Trypsin inhibitors in turnip (Brassica rapa L.) seeds

A method of trypsin inhibitors isolation from turnip seeds is described. Inhibitors were extracted with 0.01 N HCI, concentrated by salting out with ammonium sulfate, and purified using ion-exchange chromatography on Sp-Sephadex C-25, QAE-Sephadex A-25 and affinity chromatography on immobilized trypsin. Among the three isolated inhibitors, ITR I of molecular weight 15.9 kDa, pl. 6,4, inhibited trypsin activity only. Inhibitors ITR II and ITR Ill inhibited also chymotrypsin activity, they had similar molecular weight (about 10 kDa), but their pI is 7.5 and over 10, respectively. Arginine residue occurred in P, position of the reactive site of inhibitors ITR I and ITR III, while in ITR 11 this position was occupied by lysine residue. Electrophoresis on polyacrylamide gel revealed that each inhibitor possessed two protein fractions, probably a virgin and modified form, with the reactive site peptide bond broken by trypsin