INHIBITION OF PYROPHOSPHATASE ACTIVITY IN SOILS BY TRACE ELEMENTS1

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Abstract
Pyrophosphatase catalyzes the hydrolysis of pyrophosphatase to orthophosphate. Information on the control of pyrophosphatase activity in soil is desired. The effect of 21 trace elements on the activity of this enzyme in soils was studied. Nineteen of these elements inhibited pyrophosphatase activity and the relative effectiveness of the trace elements in inhibiting this enzyme depended on the soil. When the trace elements were compared by using 25 .mu.mol/g soil, the average inhibition in 3 soils showed that Hg(II), As(V), Mo(VI), and W(VI) were the most effective inhibitors (average inhibition > 50%). Moderately effective inhibitors were: Cd(II), Cu(II), Fe(II), B(III), and V(VI) (average inhibition between 30 and 50%). Other trace elements that inhibited pyrophosphatase activity in soils were: As(I), Cu(I), Mn(II), Ni(II), Pb(II), Sn(II), Zn(II), Al(III), As(III), Cr(III), Fe(III), Se(IV), and Ti(IV). Pyrophosphatase activity was not inhibited by Ba(II) or Co(II). The inhibitory effect of trace elements decreased when the amount of trace elements added per gram of soil was decreased 5-fold (from 25 to 5 .mu.mol/g soil). The inhibition by the divalent cations Hg, Cd, Cu, Fe, and Sn showed noncompetitive kinetics, but BO32-, MoO42-, WO42-, AsO43-, and VO2+ exhibited competitive kinetics. Among the trace elements studied, MoO42- and WO42- are the most effective inhibitors of pyrophosphatase activity in soils.

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