Ethanol-induced inhibition of leukotriene degradation by omega-oxidation

Ablation of Selenbp1 Alters Lipid Metabolism via the Pparα Pathway in Mouse Kidney

Selenium-binding protein 1 (Selenbp1) is a 2,3,7,8-tetrechlorodibenzo-p-dioxin inducible protein whose function is yet to be comprehensively elucidated. As the highly homologous isoform, Selenbp2, is expressed at low levels in the kidney, it is worthwhile comparing wild-type C57BL mice and Selenbp1-deficient mice under dioxin-free conditions. Accordingly, we conducted a mouse metabolomics analysis under non-dioxin-treated conditions. DNA microarray analysis was performed based on observed changes in lipid metabolism-related factors. The results showed fluctuations in the expression of numerous genes. Real-time RT-PCR confirmed the decreased expression levels of the cytochrome P450 4a (Cyp4a) subfamily, known to be involved in fatty acid ω- and ω-1 hydroxylation. Furthermore, peroxisome proliferator-activated receptor-α (Pparα) and retinoid-X-receptor-α (Rxrα), which form a heterodimer with Pparα to promote gene expression, were simultaneously reduced. This indicated that reduced Cyp4a expression was mediated via decreased Pparα and Rxrα. In line with this finding, increased levels of leukotrienes and prostaglandins were detected. Conversely, decreased hydrogen peroxide levels and reduced superoxide dismutase (SOD) activity supported the suppression of the renal expression of Sod1 and Sod2 in Selenbp1-deficient mice. Therefore, we infer that ablation of Selenbp1 elicits oxidative stress caused by increased levels of superoxide anions, which alters lipid metabolism via the Pparα pathway.

Biosynthesis and metabolism of leukotrienes

Leukotrienes are metabolites of arachidonic acid derived from the action of 5-LO (5-lipoxygenase). The immediate product of 5-LO is LTA4 (leukotriene A4), which is enzymatically converted into either LTB4 (leukotriene B4) by LTA4 hydrolase or LTC4 (leukotriene C4) by LTC4 synthase. The regulation of leukotriene production occurs at various levels, including expression of 5-LO, translocation of 5-LO to the perinuclear region and phosphorylation to either enhance or inhibit the activity of 5-LO. Several other proteins, including cPLA2α (cytosolic phospholipase A2α) and FLAP (5-LO-activating protein) also assemble at the perinuclear region before production of LTA4. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by ω-oxidation carried out by specific cytochrome P450s (CYP4F) followed by β-oxidation from the ω-carboxy position and after CoA ester formation. Other specific pathways of leukotriene metabolism include the 12-hydroxydehydrogenase/15-oxo-prostaglandin-13-reductase that forms a series of conjugated diene metabolites that have been observed to be excreted into human urine. Metabolism of LTC4 occurs by sequential peptide cleavage reactions involving a γ-glutamyl transpeptidase that forms LTD4 (leukotriene D4) and a membrane-bound dipeptidase that converts LTD4 into LTE4 (leukotriene E4) before ω-oxidation. These metabolic transformations of the primary leukotrienes are critical for termination of their biological activity, and defects in expression of participating enzymes may be involved in specific genetic disease.

ω-Oxidation of 20-Hydroxyeicosatetraenoic Acid (20-HETE) in Cerebral Microvascular Smooth Muscle and Endothelium by Alcohol Dehydrogenase 4

20-Carboxyeicosatetraenoic acid (20-COOH-AA) is a bioactive metabolite of 20-hydroxyeicosatetraenoic acid (20-HETE), an eicosanoid that produces vasoconstriction in the cerebral circulation. We found that smooth muscle (MSMC) and endothelial (MEC) cultures obtained from mouse brain microvessels convert [3H]20-HETE to 20-COOH-AA, indicating that the cerebral vasculature can produce this metabolite. The [3H]20-COOH-AA accumulated primarily in the culture medium, together with additional radiolabeled metabolites identified as the chain-shortened dicarboxylic acids 18-COOH-18:4, 18-COOH-18:3, and 16-COOH-16:3. N-Heptylformamide, a potent inhibitor of alcohol dehydrogenase (ADH), decreased the conversion of [3H]20-HETE to 20-COOH-AA by the MSMC and MEC and also by isolated mouse brain microvessels. Purified mouse and human ADH4, human ADH3, and horse liver ADH1 efficiently oxidized 20-HETE, and ADH4 and ADH3 were detected in MSMC and MEC by Western blotting. N-Heptylformamide inhibited the oxidation of 20-HETE by mouse and human ADH4 but not by ADH3. These results demonstrated that cerebral microvessels convert 20-HETE to 20-COOH-AA and that ADH catalyzes the reaction. Although ADH4 and ADH3 are expressed in MSMC and MEC, the inhibition produced by N-heptylformamide suggests that ADH4 is primarily responsible for 20-COOH-AA formation in the cerebral microvasculature.

