Sera of patients suffering from birch pollinosis were studied in the radio-allergo-sorbent test (RAST) for the presence of IgE antibodies to various allergens of vegetable origin. The sera selected were positive in the RAST for both birch pollen and fruits. IgE antibodies directed against at least three different cross-reacting determinants in birch pollen were detected. In addition to periodate-susceptible cross-reacting determinants, which are found on a number of glycoproteins, two non-related periodate-resistant determinants were found in birch pollen, with molecular weights of 20 and 18 kD, respectively. The 20-kD component appears to be responsible for the co-occurrence of the binding of IgE to allergens of fresh fruits, whereas the 18-kD component appears to cause the cross-reactivity among grass pollen, potato and fruits.
Human al-acid glycoprotein (AGP) was separated into a non-bound (AGP-A; 46%), a retarded (AGP-B; 39%) and a bound fraction (AGP-C; 150/,) using concanavalin A (ConA) -Sepharose chromatography. The apparent molecular masses, as determined by SDS-PAGE, of the three fractions were 43.5, 42.3 and 41.2 kDa, respectively.The occurrence of N-linked di-, tri-and tetraantennary glycans on these three molecular forms (AGP-A, -B, and -C) was studied by sequential lectin-affinity chromatography of the 14C-labelled glycopeptides. These were obtained by extensive pronase treatment followed by N-[14C]acetylation of the peptide moieties.The glycopeptides of AGP-A did not bind to ConA-Sepharose whereas for AGP-B and AGP-C 18% and 44%, respectively, of the glycopeptides were bound as diantennary structures. Glycopeptide fractions of all three forms of AGP which were not bound to ConA-Sepharose were shown to contain equal amounts of both tri-and tetraantennary glycans by chromatography with Phaseolus vulgaris leukoagglutinating lectin (L-PHA).With the assumption that each molecule contains five glycosylation sites, it could be shown that AGP-A contains no diantennary structures whereas AGP-B and AGP-C contain one and two diantennary structures, respectively. In addition each of the molecular forms contains equal amounts of tri-and tetraantennary structures on the remaining glycosylation sites. The results of this study, therefore, exclude a uniformity of glycan chains in the three molecular forms of AGP.The degree of sialylation of each of the molecular forms was investigated by chromatography on L-PHAagarose and Ricinus communis agglutinin-I -agarose both before and after desialylation of the glycopeptides. It was shown that about 90% of the biantennary glycans of both AGP-B and AGP-C were disialylated while the remainder were monosialylated. The degree of sialylation of the tri-and tetraantennary glycans was identical for the three molecular forms. In each case, one or more terminal galactose residues occurred on at least 20% of the tri-and 65% of the tetraantennary chains.It is suggested that the decrease in the exposure of galactose residues from AGP-A to AGP-C is related to the concomittant decrease in branching of the glycans of the three molecular forms. The relevance of these findings to studies on the function of AGP during inflammatory and liver diseases is discussed. Abbreviations. AGP, ccl-acid glycoprotein; ConA, concanavalin A; L-PHA, Phaseolus vulgaris leukoagglutinating lectin; RCAI, Ricinus communis agglutinin I; CO, LO, RO, non-retarded or nonbound, and Cn, Ln, Rn (n = 1-4) retarded or bound glycopeptide fractions on ConA-Sepharose, L-PHA-agarose and RCA,-agarose, respectively. The fraction names are sometimes used sequentially, e.g. COLlR3, which indicates the fraction of glycopeptides that was not bound on ConA-Sepharose but retarded sequentially on position L1 of L-PHA-agarose and bound on position R3 of RCAI-agarose. Use of the prefix 'a' denotes that the fraction is desialylated by mild acid hydrolysis. fini...
Human alpha 1-acid glycoprotein (AGP) has been shown to modulate various cellular and humoral immune reactions in vitro. Using glycosidase-modified derivatives of AGP, the importance of its carbohydrate moiety with regard to these effects has been noted. In normal serum, three molecular AGP forms interacting differently with concanavalin A (Con A) are present. The ratio of these forms is often changed during various physiopathological conditions. In this study, we could show that differences exist between the three AGP forms with regard to their immunomodulatory effectiveness. At physiological concentrations, the Con A-nonreactive variant AGP-A induced a stronger inhibition of the anti-CD3 stimulated lymphocyte proliferation than the other forms. Interestingly, AGP-A was also found to be responsible for the stimulation of lymphocyte proliferation induced by low AGP concentrations in vitro. Both immunomodulatory effects of AGP were abrogated by desialylation of the glycoprotein. These results support an immunomodulatory role of AGP in conditions characterized by a changed ratio of the differently glycosylated AGP forms.
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