Immobilization of jack bean urease on hydroxyapatite: urease immobilization in alkaline soils

Jack bean urease was adsorbed and immobilized on hydroxyapatite, a stable calcium phosphate mineral commonly found in alkaline soils. The stability and kinetic properties of the resulting complex were investigated to derive a model that explains the stabilisation of urease in alkaline soil. The adsorption isotherms revealed: (1) an increase of the affinity of urease for hydroxyapatite with increasing ionic strength at constant pH and (2) a decrease of affinity with increasing pH at constant salt concentration. The optimum pH for enzymatic activity increased from 7 to 8 upon adsorption. The Michaelis–Menten parameters for free urease (Vmax=230.7 U mg−1; Km=7.45 mm) and for immobilized urease (Vmax=152.9 U mg−1; Km=6.89 mm) showed a moderate decrease of enzyme specific activity, but little change of substrate affinity. Urease adsorption resulted in a 2-fold increase in enzyme stability with time and a 7-fold increase in resistance to proteolytic hydrolysis. The results presented indicate that hydroxyapatite could have a significant role in the stabilisation of extracellular urease in alkaline soils.