Leukotriene C4 metabolism by hepatoma cells deficient in the uptake of cysteinyl leukotrienes

Increased excretion of endogenous urinary leukotriene E4 in extrahepatic cholestasis

The cysteinyl leukotrienes LTC4, LTD4 and LTE4 are potent lipid mediators eliminated from the blood circulation mainly due to uptake by the liver and the kidneys. In man hepatobiliary elimination of cysteinyl leukotrienes predominates over renal excretion. In the present study, the urine from patients with extrahepatic cholestasis (n = 25) and age- and sex-matched healthy control subjects (n = 25) was analyzed for endogenous LTE4, the predominant metabolite of LTC4 excreted into urine. LTE4 was separated by reversed-phase high-performance liquid chromatography and subsequently quantified by enzyme immunoassay. Healthy subjects excreted a median concentration of 14 nmol LTE4/mol creatinine (range 5-24 nmol/mol creatinine). Its median concentration increased significantly to more than 5-fold higher levels to 74 nmol LTE4/mol creatinine (range 52-93 nmol/mol creatinine) in patients with extrahepatic cholestasis (P < 0.01). These results indicate that extrahepatic cholestasis leads to a compensatory diversion of cysteinyl leukotriene elimination to the kidney with subsequent increased excretion of LTE4 into urine.

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.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

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.

Defective drug uptake contributing to multidrug resistance in hepatoma cells can be evaluated in vitro

In clinical practice the acquired or de novo resistance of tumors to antitumor chemotherapy remains a big problem. However, in the past few years some progress has been made in understanding the two principal mechanisms: metabolic alterations leading to a reduced cytostatic or cytotoxic effect of drugs, and reduced accumulation of drugs within the tumor cells [15, 34, 35]. The second phenomenon is usually attributed to the ability of tumor cells to accelerate the efflux of various xenobiotics. This phenomenon is considered primarily responsible for the development of multidrug resistance (MDR). However, loss or impairment of drug uptake by the tumor cells may also contribute to resistance to antitumor drugs. This paper focuses on recent findings with hepatoma cells, which support this view.

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."

Direct photoaffinity labeling of leukotriene binding sites

Due to their conjugated double bonds the leukotrienes themselves are photolabile compounds and may therefore be used directly for photoaffinity labeling of leukotriene binding sites. Cryofixation eliminates unspecific labeling taking place in solution by photoisomers and photodegradation products of leukotrienes. After fixation of receptor ligand interactions by shock-freezing of the samples, irradiation-induced highly reactive excited states and/or intermediates can form covalent bonds with the respective binding site in the frozen state. After cryofixation of a solution of albumin incubated with [3H8]leukotriene E4, irradiation at 300 nm resulted in time-dependent incorporation of radioactivity into the protein. Photoaffinity labeling of rat as well as of human blood serum with [3H8]leukotriene E4 after cryofixation revealed that only one polypeptide with an Mr of 67,000 was labeled. This polypeptide was identified as albumin. Photoaffinity labeling of rat liver membrane subfractions enriched with sinusoidal membranes resulted in the labeling of a polypeptide with an apparent Mr of 48,000, whereas no polypeptide was predominantly labeled in the subfraction enriched with canalicular membranes. Photoaffinity labeling of isolated hepatocytes disclosed different leukotriene E4 binding polypeptides. In the particulate fraction of hepatocytes a polypeptide with an apparent Mr of 48,000 was labeled predominantly, whereas in the soluble fraction several polypeptides were labeled to a similar extent. One of these, with an apparent Mr of 25,000, was identified as subunit 1 of glutathione transferases by immunoprecipitation. The method of direct photoaffinity labeling in the frozen state after cryofixation using leukotrienes as photoactivatable compounds, as exemplified by leukotriene E4, may be most useful for the identification and characterization of various leukotriene binding sites, including receptors, leukotriene-metabolizing enzymes, and transport systems.

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.

