Effects of soil organic matter extracted from soil on acid phosphomonoesterase

Extract of soil organic matter (SOM) at concentrations of 0.05, 0.5, 2.5, 5 and 50 mg l−1 inhibited commercial acid phosphomonoesterase activity by 10.5, 13.3, 65.5, 77.2 and 89.3%, respectively. The inhibition was not due to adsorption of the product, because p-nitrophenol was not adsorbed on the SOM extract. At concentrations of 0.05 and 0.5 mg l−1 the SOM extract affected the enzyme kinetics considerably: apparent Km increased significantly, from 1.76 to 2.15 and 3.94 mm, respectively, but Vmax changed only slightly. With higher concentrations of the SOM extract (2.5–50 mg l−1) the reaction no longer followed Michaelis-Menten kinetics. The observed interaction with SOM and acid phosphomonoesterase might be ecologically interesting, especially in soils with high SOM concentrations, because of the importance of phosphomonoesterases in the mineralization of organic P esters in soils and rhizospheres.