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The puromycin-sensitive aminopeptidase was found to be resistant to proteolysis by trypsin, chymotrypsin, and protease V8 but was cleaved into an N-terminal 60-kDa fragment and a C-terminal 33-kDa fragment by proteinase K. The two proteinase K fragments remain associated and retained enzymatic activity. Attempts to express the 60-kDa N-terminal fragment in Escherichia coli produced inclusion bodies. A hexa-histidine fusion protein of the 60-kDa N-terminal fragment was solubilized from inclusion bodies with urea and refolded by removal of the urea through dialysis. The refolded protein was devoid of aminopeptidase activity as assayed with arginine–β-naphthylamide. However, the refolded protein bound the substrate dynorphin A(1-9) with a stoichiometry of 0.5 mol/mol and a K0.5 value of 50 μM. Dynorphin A(1-9) binding was competitively inhibited by the substrate dynorphin B(1-9), but not by des-Tyr1-leucine-enkephalin, a poor substrate for the enzyme. [Copyright &y& Elsevier]