2.4. Purification of xylanases
The crude culture filtrate (200 mL) was dialyzed against 0.05 M Tris–
HCl buffer pH 7.0 for 8 h,with buffer changes each 2 h, in order to exclude
small molecules and obtain a buffered solution in this pH. This sample
was submitted to ion exchange chromatography on a DEAE Sephadex
A-50 column (1.1 × 14.5 cm), previously equilibrated with the same
dialysis buffer; at 50 mL/h flow rate. Adsorbed proteins were then
eluted with a 0.0–0.5 M NaCl linear gradient in the same buffer. Proteins
were detected by reading absorbance at 280 nm and xylanase activity
was assayed in the collected 3 mL fractions. Fractions with high
xylanase activity were pooled and submitted to electrophoresis
(SDS-PAGE). The sample corresponding to the not retained fractions
was further dialyzed against 0.05 M ammonium acetate buffer pH 6.8
for 8 h, with buffer changes each 2 h, and then lyophilized. The sample
was re-dissolved in a small volume of this buffer and applied to size
exclusion chromatography on a Sephadex G-75 (1.8 × 60.5 cm) column
equilibrated and eluted in the same buffer, at 19 mL/h flow rate.
Proteins were detected by reading absorbance at 280 nm and xylanase
activity was assayed in the collected 3 mL fractions. Fractions with high
xylanase activity were pooled and submitted to electrophoresis
(SDS-PAGE). All purification steps were carried out at 4°C.