Mechanisms of leukotriene C4 synthase regulation

Leukotrienes are potent lipid mediators of inflammation. A member of the supergene family of Membrane Associated Proteins in Eicosanoid and Glutathione metabolism (MAPEG), LTC4 synthase catalyzes the first committed step in the biosynthesis of the biologically active cysteinyl leukotrienes. Pharmacological cross-reactivity between LTC4 synthase and other proteins in the five-lipoxygenase pathway was assessed, leading to the identification of L-699,333 as the most potent LTC4 synthase inhibitor amongst the compounds available, previously reported as a nanomolar five-lipoxygenase inhibitor and a weak LTC4 synthase inhibitor. Compounds that bind competitively to the arachidonic acid binding sites on five-lipoxygenase and five-lipoxygenase-activating protein, were found to recognize motifs that are weakly conserved on the binding site of LTC 4 synthase. L-699,333 was used to study LTC4 synthase catalysis revealing a random, rapid equilibrium mechanism, with strong substrate inhibition imparted by LTA4. Further studies showed that inhibition of LTC 4 synthase activity induced by cellular activation by phorbol esters, was due to direct phosphorylation of the enzyme. The dose-dependent phosphorylation event was found to be specific to TBP-1 cells, time-dependent with implications of negative feedback, influenced by the cellular density, and mediated by protein kinase C. These novel insights into LTC4 synthase biology further our understanding of this attractive therapeutic target against bronchial asthma