Omega-oxidation of cysteine-containing leukotrienes by rat-liver microsomes. Isolation and characterization of omega-hydroxy and omega-carboxy metabolites of leukotriene E4 and N-acetylleukotriene E4

Metabolomics analysis of maternal serum exposed to high air pollution during pregnancy and risk of autism spectrum disorder in offspring

BACKGROUND

Previously, numerous epidemiologic studies reported an association between autism spectrum disorder (ASD) and exposure to air pollution during pregnancy. However, there have been no metabolomics studies investigating the impact of pregnancy pollution exposure to ASD risk in offspring.

OBJECTIVES

To identify differences in maternal metabolism that may reflect a biological response to exposure to high air pollution in pregnancies of offspring who later did or did not develop ASD.

METHODS

We obtained stored mid-pregnancy serum from 214 mothers who lived in California's Central Valley and experienced the highest levels of air pollution during early pregnancy. We estimated each woman's average traffic-related air pollution exposure (carbon monoxide, nitric oxides, and particulate matter <2.5 μm) during the first trimester using the California Line Source Dispersion Model, version 4 (CALINE4). By utilizing liquid chromatography-high resolution mass spectrometry, we identified the metabolic profiles of maternal serum for 116 mothers with offspring who later developed ASD and 98 control mothers. Partial least squares discriminant analysis (PLS-DA) was employed to select metabolic features associated with air pollution exposure or autism risk in offspring. We also conducted extensive pathway enrichment analysis to elucidate potential ASD-related changes in the metabolome of pregnant women.

RESULTS

We extracted 4022 and 4945 metabolic features from maternal serum samples in hydrophilic interaction (HILIC) chromatography (positive ion mode) and C18 (negative ion mode) columns, respectively. After controlling for potential confounders, we identified 167 and 222 discriminative features (HILIC and C18, respectively). Pathway enrichment analysis to discriminate metabolic features associated with ASD risk indicated various metabolic pathway perturbations linked to the tricarboxylic acid (TCA) cycle and mitochondrial function, including carnitine shuttle, amino acid metabolism, bile acid metabolism, and vitamin A metabolism.

CONCLUSION

Using high resolution metabolomics, we identified several metabolic pathways disturbed in mothers with ASD offspring among women experiencing high exposure to traffic-related air pollution during pregnancy that were associated with mitochondrial dysfunction. These findings provide us with a better understanding of metabolic disturbances involved in the development of ASD under adverse environmental conditions.

Cytochrome P450 ω-Hydroxylases in Inflammation and Cancer

Cytochrome P450-dependent ω-hydroxylation is a prototypic metabolic reaction of CYP4 family members that is important for the elimination and bioactivation of not only therapeutic drugs, but also endogenous compounds, principally fatty acids. Eicosanoids, derived from arachidonic acid, are key substrates in the latter category. Human CYP4 enzymes, mainly CYP4A11, CYP4F2, and CYP4F3B, hydroxylate arachidonic acid at the omega position to form 20-HETE, which has important effects in tumor progression and on angiogenesis and blood pressure regulation in the vasculature and kidney. CYP4F3A in myeloid tissue catalyzes the ω-hydroxylation of leukotriene B4 to 20-hydroxy leukotriene B4, an inactivation process that is critical for the regulation of the inflammatory response. Here, we review the enzymology, tissue distribution, and substrate selectivity of human CYP4 ω-hydroxylases and their roles as catalysts for the formation and termination of the biological effects of key eicosanoid metabolites in inflammation and cancer progression.

Unraveling the genetic basis of aspirin hypersensitivity in asthma beyond arachidonate pathways

Although aspirin-exacerbated respiratory disease (AERD) has attracted a great deal of attention because of its association with severe asthma, it remains widely under-diagnosed in the asthmatic population. Oral aspirin challenge is the best method of diagnosing AERD, but this is a time-consuming procedure with serious complications in some cases. Thus, development of non-invasive methods for easy diagnosis is necessary to prevent unexpected complications of aspirin use in susceptible patients. For the past decade, many studies have attempted to elucidate the genetic variants responsible for risk of AERD. Several approaches have been applied in these genetic studies. To date, a limited number of biologically plausible candidate genes in the arachidonate and immune and inflammatory pathways have been studied. Recently, a genome-wide association study was performed. In this review, the results of these studies are summarized, and their limitations discussed. In addition to the genetic variants, changes in methylation patterns on CpG sites have recently been identified in a target tissue of aspirin hypersensitivity. Finally, perspectives on application of new genomic technologies are introduced; these will aid our understanding of the genetic pathogenesis of aspirin hypersensitivity in asthma.

Genetics of hypersensitivity to aspirin and nonsteroidal anti-inflammatory drugs

Various hypersensitivity reactions have been reported with aspirin and nonsteroidal anti-inflammatory drugs. Hypersensitivity can occur regardless of a chemical drug structure or its therapeutic potency. Allergic conditions include aspirin-exacerbated respiratory disease (AERD or aspirin-induced asthma), aspirin-induced urticaria/angioedema (AIU), and anaphylaxis. Several genetic studies on aspirin hypersensitivity have been performed to discover the genetic predisposition to aspirin hypersensitivity and to gain insight into the phenotypic diversity. This article updates data on the genetic mechanisms that govern AERD and AIU and summarizes recent findings on the molecular genetic mechanism of aspirin hypersensitivity.

