Wheat flour is added to and mixed with water to hydrate and associate gliadin and glutenin to form gluten. In addition to various minor interactions such as hydrogen bonding and hydrophobic interaction for proper gluten formation, it is acknowledged that disulfide bonds (SS bonds) between cysteine residues are important. Further, in terms of breeding, genes encoding proteins having excellent SS bond-forming ability, represented by Glu-D1d, are critical. There has been much research on endogenous redox substances and enzymes in the mechanism of SS bond formation in the manufacturing process; however, consensus regarding the relationship has not been obtained. Protein disulfide isomerase (PDI, EC 5.3.4.1) is present in the endoplasmic reticulum, and is a molecular chaperone that forms intramolecular (interstitial) SS bonds during protein expression. When PDI forms SS bonds in gluten, it is reduced (PDIred) and loses its oxidizing ability. It is known that oxidative regeneration to oxidized PDI (PDIoxi) by endoplasmic reticulum oxide reductase 1 (ERO1) occurs in vivo, and ascorbic acid (AA) as a dough-improving agent is involved in this regeneration in wheat flour dough. AA is converted to dehydroascorbic acid (DHA) by ascorbate oxidase in the dough. When examining the effect of DHA on PDIred, conversion to PDIoxi progresses efficiently, and since it catalyzes SS bond formation in substrate protein, it is inferred that the dough-improving effect by AA is brought about in cooperation with PDI.
