Leukotrienes and lipoxins: structures, biosynthesis, and biological effects

Arachidonic acid is released from membrane phospholipids upon cell stimulation (for example, by immune complexes and calcium ionophores) and converted to leukotrienes by a 5-lipoxygenase that also has leukotriene A4 synthetase activity. Leukotriene A4, an unstable epoxide, is hydrolyzed to leukotriene B4 or conjugated with glutathione to yield leukotriene C4 and its metabolites, leukotriene D4 and leukotriene E4. The leukotrienes participate in host defense reactions and pathophysiological conditions such as immediate hypersensitivity and inflammation. Recent studies also suggest a neuroendocrine role for leukotriene C4 in luteinizing hormone secretion. Lipoxins are formed by the action of 5- and 15-lipoxygenases on arachidonic acid. Lipoxin A causes contraction of guinea pig lung strips and dilation of the microvasculature. Both lipoxin A and B inhibit natural killer cell cytotoxicity. Thus, the multiple interaction of lipoxygenases generates compounds that can regulate specific cellular responses of importance in inflammation and immunity.

Lipoxin formation during human neutrophil-platelet interactions. Evidence for the transformation of leukotriene A4 by platelet 12-lipoxygenase in vitro

Human neutrophils from peripheral blood may physically interact with platelets in several settings including hemostasis, inflammation, and a variety of vascular disorders. A role for lipoxygenase (LO)-derived products has been implicated in each of these events; therefore, we investigated the formation of lipoxins during coincubation of human neutrophils and platelets. Simultaneous addition of FMLP and thrombin to coincubations of these cells led to formation of both lipoxin A4 and lipoxin B4, which were monitored by reversed-phase high pressure liquid chromatography. Neither stimulus nor cell type alone induced the formation of these products. When leukotriene A4 (LTA4), a candidate for the transmitting signal, was added to platelets, lipoxins were formed. In cell-free 100,000 g supernatants of platelet lysates, which displayed 12-LO activity, LTA4 was also transformed to lipoxins. Platelet formation of lipoxins was inhibited by the LO inhibitor esculetin and partially sensitive to chelation of Ca2+, while neither acetylsalicylic acid nor indomethacin significantly inhibited their generation. In contrast, neutrophils did not transform LTA4 to lipoxins. Cell-free 100,000 g supernatants of neutrophil lysates converted LTA4 to LTB4. These results indicate that neutrophil-platelet interactions can lead to the formation of lipoxins from endogenous sources and provide a role for platelet 12-LO in the formation of lipoxins from LTA4.

Single protein from human leukocytes possesses 5-lipoxygenase and leukotriene A4 synthase activities

The activity of leukotriene A4 (LTA4) synthase in crude human leukocyte homogenates was found to have a similar requirement for Ca2+ and ATP as had been noted previously for 5-lipoxygenase activity. Purification of the 5-lipoxygenase using ammonium sulfate fractionation, AcA 44 gel-filtration chromatography, and HPLC on anion-exchange and hydroxyapatite columns demonstrated that LTA4 synthase activity copurified with the 5-lipoxygenase with similar recoveries and increases in specific activity. Furthermore, the two enzymatic activities coeluted exactly on three different HPLC systems. Maximal activity of purified LTA4 synthase required the addition of three nondialyzable stimulatory factors, two of which were cytosolic and one of which was membrane-bound. These findings were identical for 5-lipoxygenase activity. When incubated with arachidonic acid, the purified 5-lipoxygenase converted approximately equal to 15% of its endogenously generated 5-hydroperoxyicosatetraenoic acid (5-HPETE) to LTA4. LTA4 production was more efficient when the enzyme utilized 5-HPETE generated from arachidonic acid than when 5-HPETE was exogenously supplied as substrate. These findings suggest that a single protein from human leukocytes possesses 5-lipoxygenase and LTA4 synthase activities and that the synthesis of LTA4 from 5-HPETE is controlled by the same complex multicomponent system that regulates the 5-lipoxygenase reaction.

