ester, trimethylsilyl ether derivatives of lipoxin A4, lipoxin B4 and pentadeuterolipoxin A4 and have characterized these products by electron-capture negative ion chemical ionization gas chromatography/mass spectrometry (NICI GC/MS). Lipoxin A4 (5S,6R,15S-trihydroxy-7,9,13-trans-11-cis-eicosa-tetraenoic acid; LXA4) was quantified following extraction from whole blood by stable isotopic dilution utilizing deuterium-labeled LXA4 as internal standard and selected ion monitoring of the [M--pentafluorobenzyl] anions. Studies with a second tritiated internal standard (e.g. [11,12-3H]LXA4) also showed that the recovery of LXA4 was greater than 80% following solid-phase extraction from whole blood, and greater than 90% from isolated cells. In addition, neither isolated neutrophils nor platelets oxidatively metabolized [11,12-3H]LXA4 when incubated in the presence or absence of stimuli. Whole blood incubated with either the ionophore of divalent cations (A23187), thrombin, or thrombin plus the chemotactic peptide formylmethionyl-leucine-phenylalanine generated both LXA4 and thromboxane, which were quantified by stable isotope dilution. The ratio of thromboxane to LXA4 formed by stimulated whole blood ranged from approximately 2:1 to 20:1. These results indicate that the lipoxins display suitable characteristics as their respective pentafluorobenzyl ester, trimethylsilyl ether derivatives for quantification by electron-capture NICI GC/MS. Moreover, they provide evidence that LXA4 can be generated from endogenous sources in whole blood following exposure to physiologically relevant stimuli.

Cytochrome P-450-dependent omega-oxidation of leukotriene B4 in rodent and human epidermis

Leukotriene B4 (LTB4,5-12-dihydroxy 6,8,10,14-eicosatetraenoic acid), an enzyme-catalyzed oxidation product of arachidonic acid, is a major inflammatory mediator. Human polymorphonuclear leukocytes and rodent hepatic microsomes catabolize LTB4 to 20-OH-LTB4 and 20-COOH-LTB4, which is mediated by a cytochrome P-450 catalyzed reaction termed the LTB4 omega-hydroxylase. In this study we investigated the catabolism of LTB4 in rat, guinea pig, and human epidermis. The incubation of 3H-LTB4 (9 microM) for 60 min in the presence of oxygen, NADPH, and epidermal microsomes prepared from neonatal fat (3.0 mg) or adult guinea pig (2.6 mg) resulted in the formation of 20-OH-LTB4 and 20-COOH-LTB4. Metabolite identification was based on co-chromatography on high pressure liquid chromatography with highly purified reference standards. The formation of 20-OH-LTB4 and 20-COOH-LTB4 was accompanied by the disappearance of LTB4. The rate of formation of 20-OH-LTB4 was 9-12-fold higher than that of 20-COOH-LTB4. Product formation was negligible with boiled microsomes, required NADPH and oxygen, was linear with respect to incubation time and protein, and was maximal at pH 7.4. LTB4-omega-hydroxylase activity was inhibited (greater than 90%) by carbon monoxide or 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF-525A) (1 mM), whereas alpha-naphthoflavone produced only moderate (13%) or no effects. Topical application of 3-methylcholanthrene and other conventional inducers of epidermal monooxygenase activities to neonatal rats (100 mg/kg, single treatment) did not result in an increase in epidermal LTB4-omega-hydroxylase activity. The addition of 3H-LTB4 (30 nmoles) to primary human keratinocytes followed by incubation at 37 degrees C resulted in time-dependent disappearance of LTB4 and appearance of 20-OH-LTB4 and 20-COOH-LTB4 in the medium. These results suggest that LTB4 is catabolized by the cytochrome P-450-dependent enzyme system in rodent and human skin and that this may participate in modulating the effects of this proinflammatory lipid in this tissue.

Hepatic microsomal metabolism of leukotriene B4 in rats: biochemical characterization, effect of inducers, and age- and sex-dependent differences

1. The cytochrome P-450-dependent metabolism of leukotriene B4 (LTB4) by rat hepatic microsomes was characterized. Hepatic microsomes were found to metabolize LTB4 to 20-hydroxy-LTB4 and 20-carboxy:LTB4. The rate of formation of 20-hydroxy-LTB4 (14.6 pmol/min per mg protein) was 5.8-fold higher than that of 20-carboxy-LTB4 (2.5 pmol/min per mg protein). 2. LTB4 omega-hydroxylase activity required NADPH and oxygen indicating that the reaction is mediated by a mono-oxygenase system. The omega-hydroxylase activity was optimal at pH 7.4 and product formation was linear with respect to time of incubation and protein concentration. The reaction was significantly inhibited by carbon monoxide (89%), SKF 525-A (1 mM), and metyrapone (0.1 mM) whereas alpha-naphthoflavone had only marginal inhibitory effects. The apparent Km and Vmax of LTB4 omega-hydroxylase were 4 microM and 19.6 pmol/min per mg protein, respectively. 3. Ontogenic studies revealed that LTB4 omega-hydroxylase activity was low in 4-day-old rats and that there was a steady increase in enzyme activity as the animal matured. 4. Phenobarbital, 3-methylcholanthrene or Aroclor 1254 treatment of rats did not induce LTB4 omega-hydroxylase activity whereas clofibrate resulted in 61% induction in enzyme activity. No significant sex-dependent differences were observed. 5. It is concluded that hepatic metabolism of LTB4 may afford an effective mechanism for limiting many of the pro-inflammatory effects of circulating leukotrienes.

