Structural Models of Limit Dextrinase Inhibitors from Barley

Two limit dextrinase inhibitors (designated low and high pI inhibitors) isolated from barley had a high sequence homology with a protein sequence coded by a mRNA previously found in barley and thought to code for alpha -amylase-trypsin inhibitor. The inhibitors did not inhibit trypsin or alpha -amylases from malt, T. molitor (yellow mealworm) or pig pancreas under the experimental conditions where they inhibited limit dextrinase. Each inhibitor was heterogeneous and appeared to have «ragged» carboxyl ends. A molecular model of the inhibitor proteins was developed which showed that of the nine cysteine residues in the molecules, eight were likely to be involved in intramolecular disulphide bonds, leaving one free sulphydryl group. The low pI inhibitor had a major component of molecular mass 12 928 that contained a glutathione residue, probably bound to the free sulphydryl group. The high pI inhibitor had two major components with molecular masses of 12 686 and 12 744 and these may contain cysteine residues bound to the free sulphydryl group. Further analysis of the model showed structural differences between the limit dextrinase inhibitors and alpha -amylase-trypsin inhibitors of the same family.