Cysteinyl-leukotrienes and the liver

Leukotrienes are potent biological mediators implicated in an increasing number of disease processes. This review outlines the basic biology of leukotrienes and discusses recent developments in our understanding of the specific role of cysteinyl-leukotrienes (cLTs) in cholestasis, hepatic inflammation, portal hypertension, and the pathogenesis of the hepatorenal syndrome (HRS).

Urinary metabolites of leukotriene B4 in the human subject

Leukotriene B4 (LTB4) is a potent chemoattractant for neutrophils and is thought to play a role in a variety of inflammatory responses in humans. The metabolism of LTB4 in vitro is complex with several competing pathways of biotransformation, but metabolism in vivo, especially for normal human subjects, is poorly understood. As part of a Phase I Clinical Trial of human tolerance to LTB4, four human subjects were injected with 150 nmol/kg LTB4 with one additional subject as placebo control. The urine of the subjects was collected in two separate pools (0-6 and 7-24 h), and aliquots from these urine collections were analyzed using high performance liquid chromatography, UV spectroscopy, and negative ion electrospray ionization tandem mass spectrometry for metabolites of LTB4. In the current investigation, 11 different metabolites of LTB4 were identified in the urine from those subjects injected with LTB4, and none were present in the urine from the placebo-injected subject. The unconjugated LTB4 metabolites found in urine were structurally characterized as 18-carboxy-LTB4, 10,11-dihydro-18-carboxy-LTB4, 20-carboxy-LTB4, and 10,11-dihydro-20-carboxy-LTB4. Several glucuronide-conjugated metabolites of LTB4 were characterized including 17-, 18-, 19-, and 20-hydroxy-LTB4, 10-hydroxy-4,6,12-octadecatrienoic acid, LTB4, and 10,11-dihydro-LTB4. The amount of LTB4 glucuronide (16.7-29.4 pmol/ml) and 20-carboxy-LTB4 (18.9-30.6 pmol/ml) present in the urine of subjects injected with LTB4 was determined using an isotope dilution mass spectrometric assay before and after treatment of the urine samples with beta-glucuronidase. The urinary metabolites of LTB4 identified in this investigation were excreted in low amounts, yet it is possible that one or more of these metabolites could be used to assess LTB4 biosynthesis following activation of the 5-lipoxygenase pathway in vivo.

Leukotriene B4 omega-side chain hydroxylation by CYP4F5 and CYP4F6

Leukotriene B(4) (LTB(4)) is a lipid mediator that plays an important role in inflammation. Metabolism of LTB(4) by cytochrome P450 (CYP) enzymes belonging to the CYP4F subfamily is considered to be of importance for the regulation of inflammation. This study investigates LTB(4) metabolism by recombinant rat CYP4F5 and CYP4F6 expressed in a yeast system and by microsomes isolated from rat organs expressing CYP4F mRNA. CYP4F6 was found to convert LTB(4) into 19-hydoxy- and 18-hydroxy-LTB(4) with an apparent K(m) of 26 microM, and CYP4F5 was found to convert LTB(4) primarily into 18-hydroxy-LTB(4) with an apparent K(m) of 9.7 microM. The rate of formation of 18-hydroxy-LTB(4) by CYP4F5 was surprisingly high. At a substrate concentration of 30 microM, the rate of formation was about 15 nmol/min/mg microsomal protein, approximately 30 times faster than the reaction catalyzed by CYP4F6. Analysis of LTB(4) metabolism by microsomes isolated from various tissues from the rat suggests that CYP4F5 and CYP4F6 are active in the lung and to some extent in the brain, kidney, and testis. CYP4F5 and CYP4F6, due to their capacities to metabolize LTB(4), may play important roles in modulating inflammatory response in these organs.

