Several inhibitors of LTA4 hydrolase from human leukocytes were prepared to probe the active site of the enzyme.Leukotriene (LT) A4 hydrolase catalyzes the hydrolysis of the arachidonic acid-derived allylic epoxide LTA4 (5(S)-5,6-oxido-7,9-trans-l1,14-cis-eicosatetraenoic acid) to produce the dihydroxy fatty acid leukotriene(LT) B4 (SS,12R-dihydroxy-6,14-cis-8,l~?~u~-eicosate~aenoic acid) (eq l),one of the physiologically important terminal products in the arachidonic acid biosynthetic pathway.'2 The enzyme has been purified to homogeneity from various sources as a water-soluble, monomeric protein with a molecular weight of about 700.3 The genes coding for the human enzyme from placenta and spleen have been cloned and sequenced.4 The mechanism of LTA4 hydrolase has not been well documented, but the recent studies have shown the similarity between this enzyme and some zinc metalloenzymes as the LTA4 hydrolase contains one zinc ion5,6 essential for the activity, and also exhibits aminopeptidase activity. 6 Further studies suggest that the peptidase and epoxide hydrolase activities of this enzyme occnr at the same active site.6It is of great interest to develop inhibitors of LTA4 hydrolase as potential antiinflammatory agents since LTB4 is a proinflammatory mediator which stimulates adhesion of circulating neutrophils to vascular endothelium and directs their migration toward sites of inflammation. LTA4 hydrolase is irreversibly inhibited by its substrate LTA4 778 and substrate analogues, e.g. LTA3 and LTA5.8-10 Bestatin, a naturally occurring, D-phenylalanine-derived norstatine-type aminopeptidase dipeptide inhibitor,11 was reported12 as a reversible inhibitor of LTA4 hydrolase. It showed inhibition for both aminopeptidase and epoxide hydrolase activities of this enzyme. Among other inhibitors tested, only captopril had the same inhibitory potency as bestatin.12 The inhibition activity of bestatin against LTA4 hydrolase prompted us to study the other Land D-phenylalanine-derived norstatine-type of compounds as inhibitors. We speculate that the peptidase activity of LTA4 hydrolase is like that of Zn++- containing thermolysin mechanically. Norstatine type of peptide isosteres, therefore, should be good inhibitors. The inhibition of captopril may be due to a strong interaction between the mercapto group and the zinc ion at the enzyme's active site. We envisage that better inhibitors may be designed if this binding/coordination interaction can be optimized.