ADSORPTION, ACTIVITY AND KINETIC-PROPERTIES OF UREASE ON MONTMORILLONITE, ALUMINUM HYDROXIDE AND AL(OH)X-MONTMORILLONITE COMPLEXES

The adsorption of urease on a montmorillonite (M), a non-crystalline aluminium hydroxide (AL) and an Al(OH)x-montmorillonite complex (AM) as well as the activity, the kinetics and the stability of the enzyme-clay mineral complexes were studied. The equilibrium adsorption isotherms of urease on clay minerals fitted both the Langmuir or the Freundlich equations. The Langmuir adsorption isotherm of the enzyme on M was of H type ("high affinity") whereas the isotherms on AL and AM were of L type ("Langmuir"). On adding up to 21.2 mg of enzyme g-1 clay, the amount of urease held on the clay minerals followed the order M > AM > AL throughout the pH range explored (4.0-9.0). The adsorption of urease on M, AM and AL was differently affected by pH. The specific activity of enzyme immobilized on M and AM was relatively high (71 and 64% respectively) as compared to that of the free enzyme; in contrast the specific activity of urease adsorbed on AL was considerably reduced (15%). The free and immobilized urease showed similar pH- and temperature-activity profiles and both states obeyed Michaelis-Menten kinetics. The V(max) and K(m) parameters, as well as the thermal stability of adsorbed urease were always lower than those of the free urease, whereas the proteolytic stability of urease held on AL was higher than that of the enzyme free or adsorbed on M and AM. Finally, it was ascertained that the covering of the surfaces of montmorillonite with different amounts of OH-Al species reduced the quantity as well as the activity of adsorbed enzyme.