Purification of Kunitz soybean trypsin inhibitor using affinity chromathography

soybean trypsin inhibitor, a single chain protein with 181 amino acids and two disulfide bonds, has special tendency for pancreatic trypsin enzyme. This protein exists in soybean abundantly and showes high resistance to heating and chemicals. Various procedures such as combination of different chromatographic methods, immobilized metal affinity chromatography and preparative electrophoresis have been used to purify this protein. Pulverized soybean was defatted with methanol and the remainder was extracted after being solved in dionized water. Storage proteins in solution was eliminated by use of isollectric precipitation method. Trypsin inhibitor riched fraction was adsorbed on an affinity column with trypsin as the ligand. After washing unbonded proteins, trypsin inhibitor was eluted from the collumn by decreasing pH. Purity and activity of the inhibitor was assessed by SDS-PAGE and an spectrophotometric method, respectively. SDS-PAGE demonstrated a protein with a single band and molecular weight of 20 KDa with purity of more than 99 percent under reducing and non-reducing conditions. Activity of the purified inhibitor protein was 3600 TlU/mg. The results showed that overloading of the affinity collumn to gether with chromatography at pH of 6 to 8 increased the purity. The proposed method is simple and efficient for preparation of large amounts of pure trypsin inhibitor. In addition, this method does not need any further steps such as dialysis