Gonadotropin releasing-hormone receptors: photoaffinity labeling with an antagonist.

A photoaffinity antagonist of gonadotropin releasing hormone (GnRH), D pGlu-D-Phe-D-Trp-Ser-D-Lys6(N epsilon-azidobenzoyl)-Leu-Arg-Pro-Gly-NH2 (photoaffinity antagonist) was prepared by reacting [D-pGlu1, D-Phe2, D-Trp3, D-Lys6]GnRH with the N-hydroxysuccinimide ester of 4-azidobenzoic acid. The analog appeared homogeneous when analyzed by thin-layer chromatography and its photoreactivity was demonstrated by spectral changes when exposed to light. The photoaffinity antagonist retained high affinity binding to the GnRH receptor of pituitary membrane preparations and exhibited antagonistic activity when assayed in vitro in whole pituitaries. Pituitary membrane preparations were incubated with the radioactive photoaffinity GnRH antagonist and irradiated with light. Sodium dodecyl sulfate gel electrophoresis after solubilization and reduction showed the specific labeling of a single specific protein with an apparent molecular weight of 60,000 daltons. These results indicate that GnRH agonists and antagonists bind to the same receptor.