Biliary and urinary excretion of peptide leukotrienes in the domestic pig

ALOX5 gene variants affect eicosanoid production and response to fish oil supplementation

The objective of this study was to determine whether 5-lipoxygenase (ALOX5) gene variants associated with cardiovascular disease affect eicosanoid production by monocytes. The study was a randomized, double-masked, parallel intervention trial with fish oil (5.0 g of fish oil daily, containing 2.0 g of eicosapentaenoic acid [EPA] and 1.0 g of docosahexaenoic acid [DHA]) or placebo oil (5.0 g of corn/soy mixture). A total of 116 subjects (68% female, 20-59 years old) of African American ancestry enrolled, and 98 subjects completed the study. Neither ALOX5 protein nor arachidonic acid-derived LTB4, LTD4, and LTE4 varied by genotype, but 5-hydroxyeicosatetraenoate (5-HETE), 6-trans-LTB4, 5-oxo-ETE, 15-HETE, and 5,15-diHETE levels were higher in subjects homozygous for the ALOX5 promoter allele containing five Sp1 element tandem repeats ("55" genotype) than in subjects with one deletion (d) (three or four repeats) and one common ("d5" genotype) allele or with two deletion ("dd") alleles. The EPA-derived metabolites 5-HEPE and 15-HEPE and the DHA-derived metabolite 17-HDoHE had similar associations with genotype and increased with supplementation; 5-HEPE and 15-HEPE increased, and 5-oxo-ETE decreased to a greater degree in the 55 than in the other genotypes. This differential eicosanoid response is consistent with the previously observed interaction of these variants with dietary intake of omega-3 fatty acids in predicting cardiovascular disease risk.

Tissue distribution, elimination and metabolism of [3H]-leukotriene C4 in the American bullfrog, Rana catesbeiana

Tissue distribution, elimination, and metabolism of [3H]-leukotriene C4 were studied at 2.5 hours after injection in the conscious and anesthetized American bullfrog, Rana catesbeiana. Conscious frogs were injected via the dorsal lymph sac or the sciatic vein. Anesthetized frogs were injected via the abdominal vein. The organs containing the greatest percent of injected radioactivity at 2.5 hours after injection were liver, small intestine and kidney. Route of injection and anesthesia appears to alter distribution and elimination of leukotrienes. [3H]-leukotrienes were eliminated into bladder water and bile. In addition, 7.8 +/- 2.2 and 5.2 +/- 2.5 percent of the injected radioactivity was found in the pan water bathing the ventral surface of the venously and dorsally injected conscious frogs, respectively, suggesting transfer of radioactivity across the skin. At 2.5 hours, polar metabolites represented 50% of the radioactivity found in liver, bile, and bladder water. These polar metabolites were determined to be 18-carboxy-19,20-dinor-leukotriene E4, 20-carboxy-leukotriene E4, and 20-hydroxy-leukotriene E4. Of the non-oxidized leukotrienes, bile contained mainly LTD4 while bladder water contained primarily LTE4. N-acetyl LTE4 was not detected in any samples. The tissue distribution, elimination and metabolism of leukotrienes in the bullfrog was similar to mammalian studies and suggests evolutionary conservation of leukotriene processing.

Influence of leukotriene D4 on arterial pressure and gastrointestinal electrical activity in the conscious piglet

In 6 conscious weaned piglets with implanted electrodes in the corpus and antrum of the stomach, the duodenum, jejunum, ileum and caecum the influence of intravenous infusion of leukotriene (LT)D4, 0.1 and 1 microgram kgmin for 10 min, on mean arterial pressure and gastrointestinal electrical activity was examined. LTD4 induced a significant increase in arterial pressure. Gastrointestinal electrical activity, however, was little influenced, since only the antrum pylori revealed a transient decrease.

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.

Application of reversed phase high performance liquid chromatography to the analysis of sulphidopeptide leukotrienes in pig bile

Reversed phase HPLC methodology has been developed for separation of peptide leukotrienes and indomethacin in porcine bile. Reproducible recoveries were obtained using radioactive leukotrienes ([3H]LTC4, 57.1 +/- 2.5%; [3H]LTE4, 62.7 +/- 1.9%; [3H]LTD4, 54.3 +/- 2.7%). Radioimmunoassay of column eluant demonstrated that as little as 300 pg of exogenous leukotrienes could be measured in bile fluids, with similar recoveries. Analysis of bile sampled 60-90 min after initiation of experimental endotoxic shock in indomethacin treated pigs revealed a leukotriene concentration of 5.24 +/- 1.16 ng/mL(LTD4). This was significantly greater (p less than 0.05, n = 3) than that observed in samples collected prior to endotoxin (0.42 +/- 0.23 ng/mL), or from untreated animals (0.85 +/- 0.51 ng/mL). This method is thus applicable to investigation of the role of 5-lipoxygenase products in porcine models of human disease, including shock conditions such as endotoxaemia, during cyclooxygenase inhibition by indomethacin.

