Wheat Seed Carboxypeptidase and Joint Action on Gliadin of Proteases from Dry and Germinating Seeds

Abstract

A carboxypeptidase preparation, homogeneous according to polyacrylamide gel electrophoresis and ultracentrifugation, was obtained from wheat seeds. The isolation procedure included (NH4)2SO4 fractionation, gel-filtration on Sepharose-6B and affinity chromatography on CABS-Sepharose. Mr of the enzyme determined by gel-filtration was 126 000. The enzyme consisted of two non-identical subunits of Mr 60 000 and 63 000. The pl of the carboxypeptidase was 5.7. The inhibitory analysis revealed that the isolated enzyme is a serine carboxypeptidase. The carboxypeptidase preferentially hydrolysed N-substituted dipeptides with aromatic amino acid residues at the C-terminus and showed weak hydrolysing activity towards gliadin. The combined action of carboxypeptidase and aspartic proteinase from dry wheat seeds led to an increase in the degree of proteolysis of the storage protein compared to that resulting from the sum of the action of individual enzymes. The cysteine proteinase from germinated wheat seeds caused complete degradation both of gliadin, which had first been treated with proteases of dry seeds (aspartic proteinase plus carboxypeptidase), and of untreated gliadin. However, when gliadin had first been hydrolysed with dry seed proteases, the rate of its proteolysis with cysteine proteinase increased 3–4 times compared to the non-hydrolysed gliadin. The data indicate the importance of preliminary modification of gliadin with dry wheat proteases, which, apparently, enhances the supply of nutritive substances to the embryo in the course of seed germination.