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The biological and binding activities of pregnant mare serum gonadotrophin (PMSG) were compared with those of highly purified FSH and LH from the pituitary gland of the same species. Pregnant mare serum gonadotrophin showed activity in bioassays considered to be specific for both FSH (e.g. the Steelman–Pohley ovarian augmentation test and cyclic AMP production by rat seminiferous tubules) and LH (androgen production by rat Leydig cells), as well as activity in a variety of radioreceptor assay systems previously considered to be specific for one of the two types of gonadotrophin. The potency of PMSG was high compared with that of purified ovine FSH or LH standards in all assays but PMSG was considerably less active than equine FSH and LH in vitro. In radioreceptor assays employing rat, pig and horse tissues, the activity of PMSG was equivalent to only 1–5% of equine FSH in competing for FSH-binding sites and only 3–35 % of equine LH in competing for LH-binding sites. Pregnant mare serum gonadotrophin was least active in homologous binding assays with horse testis and equine LH as radioligand. In the rat Leydig cell bioassay, the activity of PMSG was only 2·0% that of equine LH. Furthermore, in some assays equine LH was found to resemble PMSG in exhibiting a high degree of FSH-like activity that could not be accounted for by cross-contamination. The FSH immunoactivity of equine LH was less than 0·5% that of equine FSH, but equine LH was up to 63% as potent as equine FSH in competition for FSH-binding sites and it was 20% as active in the Steelman–Pohley ovarian augmentation bioassay. Equine LH did not, however, show the expected activity in the cyclic AMP production bioassay. Thus, the FSH-binding sites and physiological receptors may not be identical. Overall, comparison of PMSG with pituitary gonadotrophins from homologous species shows that the apparent dual activity of PMSG may not be a unique feature of this pregnancy hormone since equine LH also exhibits some FSH activities. The chemical resemblance between PMSG and equine LH is noteworthy in this regard.
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