CELLULASE ACTIVITY OF SOILS

Cellulases are a group of enzymes that catalyze the degradation of cellulose, a polysaccharide built of beta-1,4 linked glucose units. This group consists of endo-1,4-glucanase (EC 3.2.1.4), exo-1,4-glucanase (EC 3.2.1.91), and beta-D-glucosidase (EC 3.2.1.21). The products of cellulose degradation are glucose, cellobiose, and higher molecular weight oligosaccharides. An accurate and precise method for the assay of cellulase activity in soil was developed. It involves determination of the reducing sugars produced when a soil sample is incubated with acetate buffer (50 mM, pH 5.5), carboxymethyl cellulose (CMC), and toluene at 30 degrees C for 24 h. Results showed that the optimal pH of cellulase activity was 5.5. Reducing sugars produced from cellulase activity increased linearly up to 7 days, and they increased with increasing amounts of soil up to 7 g. The substrate concentration-activity curves of cellulase activity in soils obeyed the Michaelis-Menten equation. In two soils tested, the Michaelis constant (K-m) ranged from 9.7 to 21.4 g CMC 1(-1). Cellulase activity in soils was maximal at temperatures ranging from 50 to 60 degrees C. The activation energy values of the reaction catalyzed by cellulase in four soils ranged from 21.7 to 28.0 kJ mol(-1). Soil cellulase was denatured at temperatures ranging from 60 to 70 degrees C; from 60 to 80% of total cellulase activity remained after heating air-dried soil samples at 100 degrees C for 2 h. The corresponding values for held-moist soils were 18-23%. Toluene treatment markedly increased the cellulase activity values over the nontreated samples. Reducing sugar values increased significantly when air-dried or field-moist soil samples were incubated as described but without CMC, suggesting a role for cellulase in degradation of the native substrates.