Wheat is an important cereal crop as it is used to feed large amounts of the World’s population. The protein in wheat is deficient in the amino acid lysine, as is similar with all cereal crops. The principal protein in wheat flour and other cereals from the Triticale genus is gliadin, a group of structurally similar proteins that contain most of the amino acid content of the wheat. Gliadin is a component of gluten. Gluten in wheat is found in the endosperm, and composed of the structural proteins gliadin and glutenin, conjugated to polysaccharides. Gluten is important nutritionally because it is required to give dough its sticky consistency, and as such is pivotal in bread making. In addition, gliadin is problematic for those with coeliac disease who suffer from gastrointestinal reactions to the protein. This leads to gut symptoms such as bloating, pain, discomfort and diarrhea, as well as central nervous system induced symptoms such as vomiting and migraine headaches. Around 1 % of Westerners may suffer from coeliac disease.
The digestion of gliadin has been investigated by researchers to measure the digestion products and elucidate possible components that may be responsible for disorders associated with gliadin consumption. As well as coeliac disease, both schizophrenia and dermatitis herpetiformis have been linked to maldigestion of the gliadin component of gluten. For example, in one study1, researchers used in vitro digestion assays to digest gliadin using the human gastrointestinal enzymes pepsin, trypsin and pancreatin. The results of this digestion resulted in amino-terminal pyrogultamic acid peptides, and as hydrolysis of gliadin proceeded, these peptides increased in number. These digestion products were then further digested with pyrrolidonecarboxylate peptidase to produce pyroglutamic acid. The stability of lysosomal membranes was then tested when exposed to synthetic L-pyrogultamic acid and selected fragments of the gliadin digests. Both treatments were able to rupture lysosomal membranes.
The results of this study confirm that pyroglutamic acid peptides are a digestion product of gliadin when exposed to human gastrointestinal enzymes. These pyroglutamic acid peptides can be further digested to pyroglutamic acid. Further addition of pyroglutamic acid to lysosomes, destabilises their membranes and causes them to rupture. Some researchers have suggested that that coeliac disease results from an inborn error of metabolism that results in a deficiency of a particular peptidase to digest gliadin completely with an immune response to the undigested peptides. However, another possibility is that products of gliadin digestion are able to destabilise lysosomal membranes and release the enzymes contained within to the enterocytes of the gut. These enzymes, many of them digestive in nature could then digest the enterocytes from within, damaging the surface of the gut lining, and producing the symptoms of coeliac disease. Certainly results from this study suggest that the digestion products of gliadin could rupture lysosomes.
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