Bestatin, an inhibitor of aminopeptidases, was also a potent inhibitor of leukotriene (LT) A4 hydrolase. On isolated enzyme its effects were immediate and reversible with a Ki = 201 +/- 95 mM. With erythrocytes it inhibited LTB4 formation greater than 90% within 10 min; with neutrophils it inhibited LTB4 formation by only 10% during the same period, increasing to 40% in 2 h. Bestatin inhibited LTA4 hydrolase selectively; neither 5-lipoxygenase nor 15-lipoxygenase activity in neutrophil lysates was affected. Purified LTA4 hydrolase exhibited an intrinsic aminopeptidase activity, hydrolyzing L-lysine-p-nitroanilide and L-leucine-beta-naphthylamide with apparent Km = 156 microM and 70 microM and Vmax = 50 and 215 nmol/min/mg, respectively. Both LTA4 and bestatin suppressed the intrinsic aminopeptidase activity of LTA4 hydrolase with apparent Ki values of 5.3 microM and 172 nM, respectively. Other metallohydrolase inhibitors tested did not reduce LTA4 hydrolase/aminopeptidase activity, with one exception; captopril, an inhibitor of angiotensin-converting enzyme, was as effective as bestatin. The results demonstrate a functional resemblance between LTA4 hydrolase and certain metallohydrolases, consistent with a molecular resemblance at their putative Zn2(+)-binding sites. The availability of a reversible, chemically stable inhibitor of LTA4 hydrolase may facilitate investigations on the role of LTB4 in inflammation, particularly the process termed transcellular biosynthesis.