ASPARTASE ACTIVITY OF SOILS

The enzyme aspartase [L-aspartate ammonia-lyase, Enzyme Commission (EC ) 4.3.1.1] catalyzes the hydrolysis of L-aspartate producing fumarate and NH3. The detection of this enzyme in soils is reported, and a simp le and precise method is described for assaying its activity in soils. The method involves determination, by steam distillation, of the NH4-N produced when soil is incubated with potassium aspartate in 0.1 M T HAM [tris(hydroxymethyl)aminomethane] buffer (pH 8.5) and toluene at 3 7 degrees C for 24 h. Results showed that soil aspartase has its optim um activity at buffer pH 8.5 and is inactivated at temperatures above 40 degrees C. Preheating soil samples for 2 h before assay of aspartas e activity showed that the enzyme is stable up to 40 degrees C in fiel d-moist samples and up to 70 degrees C in air-dried samples. The K-m v alues of the aspartase activity in three soils ranged from 0.173 to 0. 208 M. The Arrhenius equation plot for aspartase activity in three soi ls was linear between 20 and 40 degrees C. The activation energy value s of the reaction catalyzed by this enzyme ranged from 40.1 to 50.7 kJ mol(-1), and the temperature coefficients (Q(10)) ranged from 1.50 to 2.44(avg. = 1.89). Treatment of soils with formaldehyde, dimethylsulf oxide, and HgCl2 decreased the aspartase activity but toluene increase d this activity. The use of sulfhydryl reagents suggested that a free sulfhydryl moiety is present in the active sites of this enzyme. The c oefficients of variation of the proposed method for assay of aspartase activity in soils was <2.5%.