New Mass Spectrometric Approach to Quantify the Major Isoallergens of the Apple Allergen Mal d 1

[Image: see text] Patients who suffer from birch pollinosis often develop adverse reactions to the consumption of fresh apples due to the structural similarity of the allergens Bet v 1 and Mal d 1 from birch and apples, respectively. A different allergenic potential for Mal d 1 isoallergens is postulated, but approaches to quantify the Mal d 1 isoallergen-specific are missing. Therefore, a bottom-up proteomics approach was developed to quantify Mal d 1 by stable isotope dilution and microHPLC-QTOF analyses. Marker peptides for individual isoallergens (Mal d 1.01–Mal d 1.03 and Mal d 1.06), combinations thereof (Mal d 1.01 + 1.02, Mal d 1.02 + 1.06, and Mal d 1.04 + 1.05), and two global marker peptides, comprising Mal d 1.01 + 1.02 + 1.04 + 1.05 and Mal d 1.03 + 1.06 + 1.07 + 1.08 + 1.09, were identified. By the use of an extraction standard (r-Mal d 1_mut), an optimized protocol for extraction and tryptic digestion of apple proteins was developed, and the variety-specific extraction efficiency was monitored for the flesh and peel of apples. The Mal d 1 contents in flesh and peel of five commercial apple breeds and four apple varieties from orchard meadows were quantified isoallergen-specific.