Properties of acid phosphatase–tannic acid complexes formed in the presence of Fe and Mn

The preparation and characterisation of synthetic enzyme complexes simulating those usually encountered in soil were investigated. Complexes were prepared at 30°C by the interaction of acid phosphatase and tannic acid in the presence and absence of Fe and Mn ions (as chlorides) and oxides, with and without montmorillonite. In comparison with phosphatase–tannic acid complexes, the enzymatic complexes formed in the presence of Fe oxide showed higher activity levels (>70%). Non significant increases were measured in the presence of Fe and Mn ions and Mn oxide. In the presence of montmorillonite, a further increase in the activity of the resulting complexes was measured. For example, the activity of complexes formed with Fe3+ or Mn2+ increased by 25 and 28%, respectively. In all cases, the kinetics of the immobilised enzyme conformed to the Michaelis–Menten equation and the Vmax values were significantly (P=0.001) lower and the Km values significantly higher than those of the free enzyme. For example, a two-fold reduction in the Vmax and four-fold increase in the Km parameters were measured for phosphatase complexed with tannic acid. In the presence of montmorillonite, no significant changes of Vmax values were measured whereas a detectable reduction of Km values was observed. Immobilised phosphatase showed a greater sensitivity to increases in temperature, but higher stability to proteolytic attack. Complexes with Fe ions and MnO2 were the most stable.