Interaction of [125I]LH-RH and other oligopeptides with plasma membranes of rat anterior pituitaries.

The specific binding of [125I]LH-RH to isolated plasma membranes of rat pituitaries was investigated. The binding process was found to be highly specific, temperature-dependent and saturable. The dissociation constant as caluclated by three different methods was approximately 1.3 . 10(-8) M, indicating a single type of binding sites. Maximal binding capacity was 1 . 10(-12 moles/mg protein (= 2 ng LH-RH/pituitary gland), and the number of binding sites was calculated to be 6 . 10(11) per mg membrane protein (=1 . 10(10) binding sites/pituitary gland). When diluted with ice-cold buffer the dissociation of specifically bound LH-RH occurred very rapidly (half-life 3.17 min) with a rate constant of 0.219 min-1. The dissociation process followed first-order kinetics. Specificity of binding was demonstrated by dose-dependent competition of unlabelled LH-RH, the highly potent analogue D-glutamine-(cyclohexyl)6-LH-RH-nonapeptide-ethylamide and the fragment of an analogue (6-D-Ser(TBu))-LH-RH-(3-9)-heptapeptide-ethylamide with the binding [125I]LH-RH, while angiotensin I, II, oxytocin and bacitracin did not compete. The affinities of LH-RH and the analogue to the binding sites of the pituitary plasma membranes were not consistent with the respective biological activities.