Leukotriene C-4 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 c atalyzes the conjugation of electrophilic compounds with glutathione, as we ll as the reduction of lipid hydroperoxides, Here we report that leukotrien e C-4 is a potent inhibitor of MGST-1. Leukotriene C-4 was found to be a ti ght-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 compet itive with respect to glutathione and non-competitive toward the second sub strate, 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), Leu kotriene C-4 analogues, which have been developed as antagonists of leukotr iene receptors, were found to display varying degrees of inhibition of MGST -1, In particular, the cysteinyl-leukotriene analogues SKF 104,353, ONO-107 8, 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 antago nize FLAP) also displayed significant inhibition (e.g. IC50 for BAYx1005 wa s 58 mu M). In contrast, selective 5-lipoxygenase inhibitors such as zileut on only marginally inhibited activity at high concentrations (500 mu M). Ou r discovery that leukotriene C-4 and drugs developed based on its structure are potent inhibitors of MGST-1 raises the possibility that MGST-1 influen ces the cellular processing of leukotrienes. These findings may also have i mplications for the effects and side-effects of drugs developed to manipula te leukotrienes.