The kinetics of heavy metal ions inhibition of jack bean urease was studied by progress curve analysis in a reaction system without enzyme-inhibitor preincubation. The inhibition was found to be biphasic with an initial, small inhibitory phase changing over the time course of 5-10 min into a final linear steady state with a lower velocity. This time-dependent pattern was best described by mechanism B of slow-binding inhibition, involving the rapid formation of an EI complex that subsequently undergoes slow conversion to a more stable EI* complex. The kinetic parameters of the process, the inhibition constants Ki and Ki* and the forward k5 and reverse k6 rate constants for the conversion, were evaluated from the reaction progress curves by nonlinear regression treatment. Based on the values of the overall inhibition constant Ki*, the heavy metal ions were found to inhibit urease in the following decreasing order: Hg2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+ > Pb2+ > Co2+ > Fe3+ > As3+. With the Ki* values as low as 1.9 nM for Hg2+ and 7.1 nM for Cu2+, 100-1000 times lower than those of the other ions, urease may be utilized as a bioindicator of the trace levels of these ions in environmental monitoring, bioprocess control or pharmaceutical analysis.

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The purpose of this study was to verify the correlation between the activity of urease and the content of nickel in soil of temperate climate in relation to the land management. Moreover, the metal speciation was taken into account in order to search for the above-mentioned correlation. Arable lands, forested lands, and wastelands were analyzed. The basic soil parameters were determined such as pH, clay fraction content, and organic matter content. The speciation of nickel was studied by using BCR (Bureau Communitaire de Reference) sequential extraction procedure and flame atomic absorption spectrometry, while the urease activity was determined spectrophotometrically. The pseudo-total content of Ni in every sample was below 30 mg kg−1 of dry soil. The dominant form of nickel in the soil samples was the residual form. Although the urease activity varied slightly between the samples, the differences turned out to be statistically insignificant. However, the highly positive correlations between the urease activity, organic matter content, and the pseudo-total content of Ni were found and discussed. Moreover, the positive correlations between the urease activity and two geochemical forms of nickel, namely, active and residual form, were confirmed. The results of performed experiments prove that the method of land management does not significantly affect either the topsoil urease activity or the nickel distribution in the case of the extensive agriculture and forestry. However, the existence of at least two forms of the active soil urease was proposed. The first one―contained in the soil solution or loosely adsorbed on the soil particulates and the second one―strongly adsorbed onto the clay minerals. Also the complex role of the organic matter in protecting urease from external factors was presented. Finally, it was postulated that the nickel content in soil may be the indicator of the soil urease activity.