Fluorescein diacetate hydrolase (FDA-H) is an accurate biochemical method measuring the total microbial activity in soil, which indicates soil quality under ambient environmental changes such as pesticide parathion (PTH). However, the influence of PTH on the kinetics of FDA-H is still unknown. In this study, fifteen farmland soils were exposed to acute PTH pollution to investigate how the kinetic characteristics of FDA-H change with PTH concentration. Results showed that PTH strongly inhibited the FDA-H activities. The values of maximum reaction velocity (Vmax) ranged from 0.29 to 2.18 × 10-2 mM g-1 soil h-1 and declined by 42.30%-71.01% under PTH stress. The Michaelis constant (Km) values ranged between 2.90 and 14.17 × 10-2 mM and exhibited three forms including unchanged, increased (38.16-242.65%) and decreased (13.41-39.23%) when exposed to PTH. Based on the changes in two kinetic parameters, the inhibition of PTH on FDA-H was classified as three types, i.e., noncompetitive, linear mixed and uncompetitive inhibition. The competitive inhibition constant (Kic) and noncompetitive constant (Kiu) ranged from 0.064 to 0.447 mM and 0.209 to 0.723 mM, respectively, which were larger than the Km in values. The catalytic efficiency (Vmax/Km) of FDA-H is a sensitive integrated parameter to evaluate the PTH toxicity due to the higher inhibition ratio than the Vmax. The PTH toxicity to FDA-H decreased with increase of soil organic matter and total nitrogen contents. This implied that the PTH toxicity could be alleviated by an increasing content of soil organic matter due to its buffering capacity to PTH. Besides, soils with a higher content of total nitrogen could provide stable environment for FDA-H to maintain its functionality under PTH pollution. Thus, the results of this study have great implications to the risk assessment of parathion in soils.
Keywords: Enzyme kinetics; Inhibition constants; Parathion; Soil FDA hydrolase; Soil properties.
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