Structure and stability of alpha-gliadin under INFOGEST-standard conditions

Celiac disease is a chronic small bowel autoimmune enteropathy. Celiac disease is triggered by gluten ingestion in genetically predisposed individuals. So far, the only available treatment is to practice a gluten-free diet, that highly compromises the quality of life of celiac patients. Gluten is composed by prolamins, a major structural component found in cereals, such as wheat. Previous studies suggest that gluten immunogenicity is mostly related to the presence of a certain family of prolamins: gliadins. Gliadins are proline-rich proteins with high resistance to digestive enzymes and proteolytic cleavage.

Hence, to better understand how human digestive conditions influence gliadin cleavage, structure, and stability, we have submitted wheat alpha-gliadin to the INFOGEST digestive conditions (standardized protocol based on an international consensus for in vitro simulation of gastrointestinal food digestion). Wheat alpha-gliadin and/or its derived peptides were characterized, before and after digestion, by making use of several spectroscopic methods: circular dichroism (CD), fluorescence, and Nuclear Magnetic Resonance (NMR).

Non-digested wheat alpha-gliadin showed poorly soluble β-sheet rich structures at Salivary Simulated Fluid (SSF) conditions. In turn, at Gastric Simulated Fluid (SGF) conditions, the protein also showed an overall decrease in secondary structure through the increase of unstructured regions explaining the low solubility of the protein, and its high tendency to aggregate. After enzymatic digestion, alpha-gliadin peptides showed different secondary structure features and low conformational stability, that can be related to the in vivo immunogenicity triggered by this protein.