Michaelis constant (K m ) of acid phospatase as affected by montmorillonite, illite, and kaolinite clay minerals

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Abstract
The influence of Ca homoionic clay minerals (montmorillonite, illite, and kaolinite) on the activity,K m , andV m values of acid phosphatase was examined in model experiments. At each substrate (p-nitrophenyl phosphate) level tested, the addition of increasing amounts of clays (50, 100, and 150 mg, respectively) decreased the activity and increased theK m value from 1.43×10−3 m PNP (in the soluble state) to 82.3×10−3M (montmorillonite), 8.02×10−3 m (kaolinite), and 7.65×10−3 m (illite) at the 150 mg level. The maximum enzyme reaction velocity (V m ) remained nearly constant at different amounts of kaolinite and illite, but increased remarkably with rising quantities of montmorillonite. Apparently, the substrate affinity of sorbed acid phosphatase is significantly lower with montmorillonite than with kaolinite or illite. This may be ascribed to an intensive sorption of both substrate and enzyme to the surface as well as to interlattice sites of montmorillonite.