Enzyme with dual lipoxygenase activities catalyzes leukotriene A4 synthesis from arachidonic acid

When arachidonic acid was incubated with homogenates of potato tubers, two isomers of 6-trans-leukotriene B4, epimeric at C-12, were formed in addition to the major product, (5S-hydroperoxy-6-trans-8,11,14-cis-icosatetraenoic acid (5-HPETE). To elucidate the mechanism of biosynthesis of the dihydroxy-acids, the lipoxygenase from the potato tubers was purified to apparent homogeneity by a combination of conventional chromatographic procedures and high-performance liquid chromatography equipped with a chromatofocusing column (Mono-P). The purified lipoxygenase acted on arachidonic acid and bishomo-gamma-linolenic acid to yield (5S)-hydroperoxy- and (8S)-hydroperoxyicosanoids, respectively. Furthermore, the purified enzyme converted 5-HPETE to leukotriene A4, with the presence of the epoxide intermediate being demonstrated by 18O2 experiments, methanol trapping, as well as further conversion to leukotriene B4 by the purified leukotriene A4 hydrolase. Several experiments, including those with lipoxygenase inhibitors, heat treatment, and competitive inhibition, indicated that both the 5-lipoxygenase and leukotriene A4 synthase activities resided in the same protein and that the formation of leukotriene A4 from 5-HPETE was catalyzed by the 8-lipoxygenase activity of the enzyme.

Human endothelial cells stimulate leukotriene synthesis and convert granulocyte released leukotriene A4 into leukotrienes B4, C4, D4 and E4

Incubation of human endothelial cells with leukotriene A4 resulted in the formation of leukotrienes B4, C4, D4 and E4. Endothelial cells did not produce leukotrienes after stimulation with the ionophore A23187 and/or exogenously added arachidonic acid. However, incubation of polymorphonuclear leukocytes with ionophore A23187 together with endothelial cells led to an increased synthesis of cysteinyl-containing leukotrienes (364%, mean, n = 11) and leukotriene B4 (52%) as compared to leukocytes alone. Thus, the major part of leukotriene C4 recovered in mixed cultures was attributable to the presence of endothelial cells. Similar incubations of leukocytes with fibroblasts or smooth muscle cells did not cause an increased formation of leukotriene C4 or leukotriene B4. The increased biosynthesis of cysteinyl-containing leukotrienes and leukotriene B4 in coincubation of leukocytes and endothelial cells appeared to be caused by two independent mechanisms. First, cell interactions resulted in an increased production of the total amount of leukotrienes, suggesting a stimulation of the leukocyte 5-lipoxygenase pathway, induced by a factor contributed by endothelial cells. Secondly, when endothelial cells prelabeled with [35S]cysteine were incubated with either polymorphonuclear leukocytes and A23187, or synthetic leukotriene A4, the specific activity of the isolated cysteinyl-containing leukotrienes were similar. Thus, transfer of leukotriene A4 from stimulated leukocytes to endothelial cells appeared to be an important mechanism causing an increased formation of cysteinyl-containing leukotrienes in mixed cultures of leukocytes and endothelial cells. In conclusion, the present study indicates that the vascular endothelium, when interacting with activated leukocytes, modulates both the quantity and profile of liberated leukotrienes.

Erythrocyte-neutrophil interactions: formation of leukotriene B4 by transcellular biosynthesis

