Purification and characterization of the gonadotropin secreted by cultured horse trophoblast cells.

Equine chorionic gonadotropin (eCG) secreted by horse trophoblast cells cultivated in vitro was isolated and its chemical, immunochemical, and biological characteristics were compared to those of the gonadotropin secreted in vivo and isolated from PMS. It was also compared with eCG isolated from the tissue of origin, the endometrial cups. The gonadotropin secreted in vitro had a smaller molecular size, contained appreciably less carbohydrate, and showed different amino-terminal residues from that secreted in vivo. In addition, there were significant differences in amino acid composition between eCG secreted in vivo and that secreted in vitro. The reactivity of eCG (isolated from the medium of cultured trophoblast cells) in homologous eCG (isolated from PMS) and equine LH and FSH RIAs suggested close antigenic similarities to eCG isolated from PMS. However, compared to serum-derived eCG, eCG secreted in vitro showed reduced activity in both LH (13%) and FSH (9-24%) bioassays and in LH and FSH radioreceptor assays (40-50%). The differences in bioassay potencies may be accounted for in part by the lower sialic acid content of the gonadotropin secreted in vitro. With respect to both bioactivity and chemical composition, the eCG secreted in vitro more closely resembled the eCG isolated from endometrial cups than that isolated from serum.