The kinetic parameters Kcat. and Km were determined for the hydrolysis of some arginine naphthylamides by human cathepsin B. 2. A new and efficient synthesis of Z-Arg-Arg-NNap (benzyloxycarbonyl-L-arginyl-L-arginine 2-naphthylamide) was developed. 3. Z-Arg-Arg-NNap was a specific and sensitive substrate for cathepsin B, and was used for kinetic studies. 4. Values of kcat. were maximal in the pH range 5.4--6.2, and depended on a single ionizing group of pKa 4.4. 5. Leupeptin was a purely competitive inhibitor of human cathepsin B. 6. The effect of pH on the apparent inhibitor constant, Ki (app.), was determined. Ki (app.) was pH-independent in the range pH 4.3--6.0, with the mean value 7 x 10(-9) M.
