Thermokinetic method for faster enzyme-catalyzed reactions

This paper describes the application of dynamic microcalorimetry to determine the thermokinetics of short-time enzymatic reactions. A thermokinetic double-parameter method for faster enzyme-catalyzed reactions is proposed on the basis of the double-parameter theoretical model of the conduction calorimeter. By analyzing the calorimetric curves of faster enzyme-catalyzed reactions, this method can be conveniently used to calculate both molar reaction enthalpy (Delta(r)H(m)) and kinetic parameters (K-m, upsilon(max)) of these reactions. Thermokinetics of the oxidation of xanthine catalyzed by xanthine oxidase in the presence of excess oxygen has been studied using microcalorimetry. This faster enzyme-catalyzed reaction obeyed the Michaelis-Menten kinetics, and the Michaelis constant (K-m) for xanthine and the overall molar reaction enthalpy (Delta(r)H(m)) of this oxidation were determined by this method to be 1.04 x 10(-3) mol dm(-3) and -5.25 kJ mol(-1), respectively, at 298.15 K and pH 7.5. The reliability of the double-parameter method for faster enzyme-catalyzed reactions was verified by the experimental results. (C) 1998 Elsevier Science B.V.