Characterization of β-lactamase activity using isothermal titration calorimetry

Background Hydrolysis of beta-lactam antibiotic by beta-lactamase is the most common mechanism of beta-lactam resistance in clinical isolates. Timely detection and characterization of beta-lactamases are therefore of utmost biomedical importance. Conventional spectrophotometric method is time-consuming and cannot provide thermodynamic information on beta-lactamases. Methods: A new assay was developed for the study of beta-lactamase activity in protein solutions (Metallo-beta-lactamase L1) and in clinical bacterial cells, based on heat-flow changes derived from enzymatic hydrolysis of beta-lactams using isothermal titration calorimetry. Results: (1) The thermokinetic parameters of three antibiotics (penicillin G, cefazolin and imipenem) and the inhibition constant of an azolylthioacetamide inhibitor were determined using the calorimetric assay. The results from the calorimetric assays were consistent with the data from the spectrophotometric assay. (2) The values of heat change in the calorimetric assay using two clinical Escherichia coli strains correlated well with their antibiotic susceptibility results from the broth dilution experiment. The subtypes of beta-lactamase were also determined in the calorimetric assay. Conclusions: The ITC assay is a reliable and fast method to study beta-lactamase enzyme kinetics and inhibition. It can also provide thermodynamic information on antibiotic hydrolysis, which has been taken advantage of in this work to study beta-lactamase activity in two clinical Escherichia call isolates. General significance: As the first calorimetric study of beta-lactamase activity, it may provide a new assay to assist biomedical validation of new beta-lactamase inhibitors, and also has potential applications on rapid antibiotic susceptibility testing and screening beta-lactamase producing bacteria.