Studies on a Low Molecular Weight Testicular Factor Which Inhibits Binding of FSH to Receptor

Abstract

Buffer (0.05M TRIS, pH 7.5) or water extracts of testes from mature rats inhibited the in vitro binding of 125I-hFSH [human follicle stimulating hormone] to particulate rat testicular receptors. The factor responsible for FSH binding inhibition (FSH-BI) did not seem receptor related, since repeated washings of testicular receptors failed to decrease hormone binding capacity. FSH-BI activity in extracts of rat testes was markedly (60%) decreased by dialysis and could be demonstrated in the dialysate. FSH-BI in buffer reconstituted testicular dialysates was heat labile, losing 50% activity after heating for 10 min at neutral pH in a boiling water bath. The testicular FSH-BI passed through a membrane having a MW exclusion of 8000, did not pass a membrane on ultrafiltration having a MW exclusion of 500 and eluted from a column of Sephadex G-25 in a position similar to that of bacitracin, MW about 1400. Preincubation of highly purified rat testicular membranes with such FSH-BI fractions resulted in a significant (76-98%) decrease in specific binding of subsequently added 125I-hFSH compared to that occurring when preincubation was with buffer alone. The same fractions when added to testicular membranes containing preformed hormone receptor complex promoted a lesser degree of 125I-hFSH dissociation (15-39%) compared to buffer controls. FSH-BI activity was also found in buffer extracts of rat liver and kidney. The presence of an FSH binding inhibitor in target tissue, such as rat testes, may have relevance with regard to mechanism of hormone action and its control. Although this remains to be determined, the potential usefulness of such a binding inhibitor, regardless of source or role in normal testicular physiology is such that further characterization of this factor, as yet unidentified, would seem warranted.