Background: The stability of food allergens to proteolysis and thermal denaturation contributes considerably to their allergenicity. Methods: Mal d 2, an allergenic thaumatin-like protein (TLP) from apple, was isolated and purified by anion exchange chromatography. Its IgE reactivity was tested by ELISA and immunoblotting using sera from apple allergic patients. The proteolytic stability of Mal d 2 was investigated using two in vitro digestion models. Effects of thermal treatment at different pH values on the secondary structure of the protein were recorded by circular dichroism. The level and IgE reactivity of Mal d 2 present in industrially prepared foods were assessed. Results: Purified Mal d 2 consisted of two isoforms. Both harbored carbohydrate moieties and bound serum IgE from apple allergic individuals. Mal d 2 showed remarkable stability to proteolysis and thermal treatments. The allergen remained intact after 2 h each of gastric and subsequent duodenal digestion retaining its full IgE-binding capacity. Mal d 2 was unfolded at neutral and acidic pH at 70°C. Refolding after cooling was only observed at acidic pH. Mal d 2 detected by an anti-TLP antibody in cloudy apple juice did not bind IgE of a serum pool of apple allergic patients. Conclusion: Our findings suggest that Mal d 2 maintains its structure in the gastrointestinal tract, a feature essential for sensitizing the mucosal immune system and provoking allergic reactions.

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