Digestion of Soybean Protein by the Proteolytic Enzymes of Aspergillus sojae

When the usual assay method of proteinase using milk casein as substrate is applied to the crude extract of wheat bran culture of Aspergillus sojae KS, over 90% of the total activity at pH 7 to 8 is occupied by that of alkaline proteinase. However, lower hydrolytic activity of purified alkaline proteinase than that of crude extract was observed not only in the digestion of soybean meal but also in the digestion of soybean protein, in spite of the fact that each enzyme solution had the same proteolytic activity on milk casein. From the experiments to fractionate crude extract by chromatography on DEAE-Sephadex A-50, neutral proteinase I and II, whose contribution to the hydrolysis of milk casein was estimated to be under 10% of the total activity of crude extract, were suggested to have almost comparable effect to alkaline proteinase in the digestion, determined by the increase of 0.4 m TCA-soluble nitrogen, of soybean protein by crude extract. Based on the rapid increase of 0.4 m TCA-soluble nitrogen and slight increase of Formol-titration value, it seems that both neutral proteinase I and II act as endo-type enzyme similar to alkaline proteinase and are not effective in the liberation of low molecular peptides or amino acids.