Short-term assays of soil proteolytic enzyme activities using proteins and dipeptide derivatives as substrates

Conditions are described for the rapid and precise assay of soil proteases, using proteins and dipeptide derivatives as substrates in the absence of added bacteriostatic agents. The rate of substrate hydrolysis was proportional to the soil concentration; the release of amino compounds per unit weight of soil was directly related to the incubation time. The soils investigated varied widely in their pH, texture, organic matter content, and total exchangeable cations, but nevertheless exhibited optimal protease activity near pH 8.0 and 60°C, and were consistent in their preferential hydrolysis of dipeptide derivatives containing amino acids with hydrophobic side chains. However, soils varied widely in their relative activities towards a given dipeptide derivative and towards benzoyl arginine amide (BAA), a cationic substrate used in the assay of ‘trypsin-like’ enzymes. Benzyloxycarbonyl (Z) phenylalanyl leucine was hydrolysed most rapidly by all soils investigated. Activities towards Z-phenylalanyl leucine far exceeded those towards protein substrates and were highly correlated with the clay contents of the soils but not well correlated with the organic matter contents.