Kinetic behaviour of alkaline phosphatase desorbed from different clay minerals

The kinetic (K_m, V_max) of alkaline phosphatase (AP) desorbed from different Ca-homoionic clay minerals (montmorillonite, illite, and kaolinite) by extraction with Tris-Malate-Citrate buffer solution (pH 9.6) was studied in model experiments. After extraction (shaking for 15 min.) the K_m and V_max were measured in the extract, the remaining sediment and in the whole set-up. With kaolinite and illite, V_max of the desorbed AP was lower than that of the sediment. However, with montmorillonite, V_max of AP in the extract and whole system increased if compared to the control, but decreased in the sediment. The K_m of desorbed AP increased from 4.3 × 10^?3 (control) to 5.0 × 10^?3 M (illite), 5.4 × 10^?3M (kaolinite), and 5.5 × 10^?3M (montmorillonite). These values were lower than those obtained with the various sediments and whole experimental systems. An aberrant behaviour was recorded with the illite sorbed AP which showed an increase in affinity towards the substrate. Generally speaking, AP desorbed from clays may be reduced in its affinity towards the substrate p-nitrophenylphosphate by residual inhibitor and/or conformational change of the enzyme.