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The anterior lobe of the pituitary gland secretes two gonadotropic hormones, lutropin (luteinizing hormone, LH) and follitropin (follicle-stimulating hormone, FSH) (1), which stimulate specific cells of the testis and ovary. The biosynthesis and secretion of these hormones is influenced by a number of stimulating and inhibiting factors. The hypothalamic gonadotropin-releasing hormone (GnRH) stimulates the secretion of both LH and FSH (2), whereas steroid hormones suppress peripheral concentrations of gonadotropins by inhibition of GnRH release into the hypothalamic-hypophysial portal circulation (3) and by direct effects on the pituitary gonadotrophs (4,5).

The glycoprotein hormones, thyrotropin (TSH), lutropin (LH), follitropin (FSH), and human choriogonadotropin (hCG), are a particularly interesting group with which to study structure—function relationships because they share a common subunit (α) and exhibit homologous linear sequences in their hormone-specific (s) subunits (see Figs. 1 and 2). After the initial determination of the primary structures of bovine TSH (bTSH) and ovine LH (oLH), considerable effort in many laboratories has been directed toward assessment of their secondary and tertiary structures with the goal of elucidating the structural requirements of hormone—receptor interactions. Interesting and valuable information concerning the domains of another hormone, insulin, has been obtained (e.g., Pullen et al., 1976) based on crystallographic data (Blundell et al., 1972) and chemical modification studies, and it is hoped that similar data will be forthcoming with the glycoprotein hormones even without information on the exact structure and topography at cell-surface receptors for polypeptide hormones.

Lutropin (LH) and follitropin (FSH) are two members of a family of structurally related polypeptide hormones, that also includes pituitary thyrotropin (TSH) and placental human chorionic gonadotropin (hCG). Each hormone of this family contains two non identical, non covalently linked subunits, α and β. The primary structure of the a subunits is identical among hormones within a species, whereas the primary structures of the β subunits differ greatly and confer to the hormones their specific biological activities (Pierce and Parsons, 1981). Although they are not identical, the β subunits show enough homology in their primary structures to suggest that they arose by duplications and mutations of a single ancestral gene (Fontaine and Burzawa-Gerard, 1977).

Smith demonstrated in 1926 that hypophysectomy in rats resulted in the atrophy of the gonads. The occurrence of gonadotropic hormone activity in human urine led Aschheim and Zondek (1928) to indicate that the anterior pituitary gland might be the source of gonadotropins. Between 1928 and 1932, Smith and Engle (1927a-c), Collip et al. (1933), and Fevold et al. (1931) extended studies on pituitary gonadotropins in rats, mice, and sheep, and on human pregnancy urine. Lack of complete separation of follicle-stimulating hormone (FSH, follitropin) and luteinizing hormone (LH) suggested that one gonadotropin may exist with dual activity. The development of the human chorionic gonadotropin (hCG) augmentation assay for FSH (Steelman and Pohley, 1953) and the ovarian ascorbic acid depletion test for LH (Parlow, 1961) further accelerated the purification of FSH from LH. The development of more refined techniques for the separation of proteins and mostly the collection of a large number of glands by the National Pituitary Agency in the United States led to the isolation of human FSH (hFSH) in quantities sufficient for structure-function studies.

Detailed investigations of the structure and function of many hormones have been responsible for the design of structural analogues with desired agonistic or antagonistic activities. In the case of several small and large peptides and reasonably small proteins with hormonal activity, it has been possible to chemically synthesize analogues with desired properties using currently available synthetic technology and purification methods. The problem becomes more complicated when dealing with complex proteins such as the gonadotropins (LH, lutropin, luteinizing hormone; FSH, follitropin, follicle-stimulating hormone; hCG, human chorionic gonadotropin; PMSG, pregnant mare serum gonadotropin), which have 15–45% carbohydrate content. The last decade recorded significant developments related to the structure and function of the gonadotropic hormones (Sairam and Papkoff, 1974; Ward, 1978). Many investigations (see Ward, 1978) showed the relative importance of the α and β subunits and their amino acid side chains in the function of the hormones. Little was known concerning the role of the bulky oligosaccharide chains attached to the polypeptide backbone of the two subunits at one or more locations. This question lends itself to be studied in two possible ways in which the carbohydrates could be specifically removed by enzymatic or chemical means, without altering the polypeptide portion of the hormone.

