Biological and immunoactive substances resemblin chorionic gonadotropin are present in full-term horse and zebra placentas

This study describes the presence of immunoactive and bioactive eCG-like material in full-term placentas of both domestic horses and zebras. Term placental extracts were immunoreactive in an LH monoclonal antibody RIA, and methods successfully used previously for the purification of eCG and eLH were employed to further concentrate the immunoreactive materials to the point where additional characterization studies could be performed. Sufficient equine material was obtained to perform a final fractionation on a concanavalin A Sepharose column yielding an unadsorbed fraction, e17A, and an adsorbed fraction, e17B. There was insufficient zebra material, z5D, for this step. HPLC gel filtration coupled with LH immunoassays of the column eluates showed all the final placental fractions to be highly heterogeneous, but a discrete peak of immunoactivity was found in one of the two equine fractions (el7B) and in the zebra fraction (z5D). The HPLC gel filtration elution volumes for el7B and z5D suggest that they have a smaller molecular size than either eCG or eLH but almost the same size as ovine LH. Both el7B and z5D were bioactive in the rat Leydig cell assay for LH but low in potency compared to eCG or eLH; el7A was inactive at very high doses (5 microgram). This latter fraction, however, cross-reacted in an eCG alpha RIA to a much greater extent (6 times) than el7B, suggesting that it may be an incompletely formed or degraded alpha subunit. RIAs for LH, eCG, and eCG beta suggest that epitopes distinctive for these molecules are also present or similar to those in the term placental materials. Insofar as the endometrial cups, the placental source of chorionic gonadotropin in equids, disappear long before the end of pregnancy, the results suggest that other cell types may be responsible for the presence of the eCG-like material in the term placentas of the horse and the zebra.