Activity and stability of jack bean urease in the presence of peat humic acids obtained using different extractants

The effects of two humic acid extractants, 0.1 M Na4P2O7 plus 0.1 M NaOH (NaPP) and 0.1 M NaOH (NaOH), on the activity and stability of a humic-urease complex were studied. The two humic acids isolated (HANaPP and HANaOH) exhibited different elemental compositions, metal concentrations and structural modifications in the FT-IR spectra. Depending on the pH, HANaPP and HANaOH influenced both the urease activity and urease kinetic parameters (Vmax and Km) in the same way. They inhibited urease activity between pH 6 and 7 and reduced the Vmax of the reaction at pH 6 and 7. The presence of humic acids improved the affinity of the enzyme for the substrate (Km). The stability of the urease with time, and in the presence of pronase, was improved by HANaPP and HANaOH with respect to free enzyme. These results confirm the importance of the interaction of urease with humic acids as a fundamental gateway for extracellular urease stabilisation. Since no difference in the extent of urease inhibition and urease stabilisation was observed for the two humic acids, it may be concluded that neither urease activity nor stability are influenced by the humic acid extractant used.