Association analysis of N-acetyl transferase-2 polymorphisms with aspirin intolerance among asthmatics

AIMS

Cysteinyl leukotrienes are inactivated by acetyl coenzyme A-dependent N-acetyltransferase (NAT). Thus, functional alterations of the NAT gene may contribute to the risk of aspirin-intolerant asthma.

MATERIALS & METHODS

Asthmatics (n = 438) were categorized into aspirin-intolerant asthma (15% or greater decrease in the forced expiratory volume in 1 s or cutaneous reactions, n = 170) or aspirin-tolerant asthma (n = 268) groups. In total, 14 polymorphisms of the NAT2 gene were genotyped by a single-base extension method.

RESULTS

The distributions of all loci of the 14 SNPs were in Hardy-Weinberg equilibrium (p > 0.05). Among the 14 SNPs, six common SNPs (minor allele frequency >5%) in a Korean population were used for haplotype construction and further statistical analysis. The logistic regression analysis demonstrated that NAT2 -9246G>C and haplotype 2 (TCACGG) were significantly associated with the risk of aspirin-intolerant asthma. The rare allele frequencies of the SNP and Ht2 were significantly higher in the aspirin-intolerant asthma group than in the aspirin-tolerant asthma group (p(corr) = 0.03 and p(corr) = 0.02 in codominant model).

CONCLUSION

In a large genetic epidemiology study of aspirin-intolerant asthma in a Korean population, genetic polymorphisms of NAT2 were found to be related to a risk of aspirin hypersensitivity among asthmatics.

Prostaglandin and leukotriene omega-hydroxylases

Omega and subterminal hydroxylations of prostaglandins (PGs), leukotriene B4 (LTB4) and some related eicosanoids are catalyzed by the cytochrome P450 (CYP) enzymes belonging to the CYP4A and CYP4F subfamilies. CYP4A4, which is induced in pregnant rabbits, is the only elucidated PGE omega-hydroxylase within the CYP4A subfamily. CYP4F3 is the most tissue specific and most efficient LTB4 omega-hydroxylase, judging from its restricted localization in human polymorphonuclear leukocytes (PMN) and its very low Km value for LTB4. CYP4F2 is widely distributed in human liver and other tissues, and catalyzes omega-hydroxylation of various lipoxygenase-derived eicosanoids as well as LTB4, with relatively comparable and high Km values. CYP4F3B is very similar to CYP4F2 in its tissue localization and its Km value for LTB4. Human seminal vesicle CYP4F8 is the first elucidated hydroxylase with substrate specificity for PG endoperoxides, whereas ram seminal vesicle CYP4F21 is the only elucidated PGE omega-hydroxylase within the CYP4F subfamily [corrected]. Rat CYP4F1, CYP4F4 and CYP4F5, and mouse Cyp4f14 have LTB4 omega-hydroxylase activity. Three additional human, four mouse, and one fish members of the CYP4F subfamily have been identified.

Enhanced urinary excretion of cysteinyl leukotrienes in patients with acute alcohol intoxication

BACKGROUND & AIMS

Leukotrienes are proinflammatory mediators. Ethanol inhibits the catabolism of both cysteinyl leukotrienes (leukotriene E(4) [LTE(4)] and N-acetyl-LTE(4)) and leukotriene B(4) (LTB(4)) in hepatocytes. We examined the metabolic derangement of leukotriene inactivation by ethanol in humans in vivo.

METHODS

LTE(4), N-acetyl-LTE(4), LTB(4), and 20-hydroxy-LTB(4) were quantified in urine samples from 16 patients with acute alcohol intoxication (mean blood ethanol, 75 mmol/L). In 9 healthy volunteers, urinary LTE(4) was determined before and after ethanol consumption (mean blood ethanol, 14 mmol/L).

RESULTS

The excretion of LTE(4) during alcohol intoxication was 286 compared with 36 nmol/mol creatinine in healthy subjects (P < 0.01); the corresponding values for N-acetyl-LTE(4) were 101 and 11 nmol/mol creatinine, respectively (P < 0.001). This excretion of cysteinyl leukotrienes decreased when the blood ethanol concentration returned to normal. LTB(4) and 20-hydroxy-LTB(4) were detectable only in patients with excessive blood ethanol concentrations (mean, 95 mmol/L). In healthy volunteers, LTE(4) excretion increased 3-5 hours after ethanol consumption (mean peak concentration of 1.5 nmol/L compared with 0.5 nmol/L for basal values; P < 0.005).

CONCLUSIONS

Ethanol at high concentration induces increased leukotriene excretion into urine. These changes are consistent with inhibition of leukotriene catabolism and inactivation induced by ethanol, as well as with a higher leukotriene formation caused by ethanol-induced endotoxemia.

