Adaptation of a continuous, calorimetric kinetic assay to study the agmatinase-catalyzed hydrolytic reaction

Ureohydrolases are members of the metallohydrolase family of enzymes. Here, a simple continuous assay for agmatinase (AGM) activity was established by following the degradation of agmatine to urea and putrescine using isothermal titration calorimetry (ITC). ITC is particularly useful for kinetic assays when substrates of interest do not possess suitable chromophores that facilitate the continuous spectrophotometric detection of substrate depletion and/or product formation. In order to assess the accuracy of the ITC-based assay, catalytic parameters were also determined using a discontinuous, colorimetric assay. Both methods resulted in comparable kinetic parameters. From the colorimetric assay the k_cat and K_M values are 131 s^-1 and 0.25 mM, respectively, and from the ITC assay the corresponding parameters are 30 s^-1 and 0.45 mM, respectively. The continuous ITC-based assay will facilitate functional studies for an enzyme that is an emerging target for the development of addiction treatments.