Comparative binding of FSH to chicken and rat testis

The binding characteristics of two highly purified preparations of follicle-stimulating hormone (FSH), one from equine (e) and one from porcine (p), were compared in a chicken testis radioreceptor assay. A two-component binding model was adequate to explain the binding of eFSH or pFSH to the chicken testis homogenate (CTH) FSH binding sites in the Scatchard study. The affinity of eFSH for the FSH binding site of CTH was 10-fold greater than that of pFSH by Scatchard analysis. This is consistent with the observation that in competitive protein binding studies approximately 10-fold greater quantities of pFSH were required to displace [125I] eFSH from the CTH FSH binding site than eFSH. Qualitatively, the order of relative potencies for various gonadotropin preparations was the same in either the CTH or a rat testis receptor radioligand assay. However, quantitatively, higher potency estimates were obtained with the CTH. The ability of LH preparations to displace radioiodinated pFSH or eFSH from CTH was generally very low with the notable exception of equine LH. The relative potency of highly purified eLH was approximately equal to that of the NIH-FSH-S16 standard (0.906 x NIH-FSH-S16) indicating that in the chicken, as in the rat, (Bousfield and Ward, Biochim. Biophys. Acta 885: 327, 1986), eLH has significant FSH activity. It is concluded that the chicken testis is a convenient and suitable source of FSH receptor (binding sites) for the bioassay of FSH, but that quantitative estimates of potency are not directly comparable when different sources of receptor are used. Finally the data show that eFSH has intriguing structural attributes which provide higher affinity to the chicken testis receptor than pFSH.