The effect of arsenic on soil intracellular and potential extracellular β-glucosidase differentiated by chloroform fumigation.

Arsenic (As) contamination of soil is a global issue of serious ecological and human health concern. For better use of soil enzymes as biological indicators of As pollution, the response of soil β-glucosidase in different pools of soil (total, intracellular and potential extracellular) to As(V) stress was investigated. Chloroform fumigation method was employed to distinguish the intracellular and potential extracellular β-glucosidase in three soils. The intracellular and potential extracellular β-glucosidase accounted about 79% and 21% of the total β-glucosidase activity in the tested soils. Moreover, it was found that the response of these three enzyme pools to As(V) pollution was different. Under the stress of 400 mg kg-1 As(V), the β-glucosidase activities decreased by 69%, 79%, and 28% for the total, intracellular and potential extracellular pools, respectively. The calculated median ecological dose (ED50) showed the highest value for potential extracellular β-glucosidase (19.55-27.63 mg kg-1 for total, 18.49-27.42 mg kg-1 for intracellular, and 32.27-52.69 mg kg-1 for potential extracellular β-glucosidase). As(V) exhibited an uncompetitive inhibition for total and intracellular β-glucosidase and non-competitive inhibition for potential extracellular enzyme. The inhibition constant (Kiu) is biggest for potential extracellular β-glucosidase among the three enzyme pools (0.61-0.79 mmol L-1 for total, 0.34-0.36 mmol L-1 for intracellular, and 4.01-23.90 mmol L-1 for potential extracellular β-glucosidase). Thus, compared to potential extracellular β-glucosidase, the total and intracellular β-glucosidases are more suitable for their use as sensitive indicators of As(V) pollution.