Defective hepatobiliary leukotriene elimination in patients with the Dubin-Johnson syndrome

The Dubin-Johnson syndrome (DJS) is characterized by a hereditary conjugated hyperbilirubinemia and a typical dark pigment accumulation in liver parenchymal cells. In the present study the renal excretion of leukotrienes in five patients with histologically established DJS and five age- and sex-matched healthy subjects was investigated. Endogenous urinary leukotrienes were separated by high-performance liquid chromatography and subsequently quantified by immunoassays and gas chromatography-mass spectrometry. Patients with DJS excreted significantly (P < 0.01) greater amounts of cysteinyl leukotriene, LTE4 (8-fold), the omega-oxidation product omega-carboxy-LTE4 (15-fold) and the beta-oxidation metabolite omega-carboxy-tetranor-LTE3 (26-fold) into urine than healthy controls. These results imply that in DJS leukotriene elimination into bile is defective, leading to a compensatory renal leukotriene elimination and a typical excretion pattern of urinary leukotriene metabolites. Analysis of endogenous urinary leukotrienes seems to be a new approach to the noninvasive diagnosis of this disease.

Cysteinyl leukotrienes in the urine of patients with liver diseases

The significance of cysteinyl leukotrienes was investigated in patients with liver diseases by measurements of leukotriene E4 and N-acetyl-leukotriene E4 in urine. A marked increase of renal cysteinyl leukotriene excretion was observed in patients with cirrhosis without and with ascites, intrahepatic cholestasis, and obstructive jaundice as compared with healthy subjects (leukotriene E4: means 82, 264, 221 and 142 versus 40 nmol/mol creatinine, respectively; N-acetyl-leukotriene E4: means 25, 64, 61 and 47 versus 13 nmol/mol creatinine, respectively). The urinary concentration of leukotriene E4 was positively correlated with the one of N-acetyl-leukotriene E4 (r = 0.81, p < 0.001). In patients with cirrhosis, the excretion of cysteinyl leukotrienes was strongly increased in patients in Child-Turcotte stage C as compared with those in Child-Turcotte stages A and B. In patients with intrahepatic cholestasis and in those with obstructive jaundice, the excretion of leukotriene E4 plus N-acetyl-leukotriene E4 was positively correlated with total serum bilirubin. In patients with cirrhosis and in those with obstructive jaundice, the cysteinyl leukotrienes in urine were negatively correlated with creatinine clearance. The elevated renal excretion of cysteinyl leukotrienes decreased after biliary drainage in patients with obstructive jaundice. These data support the concept that increased urinary excretion of cysteinyl leukotrienes in patients with cirrhosis is due to a reduced functional liver mass and that in patients with cholestasis it is mainly due to an impaired elimination into the biliary tract that results in a diversion to renal excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Impaired degradation of leukotrienes in patients with peroxisome deficiency disorders

The degradation of leukotrienes by beta-oxidation from the omega-end proceeds in peroxisomes (Jedlitschky et al. J. Biol. Chem. 1991. 266:24763-24772). Peroxisomal degradation of leukotrienes was studied in humans by analyses of endogenous leukotrienes in urines from eight patients with biochemically established peroxisome deficiency disorder and eight age- and sex-matched healthy infant controls. Leukotriene metabolites were separated by high-performance liquid chromatography, quantified by radioimmunoassays, and identified as well as quantified by gas chromatography-mass spectrometry. Urinary leukotriene E4 (LTE4) and N-acetyl-LTE4 excretions, relative to creatinine, were increased > 10-fold in the patients in comparison to healthy infants. The beta-oxidation product omega-carboxy-tetranor-LTE3 averaged 0.05 mumol/mol creatinine in the controls but was not detectable in the patients. However, omega-carboxy-LTE4 (median 13.6 mumol/mol creatinine) was significantly increased in the patients' urine, whereas LTB4 (median 0.07 mumol/mol creatinine) and omega-carboxy-LTB4 were detected exclusively in the urines of the patients. These data indicate an impairment of the inactivation and degradation of both LTE4 and LTB4 in patients with peroxisomal deficiency. The increased levels of the biologically active, proinflammatory mediators LTE4 and LTB4 might be of pathophysiological significance in peroxisome deficiency disorders. This is the first and so far only condition with a pronounced urinary excretion of omega-carboxy-LTE4, omega-carboxy-LTB4, and LTB4. This impaired catabolism of leukotrienes and the altered pattern of metabolites may be of diagnostic value. These findings underline the essential role of peroxisomes in the catabolism of leukotrienes in humans.

Formation of a novel 20-hydroxylated metabolite of lipoxin A4 by human neutrophil microsomes

Lipoxin A4 (LXA4) is a biologically active compound produced from arachidonic acid via interactions of lipoxygenases. Incubation of LXA4 either with human neutrophils or with the neutrophil microsomes leads to formation of a polar compound on a reverse-phase high-performance liquid chromatography. We have identified the metabolite as 20-hydroxy-LXA4, a novel metabolite of arachidonic acid, on the basis of ultraviolet spectrometry and gas chromatography-mass spectrometry. The LXA4 omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, by antibodies raised against NADPH-cytochrome P-450 reductase, or competitively by leukotriene B4 (LTB4) and LTB5, substrates of LTB4 omega-hydroxylase. These findings indicate that the formation of 20-hydroxy-LXA4 is catalyzed by a neutrophil cytochrome P-450, the LTB4 omega-hydroxylase.

