Partial purification and properties of proteases from defatted soybean flour

Four chromatographically different proteases were partially purified from defatted soybean flour, and their pH optima were around 5.0 to 5.6 using casein as the substrate. These soybean proteases were designated S1, S2, S3 and S4 according to their order of elution from a DEAE-cellulose column. Each gave a single peak of caseinolytic activity on a Sephadex G-200 column chromatogram, and corresponded to the molecular weights of about 50,000(S1), 35,000(S2), 60,000(S3) and 200,000(S4). The proteases could hydrolyze casein and poly-Glu. alpha-Casein was more rapidly hydrolyzed than beta-casein, but the esters or dipeptide could not be hydrolyzed. Aliquots of 10(-3) M Hg2+, Cu2+ and Zn2+ inhibited the caseinolytic activities by 70% to 90%, while other cations, Mn2+, Mg2+, Ca2+ and Ni2+, at the same concentration did not. SPI (10(-5) M) inhibited 80--90% of their activities, and EPNP (10(-5) M) inhibited their activities 30--60%, but DFP (10(-3) M), SSI (10(-3) M), PCMB (10(-4) M), NEM (10(-3) M) and EDTA (10(-3) M) were not inhibitory. The above results indicate that proteases S1, S2, S3 and S4 from defatted soybean flour can be classified as acid proteases.