Mechanism of action of cysteinyl leukotrienes on glucose and lactate balance and on flow in perfused rat liver. Comparison with the effects of sympathetic nerve stimulation and noradrenaline

Rat livers were perfused at constant pressure via the portal vein with media containing 5 mM glucose, 2 mM lactate and 0.2 mM pyruvate. 1. Leukotrienes C4 and D4 enhanced glucose and lactate output and reduced perfusion flow to the same extent and with essentially identical kinetics. They both caused half-maximal alterations (area under the curve) of carbohydrate metabolism at a concentration of about 1 nM and of flow at about 5 nM. The leukotriene-C4/D4 antagonist CGP 35949 B inhibited the metabolic and hemodynamic effects of 5 nM leukotrienes C4 and D4 with the same efficiency, causing 50% inhibition at about 0.1 microM. 2. Leukotriene C4 elicited the same metabolic and hemodynamic alterations with the same kinetics as leukotriene D4 in livers from rats pretreated with the gamma-glutamyltransferase inhibitor, acivicin. 3. The calcium antagonist, nifedipine, at a concentration of 50 microM did not affect the metabolic and hemodynamic changes caused by 5 nM leukotriene D4. The smooth-muscle relaxant, nitroprussiate, at a concentration of 10 microM reduced flow changes, without significantly affecting the metabolic alterations. 4. Leukotriene D4 not only reduced flow; it also caused an intrahepatic redistribution of flow, restricting some areas from perfusion. Thus, leukotrienes increased glucose and lactate output directly in the accessible parenchyma and, in addition, indirectly by washout from restricted areas during their reopening upon termination of application. 5. The phospholipase A2 inhibitor, bromophenacyl bromide, but not the cyclooxygenase inhibitor, indomethacin, at a concentration of 20 microM reduced the metabolic and hemodynamic effects of 5 mM leukotriene D4. 6. Stimulation of the sympathetic hepatic nerves with 2-ms rectangular pulses at 20 Hz and infusion of 1 microM noradrenaline increased glucose and lactate output and decreased flow, similar to 10 nM leukotrienes C4 and D4. The kinetics of the metabolic and hemodynamic changes caused by the leukotrienes differed, however, from those due to nerve stimulation and noradrenaline. 7. The leukotriene-C4/D4 antagonist, CGP 35949 B, even at very high concentrations (20 microM) inhibited the metabolic and hemodynamic alterations caused by nerve stimulation or noradrenaline infusion only slightly and unspecifically.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Hepatocyte heterogeneity in response to icosanoids. The perivenous scavenger cell hypothesis

1. The metabolic and hemodynamic effects of prostaglandin F2 alpha, leukotriene C4 and the thromboxane A2 analogue U-46619 were studied during physiologically antegrade (portal to hepatic vein) and retrograde (hepatic to portal vein) perfusion and in a system of two rat livers perfused in sequence. 2. The stimulatory effects of prostaglandin F2 alpha (3 microM) on hepatic glucose release, perfusion pressure and net Ca2+ release were diminished by 77%, 95% and 64%, respectively, during retrograde perfusion when compared to the antegrade direction, whereas the stimulation of 14CO2 production from [1-14C]glutamate by prostaglandin F2 alpha (which largely reflects the metabolism of perivenous hepatocytes) was lowered by only 20%. Ca2+ mobilization and glucose release from the liver comparable to that seen during antegrade perfusion could also be observed in retrograde perfusions; however, higher concentrations of the prostaglandin were required. 3. The glucose, Ca2+ and pressure response to leukotriene C4 (20 nM) or the thromboxane A2 analogue U-46619 (200 nM) of livers perfused in the antegrade direction were diminished by about 90% during retrograde perfusion. Sodium nitroprusside (20 microM) decreased the pressure response to leukotriene C4 (20 nM) and U-46619 (200 nM) by about 40% and 20% in antegrade perfusions, respectively, but did not affect the maximal increase of glucose output. 4. When two livers were perfused antegradely in series, such that the perfusate leaving the first liver (liver I) entered a second liver (liver II), infusion of U-46619 at concentrations below 200 nM to the influent perfusate of liver I increased the portal pressure of liver I, but not of liver II. At higher concentrations of U-46619 there was also an increase of the portal pressure of liver II and with concentrations above 800 nM the pressure responses of both livers were near-maximal [19.6 +/- 0.8 (n = 7) cm H2O and 16.5 +/- 1.1 (n = 8) cm H2O for livers I and II, respectively]. There was a similar behaviour of glucose release from livers I and II in response to U-46619 infusion. When liver I was perfused in the retrograde direction, a significant pressure or glucose response of liver II (antegrade perfusion) could not be observed even with U-46619 concentrations up to 1000 nM. 5. Similarly, the perfusion pressure increase and glucose release induced by leukotriene C4 (10 nM) observed with liver II was only about 20% of that seen with liver I.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of leukotriene D4 catabolism by D-penicillamine

