The initial rates of carboxybenzoyl-alanyl-L-leucyl-amide (Z-L-Ala-L-Leu-NH2) synthesis from carboxybenzoyl-L-alanine (Z-L-Ala) and L-leucineamide (L-Leu-NH2) and Z-L-Ala-L-Leu-NH2 hydrolysis in a homogeneous dimethyl sulfoxide-aqueous buffer solution [1:1 (v/v)] system catalyzed by PST-01 protease from Pseudomonas aeruginosa were measured under a wide range Of Z-L-Ala, L-Leu-NH2 and Z-L-Ala-L-Leu-NH2 concentrations. The initial rates of the synthetic reaction, in which Z-L-Ala-L-Leu-NH2 was produced from Z-L-Ala and L-Leu-NH2, were inhibited by the substrates. Furthermore, the initial rates of the synthetic reaction were not inhibited by the product Z-L-Ala-L-Leu-NH2, and those of the hydrolytic reaction were inhibited by Z-L-Ala and L-Leu-NH2. All the initial rate data of the synthetic and hydrolytic reactions were well correlated with the rate equation derived based on the proposed reaction scheme. (C) 2004 Wiley Periodicals, Inc.