Protein—carbohydrate complexes in gluten: Fractionation and proximate composition

Abstract

The proportions and compositions of carbohydrates associated with the alcohol-soluble (gliadin) and the alcohol-insoluble (glutenin) fractions of a gluten preparation were determined. The alcohol-soluble fraction contained 0·6% (w/w) carbohydrate comprising 55·5% galactose, 20·7% glucose, 16·1% arabinose, 6·2% mannose and 1·2 % xylose. The alcohol-insoluble fraction contained 17·0% carbohydrate comprising 97% glucose and only trace amounts of arabinose, xylose, galactose and mannose. The soluble and the insoluble fractions contained 90 and 83% protein respectively. Gel filtration chromatography of the soluble fraction on Sephadex G-200 resulted in five protein peaks and one carbohydrate peak. The carbohydrate component was eluted with the highest mol. wt. protein peak (at the void volume) and contained 2·9% (w/w) of carbohydrate. The monosaccharide composition of this peak was similar to that of the alcohol-soluble fraction but contained somewhat higher levels of galactose and lower levels of glucose and arabinose. The monosaccharide compositions of the gluten sub-fractions suggested a degree of specificity in the assocation of the non-starch carbohydrates with gluten since the predominant non-starch polysaccharides in flour are arabino-xylans. Polyacrylamide gel electrophoresis at pH 3·1 confirmed that the protein in the carbohydrate-containing peak (Peak I) of the soluble fraction was of high apparent mol. wt. Sodium dodecyl sulfate—polyacrylamide gel electrophoresis of unreduced and reduced fractions showed that peak I contained protein bands normally associated with the glutenin fraction.