Interaction of human chorionic gonadotropin with membrane components of rat testes

Abstract

The effects of down-regulation of hCG [human chorionic gonadotropin] receptors on interaction of hCG with rat testis membrane components was studied. Rat testes contained a complex ganglioside pattern that did not appear to change qualitatively or quantitatively after injection of hCG into the animals, although testis membranes from hCG-treated rats lost their capacity to bind 125I-labeled hCG (125I-hCG). Gangliosides extracted from testes of control and treated animals were equally effective inhibitors of 125I-hCG binding to testis membranes. Binding inhibition was observed only under conditions (pH 6.0, low ionic strength) such that unlabeled hCG (> 2500-fold excess) did not block 125I-hCG binding, and 125I-hCG bound similarly to testis membranes from control and treated rats. Under conditions such that hCG binding was specific (blocked by 250-fold excess of unlabeled hCG), testis gangliosides were noninhibitory. Liposomes containing gangliosides from the testes of control or hCG-treated rats bound similar small amounts of 125I-hCG. These same liposomes bound 50 and 1000 .times. more thyrotropin and cholera toxin, respectively, than hCG. Oligosaccharides derived from gangliosides did not inhibit 125I-hCG binding to testis membranes nor did they alter fluorescence of hCG conjugated with fluorescent probes; the gangliosides themselves were inhibitory and enhanced fluorescence intensity of hCG derivatives. Exposure of testis membranes from hCG-treated rats to 4 M MgCl2, which displaces bound hCG, did not restore their ability to bind 125I-hCG. When membranes were solubilized with Triton X-100, a solubilized receptor was detected from testis membranes of control but not hCG-treated rats. Apparently gangliosides do not represent the primary binding determinants of hCG receptors.