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Arylamidase activity of soils

The enzyme amino acid arylamidase [α-aminoacyl-peptide hydrolase (microsomal) EC 3.4.11.2] catalyzes the release of an N-terminal amino acid from peptides, amides, or arylamides. Because of the presence of such substrates in soils, it is likely that this enzyme is involved in N mineralization. We report on the detection of the activity of this enzyme in soils and describe a precise and accurate method for its assay. This method involves colorimetric determination of the β-naphthylamine produced when soil is incubated with L-leucine β-naphthylamide in 0.1 M THAM [tris(hydroxymethyl)aminomethane] buffer (pH 8.0) at 37°C for 1 h. The β-naphthylamine that is produced is extracted with ethanol and converted into an azo compound by reacting with p-dimethylaminocinnamaldehyde, and the absorbance of the color is measured at 540 nm. This enzyme has its optimal activity at pH 8.0 and is inactivated at temperatures above 60°C. Preheating soil samples for 2 h at temperatures ranging from 20 to 120°C before assay showed that this enzyme is stable up to 40°C in field-moist soils and up to 60°C in air-dried samples. The K m values of arylamidase activity in seven surface soils ranged from 0.19 to 0.35 mM. The temperature response followed Arrhenius equation over the range from 20 to 50°C. The activation energy values ranged from 30.6 to 49.8 kJ mol -1 for field-moist soils and from 26.2 to 32.4 kJ mol -1 for their air-dried counterparts. The means of temperature coefficients (Q 10 ) ranged from 1.32 to 1.71 (avg. = 1.44). Several compounds inhibited the activity of this enzyme in soils.