The effect of ionic environment in specific FSH binding to plasma membrane receptor

Studies were undertaken to define the effects of ionic strength and ion selectivity on specific binding of 125I-hFSH to both its particulate and Triton X-100 solubilized calf testis plasma membrane receptor. Plasma membrane vesicles migrated with a negative charge on free flow electrophoresis. There was no evidence of formation of inside-out vesicles. Specific FSH binding to the plasma membrane vesicles with both monovalent and divalent cations, used to reduce charge repulsion between the negatively charged FSH molecule and membrane vesicles, revealed similar binding maxima at an ionic strength range of 0.03 to 0.05. However, Mg2+ consistently resulted in 25-30% greater specific binding than with any other cation. That increased FSH binding suggested unmasking of specific FSH plasma membrane binding sites. However, specific FSH binding to solubilized plasma membrane vesicles was unaffected by ionic strength or ion selectivity. Enzymatic removal of sialic acid from plasma membrane vesicles significantly enhanced specific FSH binding. In previous studies, we had observed that sialic acid was not exposed within the FSH binding site. Consequently, enhanced FSH binding associated with desialylation of plasma membrane vesicles probably reflected modification of parareceptor sites which affected the conformation of the specific FSH receptor.