Extracellular protease was one of the important pathogenic factors of Stenotrophomonas maltophilia. In order to investigate the relationship of structure and function of extracellular protease, the effects of protein modification reagents on the S. maltophilia extracellular protease activity have been studied. The chemical modification of extracellular protease of S. maltophilia was studied with phenylmethanesulfonyl fluoride (PMSF), p-chloromercuribenzoate (PCMB), N-acetylimidazole (N-AI), chloramine-T (Ch-T), N-bromosuccinimide (NBS), 2-mercaptoethanol (2-ME) as the modification reagents. The results indicated that the protease was not affected by N-AI, PMSF and PCMB, but Ch-T, NBS and 2-ME could reduce the activity significantly. It implied that Ser, Tyr and sulfhydryl residues were not activity dependent groups, while Met, Trp and disulfide linkage groups played an important role in the activity of extracellular protease. Meanwhile, EDTA, Mg^2＋, Ca^2＋, Hg^2＋ and Cu^2＋ could reduce the activity obviously, which showed that the enzyme was a metalloproteinase.

Key words: Stenotrophomonas maltophilia, extracellular protease, chemical modification