Studies on the mechanism of leukotriene B4 biosynthesis in suspensions composed of neutrophils plus erythrocytes indicate that human erythrocytes convert neutrophil-derived leukotriene A4 into leukotriene B4. Leukotriene B4 formation by neutrophils in the presence of erythrocytes exceeded that from corresponding suspensions of neutrophils alone. The increase was proportional to the erythrocyte content of the suspension. The erythrocyte-dependent increase in leukotriene B4 biosynthesis did not equal the arithmetic sum of calcium ionophore-dependent biosynthesis by neutrophils plus calcium ionophore-dependent biosynthesis by erythrocytes, since erythrocytes produced no leukotriene B4 upon incubation with ionophore A23187. Erythrocytes did not stimulate 5-lipoxygenase activity within neutrophils, since the erythrocyte effect was confined to enzymatic hydration: leukotriene B4 increased coincident with decreased formation of 5,12-dihydroxyicosatetraenoic acids derived from nonenzymatic hydration. Biosynthesis of leukotriene B4 within the erythrocyte, from neutrophil-derived leukotriene A4, was established by comparing the effect of normal erythrocytes with erythrocytes containing a leukotriene A4 hydrolase that was inactivated by the substrate. In the latter case, leukotriene B4 formation increased by only 30-40%; in the former case, it increased by 100-200%. Transcellular biosynthesis of leukotriene B4 from erythrocyte-neutrophil interactions explains the paradoxical presence of leukotriene A4 hydrolase within erythrocytes, a cell incapable of synthesizing leukotriene A4; affords a mechanism to overcome rate limitations or "suicide inactivation" of leukotriene A4 hydrolase in neutrophils; exploits a cryptic capacity within erythrocytes, provisionally dormant cells in terms of icosanoid biosynthesis; indicates that the biosynthetic capacity of cell combinations is not necessarily equivalent to the sum of their separate capacities.

Transcellular conversion of endogenous arachidonic acid to lipoxins in mixed human platelet-granulocyte suspensions

Incubation of mixed human platelet/granulocyte suspensions with ionophore A23187 led to a platelet dependent formation of several lipoxin isomers from endogenous substrate. The major metabolite coeluted with authentic lipoxin A4 (5(S), 6(R), 15(S)-trihydroxy-7,9,13-trans-11-cis-eicosatetraenoic acid) in several HPLC-systems and showed an identical UV-spectrum. Furthermore, a similar profile of lipoxins was formed in pure platelet suspensions incubated with exogenous leukotriene A4 (5(S) -5, 6-oxido-7,9-trans-11,14-cis-eicosatetraenoic acid). The conversion of exogenous leukotriene A4 to lipoxin A4 was markedly increased in the presence of ionophore A23187.

A second pathway of leukotriene biosynthesis in porcine leukocytes

Incubation of suspensions containing polymorphonuclear and eosinophilic leukocytes with arachidonic acid led to the formation of two pairs of diastereomeric 8,(15S)-dihydroxy-5,9,11,13-icosatetraenoic acids and two erythro-14,15-dihydroxy-5,8,10,12-icosatetraenoic acids. The structures were elucidated by ultraviolet spectroscopy and gas chromatography--mass spectrometric analysis of several derivatives of each compound, catalytic hydrogenation, oxidative ozonolysis with steric analysis of alcohols, and comparison to reference compounds prepared by chemical synthesis. Experiments with 18O2 and H218O indicated that in all six compounds the hydroxyl group at C-15 was derived from molecular oxygen. Two of the diastereomeric 8,15-dihydroxy acids incorporated H218O at C-8, while the other two 8,15-dihydroxy products (also diastereomers) and the 14,15-dihydroxy compounds (geometric isomers) incorporated 18O2 at C-8 and C-14, respectively, in addition to C-15. Two of the 8,15-dihydroxy acids are formed by reaction of water with an unstable allylic epoxide intermediate, (14S,15S)-oxido-5,8,10,12-icosatetraenoic acid; the two 14,15-dihydroxy acids are proposed to be formed by reaction of activated molecular oxygen with the same epoxide, which in turn originates via 15S oxygenation of arachidonic acid.

