Summary

The gonadotropin receptors associated with plasma membrane fractions were solubilized by detergents, including Triton X-100, Lubrol WX, Lubrol PX and sodium deoxycholate before and after equilibration with ~:SI-labelled human chorionic gonadotropin. The binding activity remained in solution even after centrifugation at 300 000 X g for 3 h. The solubilized gonadotropin receptor or gonadotropin receptor complex was characterized by gel filtration and sucrose density gradient centrifugation. Sucrose density gradient centrifugation of sol-ubflized gonadotropin-receptor complex in the presence of Triton X-100 had a sedimentation coefficient of 6.5 S whereas the solubilized uncomplexed recep-tor had a sedimentation coefficient of 5.1 S. In the absence of the detergent, solubflized hormone receptor complex from plasma membrane fractions I and II sedimented with an apparent sedimentation coefficient of 6.6 S and 7.4 S, respectively. Similarly, the free receptor also showed higher sedimentation profile with an apparent sedimentation coefficient of 6.7 S for fraction I and 7.2 S for fraction II. Treatment of plasma membranes with phospholipase A and C inhibited the binding of 12SI-labelled human chorionic gonadotropin a dose dependent manner, whereas phospholipase D was without any effect. Doses of 1.4 mI.U. of phospholipase A or 0.6 mI.U. of phospholipase C were required to produce 50 % inhibition of the binding activity. These phospho-lipases had no effect on the preformed ~2SI-labelled human chorionic gonado-tropin-receptor complex nor on the sedimentation profile of solubilized gonad-otropin receptor complex