Molecular cloning of the gene and the crystal structure of the prolyl aminopeptidase [EC 3. 4. 11. 5] from Serratia nmrcescens have been studied by us [J. Bioehem. 122, 601-605 (1997); J. Bioehem. 126, 559-565 (1999)]. Through these studies, Phe139, Tyr149, Glu204, and Arg136 were estimated to be concerned with substrate recognition. To elucidate the details of the mechanism for the substrate specificity, the site-directed mutagenesis method was applied. The F139A mutant showed an 80-fold decrease in catalytic efficiency (k_cat/K_m), but the Y149A mutant did not show a significant change in catalytic efficiency. The catalytic efficiency of the E204Q mutant was about 4% of that of the wild type. The peptidase activity of the mutant (R136A) was markedly decreased, however, arylamidase activity with Pyr-βNA was retained as in the wild-enzyme. From these results, it was clarified that the pyrrolidine ring and the amino group of proline at the SI site were recognized by Phe139 and Glu204, respectively. P1' of a substrate was recognized by Arg136. On the other hand, the enzyme had two cysteine residues. Mutants C74A and C271A were inhibited by PCMB, but the double mutated enzyme (C74/271A) was resistant to it.