Isolation and characterization of 15-hydroxylated metabolites of leukotriene C4

Conversion of 5,6-dihydroxyeicosatetraenoic acids. A novel pathway for lipoxin formation by human platelets

Leukotriene A4 may be metabolized to 5(S),6(R)- and 5(S),6(S)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acids by enzymatic or non-enzymatic hydrolysis. Incubation of human platelet suspensions with these dihydroxy acids led to the formation of lipoxin A4 and 6(S)-lipoxin A4 via lipoxygenation at C-15. Furthermore, human platelets converted the two 5(R),6(S)- and 5(R),6(R)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acids to tetraene-containing trihydroxyeicosatetraenoic acids. In contrast, leukotrienes C4, D4 and E4 were not transformed to cysteinyl-lipoxins. Time-course studies of leukotriene A4 metabolism in human platelet suspensions indicated lipoxin formation via two pathways: (i) direct conversion of leukotriene A4, leading to formation of the lipoxin intermediate 15-hydroxy-leukotriene A4; and (ii) 15-lipoxygenation of the 5(S),6(R)- and 5(S),6(S)-dihydroxyeicosatetraenoic acids. The results demonstrate that lipoxygenation at C-15 of 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acids may be an alternative novel pathway for platelet-dependent lipoxin formation.

Determination of picomole amounts of lipoxins C4, D4 and E4 by high-performance liquid chromatography with electrochemical detection

A method for the sensitive determination of the sulphopeptide lipoxins (LXs) C4, D4 and E4 by high-performance liquid chromatography with subsequent electrochemical detection is described. The best results were obtained when the analysis was carried out with the solvent system methanol-water-trifluoroacetic acid (66:34:0.008, v/v/v). The acquired half-wave potentials were different for all investigated compounds: +1.18 V for LXC4 +1.3 V for LXD4 and +1.25 V for LXE4. The detection limits of LXC4, LXD4 and LXE4, based on a signal-to-noise ratio of 3:1, were found to be 200-700 fmol. Although sulphopeptide lipoxins possess a high molar absorptivity, electrochemical detection still is three times more sensitive than ultraviolet detection.

New series of lipoxins isolated from human eosinophils

Granulocytes from human eosinophilic donors were incubated with arachidonic acid or 15-hydroxyeicosatetraenoic acid (15-HETE) and stimulated with the ionophore A23187. The eicosanoids were extracted with reversed-phase cartridges and subjected to RP-HPLC analysis. When extracts from eosinophil-enriched populations were analysed and compared with extracts from human neutrophils, three additional peaks were detected which coeluted with 15-hydroxy-delta 13-trans-15H derivatives of leukotriene C4, D4 and E4 in different HPLC systems. The recorded absorbance spectra of the eluted compounds and the standards were identical and showed a maximum at 307 nm which is characteristic for a conjugated tetraene system with a bathochromic shift by the sulfur moiety in alpha-position to the tetraene system. The compound which coeluted with the 15-hydroxy-LTC4 standard was treated with gamma-glutamyltransferase and converted to the corresponding leukotriene D4 derivative. The results indicate that interaction between the 5- and 15-lipoxygenase pathways leads to the formation of a new series of arachidonic acid metabolites in human eosinophils. Since the biosynthetic route is similar to that of lipoxin A4 and lipoxin B4, we suggest the trivial names lipoxin C4, D4 and E4.

Enzymatic properties of the 15-lipoxygenase of human cultured keratinocytes

The arachidonic acid 15-lipoxygenase or linoleic acid omega-6 lipoxygenase of human neonatal foreskin cultured keratinocytes converts arachidonic acid to 15-hydroxy-eicosatetraenoic acid and linoleic acid to 13-hydroxy-linoleic acid. A mean of 93% of the 15-lipoxygenase activity in sonicates of cultured keratinocytes was recovered in the 400,000 X g supernatant, attesting to the cytosolic localization of this enzyme. Optimal 15-lipoxygenase activity in the 400,000 X g supernatant was expressed at pH 6.7-7.3 and in the presence of calcium at a concentration of 2 mM or higher. Keratinocyte 15-lipoxygenase metabolized arachidonic acid (Km = 10.6 microM) and linoleic acid (Km = 9.5 microM) with similar efficiency. Nordihydroguaiaretic acid and 5,8,11,14-eicosatetraynoic acid both inhibited the conversion of arachidonic acid to 15-HETE with respective 50% inhibitory concentrations of 2.0 microM and 0.9 microM, while ATP, GTP, and cyclic AMP had no effect on activity at pH 6.8-7.2. The enzymatic properties of human keratinocyte 15-lipoxygenase thus resemble those of PMN leukocyte 15-lipoxygenase and the mediators generated may contribute to the regulation of cutaneous sensation and inflammation.

Omega-hydroxylation of N-acetylleukotriene E4 by rat liver microsomes

Previous investigations have demonstrated metabolism of leukotriene (LT) C4 in vivo involving transformations of the tripeptide, but not the fatty acid part, yielding N-acetyl LTE4 as a main biliary metabolite in the rat. In addition, several polar metabolites were detected in the same studies. The present report describes the characterization of a metabolite of N-acetyl LTE4 formed during incubations with rat liver microsomes. The structure, 5,20-dihydroxy-6-s-(2-acetamido-3-thiopropionyl)-7,9-trans-11, 14-cis-eicosa-tetraenoic acid, of this metabolite showed that it is formed by hydroxylation of the fatty acid part. Preliminary evidence indicates that it is one of several polar metabolites formed in vivo.

Allergen induced histamine release and immunoreactive-leukotriene C4 generation from leukocytes in mite sensitive asthmatic patients

Leukocytes from mite sensitive asthmatic patient were challenged with the allergen and the supernatant was assayed for histamine and immunoreactive-leukotriene C4 (i-LTC4). The release of histamine was quantitated by an automated fluorometric technique and i-LTC4 was determined using a commercial radioimmunoassay kit. The results of analysis of the supernatant by high speed liquid chromatography, together with observations of modulation of the formation by agents, indicated that i-LTC4 consisted of LTC4 with a little amount of LTD4. i-LTC4 was generated as a result of basophil activation but not derived from the other cells such as monocytes and eosinophils. Allergen induced a concentration-dependent release of histamine and i-LTC4 and the maximal release of histamine and i-LTC4 occurred at the same dose of the allergen. At optimal concentration of the allergen, basophils produced 20.4 +/- 17.9 ng of i-LTC4/10(6)-cells (mean +/- S.D., n = 39) and histamine release was 55.6 +/- 20.1% of total histamine. There was a significant correlation in the capacity of leukocytes to release histamine and i-LTC4 (r = 0.47, p less than 0.01). We found a correlation between maximal histamine release or cell sensitivity, allergen concentration for 50% histamine release, and a ratio of specific IgE to mite to total IgE in the serum, but the amount of i-LTC4 failed to correlate significantly with the ratio. The releasability and the cell sensitivity of asthmatic patients' cells to the allergen for histamine release paralleled the severity to symptoms, but this correlation was not significant in i-LTC4 generation.

Action and metabolism of circulating leukotriene C

The microvascular permeability effects of intravenously administered leukotriene C4 were assessed by fluorometric determinations of extravasated Evans blue dye in various tissues of guinea pigs. The effects were widespread, with intensity differences between different vascular beds, which in part reflected the tissue distribution of tritium-labeled leukotriene C3 in mice. Local tissue specificity favored permeability effects in the urinary tract, pulmonary airways, esophagus, and distal colon. The permeability effects may contribute to the hypotensive actions of leukotriene C4 and enhance airway obstruction by edema formation in addition to smooth muscle contraction.