Identification of an abundant 56 kDa protein implicated in food allergy as granule-bound starch synthase

Authors: Krishnan, Hari; Chen, Ming Hsuan

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2013
Publication Date: 5/15/2013
Publication URL: http://handle.nal.usda.gov/10113/601053
Citation: Krishnan, H.B., Chen, M. 2013. Identification of an abundant 56 kDa protein implicated in food allergy as granule-bound starch synthase. Journal of Agricultural and Food Chemistry. 61:5404-5409.

Interpretive Summary: It has been estimated that about 5% of young children and 2% of adults in industrialized world suffer from food allergies. Majority of the global population is dependent on rice as the main source of nutrition and energy. In comparison to other cereals, rice is considered to be hypoallergenic and rice induced allergies are not common occurrence in Europe and North America. However, clinical studies have demonstrated that exposure to rice flour or inhalation of vapors from boiling rice can induce asthma. Other symptoms associated with rice allergy include rhinoconjunctivitis, eczema and atopic dermatitis. Recently, it was demonstrated that a 56 kDa rice allergen is responsible for the rice-induced hypersensitivity. However, until now the identity of the 56 kDa allergen is not known. In this study, we have demonstrated that serum from patients allergic to corn show cross-reactivity against an abundant 56 kDa rice protein. We have purified this 56 kDa protein and identified it as granule-bound starch synthase. Additionally, we have demonstrated that waxy mutant of rice does not accumulate this 56 kDa allergen implicated in rice-induced allergy. Results from this study will aid researchers in the development of non-allergenic rice cultivars. Such rice cultivars will be a valuable source of hypoallergenic rice for allergy patients and will elevate the stature of rice as a hypoallergenic food source.

Technical Abstract: 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.