Thermodynamic parameters of enzymes in grassland soils from Galicia, NW Spain

The thermodynamic parameters of the enzymes catalase, dehydrogenase, casein-protease, α-N-benzoyl-l-argininamide (BAA)-protease, urease, Carboxymethyl (CM)-cellulase, invertase, β-glucosidase and arylsulphatase, were investigated in grassland soils from a European temperate-humid zone (Galicia, NW Spain). The effect of temperature on enzyme activity was determined at 5, 18, 27, 37, 57 and 70 °C. The temperature-dependence of the rate of substrate hydrolysis varied depending on the enzyme and soil. In general, the soil containing the least amount of organic matter (OM) showed the lowest enzyme activity for all temperatures and enzymes, whereas soils with similar OM contents showed similar levels of activity for the entire temperature range. Temperature had a noteworthy effect on the activity of oxidoreductases. Product formation in the reaction catalyzed by dehydrogenase increased with increasing temperature until 70 °C, which was attributed to chemical reduction of iodonitrotetrazolium violet (INT) at high temperatures. Catalase activity was not affected above 37 °C, which may be explained either by non-enzymatic decomposition of hydrogen peroxide or by the fact that catalase has reached kinetic perfection, and is therefore not saturated with substrate.The Arrhenius equation was used to determine the activation energy (Ea) and the temperature coefficient (Q10) for all enzymes. The values of Ea and Q10 for each enzyme differed among soils, although in general the differences were small, especially for those enzymes that act on substrates of low molecular weight. In terms of the values of Ea and Q10 and the differences established among soils, the results obtained for those enzymes that act on substrates of high molecular weight differed most from those corresponding to the other enzymes. Thus the lowest Ea and Q10 values corresponded to BAA-protease, and the highest values to CM-cellulase and casein-protease. Except for catalase in one of the soils, the values of Ea and Q10 for the oxidoreductases were similar to those of most of the hydrolases. In general, the effect of temperature appeared to be more dependent on the type of enzyme than on the characteristics of the soil.