BINDING OF GONADOTROPHIN RELEASING HORMONE (GnRH) BY BOVINE ANTERIOR PITUITARY PLASMA MEMBRANES AND PURIFIED GnRH RECEPTOR PROTEIN (GnRH.R)

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Abstract
[125I]-(GnRH) concentration dependence binding curves using isolated bovine anterior pituitary plasma membranes, intact and solubilized and purified GnRH receptor protein, are compared. In all instances the concentration dependence binding curves had a stepwise character here interpreted as multisigmoid, with several steep increases and plateaus. These curves are compatible with the existence of several binding sites for GnRH. Purification of the GnRH receptor protein (GnRH.R) resulted in about 500,000-fold increase of binding activity and yielded a single protein species on polyacrylamide gel electrophoresis in SDS [sodium dodecyl sulfate], of estimated MW 60,000. Similarity of GnRH binding by the purified protein with that of intact and solubilized plasma membranes suggested that a single protein was responsible for the binding in each instance. Thus, heterogeneity of GnRH binding is likely attributable to phase transitions of a single receptor protein into different allosteric forms. Positive cooperativity is probably involved in the studied system.