Characterization and Origin of Protease Activity in Cultivated Soils
Most organisms, especially non-nitrogen fixers, must decompose to low molecular nitrogenous compounds in order to assimilate nitrogen. Hydrolysis of proteins, like ammonification and nitrification, is an important process in the nitrogen cycle with plays an essential role in soil fertility. Soils have sufficient levels of proteolytic activity to support the nitrogen metabolism in situ in soils. Most soils contained a neutral metalloprotease when protease activity was assayed using N-benzyloxycarbonyl-L-phenylalanyl-L-tyrosyl-Lleucine (Z-Phe-Tyr-Leu) as a substrate. The isoelectric points of the main metalloprotease component in the extract of samples from an Andosol in a tomato field and a Gray Lowland soil in a paddy field were estimated to be 4.9 and 2.9, respectively. The molecular weight of the main metalloprotease component from the Andosol and the Gray Lowland soil was estimated to be 47×103 and 37×103, respectively. The metalloprotease split preferentially the peptide bonds with hydrophobic amino acid residues. Characteristics of the protease activity of paddy field soil under double cropping conditions (rice-wheat) were intermediate between those of an upland soil and a paddy soil under monoculture of rice. Proteolytic Bacillus spp. was a major source of soil protease in water-logged paddy fields.