Metabolism and excretion of exogenous [3H]-LTC4 in primates

Four novel omega- and beta-oxidation (from the omega end) products of peptide leukotrienes, 20-hydroxy and 20-carboxy-LTE4, 18-carboxy-19, 20-dinor-LTE4 and 16-carboxy-17,18,19,20-tetranor-14,15-dihydro-LTE4 were prepared by total synthesis and used as standards for identification of biliary and urinary metabolites in the cynomolgus monkey. After intravenous administration 14, 15-[3H] leukotriene C4 (10 microCi kg-1) was partially metabolized in and rapidly cleared from the vascular circulation. This resulted, within 24 hours, in significant urinary excretion (14.8 +/- 2.1%, n = 4), consisting largely of material more polar than LTE4 (61% of urinary excretion) as shown by reverse phase HPLC. The polar fraction demonstrated two predominant metabolites which coeluted in several HPLC solvent systems with synthetic 16-carboxytetranordihydro-LTE4 (major component) and 18-carboxydinor-LTE4 (minor component). Characterization of the major polar metabolite as 16-carboxytetranordihydro-LTE4 was substantiated by conversion to its N-acetylated derivative. The absence of the 14, 15 double bond was confirmed by product analysis of oxidative ozonolysis. In a single animal, the bile duct was cannulated, with significant biliary excretion of radioactivity demonstrated over 4 hours (58.6% recovery). The predominant polar biliary metabolites were also identified as the 18-carboxydinor and 16-carboxytetranordihydro derivatives of LTE4 mentioned above. These data suggest that beta-oxidation products generated from the omega-carboxyl end of the 20-carboxy-LTE4 are important products of [3H] LTC4 metabolism in the monkey. Quantitation of these urinary metabolites may be an important index of in vivo leukotriene production.

Metabolism of leukotriene C4 in the anesthetized guinea pig

The distribution and metabolism of [3H] leukotriene (LT)C4 has been studied in the anesthetized guinea pig. The intravenous administration of [3H] LTC4 (1 muCi/kg) to seven guinea pigs showed a rapid vascular clearance of radioactivity with significant metabolism evident at the 15 sec and 1 min time points with material chromatographing like LTC4 (45.6 +/- 7.5%, 35.0 +/- 4.4%). LTD4 (18.4 +/- 5.1%, 33.2 +/- 4.4%) as well as polar material (25.5 +/- 6.0%, 29.7 +/- 4.7%) respectively. The biliary recovery of radioactivity was found to be 74.5 +/- 5.5% n = 4, over 120 min in the guinea pig with less than 1% of radioactivity present in the urine. Examination of the metabolic profile of the biliary radioactivity showed total conversion of LTC4 to LTD4 which was the major metabolite at early time points, and LTD4 as well as LTE4 at later time points. Significant radioactivity which increased with time was also present at the solvent front of the chromatogram indicating the presence of polar biliary metabolites. These results show that the major route of elimination of peptide leukotrienes is through the bile duct in the anesthetized guinea pig and that LTD4 is the major eliminated metabolite in this model.

Leukotrienes. Biology and role in disease

Leukotrienes are a novel group of chemical messengers derived from arachidonic acid. They are produced by several different tissues by processes linked to phospholipid flux in response to specific stimuli. The leukotrienes interact with specific receptors in target cell membranes to initiate a response. Most of these responsive cells are derived from bone marrow, skin, smooth muscle, and vascular endothelium. Leukotrienes are powerful mediators of inflammation and smooth muscle contraction, and there is increasing evidence that they are important factors in immune-mediated disease. Several available effective antiinflammatory drugs may act partially by inhibiting the production of leukotrienes.

[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.

