The following tests were performed in order to clarify the functional mechanism of X-prolyldipeptidyl-aminopeptidase (XPAP) on the degradation of soybean proteins during a model system of soy sauce fermentation. The system consisted of cookedsoybean flour, phosphate buffer (pH 6.0), toluene, and either the crude enzyme extract of wheat-bran
koji made by
Aspergillus oryzae RIB 915 or the extract from which XPAP activity was removed. It was prepared by separating a fraction of XPAP activity from the extract on Sephacryl S-300 gelfiltration. After 60 h reaction of the system at 30°C, its amino acid contents and compositions were determined. The hydrolyzate of the soybean flour hydrolyzed with the XPAP activity-free extract showed only 56 g of amino acids, while that hydrolyzed with the enzyme extract containing XPAP activity did a large amount (195 g). In regard to amino acid compositions, the hydrolyzate formed by the XPAP activity-free extract contained only small amounts of glutamic acid, aspartic acid, and proline. While, the hydrolyzate formed by the enzyme extract containing XPAP activity had 30 times as large glutamic acid amount (22 .8 g/kg soybean flour), 10 times as large aspartic acid amount (7.5 g/kg), and 30 times as large proline amount (9.6 g/kg), respectively, as those formed by the XPAP activity-free extract. It can be concluded that XPAP would play one of major roles in the hydrolysis of soybean proteins for the formation of flavoring amino acids.
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