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Amidase Activity in Soils: II. Kinetic Parameters¹

ABSTRACT

Studies to determine the kinetic parameters of the amidasecatalyzed reaction in soils showed that the K_m values of formamide, acetamide, and propionamide for this enzyme are similar to those of the same enzyme isolated from microorganisms. Application of the three linear transformations of the Michaelis-Menten equation indicated that the apparent K_m constants of the three substrates (formamide, acetamide, and propionamide) varied among the soils studied, but the results obtained by the three plots were similar. By using the Lineweaver-Burk plot, the K_m values of formamide, acetamide, and propionamide in eight soils ranged from 6.7 to 17.9mM (avg. 12.3), 4.0 to 5.1mM (avg. 4.6), and 10.1 to 20.2mM (avg. 14.5), respectively. The K_m value is the lowest and affinity constant is the highest at optimum pH of amidase activity. With the substrate used in parentheses, the V_max values of the eight soils ranged from 138 to 438 µg NH₄-N released/g soil per 24 hours (formamide), from 13 to 43 µg NH₄-N released/g soil per 24 hours (acetamide), and from 35 to 105 µg NH₄-N released/g soil per 24 hours (propionamide). The activation energy values for the amidase activity, expressed in kJ/mol, ranged from 43.3 to 49.8 (avg. 46.9), from 43.2 to 55.5 (avg. 50.0), and from 22.6 to 29.8 (avg. 26.5) using formamide, acetamide, and propionamide as substrates, respectively. The average temperature coefficient (Q₁₀) of the amidase-catalyzed reaction in the eight soils studied for temperature ranging from 10 to 60°C was 1.70 for formamide, 1.73 for acetamide, and 1.42 for propionamide.

¹ Journal Paper no. J-9593 of the Iowa Agric. Home Econ. Exp. Stn., Ames. Projects 2082 and 2112. Presented before Div. S-3, Soil Sci. Soc. Am. Chicago, Illinois, 7 Dec. 1978.

² Graduate Research Assistant and Professor of Soil Biochemistry, respectively, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication July 9, 1979. Accepted for publication January 24, 1980.