Peroxisomal leukotriene degradation: biochemical and clinical implications

Degradation of the cysteinyl leukotrienes LTE4 and N-acetyl-LTE4, and of LTB4 by beta-oxidation from the omega-end has been recognized as an important pathway in the inactivation of these mediators. The contribution of peroxisomes to leukotriene degradation and inactivation was studied in isolated hepatocytes, in isolated liver peroxisomes, and in patients with inherited peroxisome deficiency. (1) Isolated hepatocytes from rats pretreated with the peroxisome proliferator clofibrate produced highly increased amounts of beta-oxidation products derived from omega-carboxy-LTB4 and omega-carboxy-N-acetyl-LTE4 as compared to normal hepatocytes. (2) Isolated peroxisomes purified from normal and clofibrate-treated liver produced omega-carboxy-dinor-LTB4 and omega-carboxy-tetranor-LTB3 when nucleotide cofactors, including CoA, ATP, NAD+, FAD, and NADPH, were added. beta-Oxidation of the cysteinyl leukotriene omega-carboxy-N-acetyl-LTE4 was observed only with isolated peroxisomes together with a microsome fraction providing an acyl-CoA synthetase activity. (3) Peroxisomal leukotriene-binding proteins were identified by photo-affinity labeling with omega-carboxy-[3H]leukotrienes and precipitation of labeled polypeptides with antibodies against enzymes of the peroxisomal beta-oxidation system. (4) Peroxisomal degradation of leukotrienes in humans was studied by analyses of endogenous leukotrienes and their catabolites in urine from patients with an inherited peroxisomal deficiency disorder (Zellweger syndrome) and healthy infant controls. Urinary LTE4, relative to creatinine, was increased 10-fold in the patients, whereas the beta-oxidation product omega-carboxy-tetranor-LTE3 was only detectable in healthy infants. In addition, LTB4 was exclusively detected in the urine of patients with peroxisome deficiency. The increased levels of biologically active, proinflammatory mediators might be of pathophysiological significance. In addition, the altered pattern of leukotriene metabolites in urine may be of diagnostic value. The measurements in these patients underline the essential role of peroxisomes in the catabolism and inactivation of leukotrienes in humans.

Halothane metabolism. Impairment of hepatic omega-oxidation of leukotrienes in vivo and in vitro

Omega-oxidation of leukotrienes is the initial step of hepatic degradation and thus inactivation of these proinflammatory mediators. Omega-oxidation is followed by beta-oxidation of leukotrienes from the omega-end. After exposure of rats to a single dose of the anesthetic agent halothane, a transient decrease in leukotriene omega-oxidation was induced both in vivo and in vitro. In untreated rats, 44.1 +/- 6.0% of N-[3H]acetylleukotriene E4 injected intravenously was recovered unchanged in bile collected for 60 min in vivo; 46.5 +/- 3.0% was recovered as omega-/beta-oxidation products, of which 24.7 +/- 4.5% were associated with beta-oxidation products only (mean +/- SEM; n = 5). In rats receiving a single dose of halothane 18 h before the experiment, recovery of unchanged N-[3H]acetylleukotriene E4 was significantly increased to 79.8 +/- 4.8%, while the fraction of omega-/beta-oxidation products decreased to 9.0 +/- 1.7% (n = 5); 90 h after exposure to halothane, N-[3H]acetylleukotriene E4 recovery decreased to 30.0 +/- 3.0% and omega-/beta-oxidation products amounted to 49.1 +/- 3.8%; the fraction of beta-oxidation products was significantly increased to 43.1 +/- 3.4% (n = 5). Ten days after exposure of rats to halothane, the recoveries of N-[3H]acetylleukotriene E4, of omega-/beta-oxidation products, and of beta-oxidation products alone, returned to almost normal values. Microsomal fractions obtained from rat hepatocytes catalyzed the NADPH- and O2-dependent leukotriene omega-oxidation in vitro. The formation of omega-hydroxy-metabolites of leukotriene B4, leukotriene E4, and N-acetylleukotriene E4 was decreased by 50% in microsomal fractions obtained from rats 18 h and 90 h after halothane treatment, and returned back to control levels in microsomal fractions obtained 10 days after halothane treatment. The Km value of leukotriene B4 omega-oxidation revealed no significant change in enzyme affinity towards leukotriene B4; in contrast, as reflected by the reduction of the Vmax value by 65%, a decrease in the amount of the active enzyme in microsomes obtained from rats 18 h after halothane treatment was observed. Halothane-metabolism-dependent trifluoroacetylation of hepatic proteins may mediate this process. Thus, the time course of the density on immunoblots of trifluoroacetylated protein adducts paralleled that of the transient decrease in leukotriene omega-oxidation. In contrast to its omega-oxidation, leukotriene B4 synthesis from 5-hydroperoxyeicosatetraenoate was not inhibited in hepatocyte homogenates obtained from rats pretreated with halothane. The data suggest that metabolism of halothane causes a transient derangement of hepatic leukotriene homeostasis in vivo.