Pharmacological activity of leukotrienes A4, B4, C4 and D4 on selected guinea-pig, rat, rabbit and human smooth muscles

The myotropic activity of leukotrienes A4, B4, C4, D4 and histamine has been evaluated on selected smooth muscle preparations. LTA4, B4, C4 and D4 were several times more potent than histamine on the guinea-pig lung parenchymal strip, while on the guinea-pig trachea, LTB4 was less active. The guinea-pig ileum either in segments or in strips of longitudinal muscles responses well to LTC4, LTD4 and histamine but not to LTA4 and LTB4. Rat and rabbit lung parenchymal strip showed very little sensitivity for leukotrienes whereas human parenchymal strips and bronchi were nearly as sensitive as the guinea-pig lung.

Evidence for a lipoxygenase mechanism in the biosynthesis of epoxide and dihydroxy leukotrienes from 15(S)-hydroperoxyicosatetraenoic acid by human platelets and porcine leukocytes

Leukocyte preparations convert the hydroperoxy icosatetraenoic acids 5(S)-HPETE and 15(S)-HPETE to the unstable leukotriene epoxides LTA4 and 14,15-LTA4. In several ways, the conversion of 5- or 15-HPETE to leukotriene epoxide bears a formal mechanistic resemblance to the reaction catalyzed by the 12-lipoxygenase in the conversion of arachidonic acid to 12(S)-HPETE. Points of similarity include enzymatic removal of a hydrogen at carbon 10, double bond isomerization, and formation of a new carbon-to-oxygen bond. In the case of 15(S)-HPETE, two 8,15- and an erythro-14,15-dihydroxy acid (8,15- and 14,15-DiHETEs), which result from incorporation of molecular oxygen into each hydroxyl group, are coproducts in the formation of 14,15-LTA4. These facts prompted us to test the hypothesis that the biosynthesis of 14,15-LTA4 and of 8,15- and 14,15-DiHETEs from 15(S)-HPETE occurs by a mechanism similar to that observed in lipoxygenase reactions. Based on the results presented here, we conclude that the biosynthesis of 14,15-LTA4 and of 8,15- and 14,15-DiHETEs from 15(S)-HPETE occurs via a common intermediate and that, moreover, the formation of these metabolites from 15(S)-HPETE is catalyzed by an enzyme with many mechanistic features in common with the 12-lipoxygenase.

Novel transcellular interaction: conversion of granulocyte-derived leukotriene A4 to cysteinyl-containing leukotrienes by human platelets

Human platelets dose-dependently converted exogenous leukotriene A4 to leukotriene C4 and efficiently metabolized this compound to leukotrienes D4 and E4. Neither of these compounds were produced after stimulation of human platelet suspensions with ionophore A23187. After LTA4 incubation of subcellular fractions, formation of leukotriene C4 was exclusively observed in the particulate fraction and was separable from the classical glutathione S-transferase activity. This suggested the presence of a specific leukotriene C4 synthase in human platelets. Addition of physiological amounts of autologous platelets to human granulocyte suspensions significantly increased ionophore A23187-induced formation of leukotriene C4. In contrast, the production of leukotriene B4 was decreased. After preincubation of platelets with [35S]cysteine, 35S-labeled leukotriene C4 was produced by A23187-stimulated platelet-granulocyte suspensions, strongly indicating a transcellular biosynthesis of this compound.

Leukotriene C4 production by murine mast cells: evidence of a role for extracellular leukotriene A4

The glutathione-containing leukotriene C4 (LTC4) is a major mediator of smooth muscle contraction and is released by mast cells when antigen interacts with cell-bound IgE. Antigen-stimulated mast cells undergo phospholipase activation. We report a pathway of LTC4 production by mast cells that does not require phospholipase activation but depends on the interaction of activated neutrophils with unstimulated mast cells, using as an intermediate extracellular leukotriene A4 (LTA4). The epoxide LTA4 is released by neutrophils and, together with leukotriene B4 and 5-hydroxyeicosatetraenoic acid, constitutes the major lipoxygenase metabolites found in supernatants of stimulated neutrophils. Five minutes after activation of neutrophils by calcium ionophore A23187 we measured 136 pmol of extracellular LTA4 per 10(7) neutrophils (range 40-300, n = 7) by trapping the epoxide with alcohols. Therefore, we conclude that LTA4 is not just an intracellular leukotriene precursor but is released as a lipoxygenase metabolite. LTA4 is known to be stabilized by albumin and is efficiently converted by mast cells into LTC4 even at low LTA4 concentrations. The LTA4 complexed to albumin is converted into LTC4 rapidly and completely within 10-15 min. More than 50% of the LTA4 presented to mast cells is metabolized to LTC4 at concentrations of LTA4 between 0.2 and 2 nmol of LTA4 per 10(7) mast cells. This observation establishes a potential physiologic role for extracellular LTA4. Therefore, interactions between various cell types that release or utilize LTA4 may provide an important metabolic pathway for the production of leukotrienes.

