Isolation and characterization of a 94,000-dalton protein with thyrotropic activity from early bovine placenta.

Abstract

A thyrotropic protein was extracted and purified from the placenta of early bovine gestations. After protein extraction, the 45-60% ammonium sulfate precipitate of maternal and fetal bovine cotyledons was found to compete with thyroid stimulating hormone (TSH) for binding to thyroid cell membranes and to mediate TSH specific biological effects including the stimulation of cyclic AMP production, iodide uptake, and thyroxine secretion. The placental thyrotropin was further purified by gel and anion exchange chromatography, followed by binding to thyroid cell membranes and elution by mild acid treatment. 400 micrograms of isolated protein with 4.5 units of TSH-like binding activity/mg of protein was recovered from the placenta of a 90-day-old bovine gestation, representing 2 X 10(-4%) of its original wet weight. The placental thyrotropin appeared to be a 94,000-dalton protein with pI 6.0 and composed of two noncovalently associated chains of 50,000 and 44,000 daltons. The placental 94,000-dalton thyrotropin bound to TSH membrane receptors and induced specific TSH-mediated biological effects, but was structurally and immunologically distinct from TSH and hypophysical or placental gonadotropins.