Structural characterization of the N-linked oligosaccharides from tomato fruit

The primary structures of the N-linked oligosaccharides from tomato fruit ( Lycopersicon esculentum) have been elucidated. For the isolation of the pro tein fraction, two procedures were employed alternatively: a low temperatur e acetone powder method and ammonium sulfate precipitation of the tomato ex tract. After peptic digestion, the glycopeptides were purified by cation-ex change chromatography; the oligosaccharides were released by N-glycosidase A and fluorescently labelled with 2-aminopyridine. Structural characterizat ion was accomplished by means of two-dimensional HPLC in combination with e xoglycosidase digestions and MALDI-TOF mass spectrometry. Two varieties as well as two stages of ripening were investigated. In all the samples, the s ame sixteen N-glycosidic structures were detected; the two most abundant gl ycans showed identical properties to those of the major N-linked oligosacch arides of horseradish peroxidase and pineapple stem bromelain, respectively and accounted for about 65-78% of the total glycan amount; oligomannosidic glycans occurred only in small quantities (3-9%). The majority of the N-gl ycans were beta 1,2-xylosylated and carried an alpha 1,3-fucose residue lin ked to the terminal N-acetylglucosamine. This structural element contribute s to cross-reactions among non-related glycoproteins and has been shown to be an IgE-reactive determinant (Tretter, Altmann, Kubelka, Marz, & Becker, 1993). The presented study gives a possible structural explanation for repo rted immunological cross-reactivities between tomato and grass pollen extra cts due to carbohydrate IgE epitopes (Petersen, Vieths, Aulepp, Schlaak, & Becker, 1996), thereby demonstrating the importance of the structural chara cterization of plant N-glycans for a more reliable interpretation of immuno logical data. (C) 1999 Elsevier Science Ltd. All rights reserved.