Radioiodinated Nondegradable Gonadotropin-Releasing Hormone Analogs: New Probes for the Investigation of Pituitary Gonadotropin-Releasing Hormone Receptors*

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Abstract
Studies of pituitary plasma membrane gonadotropin- releasing hormone (GnRH) receptors using [I25I]- iodo-GnRH suffer major disadvantages. Only a small (<25%) proportion of specific tracer binding is to high affinity sites, with more than 70% bound to low affinity sites (Ko = 1 X 106 M'). [l25I]Iodo-GnRH is also inactivated during incubation with pituitary plasma membrane preparations. Two superactive analogs of GnRH, substituted in positions 6 and 10, were used as the labeled ligand to overcome these problems. Both analogs bound to the same high affinity sites as GnRH on bovine pituitary plasma membranes, though the affinity of the analogs was higher than that of the natural decapeptide (Ka s 2.0 x 109, 6.0 x 109, and 3.0 x 108 M“1 for [D-Ser(TBu)6]des-Gly10-GnRH ethylamide, [D-Ala6]des-Gly10-GnRH ethylamide, and GnRH, respectively). The labeled analogs bound to a single class of high affinity sites ith less than 15% of the specific binding being to low affinity sites (Ka S 1 X 1 0 6 M “ ‘). The labeled nalogs were not inactivated during incubation with the pituitary membrane preparations. Using the analogs as tracer, a single class of high affinity sites (Ka = 4.0 X 109 M“1) was also demonstrated on crude 10,800 X g rat pituitary membrane preparations. Use of these analogs as both the labeled and unlabeled ligand offers substantial advantages over GnRH for investigation of GnRH receptors, allowing accurate determination of changes in their numbers and affinities under various physiological conditions.