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Identification of an Abundant 56 kDa Protein Implicated in Food Allergy as Granule-Bound Starch Synthase

Hari B. Krishnan, Ming-Hsuan Chen
Journal of agricultural and food chemistry 2013 v.61 no.22 pp. 5404-5409
allergens, anaphylaxis, antibodies, antigen-antibody reactions, clinical trials, corn, endosperm, food allergies, glutinous rice, immunoglobulin E, molecular weight, mutants, patients, peanuts, protein sources, rice bran, rice protein, soybeans, starch synthase
Rice, the staple food of South and East Asian counties, is considered to be hypoallergenic. However, several clinical studies have documented rice-induced allergy in sensitive patients. Rice proteins with molecular weights of 14-16 kDa, 26 kDa, 33 kDa and 56 kDa have been identified as allergens. Recently, it was documented that the 56 kDa rice allergen was responsible for rice-induced anaphylaxis. The 14-16 kDa allergens have been identified as a-amylase inhibitors, the 26 kDa protein as a-globulin and the 33 kDa protein as glyoxalase I. However, the identity of the 56 kDa rice allergen has not yet been determined. In this study, we demonstrate serum from patients allergic to maize shows IgE-binding to a 56 kDa protein that was present both in maize and rice, but not in the oil seeds soybean and peanut. The 56 kDa IgE-binding protein was abundant in the rice endosperm and occurred at a lower concentration in the rice bran. We have purified this protein from rice endosperm and demonstrated its reactivity to IgE antibodies from serum of maize allergic patients. The purified protein was subjected to MALDI-TOF-MS/MS analysis resulting in identification of this rice allergen as granule-bound starch synthase, a product of the waxy gene. Immunoblot analysis using protein extracts from a waxy mutant of rice revealed the absence of the 56 kDa IgE-binding protein. Our results demonstrate that the 56 kDa rice allergen is granule-bound starch synthase and raise the possibility of using waxy mutants of rice as a potential source of hypoallergenic diet for patients sensitized to the 56 kDa rice allergen.