PURIFICATION AND PRELIMINARY CHARACTERIZATION OF DDAP, A NOVEL DIPEPTIDYLAMINOPEPTIDASE FROM DICTYOSTELIUM-DISCOIDEUM

We have discovered and purified a novel dipeptidylaminopeptidase (DAP) from the cell-free broth of Dictyostelium discoideum Ax3. The enzyme is secreted in parallel with cell growth in axenic broth culture. It s hares substrate preferences with both DAP-I and DAP-LII enzymes yet is distinct from both in some physical properties. Similar to DAP-I, the D. discoideum enzyme is able to cleave a variety of dipeptides from t he amino termini of substrates. In addition, it readily cleaves substr ate sequences beginning with RR- and KK-, a property of the DAP-m clas s. The D. discoideum enzyme has a pH optimum of 3.5, a subunit molecul ar mass of 66 000 daltons, and a molecular weight of approximately 225 000 and is not significantly inhibited by cysteine or serine protease inhibitors. It is inhibited by leupeptin and trivalent cations. On th e basis of enzymological and other data presented here, we conclude th e D. discoideum enzyme does not belong to any of the previously report ed DAP classes (DAP-I, -II, -III, -IV) and propose that the class DAP- V be established with this D. discoideum enzyme as the first member.