Arylamidase activity of soils

The enzyme amino acid arylamidase [alpha-aminoacyl-peptide hydrolase (micro somal) 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, W e report on the detection of the activity of this enzyme in soils and descr ibe a precise and accurate method for its assay. This method involves color imetric determination of the beta-naphthylamine produced when soil is incub ated with L-leucine beta-naphthylamide in 0.1 M THAM [tris(hydroxymethyl)am inomethane] buffer (pH 8.0) at 37 degrees C for 1 h, The beta-naphthylamine that is produced is extracted with ethanol and converted into an azo compo und by reacting with p-dimethylaminocinnamaldehyde, and the absorbance of t he color is measured at 540 nm, This enzyme has its optimal activity at pH 8.0 and is inactivated at temperatures above 60 degrees C. Preheating soil samples for 2 h at temperatures ranging From 20 to 120 degrees C before ass ay showed that this enzyme is stable up to 40 degrees C in field-moist soil s and up to 60 degrees C in air-dried samples. The K-m values of arylamidas e activity in seven surface soils ranged from 0.19 to 0.35 mM, The temperat ure response followed Arrhenius equation over the range from 20 to 50 degre es 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 coun terparts. The means of temperature coefficients (Q(10)) ranged from 1.32 to 1.71 (avg. = 1.44), Several compounds inhibited the activity of this enzym e in soils.