Leukotriene C4 is a tight-binding inhibitor of microsomal glutathione transferase-1 -: Effects of leukotriene pathway modifiers

Microsomal glutathione transferase-1 (MGST-1) is an abundant protein that catalyzes the conjugation of electrophilic compounds with glutathione, as well as the reduction of lipid hydroperoxides, Here we report that leukotriene C-4 is a potent inhibitor of MGST-1. Leukotriene C-4 was found to be a tight-binding inhibitor, with a K-i of 5.4 nM for the unactivated enzyme, and 9.2 nM for the N-ethylmaleimide activated enzyme, This is the first tight-binding inhibitor characterized for this enzyme, Leukotriene C-4 was competitive with respect to glutathione and non-competitive toward the second substrate, CDNB, Analysis of stoichiometry supports binding of one molecule of inhibitor per homotrimer, Leukotrienes A(4), D-4, and E-4 were much weaker inhibitors of the purified enzyme (by at least 3 orders of magnitude), Leukotriene C-4 analogues, which have been developed as antagonists of leukotriene receptors, were found to display varying degrees of inhibition of MGST-1, In particular, the cysteinyl-leukotriene analogues SKF 104,353, ONO-1078, and BAYu9773 were strong inhibitors (IC50 values: 0.13, 3.7, and 7.6 mu M, respectively), In view of the partial structural similarity between MGST-1, leukotriene C-4 synthase, and 5-lipoxygenase activating protein (FLAP), it was of interest that leukotriene C-4 synthesis inhibitors (which antagonize FLAP) also displayed significant inhibition (e.g. IC50 for BAYx1005 was 58 mu M). In contrast, selective 5-lipoxygenase inhibitors such as zileuton only marginally inhibited activity at high concentrations (500 mu M). Our discovery that leukotriene C-4 and drugs developed based on its structure are potent inhibitors of MGST-1 raises the possibility that MGST-1 influences the cellular processing of leukotrienes. These findings may also have implications for the effects and side-effects of drugs developed to manipulate leukotrienes.