Pepsin-resistant 16-kD buckwheat protein is associated with immediate hypersensitivity reaction in patients with buckwheat allergy.

Background: Buckwheat is becoming popular in many countries as a health food and the incidence of buckwheat allergy is increasing in Asia. The ingestion of small amounts sometimes provokes an anaphylactic reaction. However, it remains controversial which is the major allergen responsible for such reactions.

Methods: The patients whose sera are positive for buckwheat-specific IgE antibody measured by the CAP system fluorescein-enzyme immunoassay (CAP-FEIA) were classified into two subgroups depending on the history of immediate hypersensitivity reactions (IHR). Major buckwheat allergens were identified with immunoblotting, ELISA and N-terminal amino acid sequencing. Various treatments such as pepsin digestion were added to characterize the proteins.

Results: We found that the 24-kD protein that had previously been reported to be a major allergen reacted to IgE antibodies present in sera from almost all subjects (19/20) regardless of symptoms. On the other hand, 16- and 19-kD proteins were bound with IgE antibodies present in sera from 9 of the 10 patients with IHR including 8 patients with anaphylaxis but not in sera from buckwheat-specific IgE-positive subjects without IHR. After pepsin treatment, the 16-kD protein but not the 19- and 24-kD proteins remained undigested and preserved the capacity of IgE binding. This pepsin-resistant 16-kD protein had no homology with the 24-kD protein by the N-terminal amino acid sequencing.

Conclusions: The 16-kD buckwheat protein was resistant to pepsin digestion and appeared to be responsible for IHR including anaphylaxis, while the pepsin-sensitive 24-kD protein was responsible for CAP-FEIA but not IHR.