Dissociation of receptor-bound human chorionic gonadotropin from rat testicular membranes in vitro as a high molecular weight complex and its inhibition by heavy metals and alkylating agents

Abstract

Intratesticular administration of the heavy metals, Zn 2+, Cu 2+, Cd 2+, and the alkylating agents, N-ethylmaleimide, iodocacetamine, increased the in vivo uptake of human 125I-labelled chorionic gonadotropin (CG) into rat testicular tissue. The results of gel filtrations and Scatchard plot analyses suggested that this effect is not due to an increase in the binding affinity of the hormone, but rather to an increased stability of receptor-human [ 125I]CG complex in the plasma membranes of the target cells. In order to elucidate this possibility in vitro, the release of radioactivity from testicular particles pre-labelled in vivo with human [ 125I]CG was studied under defined incubation conditions. The release was found to be dependent upon temperature and pH, with optimum at pH 8.0–8.5, being about 40% of the total radioactivity subjected to incubation at 37°C. No such release was observed from liver particles (referred to as nonspecific binding). About 80% of the radioactivity released from testicular particles was eluted from a Sephacryl S-300 column as a high molecular weight human CG comples between the intact human [ 125I]CG and the solubilized receptor- human [ 125I]CG complex. The release of this human CG complex was selectively inhibited by the addition of heavy metals and alkylating agents without affecting the release of the other particle proteins, but not by the other enzyme activity modulating agents in the incubation medium. The results show that incubation of testicular particles pre-labelled in vivo with human [ 125I]CG results in dissociation of the receptor-hormone complex and a release of a high molecular weight human CG complex. The process, which seems to proceed via fragmentation of the receptor molecule, is significantly inhibited by heavy metals and alkylating agents. These findings are compatible with our in vivo results presented here, and support the view that fragmentation of the receptor molecule is an important aspect in the removal of the receptor-bound human CG from the target cells.