Purification and characterization of Aspergillus oryzae LK-101 salt-tolerant acid protease isolated from soybean paste

A novel salt-tolerant acid protease was produced from Aspergillus oryzae LK-101 (AOLK-101). The AOLK-101 protease was purified to homogeneity by ammonium sulfate precipitation, DEAE-Sephadex A-50 and Sephadex G-100 chromatographies in order. The specific activity and the purification ratio of the purified protease were 2,301 unit/mg and 11.6 fold, respectively, with 25 kDa of molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrpphoresis (SDS-PAGE). Its optimal pH and temperature were pH 6.5 and 50A degrees C, respectively. This protease was relatively stable at pH 4.5-7.5, below 40A degrees C, and up to 10% salt concentration. The protease was moderately inhibited by Ag2+ and Zn2+, and strongly by ethylenediamide tetraacetic acid (EDTA) and phenylmethysulfonyl fluoride (PMSF), but activated by Cu2+ and Mn2+. Therefore, the AOLK-101 protease was a serine protease based on the influence of metal ions and inhibitors. K (m) , V (max) , k (cat) , and k (cat) /K (m) values of AOLK-101 protease for hammastein milk casein were 1.04 mg/mL, 124.84 unit/L, 163.5/sec, and 3.9x10(6)/m center dot sec, respectively.