Role of Carbohydrates in Thyrotropin Binding Sites

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Abstract
The role of carbohydrates in thyrotropin binding was studied by glycosidase treatment of human thyroid membranes. Removal of over 75 % of membrane sialic acid resulted in a 50 % increase of TSH binding, measured in 10 mM Tris-HCl, 50 mM NaCl, 0.1 % bSA, pH 7.4, 37 °C (buffer A). This augmentation was due to an increase in binding to high affinity sites (Ka 1 × l010M-1). The binding was highly specific and was not significantly inhibited by gangliosides. In contrast, low affinity binding of TSH was unchanged either in buffer A or in 10 mM Tris-acetate, 0.1 % bSA pH 6.0, 4 °C (buffer B) and was inhibited by gangliosides. Treatment of membranes with β-galactosidase, β-N-acetylglucosaminidase and α-L-fucosidase had little effect on TSH binding. The data suggests that membrane-associated sialic acid inhibits TSH binding to high affinity receptors and that gangliosides are not involved in this TSH-receptor interaction.