A Bioelectrocatalysis Method for the Kinetic Measurement of Thermal Inactivation of a Redox Enzyme, Bilirubin Oxidase

The thermal stability of a redox enzyme, bilirubin oxidase (BOD), has been quantitatively evaluated by measuring the inactivation kinetics of BOD at several temperatures. The enzyme activity is directly related to the mediated bioelectrocatalytic current for the BOD-catalyzed reduction of O_2. Thus, the inactivation process is measured by the time-dependent decrease in the bioelectrocatalytic current. The results reveal that the inactivation obeys first-order kinetics, whose rate constants ( k ) are determined at pH 7.0 and at 50–70°C. The half life of BOD activity, calculated from the k value at 50°C is 114 min, which is in harmony with the thermal-stability data given in a catalog by Amano Enzyme Inc. The bioelectrocatalysis method allows in situ measurements of the inactivation kinetics in the period of a few minutes at relatively high temperatures. The rate constants show a large temperature dependence, leading to a large Arrhenius activation energy ( E _A) of 221 kJ mol^-1. The activation Gibbs energy (Δ G ^≠), activation enthalpy (Δ H ^≠), and activation entropy (Δ S ^≠) are also determined.

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