Purification and partial characterization of a proteinase inhibitor from tepary bean (Phaseolus acutifolius) seeds

In a qualitative screening of 36 accessions of tepary beans seeds, all the accessions inhibit the activity of bovine trypsin and trypsin-like proteinases from the insect P. truncatus, and the majority of them inhibit alpha-amylase activity of several important insect pests. A protein proteinase inhibitor was purified from accession L-242-45, using fractional precipitation, gelfiltration, ion exchange chromatography and reverse-phase HPLC. The protein showed an apparent molecular weight of 7,100 by PAGE. However, contrary to other inhibitors previously reported, the inhibitory activity was only present in the trimeric form. The protein was characterized as a serine-proteinase inhibitor that recognized trypsin, chymotrypsin and trypsin-like proteinases, but it also recognized aspartic acid proteinases from different insects. It contained no carbohydrate residues and showed a high stability at 96C at low pH.