THE ALPHA-GALACTOSYL EPITOPE ON MAMMALIAN THYROID-CELLS

Studies on the distribution of alpha-galactosyl epitopes with the structure Gal-alpha-1 --> 3Gal-beta-1 --> 4GlcNac-R on mammalian thyroid cell membranes are of interest, since a natural antibody interacting with this carbohydrate antigen (i.e. the natural anti alpha-galactosyl IgG antibody) was found to increase in its titre in patients with autoimmune thyroid disorders. By using a radioimmunoassay for quantification of the alpha-galactosyl epitope, we found variable concentrations of this epitope on thyroid cell membranes of all nonprimate mammals and New World monkeys studied, but not in Old World monkeys and human thyroid. The absence of the identifiable alpha-galactosyl epitopes on human and Old World monkey thyroid cells correlates with diminished activity of the enzyme, alpha-1-3 galactosyltransferase, which, in other species, synthesizes the alpha-galactosyl epitopes within the Golgi apparatus. It is argued that a proportion of anti-thyroid reactivity in human normal and pathologic sera, when assayed with mammalian thyroid cells, may be attributed to natural anti alpha-galactosyl IgG antibody, which interacts with alpha-galactosyl epitopes on thyroid tissues used for the bioassay.