Fig. 1. – Proposed mechanism for the involvement of 5-LO and FLAP in the regulation of cellular leukotriene biosynthesis, and for various
ways of inhibiting the effects of cysteinyl-leukotrienes. An inflammatory stimulus (R) leads to receptor-mediated intracellular influx of calcium
(Ca++) ions , altowing cytosolic PLA2 and 5-LO to translocate from the cytosol to the cell membrane and . 5-LO binds to FLAP in the
cell membrane to make a stable complex . PLA2 cleaves AA from membrane phospholipids, and AA is converted by 5–LO in the presenc of
FLAP to 5-HPETE and . 5-HPETE is converted by 5-LO to LTA4 which is converted by LTA4 hydrolase to LTB4, or by LTC4 synthas to
LTC4, and subsequently to LTD4 and LTE4. Three sites of action for drugs are illustrated: FLAP inhibitors which prevent 5-LO binding with
FLAP; 5-LO inhibitors which inhibits 5-LO activity and cysteinyl-leukotriene receptor antagonists which inhibit the effects of cysteinylleukotrienes
on other cells. R: receptor; ATP: adenosine triphosphate; PLA2: phospholipase A2; FLAP: 5-lipoxygenase-activating protein; 5-LO:
5-lipoxygenase; AA: arachidonic acid; 5-HPETE: 5-hydroperoxy-eicosatetraenoic acid; LT: leukotriene; CysLT1: cysteinyl leukotriene receptor.