Primary hepatocytes of Tupaia belangeri as a potential model for hepatitis C virus infection

Hepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide, but the study of HCV infection has been hampered by the lack of an in vitro or in vivo small animal model. The tree shrew Tupaia belangeri is susceptible to infection with a variety of human viruses in vivo, including hepatitis viruses. We show that primary Tupaia hepatocytes can be infected with serum- or plasma-derived HCV from infected humans, as measured by de novo synthesis of HCV RNA, analysis of viral quasispecies evolution, and detection of viral proteins. Production of infectious virus could be demonstrated by passage to naive hepatocytes. To assess whether viral entry in Tupaia hepatocytes was dependent on the recently isolated HCV E2 binding protein CD81, we identified and characterized Tupaia CD81. Sequence analysis of cloned Tupaia cDNA revealed a high degree of homology between Tupaia and human CD81 large extracellular loops (LEL). Cellular binding of E2 and HCV infection could not be inhibited by anti-CD81 antibodies or soluble CD81-LEL, suggesting that viral entry can occur through receptors other than CD81. Thus, primary Tupaia hepatocytes provide a potential model for the study of HCV infection of hepatocytes.

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.

Metabolism of leukotrienes is impaired in hepatocytes from rats with thioacetamide-induced liver cirrhosis

It is likely that the hepatocellular metabolism of potent mediators of inflammation is impaired in chronic liver injury. Therefore, in this study the degradation of the leukotrienes LTC4, LTE4 and LTB4 was investigated in isolated liver parenchymal cells (LPC) from rats with thioacetamide-induced macronodular liver cirrhosis or after bile duct ligation. The degradation of LTE4 as well as the formation of N-acetyl-LTE4 was significantly delayed in LPC from macronodular cirrhotic rats but not in those from bile duct-ligated rats. LPC from macronodular cirrhotic rats eliminated LTC4 at the same rate as isolated hepatocytes from control animals. The rate of LTB4 degradation was significantly decreased by 35% in LPC from macronodular cirrhotic rats. Furthermore, the rate of LTB4 hydroxylation was significantly lower by 50% in microsomes isolated from hepatocytes of macronodular cirrhotic rats than in those from controls. In summary, one may conclude that the N-acetylation reaction of LTE4 and the hydroxylation reaction of LTB4 is impaired in LPC from rats with thioacetamide-induced macronodular cirrhosis.

Cloning and expression of a novel form of leukotriene B4 omega-hydroxylase from human liver

We have isolated and sequenced a cDNA for human liver LTB4 omega-hydroxylase. The cDNA encoded a protein of 520 amino acids with a molecular weight of 59,853 Da. The cDNA-deduced amino acid sequence showed 87.3% homology to that of human polymorphonuclear leukocytes (PMN) LTB4 omega-hydroxylase (CYP4F3). Northern blot analysis revealed that the mRNA hybridized to the specific cDNA fragment is expressed in human liver, but not in human PMN. The microsomes from yeast cells transfected with the cDNA catalyzed the omega-hydroxylation of LTB4 with a Km of 44.8 microM. These results clearly show that a new form of the CYP4F LTB4 omega-hydroxylase exists in human liver.

Leukotriene B4 formation upon halothane-induced lipid peroxidation in liver membrane fractions under low O2 concentrations in vitro

Lipid peroxidation was induced in rat liver membrane fractions in vitro upon NADPH-dependent metabolic activation of the anesthetic agent halothane at low O2 concentrations. Halothane-induced lipid peroxidation was dependent on time, concentration of halothane, and the calculated O2 concentrations present in the system. Lipid peroxidation was inducible at increasing O2 concentrations up to 12 microM, decreased at higher O2 concentrations up to 48 microM, and was not detectable at normoxic conditions. Leukotriene B4 (LTB4) was identified as a product arising upon lipid peroxidation by reverse-phase high-pressure liquid chromatography combined with a radioimmunoassay. LTB4 formation was maximal under conditions of maximal lipid peroxidation at a calculated O2 concentration of 12 microM. Even at high concentrations, the 5-lipoxygenase inhibitors MK886 (10 microM), ZD2138 (20 microM), and ZM230487 (20 microM) were not inhibitory in halothane-induced lipid peroxidation nor in the associated formation of LTB4. Synthetic LTB4 was transformed into its 20-hydroxy derivative by omega-oxidation in an O2-concentration-dependent manner, being considerably reduced at the low O2 concentrations that maximally promoted lipid peroxidation. The collective evidence of these data raises the possibility that exposure to halothane might lead to peroxidation-associated net synthesis of LTB4 through 5-lipoxygenase-independent escape routes in liver tissue under physiologically or pathophysiologically low O2 concentrations.