Inhibition of the catabolism of the most biologically potent cysteinyl leukotriene, LTD4, was studied in rat hepatoma cells in vitro and in the rat in vivo. LTD4 dipeptidase, an ectoenzyme on the surface of AS-30D hepatoma cells, exhibited an apparent Km value of 6.6 microM for LTD4. D-Penicillamine and L-penicillamine inhibited this enzyme activity with apparent Ki values of 0.46 mM and 0.21 mM respectively. Bestatin, an inhibitor of the aminopeptidase activity of hepatoma cells, did not affect LTD4 hydrolysis at concentrations as high as 5 mM, indicating that the aminopeptidase did not contribute to LTD4 catabolism. In the rat in vivo, D-penicillamine also inhibited LTD4 catabolism. After intravenous injection of [3H]LTC4 an accumulation of [3H]LTD4 and a retarded formation of [3H]LTE4 were observed in the circulating blood after D-penicillamine pretreatment. Within 1 h after intravenous [3H]LTC4 injection, about 80% of the administered radioactivity was recovered in bile. After D-penicillamine pretreatment [3H]LTD4 was the major biliary leukotriene metabolite, whereas in untreated controls leukotriene metabolites more polar than LTC4 predominated in bile. After stimulation of endogenous leukotriene production in vivo by platelet-activating factor, N-acetyl-LTE4 was the major cysteinyl leukotriene detected in bile. D-Penicillamine treatment prior to platelet-activating factor resulted in the accumulation of LTD4, which under these circumstances was the major endogenous leukotriene metabolite detected in bile.

Leukotriene C4 metabolism by hepatoma cells and liver

The metabolism of the glutathionyl leukotriene LTC4 in the mercapturic acid pathway was studied in suspensions of AS-30D hepatoma cells and hepatocytes, as well as in vivo in the bile duct-cannulated rat and in primates. 1. Isolated hepatocytes actively took up cysteinyl leukotrienes and metabolized LTC4 not only to LTD4 and LTE4 but also to N-acetyl-LTE4 and to metabolites more polar than LTC4. 2. AS-30D hepatoma cells are deficient in the transport system for the uptake of cysteinyl leukotrienes. Peptide cleavage of LTC4 to LTD4 and LTE4 was catalyzed by ectoenzymes of these cells. Inactivation of gamma-glutamyltransferase by acivicin and inhibition of LTD4 dipeptidase by penicillamine largely prevented further catabolism of LTC4 and LTD4, respectively. 3. [3H]LTC4 injected i.v. into rats was rapidly eliminated from the circulating blood, taken up by the liver, and excreted into bile where 77% of the administered radioactivity was recovered within 1 hr. The biliary LTC4 metabolites included LTD4, N-acetyl-LTE4, and metabolites more polar than LTC4. 4. Inhibition of [3H]LTC4 metabolism in vivo by i.v. penicillamine shifted the pattern of biliary cysteinyl leukotrienes; an extended half-life of [3H]LTD4 was associated with a retarded formation of N-acetyl-LTE4 and of polar metabolites. 5. Endogenous cysteinyl leukotrienes elicited by trauma were measured after HPLC separation by radioimmunologic analysis in plasma and bile of rats. The biliary concentration of these leukotrienes was up to 100 times as great as in plasma. N-Acetyl-LTE4 was the predominant endogenous metabolite in rat bile. 6. In the monkey Macaca fascicularis, cysteinyl leukotrienes were predominantly eliminated from blood via the liver into bile; renal excretion amounted to about 50% of the hepatobiliary elimination. Absorption of cysteinyl leukotrienes from the intestine resulted in enterohepatic circulation of these mediators. 7. Metabolites of [3H]LTC4 injected i.v. in the monkey were analyzed in bile and urine. In addition to polar metabolites and a small percentage of [3H]LTD4, [3H]LTE4 was a predominant metabolite particularly in bile. LTE4 was also the major endogenous cysteinyl leukotriene detected by radioimmunologic analysis in monkey bile. 8. LTE4 was the predominant endogenous cysteinyl leukotriene measured in human bile in patients suffering from acute pancreatitis. The detected amounts of LTE4 may be sufficient to induce known phenomena associated with acute pancreatitis including the shock-like reaction.