A 32,000-molecular weight protein from bovine placenta with placental lactogen-like activity in radioreceptor assays

Considerable discrepancies exist in the literature concerning the size and activity of bovine placental lactogen. Our bovine placental lactogen purification preparations were 20% as active as bovine PRL (bPRL) on a per weight basis when compared to bPRL in a lactogenic radioreceptor assay. To identify the active component in these preparations, the proteins were radioiodinated and bound to membrane receptors in the presence and absence of competing hormones, bPRL, and bovine GH (bGH). After centrifugation, membrane pellets were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the gels were autoradiographed. Only one radioiodinated protein band was present. This protein was displaced in the presence of competing hormone and comigrated which a major component of the purification preparations. In radioreceptor assays the active component was as active as bPRL and bGH. From the migration of protein standards included with the radioiodinated purification preparations in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, we estimate that the protein is 32,000 mol wt--about 10,000 mol wt larger than placental lactogens isolated in other species. The possibility that the active molecule was a precursor protein was investigated by examining proteins secreted by bovine placenta tissue cultures. The binding activity in these secretions, as well as in the purification preparations, eluted between ovalbumin (43,000 mol wt) and bPRL (22,000 mol wt) under nondenaturing conditions using high performance gel filtration chromatography. Analysis of this secreted protein, also by binding to membrane receptors, showed that the protein had the same molecular weight as that isolated from the purification preparations and was specifically displaced by the same hormones.