The omega-hydroxylation of arachidonic acid by human polymorphonuclear leukocytes

Incubations of [1-14C]arachidonic acid with unstimulated human polymorphonuclear leukocytes resulted in the formation of four new metabolites in a previously described reverse-phase HPLC system. Three of these metabolites were largely suppressed in a CO/O2 (80/20, by vol.) atmosphere indicating a cytochrome-P450-dependent monooxygenase reaction. In agreement with this assumption is their NADPH/O2-dependent formation in the microsomal fraction. One metabolite was identified by gas chromatography/mass spectrometry analysis as omega-hydroxy-arachidonic acid and the two others were secondary products identified as omega-carboxy-arachidonic acid and 5,20-dihydroxy-E,Z,Z,Z-6,8,11,14-eicosatetraenoic acid. Since the affinity for arachidonate of the omega-monooxygenase was quite low and the presence of LTB4 suppressed the omega-hydroxylation of arachidonate, we conclude that the known LTB4 omega-monooxygenase is responsible for the formation of omega-hydroxy-arachidonate. It is unlikely, however, that significant concentrations of these metabolites are formed by activated polymorphonuclear leukocytes in vivo. The fourth metabolite remains tightly associated with the leukocytes but has not been further characterized.

The immunocytochemical localisation and distribution of cytochrome P-450 in normal human hepatic and extrahepatic tissues with a monoclonal antibody to human cytochrome P-450

1. The localisation and distribution of cytochrome P-450 in human tissues has been studied by immunocytochemistry using a monoclonal antibody to a major form of human hepatic cytochrome P-450, P-450hA7, which is closely related to cytochromes P-450 HLp and P-450NF. 2. Strong immunoreactivity was identified in hepatocytes, columnar absorptive epithelial cells of the small intestine, polymorphonuclear leucocytes and their precursors in the bone marrow, and in mast cells. 3. Weak immunoreactivity was present in the proximal tubules of the kidney, pancreatic acini, gall bladder epithelium, squamous epithelium and sebaceous glands of the skin, interstitial cells of the testis and luteal cells of the ovary. 4. Immunoreactivity could not be demonstrated in the adrenal gland, placenta, colonic epithelium and alveolar type II cells and Clara cells of the lung.

Characterization of leukotriene B4-omega-hydroxylase activity within human polymorphonuclear granulocytes

Human polymorphonuclear granulocytes (PMN) metabolize exogenous [3H]leukotriene B4 (LTB4) into 20-hydroxy- and 20-carboxy-[3H]LTB4. The conversion was enhanced at acidic pH values (pH 6.0-7.0). Sonication of purified PMN and subcellular fractionation by differential centrifugation showed that major LTB4-hydroxylase activity was associated with the microsomal fraction (105,000 g pellet). In contrast to intact cells, LTB4-hydroxylase activity within the microsomal fraction revealed optimal activity at neutral pH and was inhibited by a wide range of divalent cations. There was a strict requirement for the presence of suitable electron donors such as NADPH. Heterocyclic nitrogenous bases, such as imidazole and pyridine, inhibited the LTB4 conversion induced by intact PMN as well as by their microsomes. These observations combined with the spectrophotometric analysis (carbon monoxide dithionite-reduced difference spectrum) supported the assumption that LTB4-hydroxylase resembled a cytochrome P-450 enzyme. The LTB4-hydroxylase within human PMN was not identical with the cytochrome P-450 of rat liver; hepatic microsomes only showed minute conversion of LTB4.

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.

Leukotriene B4 catabolism: quantitation of leukotriene B4 and its omega-oxidation products by reversed-phase high-performance liquid chromatography

LTB4 and its omega-oxidation products may be rapidly, sensitively, and specifically quantitated by the methods of solid-phase extraction and reversed-phase high-performance liquid chromatography (HPLC), which are described in this chapter. Although other techniques, such as radioimmunoassay or gas chromatography-mass spectrometry, may be utilized for quantitative analysis of the lipoxygenase products of arachidonic acid, only the technique of reversed-phase HPLC can quantitate as many as 10 metabolites in a single analysis, without prior derivatization. In this chapter, we also reviewed the chromatographic theory which we utilized in order to optimize reversed-phase HPLC analysis of LTB4 and its omega-oxidation products. With this information and a gradient HPLC system, it is possible for any investigator to develop a powerful assay for the potent inflammatory mediator, LTB4, or for any other lipoxygenase product of arachidonic acid.

