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Aspartase Activity of Soils

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010

*Corresponding author (malit{at}iastate.edu).

ABSTRACT

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 NH⁴₊-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 (Q₁₀) ranged from 1.50 to 2.44 (avg. = 1.89). Treatment of soils with formaldehyde, dimethylsulfoxide, and HgCl₂ 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%.

Current address for Z.N. Senwo is Dep. of Plant and Soil Sciences, Alabama A&M Univ., Normal, AL 35762. Journal paper no. J-16489 of the Iowa Agric. and Home Econ. Exp. Stn.

Received for publication August 14, 1995.

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