Each plate contained the test strain with the plasmid pET26b+ or with the plasmid pETSN as a control. The selected transformants were cultured in LB medium containing 30 μg mL−1 kanamycin at 37 °C. IPTG was added to the medium at a final concentration of 0.7 mM to induce bacteria when the OD600 nm reached approximately 0.8 (Liang et al., 2007). After further induction overnight at 20 °C, cells were harvested by centrifugation, resuspended and then disrupted by sonication on ice. The supernatant of the whole-cell extracts was purified using the Ni-NTA column (Invitrogen) and DEAE Sepharose Fast Flow column

(Amersham Biosciences) according to the manufacturers’ instructions. The purification was performed at 0–4 °C. After the purification, the molecular mass of the purified enzymes was analyzed by SDS-PAGE using signaling pathway a 12.5% (w/v) polyacrylamide separating gel. The protein concentration was determined

using the BCA protein assay reagent kit (Pierce). For immunoblotting, the proteins separated by SDS-PAGE were electrically transferred onto a polyvinylidene difluoride membrane. The membrane was blocked overnight at 4 °C in the blocking buffer (5% skim milk) and then incubated for 2 h at 37 °C with 1 : 3000 diluted mouse anti-His-tag monoclonal antibody, followed by incubation for 1 h at 37 °C with 1 : 6000 diluted HRP-Goat anti-mouse IgG (H + L) (Genscript, Nanjing, China). Finally, bands were visualized using enhanced chemiluminescence Bax protein Western blotting detection reagents (Millipore). Fibrinolytic activity was determined by measuring the areas of the lysed zone on the fibrin plate (Astrup & Mullertz, 1952; Liang et al., 2007). In brief, the fibrin plate was made up of 0.4% fibrinogen, 0.6% agarose and 0.5 U mL−1 thrombin, which were dissolved in 50 mM barbitol buffer (pH 7.8) beforehand and mixed in a petri dish (9 cm in diameter). Purified enzymes were also diluted using the 50 mM barbitol buffer, and 20 μL of the samples were placed into holes which had been made previously on the fibrin plate. After measuring the dimension of the clear zone and Phospholipase D1 incubating

the plate at 37 °C for 18 h, the fibrinolytic activity was estimated using urokinase as a standard. The specific activity of the enzyme to hydrolyze fibrin was defined as urokinase units of fibrinolytic activity in each milligram of enzyme. Enzymatic kinetics were determined by measuring the release of p-nitroaniline from the chromogenic substrate suc-AAPF-pNA in 100 mM phosphate buffer (pH 8.0) containing 4% (v/v) DMSO at (37 °C ± 0.2) (Sumi et al., 1987). After incubation for 10 min at 37 ± 0.2 °C, the concentration of liberated p-nitroaniline was measured at an absorbance of 405 nm using an automatic microplate reader (Thermo Lab systems, Multiskan MK3). Kinetic parameters (Vmax and Km) were determined from initial rate measurements at different substrate concentrations ranging from 0.098 to 0.392 mM.