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).

Oxygen concentration-dependent metabolism of leukotriene B4 by hepatocyte monolayers

Leukotriene B4 was found to be metabolized by rat hepatocyte monolayers at a rate that was linear with increasing substrate concentration from 74 to 740 nM leukotriene B4. The rates of metabolism were dependent on the O2 concentration and were 315, 213, 80, and 36 pmol leukotriene B4 per min per nmol cytochrome P-450 at 20% (212 microM), 4% (42.5 microM), 2% (21.2 microM), and 1% (10.6 microM) O2, respectively. The metabolic rate was not linear with respect to O2 concentration; however, half maximal rate occurred at 4% O2, and O2 concentration found in the pericentral region of normally oxygenated liver. These results suggest that in vivo conditions of hypoxia or ischemia that lead to blood O2 concentrations less than 4% may drastically decrease hepatic clearance of leukotriene B4.

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.

NAD+-dependent conversion of 20-OH-LTB4 to 20-COOH-LTB4 by a cell-free system of human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes, but not mononuclear leukocytes, platelets, or erythrocytes, oxidized 20-hydroxy-leukotriene B4 (20-OH-LTB4) to 20-carboxy-LTB4 (20-COOH-LTB4). 20-OH-LTB4 was quantitatively converted to 20-COOH-LTB4 by the sonicate of polymorphonuclear leukocytes in the presence of NAD+, with an optimal pH of about 7.9. NADP+ could not replace NAD+. The conversion was not inhibited by 2 mM pyrazole, a potent inhibitor of alcohol dehydrogenase. When LTB4 was incubated with the sonicate in the presence of NADPH and NAD+, 20-OH-LTB4 and 20-COOH-LTB4 appeared concomitantly with the disappearance of LTB4.

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.

Induction of cytochrome P-450b,e-type isozymes by polychlorinated biphenyls in rat liver. Molecular cloning of induced mRNAs

We have studied the induction of cytochrome P-450b,e-type antigen and mRNA by phenobarbital, 4,4'-dichlorobiphenyl and 2,4,5,2',4',5'-hexachlorobiphenyl in male Wistar rat liver. Chronic treatment of rats with 4,4'-dichlorobiphenyl leads to a relatively slow, 20-fold increase in the cytochrome P-450b,e-type antigen level and to an equivalent increase in the concentration of the corresponding mRNA. Treatment of rats with phenobarbital or 2,4,5,2',4',5'-hexachlorobiphenyl results in a faster and more pronounced increase of cytochrome P-450b,e-type antigen and mRNA levels. Analysis of clones from cDNA banks showed that two types of sequences are induced by phenobarbital corresponding to cytochrome P-450b and cytochrome P-450e respectively. 2,4,5,2',4',5'-Hexachlorobiphenyl appears to induce primarily a cytochrome P-450b-type sequence. The implications of these results for the study of the mechanism of induction are discussed.

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.

Identification and functional characterization of leukotriene B4 20-hydroxylase of human polymorphonuclear leukocytes

A single reaction product was formed during the incubation of 1.5 microM (5S,12R)-dihydroxy-6,14-cis-8,10-trans-[3H]icosatetraenoic acid (leukotriene B4, LTB4) for 30 min at 37 degrees C in 10 mM potassium phosphate buffer (pH 7.5) with 100 microM NADPH and the 150,000 X g supernatant of sonicated human polymorphonuclear leukocytes (PMN). The reaction product exhibited the same mobility on reversed-phase HPLC (RP-HPLC) and TLC as standard 20-hydroxy-LTB4 (20-OH-LTB4). When the omega-oxidation product of [3H]LTB4 was eluted from a Sep-Pak, resolved by RP-HPLC, and analyzed by GC/MS, its structure was determined to be solely 20-OH-LTB4. The Km of the 20-hydroxylase for [3H]LTB4 at its optimal pH of 7.5 was 0.22 +/- 0.08 microM (mean +/- SD, n = 4) and the Vmax was 48 +/- 11 pmol/min X mg of protein (mean +/- SD, n = 4). When the concentration of [3H]LTB4 was fixed at 1.5 microM, the Km for NADPH was 1.01 +/- 0.59 microM (mean +/- SD, n = 3). The location in the 150,000 X g supernatant of the LTB4 20-hydroxylase distinguishes it from the cytochrome P-450 system of liver, lung, and kidney microsomes and from the NADPH oxidase-cytochrome b-245 system of the human PMN. The LTB4 20-hydroxylase is either a unique cytochrome P-450 or other monooxygenase.