Distribution of glutathione in Osborne fractions as affected by additions of ascorbic acid, reduced and oxidised glutathione

Kernels of the wheat class DNS were stored for 21 months at 20 degrees C. Flours were milled before in and after storage (II). Doughs from II were firmer than those from I, possibly due to the decrease of reduced glutathione (GSH) from 124 to 30 nmol/g. The two hours, as well as doughs that were prepared by the addition of GSH, oxidised glutathione (GSSG) and ascorbic acid (AA), were fractionated into albumin, globulin, gliadin, and acid-extractable and acid unextractable glutenin. During kneading of flour-water doughs, endogenous GSH was preferentially bound to the acid-extractable glutenin leading to an increase of protein-glutathione mixed disulphide (PSSG). Whereas addition of GSH to a dough enhanced the amount of PSSG in the acid-unextractable glutenin. GSSG increased the extensibility of the doughs but to a lesser extent than GSH and was mainly bound to the acid-extractable glutenin, most likely by an SS/SH interchange reaction with the SH groups of the proteins. GSH and GSSG, both in the presence of AA, also became bound into the glutenin but without an impairment of the improver action of AA on dough rheology. It is discussed that the addition of AA leads to a rapid conversion of GSH into GSSG, which then becomes attached to the glutenin. The increase of total non-peptide bound cysteine (PSSC) in the acid-extractable glutenin, which was favoured by AA, can he explained by a reaction of endogenous cystine with protein SH groups. (C) 1998 Academic Press Limited.