Protein-lipid interactions in wheat gluten: reassessment of the occurrence of lipid-mediated aggregation of protein in the gliadin fraction

Gliadin preparations, isolated by AUC extraction and pH precipitation from undefatted- and water-saturated butan-1-ol (WSB) defatted cv. Neepawa gluten, were chromatographed on Sephacryl S-200, resulting in separation into five peaks, one at the void volume (Vo) and the other four included by the resin. Defatting resulted in a diminished Vo peak, but the other peaks were unaffected. Electrophoretic analysis (SDS-PAGE) showed that the Vo fraction contained both high and low Mr subunits of glutenin but very little gliadin except for traces of omega-gliadins. Re-chromatography of the Sephacryl S-200 Vo fraction from undefatted gliadin and chromatography of whole undefatted gliadin on a larger pore size Sephacryl S-400 column resulted in the separation of two apparently UV absorbing fractions, one at the Vo, and an included fraction that was eluted over a broad area of the chromatogram following the Vo peak. In the case of whole undefatted gliadin, several overlapping peaks corresponding to lower Mr proteins were also observed. The Sephacryl S-400 Vo peak from both the gliadin preparation and the re-chromatographed Sephacryl S-200 Vo fraction were removed by defatting, and no protein was detected in those Sephacryl S-400 Vo fractions. The components included by Sephacryl S-400, which were eluted over a broad range in the chromatogram, contained high and low Mr subunits of glutenin, and the later peaks in the gliadin chromatograms contained gliadin polypeptides and lower Mr proteins. No evidence was obtained for the occurrence of lipid-mediated aggregation of gluten proteins in gliadin isolated by the pH precipitation procedure. The material that can be removed by defatting with WSB (presumably lipid) may be present in the form of large vesicles, which are detected as an apparent Vo peak because of light scattering.