title: Gonadotropin receptors in plasma membranes of bovine corpus luteum. II. Role of membrane phospholipids.
abstract: The role of phospholipids in the binding of 125I-choriogonadotropin to bovine corpus luteum plasma membranes has been investigated with the use of purified phospholipase A and phospholipase C to alter membrane phospholipids. The phospholipase C-digested plasma membrane preparation showed 85 to 90% inhibition of 125I-choriogonadotropin binding activity when 70% of the membrane phospholipid was hydrolyzed. Similarly treatment of plasma membranes with phospholipase A resulted in 45 to 55% hydrolysis of membrane phospholipid and almost 75% inhibition of receptor activity. Both these enzymes hydrolyzed membrane-associated phosphatidylcholine to a greater extent than phosphatidylethanolamine and phosphatidylserine. Phosphorylaminoalcohols of phospholiphase C end products were completely released into the medium, while phospholipase A by-products remained associated with plasma membranes. Addition of a phospholipids suspension or liposomes to plasma membranes pretreated with phospholipase A and C did not restore gonadotropin binding activity. Soluble phosphorylcholine, phosphorylethanolamine, and phosphorylserine and insoluble diglyceride products of phospholipase C action had no effect on receptor activity. In contrast, end products of the phospholipase A action, such as lysophosphatides and fatty acids, inhibited both on the membrane-associated and solubilized receptor activity. Lysophosphatidylcholine was the most effective end product inhibiting the binding of gonadotropin to the receptor, followed by lysophosphatidylethanolamine and lysophosphatidylserine. The inhibitory effects of phospholipase A or lysophosphatides were completely reversed upon removal of membrane-bound phospholipid end products by washing the membranes with defatted bovine serum albumin. However, phospholipase C inhibition could not be overcome by defatted albumin washings. Solubilization of plasma membranes with detergents which had been pretreated with phospholipase C partially restored the inhibited activity. It is concluded that the phospholipase-mediated inhibition of gonadotropin binding activity was due to hydrolysis and alterations of the phospholipid environment in the case of phospholipase C and by direct inhibition by end products in the case of phospholipase A.
mesh_terms: D000818:Animals; D002417:Cattle; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D006063:Chorionic Gonadotropin / Q000378:metabolism*; D003338:Corpus Luteum / Q000378:metabolism*; D005260:Female; D007700:Kinetics; D008563:Membrane Lipids / Q000378:metabolism*; D010740:Phospholipases; D010743:Phospholipids / Q000378:metabolism* / Q000494:pharmacology; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*
chemical_list: D006063:Chorionic Gonadotropin; D008563:Membrane Lipids; D010743:Phospholipids; D011956:Receptors, Cell Surface; D010740:Phospholipases