The glycoprotein hormones include the pituitary hormones, luteinizing hormone (lutropin, LH), follicle-stimulating hormone (follitropin, FSH), and thyroid-stimulating hormone (thyrotropin, TSH), and the placental hormones, human chorionic gonadotropin (hCG), and pregnant mare serum gonadotropin. Erythropoietin (which is probably derived from kidney or a kidney enzyme that produces it from a serum protein) is also a glycoprotein hormone. All the glycoprotein hormones so far described have the distinctive feature that they are comprised of dissimilar subunits. This dissimilarity has not been demonstrated in the case of erythropoietin, but certain data suggest that it may obtain even in this instance.

Lutropin (LH) and follitropin (FSH) are gonadotropins produced and secreted by the mammalian anterior pituitary gland. They serve to regulate gonadal function by stimulating the production of sex steroid hormones (estrogen, progesterone, and testosterone) and gametogenesis. The gonadotropins are the effectors of well-regulated gonadal development and function orchestrated in a finely tuned manner by factors from the hypothalamus. Hence, LH and FSH play critical roles in sexual development and maturation as well as normal reproductive function (Kalra and Kalra, 1985).

Thyrotropin (TSH) is a member of the glycoprotein hormone family which includes pituitary lutropin (LH) and follitropin and placental chorionic gonadotropin. As with the other glycoprotein hormones, TSH is composed of two nonconvalently linked subunits, α and TSHβ. The a subunit is common to all of the hormones and the β subunit is distinct for each hormone (1,2). Separate mRNAs which are encoded by separate genes have been identified for the TSH subunits (3–8). In the rat, there appears to be a single TSH mRNA (0.7 Kb) which is consistent with data from Chin et al. (7) that indicates a single rat TSHβ gene. On the other hand, Kourides and co-workers have suggested the possibility of two TSHβ genes in the mouse with only one being expressed (9).

The interaction of FSH with granulosa cell receptors has previously been studied by Nimrod et al. (1), Louvet and Vaitukaitis (2) and Lee and Takahashi (3) in the rat and by Zeleznik et al. (4) in the porcine. In this report, we describe the binding of FSH to bovine granulosa cells and summarize studies suggesting the presence in bovine follicular fluid of a low molecular weight inhibitor of FSH binding (FSH-BI) to bovine granulosa cell receptors.

In man, lutropin (LH) and chorionic gonadotropin (CG) belong to a group of hormones known collectively as the glycoprotein hormones. LH along with follicle-stimulating hormone (follitropin, FSH) and thyroid-stimulating hormone (TSH) is produced and secreted by the anterior pituitary gland and CG by the placenta. LH and FSH, the pituitary gonadotropins, stimulate the production of the sex steroid hormones and gametogenesis. As such, they are important for sexual development and function, and reproduction. CG, whose maximal levels appear in early pregnancy, is essential for the function of the corpus luteum (1, 2).

Lutropin (LH) is a member of a family of glycoprotein hormones that includes follitropin (FSH), thyrotropin (TSH), and a placental hormone, human chorionic gonadotropin (hCG). Each one of these hormones is built up of two dissimilar subunits α and β, attached noncovalently in the native hormone. Within a given species, the α subunit is common to all glycoprotein hormones; only the β subunits, which confer to a hormone its biological and immunological specificity, differ (Giudice and Pierce, 1978).

The interaction of follitropin (FSH, follicle-stimulating hormone) with testicular receptors has been studied most extensively in the rat model. Means and Vaitukaitis (1972) studied the binding of tritiated human FSH (hFSH) to receptors in immature rat testis. We have studied the interaction of biologically active, radioiodinated hFSH [125I]-hFSH with receptors in homogenates (Bhalla and Reichert, 1974a,b) and purified membranes from testes tubules of mature rats (Abou-Issa and Reichert, 1976). Desjardins et al. (1977) utilized radioiodinated human and rat FSH to study binding of the hormone to testis during maturation in the rat. It was found that the amount of labeled hormone bound to testis receptor increased slightly (P< 0.01) between 5 and 15 days and then remained relatively constant from 30 to 90 days. No apparent differences were noted in binding parameters between rat FSH and hFSH, although this was not rigorously examined. Considerable information is now available on the characteristics of the follitropin—receptor interaction in rat testes. A summary of reported values for K d and binding capacity is given in Table I. An outline of different conditions utilized to achieve maximum specific binding is also given in Table I. Binding of radiolabeled hFSH to rat testicular receptor is hormone- and tissue-specific and temperature- and pH-dependent, and apparently requires divalent cations for maximum binding, with no apparent specificity shown among Mg2+, Mn2+, and Ca2+ (Abou-Issa and Reichert, 1976).