Competition for the in vitro binding of radioiodinated human follicle-stimulating hormone in reptilian, avian, and mammalian gonads by nonmammalian gonadotropins

Abstract

Radioiodinated human FSH (125I-hFSH) was used to study the specificity of the gonadotropin binding sites of various reptilian, avian, and mammalian gonadal tissues by examining competitive interactions with several nonmammalian gonadotropins. All preparations of nonmammalian gonadotropins showed some activity in these radioligand assays, but wide variations in activities were evident depending on the source of the tissue and source of the hormone. Several cases of marked species specificity in binding were apparent: Nonmammalian hormones were relatively inactive with porcine granulosa cells; frog and snake hormones were relatively inactive in turtle tissues; and frog hormones were essentially inactive in avian tissues. FSH-LH specificity of binding determined with hormones of nonmammalian origin differed significantly from that previously described with the aid of mammalian gonadotropins. In particular, when tested with some tissues, three preparations of LH, from the turkey, sea turtle, and frog, were in some cases more active in competing for 125I-hFSH binding than preparations of FSH from the same species; this activity could not be readily accounted for by FSH contamination. These comparative data demonstrate the existence of considerable overlap in the binding characteristics of some species of FSH and LH; the observed differences probably reflect evolutionary changes in both gonadotropin binding sites and in the structure of the gonadotropins.