Characterization of leukotriene A4 and B4 biosynthesis

We have studied LTA4 and LTB4 synthesis in a cell-free system from RBL-1 cells. All the enzymes leading to the formation of LTB4 from arachidonic acid are localized in the soluble fraction (100,000 x g supernatant) of these cells. The formation of LTA4 and LTB4 is complete by 10 min. When we varied the arachidonic acid concentration from 1 to 300 microM, the synthesis of LTB4 leveled off at 30 microM and of LTA4 at 100 microM while 5-HETE had not reached a plateau at 300 microM. This enzyme system has the capacity to generate relatively large amounts of 5-HETE and LTA4 and only a relatively small amount of LTB4. Therefore, the rate limiting step is not the 5-lipoxygenase, the first step in the pathway, but the conversion of LTA4 to LTB4. This is in contrast to cyclooxygenase pathway where the first step is rate limiting. A second addition of arachidonic acid at submaximal concentration for LTA4 synthesis did not produce any additional LTA4 or LTB4. Further study of this phenomenon showed that the 5-lipoxygenase and LTA-synthase were inactivated with time by preincubation with arachidonic acid and that peroxy fatty acids seem to be the inactivating species.

Catalytic properties of human platelet 12-lipoxygenase as compared with the enzymes of other origins

Arachidonate 12-lipoxygenases of porcine and bovine leukocytes were different in substrate specificity and immunogenicity from the enzyme of bovine platelets (Arch. Biochem. Biophys. (1988) 266, 613). In order to extend the comparative studies on the two types of 12-lipoxygenase, we purified the enzyme from the cytosol of human platelets by immunoaffinity chromatography to a specific activity of about 0.3 mumol/min per mg protein at 37 degrees C. The purified enzyme was active with eicosapolyenoic acids and docosahexaenoic acid. Linoleic and linolenic acids were poor substrates in contrast to the high reactivity of the leukocyte enzymes with these octadecapolyenoic acids. The finding that the human platelet enzyme catalyzed 15-oxygenation of 5S-hydroxy-6,8,11,14-eicosatetraenoic acid, raised a question if lipoxins were produced by incubation of the enzyme with leukotriene A4. However, the leukotriene A4 was scarcely transformed to lipoxin isomers by 12-lipoxygenases of human and bovine platelets. In sharp contrast, the porcine and bovine leukocyte enzymes converted leukotriene A4 to various lipoxin isomers by the reaction rates of 3% and 2% of the arachidonate 12-oxygenation. Thus, 12-lipoxygenases of human and bovine platelets were catalytically distinct from the porcine and bovine leukocyte enzymes in terms of their reactivities not only with linoleic and linolenic acids, but also with leukotriene A4 as lipoxin precursor.

