Schistosoma japonicum: biochemistry and cytochemistry of dipeptidyl aminopeptidase-II-like activity in adults.

The biochemical characterization of dipeptidyl aminopeptidase II activity was investigated in the supernatant of centrifuged homogenates of adult Schistosoma japonicum using a lysine-alanine oligopeptide derivative of 4-methoxy-2-naphthylamide as a substrate. It was observed that the pH optimum of the enzyme is in the acid range, with an optimum at pH 6.3. Time and enzyme concentration studies, along with temperature studies, support the premise that the reaction is enzymatic. The Km was 3.3 X 10(-3) M, at pH 5.5 and 37 C. Tris and diisopropyl phosphofluoridate, when incorporated into the assay system at final concentrations of 500 and 2 mM, respectively, significantly inhibited the reaction by 70.9 and 75%, respectively. Leupeptin (5 X 10(-4) mM) had no effect. The results indicate that the enzyme under study in the present investigation strongly resembles mammalian dipeptidyl aminopeptidase II due to its affinity for substrate, sensitivity to Tris and diisopropyl phosphofluoridate inhibition, and pH optimum. Its inhibition by diisopropyl phosphofluoridate indicates that it may belong to the serine class of proteases. Cytochemical studies revealed reaction product in the lipid-like globules in the gastrodermis, adding further credence that these globules are lysosomal.