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 simple and precise method is described for assaying its activity in soils. The method involves determination, by steam distillation, of the NH 4 + -N produced when soil is incubated with potassium aspartate in 0.1 M THAM [tris(hydroxymethyl)aminomethane] buffer (pH 8.5) and toluene at 37°C for 24 h. Results showed that soil aspartase has its optimum activity at buffer pH 8.5 and is inactivated at temperatures above 40°C. Preheating soil samples for 2 h before assay of aspartase activity showed that the enzyme is stable up to 40°C in field-moist samples and up to 70°C in air-dried samples. The K m values of the aspartase activity in three soils ranged from 0.173 to 0.208 M. The Arrhenius equation plot for aspartase activity in three soils was linear between 20 and 40°C. The activation energy values 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, dimethylsulfoxide, and HgCl 2 decreased the aspartase activity but toluene increased this activity. The use of sulfhydryl reagents suggested that a free sulfhydryl moiety is present in the active sites of this enzyme. The coefficients of variation of the proposed method for assay of aspartase activity in soils was <2.5%.