Substrate specificity of arylamidase in soils

The enzyme amino acid arylamidase [EC 3.4.11.2] catalyzes the hydrolysis of an N-terminal amino acid from arylamides. Recently, this enzyme was detected in soils, and a method was developed 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. Because this enzyme may be involved in N mineralization and because of the presence of a number of amino acids in soils, information on the substrate specificity of arylamidase in soils is needed for a better understanding of the reactions involved in organic N mineralization in soils. In this work eight compounds containing individual amino acid moieties linked to naphthylamine were studied as substrates for arylamidase activity in six surface soils. Results showed that the optimal pH of activity varied with the amino acid moiety, ranging from 7.0 to 9.0; most of the optimal pH values, however, were at 8.0. Among the amino acid moieties studied, the means of activity in six soils decreased with the following order: alanine>leucine>serine>lysine>glycine>arginine>histidine, and proline was not hydrolyzed in soils. The chromophore linked to the amino acids L-asparagine, L-tyrosine, L-glutamic acid, and L-aspartic acid did not dissolve. Calculated from arylamidase activity at the optimal pH values, the Km, Vmax, Ea, and ΔHa values varied markedly among the soils and the amino acid moieties of the substrates studied. The means of the Q10 values ranged from 1.04 to 1.66. Results suggest that there are more than two isoforms of this enzyme in soils, and that amino acid moieties of ring compounds in soils significantly affect the rate of N mineralization in soils.