Improving medical approaches to primary CNS malignancies--retinoid therapy and more

Successfully inducing differentiation in ectodermal diseases, retinoids harbour considerable therapeutic potential in the treatment of neuroectodermal-neuroepithelial malignancies. The principal tissue retinoid, retinoic acid, can be potently upregulated in vivo by a relatively specific catabolic inhibitor, R75251 (liarozole). Both substances have been given orally over 2 years in addition to standard treatment, and have been well tolerated. Corresponding closely to plasma retinoid levels, cutaneous side effects facilitate individual dosing. We evaluate this adjuvant retinoid approach and additional efforts to improve therapy of primary CNS malignancies, including the topical administration of retinoids in gamma linolenic acid.

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.

Leukotriene uptake by hepatocytes and hepatoma cells

The uptake of tritiated cysteinyl leukotrienes (LTC4, LTD4, LTE4) and LTB4 was investigated in freshly isolated rat hepatocytes and different hepatoma cell lines under initial-rate conditions. Leukotriene uptake by hepatocytes was independent of an Na+ gradient and a K+ diffusion potential across the hepatocyte membranes as established in experiments with isolated hepatocytes and plasma membrane vesicles. Kinetic experiments with isolated hepatocytes indicated a low-Km system and a non-saturable system for the uptake of cysteinyl leukotrienes as well as LTB4 under the conditions used. AS-30D hepatoma cells and human Hep G2 hepatoma cells were deficient in the uptake of cysteinyl leukotrienes, but showed significant accumulation of LTB4. Moreover, only LTB4 was metabolized in Hep G2 hepatoma cells. Competition studies on the uptake of LTE4 and LTB4 (10 nM each) indicated inhibition by the organic anions bromosulfophthalein, S-decyl glutathione, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate, probenecid, docosanedioate, and hexadecanedioate (100 microM each), but not by taurocholate, the amphiphilic cations verapamil and N-propyl ajmaline, and the neutral glycoside ouabain. Cholate and the glycoside digitoxin were inhibitors of LTB4 uptake only. Bromosulfophthalein, the strongest inhibitor of leukotriene uptake by hepatocytes, did not inhibit LTB4 uptake by Hep G2 hepatoma cells under the same experimental conditions. Leukotriene-binding proteins were analyzed by comparative photoaffinity labeling of human hepatocytes and Hep G2 hepatoma cells using [3H]LTE4 and [3H]LTB4 as the photolabile ligands. Predominant leukotriene-binding proteins with apparent molecular masses in the ranges of 48-58 kDa and 38-40 kDa were labeled by both leukotrienes in the particulate and in the cytosolic fraction of hepatocytes, respectively. In contrast, no labeling was obtained with [3H]LTE4 in Hep G2 cells. With [3H]LTB4 a protein with a molecular mass of about 48 kDa was predominantly labeled in the particulate fraction of the hepatoma cells, whereas in the cytosolic fraction a labeled protein in the range of 40 kDa was detected. Our results provide evidence for the existence of distinct uptake systems for cysteinyl leukotrienes and LTB4 at the sinusoidal membrane of hepatocytes; however, some of the inhibitors tested interfere with both transport systems. Only LTB4, but not cysteinyl leukotrienes, is taken up and metabolized by the transformed hepatoma cells.