Mechanisms of leukotriene-induced contractions of guinea pig airways: leukotriene C4 has a potent direct action whereas leukotriene B4 acts indirectly

The leukotrienes (LT's) are a group of arachidonic acid derivatives implicated as mediators of allergic bronchoconstriction and acute inflammation. Tracheal spirals and strips of lung parenchyma from guinea pigs were used under non-flow conditions to characterize the contractions caused by LTA4, LTB4 and LTC4. Cumulative administrations of leukotrienes desensitized the lung strip, whereas non-cumulative dose-response relationships for the leukotrienes and histamine were reasonably parallel. Half maximal contractions of the lung strips were obtained at a final bath concentration of 1 nM for LTC4 and 300 nM for LTA4 or LTB4, as compared with 6 000 nM for histamine. In the trachea, LTC4 was approximately 100 times more potent than LTA4 and histamine. Leukotrienes B4 and C4, but not acetylcholine or histamine, elicited release of the bronchoconstrictive thromboxane A2 from the lung under non-flow conditions. Indomethacin blocked the contractile response to LTB4, whereas the contractile effect of LTC4 remained unaltered. The beta-adrenoceptor agonist isoproterenol and the LTC4 antagonist FPL 55712 attenuated the contraction, but not the release of thromboxane A2, induced by LTC4. Changing to a perifusion technique rendered the lung strips less sensitive to the direct action of LTC4, and released thromboxane A2 now contributed significantly to the contractile response. In addition, the perifusion experiments indicated that LTB4 released histamine as well. We conclude that the chemoattractant LTB4 is an indirectly acting bronchoconstrictor, whereas the slow reacting substance LTC4 contracts the airway muscle by a predominantly direct mechanism. The exquisite bronchoconstrictive activity of LTC4 may be unrelated to its ability to induce formation of thromboxane A2.

Leukotriene A5 is a substrate and an inhibitor of rat and human neutrophil LTA4 hydrolase

The epoxide 5(S) trans-5,6 oxido, 7,9 trans-11,14,17 cis eicosatetraenoic acid (leukotriene A5) was chemically synthesized and demonstrated to be both a substrate and an inhibitor of partially purified rat and human LTA4 hydrolase. Both rat and human LTA4 hydrolase utilized leukotriene A5 less effectively as a substrate than leukotriene A4. Incubation of leukotriene A5 (10 microM) or leukotriene A4 (10 microM) with rat neutrophils demonstrated formation of 123 pmol LTB5/min/10(7) cells and 408 pmol LTB4/min/10(7) cells respectively. Purified rat neutrophil LTA4 hydrolase incubated with 100 microM leukotriene A5 produced 22 nmol LTB5/min/mg protein and when incubated with 100 microM leukotriene A4 produced 50 nmol LTB4/min/mg protein. Human neutrophil LTA4 hydrolase incubated with 100 microM leukotriene A5 produced 24 nmol LTB5/min/mg protein and when incubated with 100 microM leukotriene A4 produced 52 nmol LTB4/min/mg protein. Leukotriene A5 was an inhibitor of the formation of leukotriene B4 from leukotriene A4 by both the rat and human neutrophil LTA4 hydrolase. Excess leukotriene A5 prevented covalent coupling of [3H] leukotriene A4 to LTA4 hydrolase suggesting inhibition may involve covalent coupling of leukotriene A5 to the LTA4 hydrolase.

Fatty acid structural requirements for leukotriene biosynthesis

Utilizing a variety of fatty acids, differing in chain length, degree and position of unsaturation, we investigated the substrate specificity for the enzymatic production of biologically active slow reacting substances (SRS) and of the other leukotrienes. A cell-free enzyme system obtained from RBL-1 cells was used in this study. The primary structural requirement observed for the conversion by this lipoxygenase enzyme system was a delta 5,8,11 unsaturation in a polyenoic fatty acid. Such fatty acids as 20:4 (5,8,11,14) 20:5 (5,8,11,14,17), 20:3 (5,8,11), 19:4 (5,8,11,14) and 18:4 (5,8,11,14) were readily converted to compounds that comigrated with 5-HETE and 5,12-DiHETE and to biologically active SRS. Chain length did not have an influence on the formatin of these hydroxyacids. Fatty acids with the initial unsaturation at delta 4, delta 6, delta 7, or delta 8 were a poor substrate for the leukotriene enzyme system. Therefore, this lipoxygenase pathway in leukocytes is quite different from the lipoxygenase in platelets which does not exhibit this specificity.

