NK activity assay
Quantitative analysis of NK activity was conducted by the fibrin plate
method, with slight modification
[17]. Briefly, bovine fibrinogen (Sigma, St. Louis, MO, USA) and
thrombin (Sigma, St. Louis, MO, USA) were dissolved in 0.1 mol
L-1 sodium phosphate buffer (PBS) at pH 7.4. An
equi-voluminal mixture of 5 g L-1 bovine fibrinogen
solution and 12 g L-1 agarose solution was then warmed
in a 45°C bath, after which 10 μL of 500 U thrombin was added to 15 mL
of the mixture solution in a 90 mm petri dish and kept at room
temperature for 1 h to form fibrin. Holes with a 2 mm diameter were made
on the fibrin plate and 10 µL of each diluted supernatant of
fermentation broth was then added. The plates were subsequently
incubated at 25°C for 16 h, after which the areas of the lysis zones on
the fibrin plates were measured and the fibrinolytic activities were
determined according to the standard curve of urokinase.