Leukotriene A4 hydrolase: an epoxide hydrolase with peptidase activity.

Purified leukotriene A4 hydrolase from human leukocytes is shown to exhibit peptidase activity towards the synthetic substrates alanine-4-nitroanilide and leucine-4-nitroanilide. The enzymatic activity is abolished after heat treatment (70 degrees C, 30 min). At 37 degrees C these substrates are hydrolyzed at a rate of 380 and 130 nmol/mg/min, respectively, and there is no enzyme inhibition during catalysis. Apo-leukotriene A4 hydrolase, obtained by removal of the intrinsic zinc atom, exhibits only a low peptidase activity which can be restored by the addition of stoichiometric amounts of zinc. Reconstitution of the apoenzyme with cobalt results in a peptidase activity which exceeds that of enzyme reactivated with zinc. Preincubation of the native enzyme with leukotriene A4 reduces the peptidase activity. Semipurified preparations of bovine intestinal aminopeptidase and porcine kidney aminopeptidase do not hydrolyze leukotriene A4 into leukotriene B4.