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Gluten: amino acids, digestion, toxic peptides

by luciano

Gliadin and Glutenin
They are the wheat proteins (gliadin, soluble in alcohol and glutenin, insoluble in alcohol) and are composed of amino acid chains (1). Gliadin is made up of the union of about 100-200 amino acids (the main cause of celiac disease), and glutenin, consisting of a combination of about 2,000-20,000 amino acids. The covalent bond that unites two amino acids also takes the name in biochemistry of “peptide bond”. A chain of multiple amino acids linked through peptide bonds takes the generic name of peptide or polypeptide or oligopeptide if the number of amino acids involved is limited; one or more polypeptides, sometimes accompanied by other auxiliary structures or ions called cofactors or prosthetic groups, constitute a protein. amino-acids (or amminoacids) are the primary structural unit of proteins. We can therefore imagine the amino acids as bricks that, united by a glue called peptide bond, form a long sequence that gives rise to a protein. Alcohol soluble cereal proteins are called: prolamines.

The wheat prolamine is gliadin, that of barley is hordein; that of rye is secaline, that of avena is avenin. The different types of prolamins contain different amino-acids and the higher the content of proline and glutamine (which are some of the amino-acids that compose it) the more the prolamine, and therefore the peptides of that cereal will be toxic (2) for the affected patient from celiac disease. The highest levels of proline and glutamine are in wheat, barley and rye. Also glutenins have some toxic sequences for celiacs but they appear to be much less active in soliciting the adverse response of the humanitarian system of man.

Grain varieties with low toxic fraction content: an opportunity for products dedicated to children

by luciano

LC/MS ANALYSIS OF GLUTEN PEPTIDES DERIVED FROM SIMULATED GASTROINTESTINAL DIGESTION OF DIFFERENT WHEAT VARIETIES: QUALITY AND SAFETY IMPLICATIONS. Sforza, Stefano & Prandi, Barbara & Bencivenni, Mariangela & Tedeschi, Tullia & Dossena, Arnaldo & Marchelli, Rosangela & Galaverna, Gianni. (2011):


“Gluten content of wheat is highly variable, depending on the plant genetics and the growing conditions. Beside short peptides, gastrointestinal digestion of gluten also produces longer ones, since the high proline content of gliadins (16-26%) and glutenins (11-13%) makes them very resistant to the degradation by digestive proteases. In the present work, a method for the extraction of the prolamine fraction was applied to different wheat varieties, followed by a simulated gastrointestinal digestion of the gliadin extracted. The peptide mixtures generated were characterized by LC/MS, and most abundant peptides were identified by low- and high-resolution multiple stage MS techniques and through synthesis of authentic standards. These peptides were also semiquantified in the different samples against a suitable internal standard. The peptide mixtures were found to be highly variable, according to the different content and type of gliadins present in wheat varieties, with strong differences among the varieties tested, both qualitatively (the sequences of the peptides generated) and quantitatively (their amount). The greatest difference was found between common and durum wheat varieties. Peptides present only in the former varieties were identified, and used as molecular markers for identifying and quantifying the presence of common wheat when added to durum wheat samples. Most of the peptides identified were also already known to be pathogenic for people affected by celiac disease, an autoimmune enteropathy triggered by gluten proteins, which develops in some genetically susceptible subjects after gluten consumption. Some samples belonging to defined varieties showed a lower amount of celiac-related pathogenic peptides upon digestion, due to a lower gliadin content. Albeit not safe for celiac patients, the use of these varieties in the formulations of baby food could be of great help for lowering the spread of the disease, since the prevalence of celiac disease seems to be promoted by an early exposure to a large amount of gluten peptides”.

Gluten and “toxic” fractions (part I)

by luciano

– the structure of the gliadin and the toxicity of some fraction –
Gluten which is a protein compound formed by the prolamine, known as gliadin in wheat and responsible for the main phenomena of adverse reactions, and glutenin present mainly in the endosperm of cereal caryopsis such as wheat, spelled, rye and barley. Gluten is formed when water, flour and yeast are mixed: gliadin and glutenin combine to form a mixture characterized by viscosity, elasticity and cohesion. Therefore the quantity and integrity of the proteins that make up the gluten present in a flour are an important index to evaluate the quality and aptitude for baking.
Gliadin and glutenin, therefore, have been the subject of numerous research both in relation to the properties concerning the rheological characteristics of the doughs and to the adverse reactions that activate the immune system. Studies have been carried out on celiac disease that have discovered who and how this pathology is caused: they are some peptides (a set of amino acids) present, especially in the gliadin that contain sequences that are toxic, ie they activate the adverse reaction in genetically predisposed subjects of the immune system. The gliadin, in turn, is composed of several sub-units and these contain the “toxic” fractions in different quantities and qualities. Not only has William Hekkins’ research highlighted how the shape and location of gliadin molecules also influence not only chemical and physical properties but also toxicity.
“The gliadin proteins are heterogenous in different regions of the molecule and consequently differ in phisical and chemical properties. About 35% of the gliadin molecule is the alfa helix form, whereas 35% are beta turns(5). The latter are concentrated in the N terminal and C terminal more apolar parts of the gliadin. The remaining part has a random structure. These form have conseguences for the immunogenecity of the different regions in the molecule. Especially beta-turns are immunogenic.” The Toxicity of wheat prolamins William TH. J. M. Hekkens Annales Nestlé 1995 n. 51.
The study also analyzed the mechanism underlying the toxicity by detecting how “the passage of undigested gliadin fragments (fragments longer than 8 amino acids) or a lower tolerance to gliadin causes the immune system to react”. It is not enough, therefore, to know how much gliadin is present in a grain, but it is necessary to have the complete screening of its sub-units (quality, quantity, and, according to the study mentioned also form and position). The study on the “structure of gliadin” could partly explain why some ancient grains (for example, the monococcus), despite having a quantity of gliadin (and in particular alpha gliadin) not inferior to modern grains, have almost zero toxic levels.
The Toxicity of wheat prolamins