The glycoprotein hormones discussed in this review belong to a family of homologousproteins which are secreted by the anterior pituitary gland of all vertebrates as well as by placental cells ofprimates and equids. The pituitary glycoprotein hormones include follitropin (follicle-stimulating hormone, FSH) and lutropin (luteinizing hormone, LH), essential regulatory elements of ovary and testis which are required for reproduction, and thyrotropin (thyroid-stimulating hormone, TSH), which is indispensable for the control of thyroid structure and function and ultimately metabolism. Placental cells of primates and equids secrete choriogonadotropin (chorionic gonadotrophin, CG), which also stimulates gonadal tissue. In addition there is aberrant production of portions of these proteins and/or complete hormone molecules by certain types of tumour cells. Each hormone is transferred via the blood stream to its target tissue, and its biological activity is dependent uponan initial binding to a specific receptoron the surface ofa target cell. This evokes a chain of events, via specific response pathways involving chemical'messengers'(such as cyclic AMP and/or calcium), which culminates in the response typical of the hormone. In the female mammal, follitropin stimulates growth of ovarian follicles, whereas in the male it promotes spermatogenesis. Subsequent stages in the female reproductive cycle are dominated by lutropin, which stimulates ovulation of the follicle and its conversion to the corpus luteum which in turnsecretes progesterone; in the male, lutropin stimulates the Leydig cells to secrete androgen. Human choriogonadotropin, which has been studied extensively, possesses the biological activity of lutropin in the main, butthe choriogonadotropin produced by the pregnant mare (known as equine choriogonadotropin, PMSG or pregnant mare serum gonadotropin) exhibits appreciable follitropin activity in many non-equine mammals in addition to its lutropin activity. Thyrotropin in both sexes stimulates growth and secretion of thyroid gland cells. The corresponding hormones of lower vertebrates often have biological activities that differ from those of mammals. In addition, there is evidence in some of the former that a single gonadotropic hormone replaces mammalian follitropin and lutropin [1].

A great deal of information is now available on the amino acid sequences of these hormones from various vertebrate species. Studies employing the use of specific antibodies, chemical or enzymic modification and site-directed mutagenesis have identified certain regions of these proteins that are important in the maintenance of structural integrity and in receptor binding and transduction of the hormonal response. The attached carbohydrate is apparently required for efficient formation of their specific folding patterns and for full expression of biological activity, and the structures of some of the individual carbohydrate side chains have recently been published. Much new information has appeared since publication of earlier reviews on structure-function relationships of these hormones [2, 3] and on structure, synthesis and function of some of their carbohydrate moieties [4]. This Review attempts to summarize currently available structural information on the protein and carbohydrate components of these hormones as well as the functions thus far established for the latter.