Comparative studies of high M(r) subunits of rye and wheat .1. Isolation and biochemical characterisation and effects on gluten extensibility

The structural features of high M(r) glutenin subunits of wheat were compared with those of analogous proteins from rye. Subunits of two rye cultivars (Danko and Halo) and of the wheat cultivar Rektor were isolated from defatted flours by extraction with 50% (v/v) aqueous propan-1-ol under reducing conditions at 60 degrees C followed by precipitation using a 60% concentration of propan-1-ol. The yields of dialysed and freeze-dried subunits were 0.33% and 0.32% (w/w of flour), respectively (rye cultivars), and 0.91% (Rektor). SDS-PAGE revealed that the rye cultivars contained at least five subunits with mobilities corresponding to the x-type subunits of wheat. Separation by RP-HPLC indicated that the rye cultivars did not differ in the qualitative composition of subunits, but in their quantitative proportions. The surface hydrophobicities of the rye subunits were significantly lower than those of wheat subunits. The amino acid compositions of single rye subunits were characterised by high contents of Glx, Gly and Pro, and they were closely related to those of wheat subunits, except that the Glx content was generally lower and the Cys content higher. Notable differences between rye and wheat subunits were found in their contributions to gluten strength. Whereas wheat subunits, reoxidised with potassium bromate and mixed with a standard wheat flour, caused a significant increase in gluten strength, reoxidised rye subunits had the opposite effect. (C) 1996 Academic Press Limited