Follitropin binding to receptors in testis. Modulation by monovalent salts and divalent cations.

The effects of monovalent salts and divalent metal ions on the interactions of radioiodinated human follitropin (IZ5I-hFSH) with membrane-bound, detergentsolubilized, or buffer-soluble receptors from calf testis were studied.Binding of "'1-hFSH to the membranebound receptor was stimulated 2-to 3-fold by Mn2+, M&+, or Ca2+ (each at 2-5 m), but was inhibited by Co2+ or Ni".Neither of these ions was capable of causing dissociation of preformed hormone receptor complexes.Addition of 10 mM EDTA resulted in a rapid, reversible dissociation of "'I-hFSH from each class of the receptor.Binding of FSH to detergent-solubilized or buffer-soluble receptor in the absence of divalent ions was negligible and was maximal at approximately 5 mM Mn2+, M$+, or Ca2+, with a midpoint of 0.8 mM.Various monovalent salts either inhibited or stimulated specific binding of FSH to the three classes of receptor.Inhibition of halides increased with ionic radius, in the order F-< C1-< I-.Among the alkali ions, Na+ was more inhibitory than Li+ or K+ a t 0.1 M. Acetate (0.1 M) was noninhibitory, while NO3-or HC03was a potent inhibitor.Stimulation of Iz5I-hFSH binding was seen at 0.1 M NH4+ ion.The effects of the various monovalent salts were primarily on receptor affinity, with the rate of dissociation being affected more than the rate of association.These effects, which are discussed in terms of their relationship to the B coefficient of viscosity, were reversible and nonspecific binding was largely unaffected.The similarity of effects of these salts or cations on the interaction of FSH with receptors in testis membranes, after detergent solubilization, and with FSH binding components soluble in the absence of detergent support the notion that the latter preparations are suitable models for the study of the receptor once removed from its membrane.The results also indicate that a detailed understanding of the effects of common inorganic ions on the interaction of FSH with receptor is essential to proper evaluation of in vitro binding studies.The interaction of many proteins with their receptors is dependent upon the presence of divalent metal ions and may be influenced by the ionic strength of the binding mixture.In some systems, such as with the hepatic galactoside lectin (l), specific binding appears to be facilitated by high salt concentrations, whereas in others (2), inhibition of binding is seen in the presence of moderate salt concentrations.We have pre-