Immunochemical Studies with Pregnant Mare Serum Gonadotropin1

Immunological relationships among pregnant mare serum gonadotropin (PMSG), its subunits and equine pituitary gonadotropins (LH and FSH) were examined. For this purpose a homologous double antibody radioimmunoassay (RIA) for PMSG was developed and characterized. Levels of PMSG in serum samples collected from 2 mares during the first trimester of pregnancy were measured in the RIA and were found to be similar to values previously reported using bioassay and heterologous RIA techniques. Compared with PMSG which has been isolated from serum, preparations of this gonadotropin isolated from endometrial cup tissue were found to give parallel but slightly less potent (×0.3–0.9) inhibition curves. Extremely small amounts of PMSG were detected in pregnant mare urine. Isolated PMSG subunits were found to have a low degree of nonparallel cross reaction in the PMSG RIA; assay of heterologous subunit combinations revealed the importance of the β-subunit for immunological recognition. Antisera to PMSG subunits were tested in additional RIAs. When PMSG-α antiserum was used, all equine gonadotropins showed parallel inhibition curves, but these preparations demonstrated much less cross reaction with PMSG-β antiserum. Equine pituitary gonadotropins showed significant cross reactions in the PMSG RIA while those of other species gave minimal and nonspecific cross reactions. These data, in conjunction with data obtained from RIAs employing antiserum to eLH and eFSH, demonstrate a very close immunological relationship between PMSG and eLH.