The committed step in the biosynthesis of cysteinyl-leukotrienes is catalyzed by leukotriene C-4 synthase as well as microsomal glutathione S-transferase (MGST) type 2 and type 3, which belong to a family of membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG). We cloned and characterized these three enzymes from the rat to allow a side-by-side comparison of structural and catalytic properties. The proteins are 79.6-86.7% identical to the human orthologs. Rat MGST3 fails to convert leukotriene A(4) into leukotriene C-4, which in turn challenges the proposed catalytic role of a conserved Arg and Tyr residue for the leukotriene C-4 synthase reaction. Comparative inhibitor studies of all three enzymes, using MK-886 and cysteinyl-leukotrienes, indicate that their catalytic centers originate from structurally. related and overlapping active sites. Hence, it seems feasible to design enzyme inhibitors, which simultaneously target several members of this protein family to yield compounds with increased anti-inflammatory action. (C) 2003 Elsevier Inc. All rights reserved.