The Extraction, Solubility, and Characterization of Two Groups of Barley Storage Polypeptides

Abstract

This paper reports further studies on the characteristics of the storage protein fraction (hordein) of barley. Hordein consists of two groups of polypeptides (termed ‘B’ and ‘C’) coded by two separate but linked loci. Whereas the ‘C’ polypeptides are readily soluble and extracted in 60% (v/v) ethanol at room temperature, the ‘B’ group is more soluble in, and therefore more efficiently extracted by, 50% (v/v) propan-1-ol or 45% (v/v) propan-2-ol at elevated temperatures and in the presence of 2-mercaptoethanol. However, the most efficient conditions for hordein extraction (50% propan-1-ol + 2% (v/v) 2-mercaptoethanol at 60 °C) also extract some contaminating non-hordein polypeptides resulting in an apparently increased lysine content of the hordein fraction. Amino acid analysis of the purified ‘B’ and ‘C’ hordein groups shows that, whereas ‘C’ hordein contains more glutamate + glutamine, proline, and phenylalanine than ‘B’ hordein, it contains only traces of lysine and sulphur amino acids in contrast to ‘B’ hordein which contains 0·5% lysine 0·6% methionine, and 2·5% cysteine. Equilibrium sedimentation analyses carried out on the purified ‘B’ and ‘C’ groups indicates that the preparations were reasonably monodisperse with molecular weights of approximately 32 000 and 52 000 respectively. These values are considerably lower than those previously determined by SDS-PAGE.