Leukotriene C4 Is a Tight-binding Inhibitor of Microsomal Glutathione Transferase-1

Abstract

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 (IC 50 values: 0.13, 3.7, and 7.6 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. IC 50 for BAYx1005 was 58 M). In contrast, selective 5-lipoxygenase inhibitors such as zileuton only marginally inhibited activity at high concentrations (500 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.

International Union of Pharmacology XXXVII. Nomenclature for Leukotriene and Lipoxin Receptors

“…3 H]LTC 4 may bind to other cellular entities, such as glutathione Stransferase (GST) as has been reported by several investigators (Sun et al, 1987;Metters et al, 1994;Bannenberg et al, 1999). In addition, LTC 4 may also bind to specific exporters in human eosinophils (Lam et al, 1992), to ATP-dependent carriers in rat liver (Ishikawa et al, 1990;Keppler et al, 1992), and to multidrugresistant protein 1 (Qian et al, 2001).…”

Characterization of two polymorphisms in the leukotriene C4 synthase gene in an Australian population of subjects with mild, moderate, and severe asthma☆

“…11 The gene encoding LTC 4 synthase is located on the long arm of chromosome 5 (5q35) 12,13 in a region implicated in asthma by linkage analysis and adjacent to a region (5q31-33) that includes a number of candidate asthma genes. 14 The LTC 4 synthase gene spans about 2.5 kilobases, containing 5 small exons and a number of putative DNA binding motifs or regulatory sites in the 59 untranslated region.…”

Reactive Intermediates and the Pathogenesis of Adverse Drug Reactions: The Toxicology Perspective

“…Leukotrine C4 and drugs developed based on this structure are potent inhibitors of microsomal glutathione transferase-1 [71]. Drugs such as carbamazapine produce an epoxide as the pharmacologically active metabolite; however, for the tricyclic antidepressant amitripyline, a study by Steimer et al [72] has shown that the combination of normal (fast) CYP2C19 function and slightly diminished CYP2D6 function leads to high concentrations of what they determined to be the toxic intermediate metabolite, nortriptyline (also an active compound), and a high risk for adverse events.…”