Effect of the leukotriene receptor antagonists FPL 55712, LY 163443, and MK-571 on the elimination of cysteinyl leukotrienes in the rat

1. Leukotriene elimination via bile and urine is an important mechanism of inactivation for these potent lipid mediators. We investigated whether the elimination of cysteinyl leukotrienes is a target for the action of leukotriene receptor antagonists. 2. Experiments were performed in male rats under deep thiopentone anaesthesia. The bile duct and the urinary bladder were cannulated. Tritium labelled leukotrienes and leukotriene receptor antagonists were given via central venous catheters. Elimination of leukotrienes produced in vivo was studied following stimulation of endogenous leukotriene biosynthesis by operative trauma. 3H-leukotriene metabolites were identified by h.p.l.c. analysis. Leukotrienes produced in vivo were measured by combined use of h.p.l.c. and RIA. 3. Under control conditions, 49 +/- 12% of the injected 3H-leukotriene radioactivity was recovered in bile and 1 +/- 0.8% in urine within 90 min. Operative trauma resulted in initial hepatobiliary secretion of 887 +/- 206 pmol kg-1 h-1 of the endogenous leukotriene metabolite N-acetyl leukotriene E4 (LTE4NAc). 4. FPL 55712 strongly inhibited hepatobiliary elimination of 3H-leukotriene radioactivity in a dose-dependent manner after i.v. injection of [3H]-LTC4, [3H]-LTD4 or [3H]LTE4, respectively. Biliary [3H]-LTD4 was reduced most effectively. The leukotriene antagonist potently prevented biliary elimination of LTE4NAc produced in vivo. Bile flow and elimination from blood into bile of [3H]-ouabain were also impaired by FPL 55712, but to a lesser extent. 5. LY 163443 reduced biliary [3H]-LTD4 after i.v. administration of [3H]-LTD4. However, the total elimination of 3H-leukotriene metabolites into bile was not significantly inhibited by the drug. 6. MK-571 reduced the biliary concentration of tracer after administration of 3H-leukotrienes most potently with respect to [3H]-LTD4. In contrast, the total recovery of 3H-leukotrienes in bile tended to increase. This is explained by a drug-induced increase in bile flow. 7. Urinary elimination of 3H-leukotrienes, quantitatively less important in the rat, was not significantly influenced by the leukotriene receptor antagonists. Recovery of 3H-leukotriene radioactivity in liver and kidneys was quantitatively insignificant. 8. From our data, we conclude that leukotriene receptor antagonists have the potential to affect leukotriene elimination by a mechanism not necessarily related to receptor blockade. Inhibition of elimination by the receptor antagonists may prolong the biological half life of leukotrienes. This effect may counteract the antagonistic properties of these drugs.

Metabolism of cysteinyl leukotrienes in monkey and man

The proinflammatory cysteinyl leukotrienes are inactivated in primates by (a) intravascular degradation, (b) hepatic and renal uptake from the blood circulation, (c) intracellular metabolism of leukotriene E4 (LTE4), and (d) biliary and renal excretion of LTC4 degradation products. We have analyzed cysteinyl leukotriene metabolites excreted into bile and urine of the monkey Macaca fascicularis and of man. In both species, hepatobiliary leukotriene elimination predominated over renal excretion. In a representative healthy human subject at least 25% of the administered radioactivity were recovered from bile and 20% from urine within 24 h. In monkey and man intravenous administration of 14,15-3H2-labeled LTC4 resulted in the biliary and urinary excretion of labeled LTE4, omega-hydroxy-LTE4, omega-carboxy-LTE4, omega-carboxy-dinor-LTE4, and omega-carboxy-tetranor-dihydro-LTE4. Small amounts of N-acetyl-LTE4 were detected in human urine only. Oxidative metabolism of LTE4 proceeded more rapidly in the monkey resulting in the formation of higher relative amounts of omega-oxidized leukotrienes in this species as compared to man. [3H]H2O amounted to less than 2% of the administered dose in monkey and human bile and urine samples. Incubation of isolated human hepatocytes with [3H2]LTC4, [3H2]LTD4, and [3H2]LTE4 showed that only [3H2]LTE4 underwent intracellular oxidative metabolism resulting in the formation of omega- and beta-oxidation products. N-Acetylated LTE4 derivatives were not detected as products formed by human hepatocytes. By a combination of reversed-phase high-performance liquid chromatography and radioimmunoassay, endogenous LTE4 and N-acetyl-LTE4 were detected in human urine in concentrations of 220 +/- 40 and 24 +/- 3 pM, corresponding to 12 +/- 1 and 1.5 +/- 0.2 nmol/mol creatinine, respectively (mean +/- SEM; n = 10). Endogenous LTD4 and LTE4 were detected in human bile (n = 3) in concentrations between 0.2-0.9 nM. Our results demonstrate that LTD4 and LTE4 are major LTC4 metabolites in human bile and/or urine and may serve as index metabolites for the measurement of endogenously generated cysteinyl leukotrienes. Moreover, omega-oxidation and subsequent beta-oxidation from the omega-end contribute to the metabolic degradation of LTE4 not only in monkey but also in man.

