Escherichia coli beta-lactamase TEM-1 is potentially useful in the antibody-directed enzyme/prodrug therapy (ADEPT), converting nontoxic prodrugs to toxic agents. The produced toxin would kill cancer cells, when the enzyme is attached to a tumor-antigen-specific antibody. However, the off-site reaction possibly occurring in the blood or normal tissues raises safety concern. In the present study, we engineered TEM-1 variants preferentially active at pH 5.8-6.2, near the pH of the acidic microenvironment of tumor. A library of randomly mutagenized variants was screened for the ability to confer an antibiotic resistance on E. coli cells in acidic growth media and not in neutral media, to isolate a variant with a Thr-to-Ile substitution at position 160. An extensive mutagenesis study was then conducted in the proximity of this position, to show that a Leu162Glu mutation also causes the acid preference. Kinetic analyses indicated that the overall activity of the wild-type TEM-1 hardly changes over a pH range from 5.8 to 7.0, whereas TEM-1(T1601) is 1.5-times as active at pH 6.2 than pH 7.0, and TEM-1(T1601) is 3.1-fold as active at pH 5.8 than pH 7.0. A further mutagenesis study suggested that a change in the overall structure of the enzyme underlies the pH dependency of the variants. (C) 2018 Elsevier Inc. All rights reserved.