Role of Stabilised Enzymes in Microbial Ecology and Enzyme Extraction from Soil with Potential Applications in Soil Proteomics

Abstract

The same enzyme can have different locations in the soil matrix being cytoplasmic, periplasmic (in Gram-negative bacteria) or attached (ectoenzymes) to the outer surface of active cells, intracellular in dead cells or cell debris, intracellular in resting cells, such as bacterial spores or protozoan cysts, or released as extracellular enzymes, which can be free in the soil solution, adsorbed on a substrate or on soil colloids (Burns 1982; Nannipieri 1994; Chap. 12). Some of these enzyme activities are not quantitatively important; e.g. “free” extracellular enzymes are supposed to be short-lived unless they are protected from proteolysis upon adsorption by soil minerals or entrapment by humic colloids (Burns 1982; Nannipieri et al. 2002). It is well established that enzyme activities of resting cells, such as bacterial spores, are very low and probably undetectable in soil enzyme assays (Atlas and Bartha 1981). A better interpretation of measurements of enzyme activities should be possible by distinguishing the extracellular activity due to enzymes adsorbed by soil minerals or entrapped by humic colloids from the activity of enzymes associated with active microbial cells (the cytoplasmic and periplasmic enzymes and the ectoenzymes; Nannipieri et al. 2002). Of thedifferent locations thatanenzymecanhave in thesoilmatrix, those associated with soil colloids have been extensively studied because they are markedly resistant to thermal denaturation and generally to proteolysis (Nannipieri et al. 2002). Thus, they can be active even under conditions limitingmicrobial activity andarenot regulatedby factors repressingorderepressing their synthesis. For this reason they are also called “stabilised” enzyme activities in soil. Research aimed at understanding the state of these enzymes in soil has been carried by two different approaches. The first concerns the study of properties of enzyme complexes prepared in the laboratory. For example, various enzymes have been adsorbed on pure clay minerals made homoionic to different cations (Stotzky 1986; Nannipieri et