Equine chorionic gonadotropin.

Cells from the chorionic girdle of the equine trophoblast invade the maternal endometrium at day 36 of gestation and become established as secretory elements known as the endometrial cups. These structures, which persist for 40-60 days, produce a gonadotropin which can be found in circulation until about day 130 of gestation. This glycoprotein has been identified in the horse and the donkey, with the former having received much better characterization. It consists of 2 noncovalently linked peptide chains; an alpha-subunit of 96 amino acids, which is common to that found in other horse glycoprotein hormones. The beta-subunit of 149 amino acids is identical to horse LH beta. Horse CG is the most heavily glycosylated of the known pituitary and placental glycoprotein hormones. The alpha-subunit has two and the beta-subunit one N-linked glycosylation site, and the beta-chain has in excess of four O-linked glycosylation sites. The N-linked glycans have some oligosaccharides that are not found on other glycoprotein hormones. The sialic component of glycosylation confers an exceptionally long half-life on CG compared to other glycoprotein hormones. Horse CG has LH-like activity in horse receptor and in vitro bioassays. In spite of the amino acid homology, it has lower LH activity than does horse LH. Its most intriguing, and as yet unexplained, characteristic is its pronounced FSH and LH activity in species other than the horse. Horse CG binds to FSH receptors of virtually all mammalian species, other than the horse, in which it has been tested and will produce biological effects peculiar to FSH. It has similar and potent interaction with LH receptors. The structural basis of this duality is not known but may be related to the region 90-110 of the beta-chain. Horse CG is believed to be constitutively expressed by the trophoblastic cells until the endometrial cups degenerate. The role of CG in equine gestation is not completely understood. It is believed to act as an LH-like hormone to induce supplementary ovulation and/or luteinization of follicles in the mare. It has not been established whether CG or the accessory corpora lutea are necessary for successful horse pregnancy. They may serve as a redundant system to assure that there is sufficient secretion of the primary corpus luteum to maintain pregnancy until the placenta assumes its role as the principal steroidogenic organ of gestation.