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.