Inhibition of leukotriene omega-oxidation by omega-trifluoro analogs of leukotrienes

omega-Oxidation with subsequent beta-oxidation from the omega-end is the major pathway for inactivation and degradation of leukotrienes. Oxidative degradation of leukotriene E4 (LTE4), N-acetyl-LTE4, and LTB4 was inhibited by the omega-trifluoro analogs of LTE4, omega-trifluoro-LTE4 (omega-F3-LTE4), and (1S,2R)-5-(3-[1-hydroxy-15,15,15-trifluoro-2-(2-1H- tetrazol-5-ylethyl-thio)pentadeca-3(E),5(Z)-dienyl+ ++]phenyl)-1H-tetrazole (LY 245769). The latter substance inhibited the oxidative degradation of LTE4 and N-acetyl-LTE4 in the rat in vivo by 50% at a dose of 7 mumol/kg body weight. In rat hepatocyte cultures both omega-trifluoro analogs interfered with the omega-oxidation of N-acetyl-LTE4 and LTB4 with IC50 values of about 4 microM. Both analogs inhibited the omega-hydroxylation in isolated rat liver microsomes with IC50 values between 16 and 37 microM. This inhibition is apparently competitive. In addition, in liver cytosol, the conversion of the omega-hydroxylated leukotrienes to omega-carboxy-LTE4 and omega-carboxy-LTB4 was inhibited by both compounds. omega-Trifluoro analogs of leukotrienes provide a new tool for interfering with the inactivation of leukotrienes in the omega-oxidation pathway.

Characteristics of sinusoidal uptake and biliary excretion of cysteinyl leukotrienes in perfused rat liver

In single-pass perfused rat liver, the sinusoidal uptake of infused 3H-labelled leukotriene (LT) C4 (10 nmol.l-1) was inhibited by sulfobromophthalein. Inhibition was half-maximal at sulfobromophthalein concentrations of approximately 1.2 mumol.l-1 in the influent perfusate and leukotriene uptake was inhibited by maximally 34%. Sulfobromophthalein (20 mumol.l-1) also decreased the uptake of infused [3H]LTE4 (10 nmol.l-1) by 31%. Indocyanine green (10 mumol.l-1) inhibited the sinusoidal [3H]LTC4 uptake by 19%. Replacement of sodium in the perfusion medium by choline decreased the uptake of infused [3H]LTC4 (10 nmol.l-1) by 56%, but was without effect on the uptake of sulfobromophthalein. The canalicular excretion of LTC4, LTD4 and N-acetyl-LTE4 was inhibited by sulfobromophthalein. In contrast, the proportion of polar omega-oxidation metabolites recovered in bile following the infusion of [3H]LTC4 was increased. Taurocholate, which had no effect on the sinusoidal leukotriene uptake, increased bile flow and also the biliary elimination of the radioactivity taken up. With increasing taurocholate additions, the amount of LTD4 recovered in bile increased at the expense of LTC4. Following the infusion of [3H]LTD4 (10 nmol.l-1), a major biliary metabolite was LTC4 indicating a reconversion of LTD4 to LTC4. In the presence of taurocholate (40 mumol.l-1), however, this reconversion was completely inhibited. The findings suggest the involvement of different transport systems in the sinusoidal uptake of cysteinyl leukotrienes. LTC4 uptake is not affected by bile acids and has a sodium-dependent and a sodium-independent component, the latter probably being shared with organic dyes. Sulfobromophthalein also interferes with the canalicular transport of LTC4, LTD4 and N-acetyl-LTE4, but not with the excretion of omega-oxidized cysteinyl leukotrienes. The data may be relevant for the understanding of hepatic leukotriene processing in conditions like hyperbilirubinemia or cholestasis.

Hypoxia and CCl4-induced liver injury, but not acidosis, impair metabolism of cysteinyl leukotrienes in perfused rat liver

Uptake, metabolism and biliary elimination of infused cysteinyl leukotrienes were investigated in single-pass perfused rat liver. Hypoxia did not impair uptake of infused [3H]leukotriene C4, but inhibited biliary excretion of radioactivity by about 50% compared with normoxic control experiments. In addition, the leukotriene metabolite pattern in bile was profoundly altered and was characterized in hypoxia by a 75% to 80% decrease of both leukotriene C4 and polar metabolites, representing omega-oxidation products, whereas the appearance of leukotriene D4 in bile was not affected. Reoxygenation was followed by a marked increase of biliary excretion of polar metabolites, indicating that leukotrienes taken up and stored in the liver cells during the hypoxic period now underwent omega-oxidation with subsequent elimination of the omega-oxidized products. Hypoxia also inhibited the biliary excretion of radioactivity after [3H]leukotriene E4 addition because of an almost complete absence of omega-oxidation products in bile, whereas N-acetyl-leukotriene E4 excretion was not affected. Induction of liver injury by carbon tetrachloride treatment decreased single-pass uptake of [3H]leukotriene C4 by 30%, and only 36% of the radioactivity taken up by the liver was eliminated into bile within 1 hr, compared with 78% in normal livers. The pattern of biliary leukotriene metabolites, however, was not significantly different. Lowering the pH in the perfusion medium from 7.4 to 7.1 had no effect on uptake, metabolism or biliary elimination of infused [3H]leukotriene C4. The data show that hypoxia and experimental liver injury, but not acidosis, impair hepatic processing of cysteinyl leukotrienes. Thus, in leukotriene-induced shock syndromes, leukotriene elimination and inactivation may be impaired giving rise to a "vicious circle."

Hepatic uptake and metabolic disposition of leukotriene B4 in rats

1. In isolated perfused rat liver and in vivo, up to 25% of [3H]leukotriene B4 was eliminated from the circulation via hepatic uptake and biliary excretion within 1 h. Total body recovery of 3H amounted to about 60% of infused [3H]leukotriene B4. 2. Hepatobiliary excretion of leukotriene B4 and its metabolites exceeded renal elimination by about 4-fold and depended, in contrast with excretion of cysteinyl leukotriene E4, upon continuous taurocholate supply. 3. Analyses of bile, liver and recirculated perfusate using h.p.l.c. indicated that the liver metabolized leukotriene B4 extensively to omega-carboxyleukotriene B4 and its beta-oxidized derivatives, and no unmetabolized leukotriene B4 appeared in bile. These results substantiate the important contribution of the hepatobiliary system with respect to the metabolic fate of leukotriene B4.

