Comparison of kinetic and thermodynamic parameters of phosphomonoesterases of soils and of corn and soybean roots

Abstract

The kinetic and thennodynamic parameters of phosphomonoesterases of soils and of corn ( Zea mays L.) and soybean ( Glycine max L.) roots were evaluated. The optimum pH values for acid phosphatase activity in corn and soybean roots were 4 and 5, respectively. Roots of these plants did not contain alkaline phosphatase activity: therefore, the alkaline phosphatase activity in agricultural soils seems solely derived from microbial and animal sources. Soils contained both acid and alkaline phosphatase activities, with acid phosphatase being predominant in acid soils and alkaline phosphatase being predominant in alkaline soils. The K m values of acid phosphatase in acid soils ranged from 1.11 to 1.88 mM and were in the same order of magnitude as those of sterile and nonsterile soybean roots (1.89-2.73 mM). The K m values of add phosphatase activity in 2 alkaline soils were 2.53 and 3.40 mM and were significantly greater than those of this enzyme in acid soils. The K m values of alkaline phosphatase activity ranged from 0.44 to 4.94 mM and did not show any particular trend with respect to soil pH. Acid phosphatase activity of plant roots, as estimated by V max , under sterile and nonsterile conditions were significantly different ( P < 0.05) within plant varieties. Temperature of inactivation of soil and root phosphatases was between 60–65°C. The mean activation energy ( E a ) for acid phosphatase of corn roots (42.1 kJ mol −1) was about 6kJ mol −1 higher than that for soybean roots but was similar to that of acid soils, E a values for acid phosphatase of 6 soils ranged from 38.4 to 50.1 kJ mol −1. The E a values of alkaline phosphatase of 3 alkaline soils averaged to 31.2 kJ mol −1 and were significantly lower than those of 3 acid soils (av. = 42.4 kJ mol −1). The heat of activation (Δ H a ) values for soil and root phosphatases showed similar trends to the E a , except that their magnitudes were 2.5 kJ mol −1 lower. The entropy of activation (δ S a ) values for acid phosphatase of corn roots ranged from −85 to −92J deg −1 mol −1 and were higher than those of soybean roots (−103 to −110J deg −1 mol −1). The Δ S a , for add phosphatase of soils ranged from −58.5 to −86.5 J deg −1 mol −1. The Δ S a , values of alkaline phosphatase of 3 acid soils ranged from −80 to −95J deg −1 mol −1 and were significantly higher ( P < 0.05) than those of alkaline soils (−104 to − 114 J deg −1 mol −1). The free energy of activation (Δ G a ) values for phosphatases of soils and corn and soybean roots ranged from 61.7 to 67.3 kJ mol −1. The relatively constant Δ G a for phosphatases suggests that the energy requirement for the overall reaction is independent of the plant root or soil used.