Isolation and characterization of genetic variants of the Kunitz soybean trypsin inhibitor

Three genetically related forms of the Kunitz soybean trypsin inhibitor have been isolated by ion-exchange chromatography. The three variants have different electrophoretic mobilities and have been designated Tia, Tib and Tic. An enzyme assay system was used to determine the association equilibrium constant of each variant for trypsin at pH 8.1 and values of 2 x 10(9) M-1, 2 x 10(7) M-1 and 3 x 10(10) M-1 were found for Tia, Tib and Tic respectively. The hydrolysis equillibrium constant at pH 6.0 for the reactive site peptide bond hydrolysis for each variant was also determined and found to be 4, 9 and 3 for Tia, Tib and Tic, respectively.