Characterization of β‐ d‐glucosidase extracted from soil fractions

One way to study the state in which stabilized extracellular enzymes persist and are active in the soil is by extraction from the soil, with subsequent fractionation of enzyme–organomineral complexes and characterization of such complexes. In order to investigate the location and characteristics of soil β‐glucosidase, three soil fractions were obtained both from real (undisturbed) soil aggregates and from structural (dispersed in water and physically disrupted) aggregates using two different granulometric procedures. The β‐glucosidase activity of the fraction was then assayed. When the aggregates were dispersed, more than 73% of activity was in the soil microaggregates with diameters of less than 50 μm (SF₅₀). These aggregates were associated with strongly humified organic matter. Solutions of diluted pyrophosphate at neutral pH liberated active β‐glucosidase from all fractions, although the efficacy of extraction varied according to the type of fraction. The SF₅₀ fraction and aggregates of 2000–100 μm obtained by sieving (SF₂₀₀₀) showed the greatest β‐glucosidase activity (34.5 and 36.0%, respectively). Micro‐ and ultrafiltration of SF₅₀ extracts increased the total β‐glucosidase activity, whereas these procedures, applied to the RF₂₀₀₀ fraction, decreased it. Humus–β‐glucosidase complexes in the SF₅₀ fraction, between 0.45 μm and 10⁵ nominal molecular weight limit ( nmwl ) (SF₅₀II) and < 10⁵nmwl (SF₅₀III) showed an optimum pH at 5.4, and in the SF₅₀I fraction (> 0.45 μm) the optimum was 4.0. The stability of β‐glucosidase in the aggregates of the smallest size SF₅₀II and SF₅₀III decreased at acid pHs. The presence of two enzymes (or two forms of the same enzyme) catalysing the same reaction with different values of Michaelis constant and maximum velocity was observed in all but one of the β‐glucosidase complexes extracted and partially purified from the SF₅₀ aggregates.