Purification and characterization of Stenotrophomonas maltophilia-derived l-amino acid ester hydrolase for synthesizing dipeptide, isoleucyl-tryptophan.

In the present study, we purified α-amino acid ester hydrolase (AEH) from cell-free extracts of the Stenotrophomonas maltophilia strain HS1. The approximately 70-kDa AEH from S. maltophilia HS1 (SmAEH) was homogeneous in sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) analyses, and was present as a tetramer in gel-filtration experiments. The activity of the SmAEH enzyme was then determined by monitoring the synthesis of the antihypertensive agent dipeptide isoleucyl-tryptophan (Ile-Trp) from isoleucyl methyl ester (Ile-OMe) and tryptophan (Trp). In these experiments, SmAEH had wide substrate specificity for acyl donors, such as Gly-OMe, β-Ala-OMe, Pro-OMe and Trp-OMe and Ile-OMe, and maximal activity were observed under conditions of pH 9.0 and 30 °C. SmAEH also showed the greatest stability at pH 9.0, whereas its activity was reduced by 40% after 10-min incubation at approximately 50 °C. In subsequent activity assays in the presence of various metal ions, Ag+ strongly inhibited enzyme activity. Finally, SmAEH activity was completely inhibited by phenylmethanesulfonyl fluoride (PMSF), suggesting that the protein is a serine protease.