Inhibition of leukotriene A4 hydrolase/aminopeptidase by captopril

Captopril ((2S)-1-(3-mercapto-2-methyl-propionyl)-L-proline) inhibited the bifunctional, Zn(2+)-containing enzyme leukotriene A4 hydrolase/aminopeptidase reversibly and competitively with Ki = 6.0 microM for leukotriene B4 formation and Ki = 60 nM for L-lysine-p-nitroanilide hydrolysis at pH 8. Inhibition was independent of pH between pH 7 and 8, the optimum range for each catalytic activity. Half-maximal inhibition of leukotriene B4 formation by intact erythrocytes and neutrophils required 50 and 88 microM captopril, respectively. In neutrophils and platelets neither 5(S)-hydroxyeicosatetraenoic acid, 12(S)-hydroxyeicosatetraenoic acid, nor leukotriene C4 formation were reduced, indicating selective inhibition of leukotriene A4 hydrolase/aminopeptidase, not 5-lipoxygenase, 12-lipoxygenase, or leukotriene C4 synthase. In whole blood, captopril inhibited leukotriene B4 formation with an accompanying redistribution of substrate toward formation of cysteinyl leukotrienes. The decrease in leukotriene B4 was more substantial than the corresponding increase in cysteinyl leukotrienes suggesting that nonenzymatic hydration predominates over transcellular metabolism of leukotriene A4 by platelets during selective inhibition of leukotriene A4 hydrolase. Enalapril dicarboxylic acid and Glu-Trp-Pro-Arg-ProGln-Ile-Pro-Pro which inhibit angiotensin-converting enzyme: angiotensin I, bradykinin, and N-[3-(2-furyl)acryloyl]Phe-Gly-Gly which are substrates; and chloride ions which activate angiotensin-converting enzyme did not modulate leukotriene A4 hydrolase/aminopeptidase activity. The results indicate that: (i) the sulfhydryl group of captopril is an important determinant for inhibition of leukotriene A4 hydrolase/aminopeptidase, probably by binding to an active site Zn2+; (ii) aminopeptidase and leukotriene A4 hydrolase display differential susceptibility to inhibition; (iii) there is minimal functional similarity between angiotensin-converting enzyme (peptidyl dipeptidase) and leukotriene A4 hydrolase/aminopeptidase; (iv) captopril may be a useful prototype to identify more potent and selective leukotriene A4 hydrolase inhibitors.