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A binding assay has been developed to measure the affinity of leukotriene synthesis inhibitors for 5-lipoxygenase-activating protein (FLAP), using human leukocyte membranes as the source of FLAP and a radioiodinated leukotriene synthesis inhibitor, 125I-L-691,831, as ligand. Linearity of specific binding of radiolabeled ligand was demonstrated with increasing protein and ligand concentrations. Saturation analysis of radioligand binding showed a Kd of 6 nM and a Bmax that, depending on the membrane preparation, varied between 8 and 53 pmol/mg of protein. An excellent correlation was shown between affinity for FLAP in the binding assay and inhibition of leukotriene synthesis in human polymorphonuclear leukocytes for compounds from two structurally distinct classes, namely indoles and quinolines. A large number of membrane-active compounds did not compete with 125I-L-691,831 binding to FLAP. In addition, direct 5-lipoxygenase inhibitors and a selection of eicosanoids were unable to compete for FLAP binding. This study validates a selective binding assay for leukotriene synthesis inhibitors whose protein target is FLAP.

An 18-kDa leukocyte membrane protein, termed 5-lipoxygenase-activating protein (FLAP), has recently been shown to be the target of two structurally distinct classes of leukotriene biosynthesis inhibitors. These classes of inhibitors are based on indole and quinoline structures and are represented by MK-886 and L-674,573, respectively. A novel class of hybrid structure based on the indole and quinoline classes of inhibitors, termed quindoles, has recently been developed. These compounds, exemplified by L-689,037, are potent inhibitors of leukotriene biosynthesis, both in vitro and in vivo. In the present study, we have developed and characterized a potent radioiodinated photoaffinity analogue of L-689,037, termed [125I]L-691,678. This compound was used in immunoprecipitation studies with FLAP antisera to show that the quindole series of leukotriene biosynthesis inhibitors interact directly with FLAP. In addition, we show that MK-886, L-674,573, and L-689,037 specifically compete, in a concentration-dependent manner, with both [125I]L-691,678 and [125I]L-669,083, a photoaffinity analogue of MK-886, for binding to FLAP. These results suggest that these three classes of leukotriene biosynthesis inhibitors share a common binding site on FLAP, providing further evidence that FLAP represents a suitable target for structurally diverse classes of leukotriene biosynthesis inhibitors.

Previous studies involving transfection of cDNAs for 5-lipoxygenase-activating protein (FLAP) and 5-lipoxygenase into osteosarcoma cells have shown that both these proteins are essential for leukotriene synthesis (Dixon, R. A. F., Diehl, R. E., Opas, E., Rands, E., Vickers, P. J., Evans, J. F., Gillard, J. W., and Miller, D. K. (1990) Nature 343, 282-284). In the present study we show that FLAP is present in a variety of cells known to produce leukotrienes, but is absent from a number of cells which do not synthesize leukotrienes. Furthermore, differentiation of the human promyelocytic HL-60 cell line towards granulocytic cells following exposure to dimethylsulfoxide is associated with the concurrent induction of both FLAP and 5-lipoxygenase and an increased capacity to synthesize leukotrienes. Cellular leukotriene synthesis in this system is functionally dependent on FLAP as shown by its inhibition by the leukotriene biosynthesis inhibitor MK-886, a compound which specifically binds to FLAP.

Several inflammatory diseases, including asthma, arthritis and psoriasis are associated with the production of leukotrienes by neutrophils, mast cells and macrophages. The initial enzymatic step in the formation of leukotrienes is the oxidation of arachidonic acid by 5-lipoxygenase (5-LO) to leukotriene A4. Osteosarcoma cells transfected with 5-LO express active enzyme in broken cell preparations, but no leukotriene metabolites are produced by these cells when stimulated with the calcium ionophore A23187, indicating that an additional component is necessary for cellular 5-LO activity. A new class of indole leukotriene inhibitor has been described that inhibits the formation of cellular leukotrienes but has no direct inhibitory effect on soluble 5-LO activity. We have now used these potent agents to identify and isolate a novel membrane protein of relative molecular mass 18,000 which is necessary for cellular leukotriene synthesis.

Leukotrienes, the biologically active metabolites of arachidonic acid, have been implicated in a variety of inflammatory responses, including asthma, arthritis and psoriasis. Recently a compound, MK-886, has been described that blocks the synthesis of leukotrienes in intact activated leukocytes, but has little or no effect on enzymes involved in leukotriene synthesis, including 5-lipoxygenase, in cell-free systems. A membrane protein with a high affinity for MK-886 and possibly representing the cellular target for MK-886 has been isolated from rat and human leukocytes. Here, we report the isolation of a complementary DNA clone encoding the MK-886-binding protein. We also demonstrate that the expression of both the MK-886-binding protein and 5-lipoxygenase is necessary for leukotriene synthesis in intact cells. Because the MK-886-binding protein seems to play a part in activating this enzyme in cells, it is termed the five-lipoxygenase activating protein (FLAP).