Novel 3-hydroxylated leukotriene b4 metabolites from ethanol-treated rat hepatocytes

Coincubations of radiolabeled leukotriene B4 (LTB4) and ethanol with isolated rat hepatocytes led to formation of one dihydroxylated and two novel β-oxidized metabolites of LTB4. The major radioactive peaks from reverse-phase-high performance liquid chromatography (RP-HPLC) eluted with material absorbing UV light maximally at 270 nm, with shoulders at 260 and 280 nm, indicating retention of the conjugated triene structure of the parent molecule in each metabolite structure. Following purification, catalytic reduction, and derivatization, mass spectrometric analysis revealed that all three metabolites were hydroxylated at the C-3 carbon atom based on characteristic ions at m/z 201 and 175 in the electron ionization mass spectra of the metabolites. Negative-ion electron capture mass spectrometry of the metabolites as pentafluorobenzyl (PFB) ester, trimethylsilyl ether derivatives aided structural characterizations while revealing interesting fragmentations. A ketene-containing ion appeared to result from the loss of both PFB groups (one as PFB alcohol), while a lactone alkoxide ion appeared to result following loss of PFB and bis (trimethylsilyl) ether. From these data three novel LTB4 metabolites were suggested to be 3,20-dihydroxy-LTB4 (3,20-diOH-LTB4), 3-hydroxy-18-carboxy-LTB4 (3-OH-18-COOH-LTB4), and 3-hydroxy-16-carboxy-LTB3 (3-OH-16-COOH-LTB3). The significance of the almost exclusive formation of these 3-hydroxylated LTB4 metabolites in the presence of ethanol is currently unknown, but may result from interrupted β-oxidation from the C-1 carboxyl moiety.

Chemotactic LTB4 metabolites produced by hepatocytes in the presence of ethanol

Ethanol in low concentrations significantly alters the hepatocyte metabolism of the neutrophil chemotactic lipid leukotriene B4 (LTB4). Two novel metabolites of LTB4 which are encountered only when ethanol is present, retained significant biological activity. One metabolite, 3-hydroxy-LTB4 increased intracellular free calcium in the human neutrophil at concentrations as low as 3 x 10(-10) M as well as induced shape change and adherence to albumin-coated latex beads at 10 nM. The 3-hydroxy-LTB4 and 3,20-hydroxy-LTB4 metabolites were also potent chemotactic agonists with an ED50 at 3.0 and 9.0 nM, respectively. These results suggest that the presence of ethanol can substantially alter inactivation of LTB4 by the liver and may mediate neutrophil accumulation into the liver, thereby contributing to the pathogenesis of alcoholic hepatitis even when LTB4 biosynthesis occurs at some site distant to the liver.

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.

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.

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.

Metabolic inactivation of leukotrienes

The metabolic inactivation of the cysteinyl leukotrienes LTC4 and LTD4 and of the chemotactic LTB4 was studied in the rat in vivo and in hepatocyte suspensions, respectively. 1. Deactivation of LTC4 via LTD4 to LTE4 was a most active process in the blood circulation, catalyzed mainly by ectoenzymes located on the internal wall of blood vessels. Uptake of cysteinyl leukotrienes by hepatocytes and kidney cells contributed to the rapid elimination of these potent mediators whenever they were released into the blood circulation. The initial half-life of LTC4 in vivo was 12 seconds. 2. omega-Oxidation leads to the formation of omega-hydroxylated and omega-carboxylated cysteinyl leukotrienes which were detected in bile and urine. Biliary metabolites included those formed by stepwise beta-oxidative degradation of omega-carboxy-N-acetyl-LTE4, yielding the dinor, tetranor, and hexanor derivative. 3. The peroxisome proliferator clofibrate strongly increased the degradation of LTE4 by omega-oxidation and subsequent beta-oxidation in vivo. The generation of new polar metabolites was detected by HPLC methods and by the use of 3H8-labeled cysteinyl leukotrienes in comparison with the 3H2-labeled precursor. 4. The metabolic degradation and inactivation of cysteinyl leukotrienes in vivo and of LTB4 in isolated hepatocytes was potently inhibited by ethanol. The site of inhibition was the oxidation of omega-hydroxy-N-acetyl-LTE4 and of omega-hydroxy-LTB4 to the respective omega-carboxylated metabolite. This inhibition led to an accumulation of the biologically active LTB4 and of N-acetyl-LTE4. The interference of leukotriene inactivation in the liver may provide a novel explanation for the ethanol-induced inflammatory reaction in acute alcoholic liver disease.