Water-Soluble Gonadotropin Receptors of the Rat Ovary*

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Abstract
Soluble receptors for LH/hCG were characterized in aqueous and low ionic strength buffer extracts of the luteinized rat ovary. These receptors are proteins with high affinity (Ka = 1010 M-1) and specificity for hCG-and LH-like gonadotropins. The water-soluble receptors are stable and retained 60–80% of their binding capacity after lyophilization. Resolution of the water-soluble receptors by gel electrophoresis demonstrated five binding species, with molecular weights of 165,000, 81,000, 48,000, 24,000 and 12,000. These findings have demonstrated that a proportion (5–10%) of ovarian LH/hCG membrane receptors can be rendered water soluble with preservation of their binding properties and that the specific binding site for LH/hCG is present in proteins much smaller than the predominant 6. 5 S (mol wt, 194,000) form extracted by nonionic detergents. The stability of these small LH-binding sites is of value for further purification and analysis of structural determinants for gonadotropin binding.