Assay for proteolytic activity The standard proteolytic activity against casein in protein purification was assayed according to the method of Satake et al. (46). The reaction mixture (165 μl) consisted of 140 μl of 1% (w/v) casein in 50 mM phosphate buffer (pH 7.5) and 25 μl of cordysobin solution (1.75 μg). The reaction was started by adding enzyme solution at 37°C. After incubation for 15 min, the reaction was stopped by adding 600 μl of 5% (w/v) trichloroacetic acid. The reaction mixture was kept on ice for another 10 min, then centrifuged at 12,000×g for 5 min at 4°C. The absorbance of the supernatant was read at 280 nm against water as a blank using a UV spectrophotometer. Protease activity was expressed in units, where one unit represented a 0.001 absorbance increase per minute in the supernatant per ml of reaction mixture under specified conditions (4).
Serine specific proteolytic activity against Suc-Leu-Leu-Val-Tyr-MCA in functional assays was measured according to the method of Jiang et al. (47). Appropriately 25 μl of diluted cordysobin (1 μg) was added to 450 μl of 10 mM Tris–HCl buffer, pH 8.0. The reaction was immediately initiated by the addition of 25 μl of 1 mM substrate and incubated at 37°C for 10 min. To stop the reaction, 750 μl of the stopping agent (methyl alcohol:isopropyl alcohol:distilled water=35:30:35, v/v) was added. The fluorescence intensity of the liberated 7-amino-4-methylcoumarin (AMC) was measured by a fluorescence microplate reader (Tecan) at an excitation wavelength of 380 nm and an emission wavelength of 450 nm. One unit of enzyme activity was defined as the amount of the enzyme to release 1 nmol of AMC/min.
Assay for proteolytic activity The standard proteolytic activity against casein in protein purification was assayed according to the method of Satake et al. (46). The reaction mixture (165 μl) consisted of 140 μl of 1% (w/v) casein in 50 mM phosphate buffer (pH 7.5) and 25 μl of cordysobin solution (1.75 μg). The reaction was started by adding enzyme solution at 37°C. After incubation for 15 min, the reaction was stopped by adding 600 μl of 5% (w/v) trichloroacetic acid. The reaction mixture was kept on ice for another 10 min, then centrifuged at 12,000×g for 5 min at 4°C. The absorbance of the supernatant was read at 280 nm against water as a blank using a UV spectrophotometer. Protease activity was expressed in units, where one unit represented a 0.001 absorbance increase per minute in the supernatant per ml of reaction mixture under specified conditions (4).Serine specific proteolytic activity against Suc-Leu-Leu-Val-Tyr-MCA in functional assays was measured according to the method of Jiang et al. (47). Appropriately 25 μl of diluted cordysobin (1 μg) was added to 450 μl of 10 mM Tris–HCl buffer, pH 8.0. The reaction was immediately initiated by the addition of 25 μl of 1 mM substrate and incubated at 37°C for 10 min. To stop the reaction, 750 μl of the stopping agent (methyl alcohol:isopropyl alcohol:distilled water=35:30:35, v/v) was added. The fluorescence intensity of the liberated 7-amino-4-methylcoumarin (AMC) was measured by a fluorescence microplate reader (Tecan) at an excitation wavelength of 380 nm and an emission wavelength of 450 nm. One unit of enzyme activity was defined as the amount of the enzyme to release 1 nmol of AMC/min.
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