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Acetylcholinesterase (ACHE) shares an overall homology of 28% with the C-terminal end of thyroglobulin (Tg). Autoantibodies recognizing both of these proteins (bireactive antibodies) have been found in patients with autoimmune thyroid diseases (AITD) and most frequently in Graves' disease (GD). The presence of bireactive antibodies has been correlated with ophthalmopathy in GD. In order to investigate the presence of common linear epitopes between AChE and Tg, we carried out an epitope mapping on the AChE-homologous region of human Tg.Forty-eight 20-mer peptides, overlapping by eight residues and covering the sequence 2171-2748 of human Tg, were synthesized according to the multipin strategy. The reactivity of IgG antibodies (protein G-Sepharose purified) from two bireactive GD sera and two with Hashimoto's thyroiditis, as well as from three healthy control sera, was tested against the synthetic peptides. Moreover, induced rabbit IgG anti-Tg antibodies (produced after hyperimmunization with human Tg and isolated on a Tg affinity column) were tested for anti-peptide reactivity and compared to that of control rabbit IgG.The pattern of anti-peptide reactivity showed mostly quantitative differences between AITD patients and healthy controls. In order to clarify which of the peptides constitute epitopes on the native protein, the anti-Tg antibodies were depleted from the total IgG on a Tg affinity column. The anti-peptide reactivity of total IgG was compared to the reactivity of anti-Tg-depleted IgG. The greatest reduction of reactivity after depletion of anti-Tg antibodies, was observed for a peptide, namely TgP15, corresponding to the …