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 m M 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 Several compounds inhibited the activity of this enzyme in soils.