Identification of actinidin as the major allergen of kiwi fruit

Background: Allergic reactions to fruits and vegetables are among the most frequent food allergies in adults. Kiwi fruit (Actinidia chinensia) is commonly involved, causing local mucosal, systemic, or both types of symptoms by an IgE-mediated mechanism. In a previous study on 30 patients allergic to kiwi, we identified a major allergen of 30 kd against which all sera tested clearly reacted. Other allergens were detected at 12, 24, and 28 kd. Objective: The aim of this study was to fully characterize the major kiwi fruit allergen of 30 kd. Methods: Allergens were separated and purified by high-performance liquid chromatography with anion-exchange columns. The purity of the single proteins was checked by sodium dodecylsulfate- polyacrylamide gel electrophoresis, and their allergenicity was checked by immunoblotting with a pool of sera from patients allergic to kiwi. The allergens were characterized by isoelectrofocusing and amino acid sequencing, and periodic acid-Schiff stain was used to detect glycoproteins. Results: Proteins of 30, 28, 24, and 17 kd were purified by high-performance liquid chromatography. IgE binding indicated the 30 kd protein, which showed an isoelectric point of 3.5, as the major allergen of kiwi. Determination of its partial amino acid sequence and comparison with the Swiss Protein Bank showed that this was actinidin, the main protein component of kiwi. The 24 and 28 kd proteins had the same N-terminal sequence, which did not correspond to any known protein. The 17 kd protein had a blocked N-terminal sequence. Conclusions: These results demonstrate that the major allergen of kiwi fruit, Act c 1, is actinidin, a proteolytic enzyme belonging to the class of thiol- proteases. Two other allergens of 24 and 28 kd appear identical on amino acid sequencing.