Urease immobilization on an alkylamine derivative of titanium (IV)‐porous silica: Kinetics and operational stability

For the purpose of immobilizing urease (urea amidohydrolase, EC 3.5.1.5) for routine determination of urea, a covalent bond immobilization method on titanium (IV) chloride activated silica supports1,2 was used. Several parameters were studied in order to optimize the residual activity upon immobilization and during operation. The kinetic constants of the immobilized enzyme form (Km=92 mmol dm−3, Vmax=752 μmole min−1 mg−1) are much higher than those of the free preparation (Km=31 mmol dm−3, Vmax=128 μmole min−1 mg−1). The optimum pH of the immobilized enzyme preparation was shifted to higher values, from pH 7·4 to 8. The optimum temperatures for the immobilized and free forms were 75 and 65°C, respectively. The activation energy of the immobilized form is four times smaller than that of the free, indicating less sensitivity to temperature. It was also observed that the immobilized enzyme exhibits a higher heat stability than the free enzyme. From these studies the conditions for optimal operation and retention of stability of the immobilized enzyme were determined. The enzyme immobilized on the alkylamine derivative of porous silica can be reused at 37°C for several hours a day with 100% retention of its initial activity for at least 1200 h of total life (i. e. operating time at 37°C plus stationary time at 4°C).