Inhibition of leukotriene omega-oxidation by isonicotinic acid hydrazide (isoniazid)

Metabolism of leukotrienes via omega-oxidation represents a major degradative and inactivating pathway of these biologically active icosanoids. Isonicotinic acid hydrazide (isoniazid) inhibited this process in rats in vivo, in the isolated perfused rat liver, and in hepatic microsomes. The in vivo catabolism of leukotriene E4 via N-acetyl-leukotriene E4 to its omega-oxidized metabolites was inhibited by 50% or 71% using single intravenous isoniazid doses of 0.6 mmol or 1.0 mmol/kg body mass, respectively. Isoniazid interfered with leukotriene catabolism at the initial omega-oxidation step, resulting in an accumulation of N-acetyl-leukotriene E4. Analogous although weaker inhibition of leukotriene omega-oxidation in vivo was observed by pretreatment with isonicotinic acid 2-isopropylhydrazide and monoacetyl hydrazine. In the isolated perfused liver, isoniazid at concentrations varying over 0.2-10 mM decreased the omega-oxidation of cysteinyl leukotrienes dose-dependently by up to 94%. omega-Oxidation of both leukotriene E4 and leukotriene B4 by rat liver microsomes was inhibited by isoniazid, isonicotinic acid 2-isopropylhydrazide, and monoacetyl hydrazine with half-maximal concentrations in the range of 5-15 mM. Our measurements indicate that the impairment of leukotriene omega-oxidation by isoniazid involves both cytochrome-P450-dependent enzyme systems responsible for omega-oxidation of leukotriene E4 and leukotriene B4. In effect, under isoniazid treatment one can expect a prolongation of the proinflammatory actions of endogenously produced leukotrienes.

Metabolism of leukotriene E4 to 5-hydroxy-6-mercapto7,9-trans-11,14-cis-eicosatetraenoic acid by microfloral cysteine-conjugate beta-lyase and rat cecum contents

Leukotriene E4 was incubated with cysteine-conjugate beta-lyase isolated from the intestinal bacterium Eubacterium limosum. The reaction was terminated by addition of iodoacetic acid or dimethyl sulfate, and the products formed were isolated by reverse-phase high-performance liquid chromatography. The structures of two adducts of a metabolite were determined by uv spectroscopy, by gas-liquid radiochromatography, and by comparisons with chemically synthesized reference compounds. They were 5-hydroxy-6-S-carboxymethylthio-7,9-trans-11,14-cis-eicosatetraeno ic acid (iodoacetic acid adduct) and 5-hydroxy-6-S-methylthio-7,9-trans-11,14-cis-eicosatetraenoic acid (dimethyl sulfate adduct) indicating that the structure of the underivatized metabolite was 5-hydroxy-6-mercapto-7,9,11,14-eicosatetraenoic acid (5,6-HMETE). The latter product is formed by beta-lyase-catalyzed cleavage of the cysteine C-S bond in leukotriene E4. Leukotriene E4 was also metabolized to 5,6-HMETE by rat cecal contents. A product formed was trapped as the iodoacetic acid derivative and identified as 5-hydroxy-6-S-carboxy-methylthio-7,9,11,14-eicosatetraenoic acid. It is concluded that intestinal leukotriene E4, originating from biliary excretion of systemic cysteinyl leukotrienes or produced in the intestine, is converted by microfloral cysteine-conjugate beta-lyase to 5,6-HMETE.

Uptake, production and metabolism of cysteinyl leukotrienes in the isolated perfused rat liver. Inhibition of leukotriene uptake by cyclosporine

1. The isolated perfused rat liver efficiently takes up cysteinyl leukotrienes (LTs) C4, D4, E4 and N-acetyl-LTE4 from circulation. More than 70% of these cysteinyl LTs are excreted from liver into bile within 1 h of onset of a 5 min infusion, while about 5% remain in the liver. About 20% of infused N-acetyl-LTE4 escapes hepatic first-pass extraction under our conditions. 2. Metabolites of LTC4 appearing in bile within 20 min of the onset of infusion include mainly LTD4 and N-acetyl-LTE4, but also omega-hydroxy-N-acetyl-LTE4 and omega-carboxy-N-acetyl-LTE4. Metabolites generated from omega-carboxy-N-acetyl-LTE4 by beta-oxidation from the omega-end represent the major biliary LTs secreted at later times. 3. Stimulation of the isolated perfused liver by the combined infusion of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and the Ca2+ ionophore A23187 results in a transient increase of endogenous cysteinyl LT production, which is independent of extrahepatic cells. 4. The immunosuppressive drug cyclosporine causes a dose-dependent inhibition of hepatobiliary cysteinyl LT excretion, probably by interference with the sinusoidal uptake system for cysteinyl LTs.