Immunopathogenetic roles of leukotrienes in human diseases

The recent definition of the pathways of generation and structures of diverse products of the lipoxygenation of arachidonic acid has established the identity of a new family of mediators of hypersensitivity and inflammation. Studies of the effects of these mediators have shown that leukotrienes C, D, and E, the constitutents of the slow-reacting substance of anaphylaxis (SRS-A), are extremely potent smooth muscle contractile and vasoactive factors. Leukotriene B is a highly active stimulus of neutrophil and eosinophil functions and suppresses the immunological capabilities of T lymphocytes. The development of specific and sensitive radioimmunoassays has permitted the detection of elevated concentrations of leukotrienes in tissues or exudates in several diseases, including asthma, diverse allergic states, adult respiratory distress syndrome, psoriasis, spondyloarthritis, and gout. The application of selective inhibitors and antagonists of leukotrienes will clarify their pathogenetic contributions in human diseases and may yield new therapeutic approaches.

Lipoxin formation in human nasal polyps and bronchial tissue

Chopped human nasal polyps and bronchial tissue produced lipoxin A4 and isomers of lipoxins A4 and B4, but not lipoxin B4, after incubation with exogenous leukotriene A4. In addition, these tissues transformed arachidonic acid to 15-hydroxyeicosatetraenoic acid. The capacity per gram of tissue to produce lipoxins and 15-hydroxyeicosatetraenoic acid was 3-5-times higher in the nasal polyps. Neither tissue produced detectable levels of lipoxins or leukotrienes after incubation with ionophore A23187 and arachidonic acid. Co-incubation of nasal polyps and polymorphonuclear granulocytes with ionophore A23187 led to the formation of lipoxins, including lipoxins A4 and B4. The results indicate the involvement of an epithelial 15-lipoxygenase in lipoxin formation in human airways.

Vascular smooth muscle cell leukotriene C4 synthesis: requirement for transcellular leukotriene A4 metabolism

Leukotriene synthesis and metabolism were studied in cultured porcine aortic smooth muscle cells (PSM). Cultures stimulated with calcium ionophore A23187, with or without exogenous arachidonic acid, did not release detectable levels of leukotriene B4, C4, D4 or E4. Those products were assayed by high-performance liquid chromatography, ultraviolet spectrometry and, in some cases, radioimmunoassay. Smooth muscle cultures were able to convert leukotriene A4 to leukotriene C4, indicating the presence of leukotriene C4 synthetase. Although this enzymatic activity has previously been found in cultured porcine aortic endothelial cells, it was not detectable in cardiac myocytes, fibroblasts from several organs or renal epithelial cells. It is known from previous work that inflammatory cells such as polymorphonuclear leukocytes (PMNL) or mast cells release leukotriene A4 when stimulated. Further, increased numbers of these cell-types are found associated with vascular tissue during several pathologic situations. Therefore, the potential for a leukocyte-smooth muscle cell interaction involving the transcellular metabolism of leukotriene A4 was assessed. Stimulation of PMNL suspensions in the presence of PSM resulted in a significant increase in total leukotriene C4 produced in comparison to either cell-type alone (255% of PMNL alone, P less than 0.05). Furthermore, after the intracellular glutathione pool of PSM was prelabelled with 35S, a PSM-PMNL coincubation produced levels of [35S]leukotriene C4 which were significantly greater (P less than 0.05) than those found after coincubating prelabelled PMNL with unlabelled PSM. These data demonstrate a PMNL-PSM interaction in which smooth muscle cell leukotriene C4 synthesis results from the transcellular metabolism of PMNL-derived leukotriene A4. Since leukotriene C4 and its metabolites are vasoconstrictors and cause increased vascular permeability, the biochemical interaction described in this report may be relevant to the pathophysiology of arterial vasospasm, atherogenesis and to the abnormalities of tissue perfusion associated with ischemic or inflammatory disorders.