Synthesis of beta-oxidation products as potential leukotriene metabolites and their detection in bile of anesthetized rat

Two novel beta-oxidation products of peptido leukotrienes, 16-carboxy-17,18,19,20-tetranor-14,15-dihydro-N-acetyl LTE4 and 18-carboxy-19,20-dinor-N-acetyl LTE4, were prepared by total synthesis and used to identify previously unknown polar rat biliary metabolites. When [3H] LTC4 and synthetic N-acetyl-LTE4 were administered intravenously to anesthetized inbred male rats, extraction of the bile and subsequent reverse-phase HPLC fractionation allowed the isolation of two novel metabolites of N-acetyl-LTE4. Comparison of U.V. spectra and coelution experiments revealed that these metabolites correspond to the above-mentioned synthetic beta-oxidation products. This was further confirmed by the coelution of the corresponding methyl esters. Oxidative ozonolysis of the metabolically produced 16-carboxy-17,18,19,20-tetranor-14,15-dihydro-N-acetyl LTE4 (major metabolite) confirmed the absence of the 14,15-unsaturation. The presence of these metabolites indicates that peptide leukotrienes undergo N-acetylation followed by omega and subsequent beta-oxidation in the anesthetized rat.

N-acetyl-leukotriene E4 is a potent constrictor of rat mesenteric vessels

N-Acetyl-leukotriene E4 administered to conscious freely moving rats produced a dose-dependent vasoconstriction in the mesenteric vessels which led to profound reduction of blood flow to the gut. Renal and hindquarter blood flow and vascular resistance were not affected even by high doses of N-Acetyl-leukotriene E4. N-Acetyl-leukotriene E4 was 10-fold more potent than the thromboxane analog U-46619 and 1000-fold more potent than prostaglandin F2 alpha but 2-5-fold less potent than leukotriene D4/E4 to induce mesenteric vasoconstriction. These data indicate that N-acetyl-leukotriene E4 is a biologically active metabolite of peptide leukotrienes, and might play a role in cardiovascular derangements mediated by leukotrienes.

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

Leukotriene E4 was metabolized to two polar products by rat liver microsomes. These products were characterized by physico-chemical and chemical techniques. The chemical structures, (5S, 6R)-5,20-dihydroxy-6S-cysteinyl-7,9-trans-11,14-cis-icosatetraenoic acid (omega-hydroxy-leukotriene E4) and (5S, 6R)-5-hydroxy-6S-cysteinyl-7,9-trans-11,14-cis-icosatetraen-1,20-d ioic acid (omega-carboxy-leukotriene E4) suggested that leukotriene E4 was transformed by an omega-hydroxylase and omega-hydroxyleukotriene E dehydrogenase in sequence. N-Acetyl-leukotriene E4 was also transformed by these enzymes, but at a rate six times lower than leukotriene E4. The products formed from N-acetylleukotriene E4 were characterized as being N-acetyl-omega-hydroxy-leukotriene E4 and N-acetyl-omega-carboxy-leukotriene E4. Other substrates were 11-trans-leukotriene E4 and N-acetyl-11-trans-leukotriene E4. In contrast, leukotrienes C4 and D4 were not converted into omega-oxidized metabolites. The leukotriene E omega-hydroxylase reaction required NADPH and molecular oxygen as cofactors, and was most rapidly catalyzed by liver microsomes. Liver cytosol, fortified with NAD+, converted omega-hydroxyleukotriene E4 and N-acetyl-omega-hydroxy-leukotriene E4 into omega-carboxy metabolites. Microsomes contained at least 18 times less omega-hydroxy-leukotriene E dehydrogenase activity than did cytosol. Liver microsomes supplemented with acetyl-coenzyme A converted omega-hydroxy and omega-carboxy-leukotriene E4 into the corresponding N-acetyl derivatives. The novel enzyme, leukotriene E omega-hydroxylase, which is described here is distinct from a previously described leukotriene B omega-hydroxylase based on substrate competition and kinetic data.

w-oxidation products of leukotriene E4 in bile and urine of the monkey

The intravenous administration of [3H]leukotriene C4 in the monkey Macaca fascicularis results in the biliary and urinary elimination of [3H]leukotriene D4 and [3H]leukotriene E4 in addition to more-polar metabolites. Separation of these polar metabolites and chromatographic comparison with synthetic w-oxidized leukotrienes indicated the in vivo formation of w-hydroxy-[3H]leukotriene E4 and w-carboxy-[3H]leukotriene E4. Time course studies of the [3H]leukotriene metabolite pattern in bile and urine showed that w-hydroxy-leukotriene E4 was decreasing as w-carboxy-leukotriene E4 and additional polar derivatives were increasing.