Ethanol-induced inhibition of leukotriene degradation by omega-oxidation

omega-Oxidation of leukotrienes is a major pathway in the degradation and inactivation of these proinflammatory mediators. Ethanol inhibited this process in vivo and in vitro. In rat liver in vivo the catabolism of LTE4 to omega-carboxylated leukotrienes was inhibited by 57% by an ethanol dose of 25 mmol/kg body mass administered intragastrically. The site of inhibition was the oxidation of omega-hydroxy-N-acetyl-LTE4 to omega-carboxy-N-acetyl-LTE4 resulting in an accumulation of omega-hydroxy-N-acetyl-LTE4 and of N-acetyl-LTE4. Analogous results were obtained for the oxidative degradation of LTB4 and omega-hydroxy-LTB4 in rat hepatocyte suspensions. Ethanol, at a concentration of 12.5 mmol/l (0.07%; by vol.), caused 68% inhibition of the oxidation of omega-hydroxy-LTB4 by 50% in hepatocyte suspensions. The conversion of omega-hydroxy-LTB4 to omega-carboxy-LTB4 by rat and human liver cytosol was inhibited by ethanol with half maximal concentrations of 100 mumols/l and 300 mumols/l, respectively. Our measurements indicate that direct interference by ethanol of the omega-oxidation of leukotrienes as well as an increased NADH/NAD+ ratio induced by ethanol led to the inhibition of leukotriene degradation. The impairment of leukotriene inactivation in the liver by ethanol may contribute to the development of the inflammatory reaction in acute alcoholic liver disease.

Analysis of cysteinyl leukotrienes in human urine: enhanced excretion in patients with liver cirrhosis and hepatorenal syndrome

The cysteinyl leukotrienes, comprising leukotriene C4 and its metabolites, are biologically most active mediators, eliminated from the blood circulation by the liver and the kidneys. The urine of normal subjects and of patients with hepatic and/or renal failure was analysed for endogenous cysteinyl leukotrienes. The leukotriene metabolites were separated by reversed-phase high-performance liquid chromatography and subsequently quantified by radioimmunoassay. Leukotriene E4 was detected in all urine samples analysed. Its mean concentration increased from 0.3 nmol l-1 in healthy subjects to 0.8 nmol l-1 in patients with liver cirrhosis. In patients with hepatorenal syndrome leukotriene E4 averaged 7.8 nmol l-1; in addition, N-acetyl-leukotriene E4 was detected in an average amount of 1.5 nmol l-1. The mean leukotriene E4/creatinine ratio in urine increased from 0.02 in healthy subjects to 0.11 in patients with liver cirrhosis and to 1.2 mumol leukotriene E4 mol-1 creatinine in patients with hepatorenal syndrome. These results indicate that cysteinyl leukotrienes may play an important role in the mediator network responsible for the development of the hepatorenal syndrome in patients with severe liver disease.

[The Heinrich-Wieland Prize presentation. Metabolism and analysis of leukotrienes in vivo]

Leukotrienes are potent mediators of inflammatory and allergic reactions involved, among others, in endotoxin action and shock, tissue trauma, acute liver injury, hepatorenal syndrome, inflammatory bowel disease, acute pancreatitis, and asthma. Studies on metabolism and analysis of these arachidonate metabolites in vivo are a prerequisite for an improved understanding of their role under physiological and pathophysiological conditions and for the development of inhibitors of leukotriene synthesis and of receptor antagonists. Leukotriene C4 and its metabolites, collectively termed the cysteinyl leukotrienes, are predominantly inactivated by the liver. Rapid hepatocellular uptake is followed by partial metabolic inactivation, comprising omega-oxidation and N-acetylation of leukotriene E4, and excretion into bile. A minor portion of the cysteinyl leukotrienes undergoes enterohepatic circulation. In all species investigated so far, hepatobiliary elimination of cysteinyl leukotrienes predominates over renal excretion. Analysis of the systemic production of cysteinyl leukotrienes in vivo has been accomplished by radioimmunological determination of species-characteristic index metabolites in bile after their separation by high-performance liquid chromatography. The mercapturate N-acetyl-leukotriene E4 is the index metabolite of choice in the rat. In man, leukotriene E4 is the predominant endogenous cysteinyl leukotriene in both bile and urine. The amounts of cysteinyl leukotrienes detected under various pathophysiological conditions may be sufficient to induce known phenomena associated with the respective disease. As shown under experimental conditions, inhibition of leukotriene synthesis or receptor antagonism can serve as therapeutic approaches.

In vivo formation of omega-oxidized metabolites of leukotriene C4 in the rat

[3H8]Leukotriene C4 was administered to germfree rats and to conventional rats having a bile duct cannula. Several radioactive metabolites were isolated. Two polar biliary metabolites from conventional rats were identified as N-acetyl-omega-carboxy-leukotriene E4 and N-acetyl-omega-hydroxy-leukotriene E4. A polar fecal metabolite from germfree rats was found to be N-acetyl-omega-carboxy-leukotriene E4. Chemical identities were established using UV spectroscopy and cochromatographies with authentic compounds in several HPLC systems. The fecal metabolite was further characterized by reductive desulfurization followed by gas-liquid-radiochromatography. The yield of the two biliary metabolites was 5% of the administered tritium after three hours and the yield of fecal N-acetyl-omega-carboxy-leukotriene E4 was 3.5% after three days.