Toluene induces hormetic response of soil alkaline phosphatase and the potential enzyme kinetic mechanism.

Hormesis of soil enzyme that involved in heavy metal has been attracting much more attention for risk assessment of heavy metal toxicity, but insufficient studies were conducted to define the hormetic responses induced by toluene or other organic pollutions. The objectives of this study were to investigate the hormetic responses of soil enzyme induced by toluene and explore the potential enzyme kinetic mechanism. Soil alkaline phosphatase (ALP) activity was regarded as the endpoint to explore the hormetic responses under different doses of toluene (0.0, 0.1, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0, 50.0 and 100.0 μL g). Subsequently, we conducted the experiments of enzymatic reaction kinetics and pure enzyme to further verify the potential mechanisms of soil ALP's hormesis. Results showed that ALP activities at 0.1-1.0 μL g toluene were significantly increased in contrast to the control (0 μL g toluene) (P < 0.05) at the exposure time of 30, 36, 48 and 54 h, with the maximum stimulation magnitudes of 24-43%. ALP activities were almost not affected by toluene (2-100 μL L) in the whole experimental period (6-54 h). Meanwhile, the values of catalytic efficiency (the radio V/K, V: maximum reaction velocity and K: Michaelis constant) and V significantly increased compared with the control, but the value of K decreased from 2.5 to 1.6. Overall, low dose toluene can induce hormesis of soil ALP. The potential reason is that low-dose toluene could enhance the combination of soil ALP and substrates. We believe that this study will provide a new viewpoint for ecological risk assessment of toluene contaminated soils.