Human Placental Lhrh Receptor: Agonist and Antagonist Labeling Produces Differences in the Size of the Non-Denatured, Solubilized Receptor

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Abstract
Membranes of human term placenta were labeled with several photosensitive and non-photosensitive analogues of LHRH. In both groups agonistic and antagonistic peptide structures were tested. The photolabeling amino acid azidophenylalanine was placed either in positions 2, 6 or 7. All compounds were specifically displacing iodinated Buserelin from human placental membranes and from rat pituitary membranes. The iodinated photolabels were displaced by cold Buserelin, some compounds however had a high non-specific binding. Photolabeling experiments on human placental membranes with radioactive photolabels produced all a band at 58,000 daltons in SDS-gel electrophoresis. Solubilization with a non-denaturing detergent and gel filtration produced a major radioactivity peak which was attributed to the LHRH receptor. All labeling experiments with agonist labels produced Kav's indifferent from each other but significantly different from the Kav's of antagonist labeled membranes. This result was confirmed with similar experiments carried out with radioactive Buserelin and a radioactive antagonist under non-photolytic conditions. It is therefore concluded that the placental LHRH receptor contains an LHRH binding component of 58,000 daltons but that the native receptor is composed of several proteins. It is also concluded that agonist occupation of the receptor induces the association of a further component which might be involved in the transmembrane signalling system. The agonist labeled, solubilized native LHRH receptor has a Stokes radius of 52 R and the same, antagonist labeled receptor a radius of 48 R.