Inhibition of arylamidase activity in soils by toluene

Toluene is used in assays of many soil enzymes, especially the hydrolases group (EC 3), because it inhibits microbial proliferation and it renders the microbial cell membrane permeable to substrate and/or reaction products. Studies showed that toluene inhibits arylamidase (EC 3.4.11.2) activity, the enzyme that catalyzes the release of an N-terminal amino acid from peptides, amides or arylamides. This enzyme plays a key role in the initial reaction of N mineralization as it is involved in the release of amino acids from soil organic matter. The assay involves colorimetric determination of the β-naphthylamine released when 1 g of soil is incubated at 37°C for 1 h with l-leucine β-naphthylamide and 0.1 M THAM buffer pH 8.0. Results showed that the inhibition of this enzyme by toluene is of mixed type; i.e. both the Km and Vmax values are affected by toluene, indicating that both the enzyme and the enzyme–substrate (ES) complex are inhibited. Kinetic and thermodynamic studies showed that the Km values of arylamidase activity in four surface soils in the presence and absence of toluene ranged from 0.28 to 0.77 and 0.19 to 0.35 mM, respectively. The corresponding Vmax values ranged from 16 to 81 and 22 to 100 mg β-naphthylamine kg−1 soil h−1. The Ea values in the presence and absence of toluene ranged from 19.3 to 27.2 and 26.2 to 32.4 kJ mol−1, respectively. The ΔHa values of the reaction catalyzed by arylamidase in soils were equally affected by toluene.