Catalytic efficiency is a better predictor of arsenic toxicity to soil alkaline phosphatase

Arsenic (As) is an inhibitor of phosphatase, however, in the complex soil system, the substrate concentration effect and the mechanism of As inhibition of soil alkaline phosphatase (ALP) and its kinetics has not been adequately studied. In this work, we investigated soil ALP activity in response to As pollution at different substrate concentrations in various types of soils and explored the inhibition mechanism using the enzyme kinetics. The results showed that As inhibition of soil ALP activity was substrate concentration-dependent. Increasing substrate concentration decreased inhibition rate, suggesting reduced toxicity. This dependency was due to the competitive inhibition mechanism of As to soil ALP. The kinetic parameters, maximum reaction velocity (V_max) and Michaelis constant (K_m) in unpolluted soils were 0.012-0.267mMh^-1 and 1.34-3.79mM respectively. The competitive inhibition constant (K_ic) was 0.17-0.70mM, which was lower than K_m, suggesting higher enzyme affinity for As than for substrate. The ecological doses, ED₁₀ and ED₅₀ (concentration of As that results in 10% and 50% inhibition on enzyme parameter) for inhibition of catalytic efficiency (V_max/K_m) were lower than those for inhibition of enzyme activity at different substrate concentrations. This suggests that the integrated kinetic parameter, catalytic efficiency is substrate concentration independent and more sensitive to As than ALP activity. Thus, catalytic efficiency was proposed as a more reliable indicator than ALP activity for risk assessment of As pollution.