Purification and characterization of bovine placental lactogen

Bovine placental lactogen (bPL) has been purified to homogeneity from bovine placental tissue. A radioreceptor assay (RRA), using a pregnant rabbit liver membrane fraction and human GH (hGH) as standard and tracer, was employed to monitor hormonal activity during the purification. The apparent molecular weight of bPL is approximately 30,000-32,000 as determined by analytical sodium dodecyl sulfate polyacrylamide gel electrophoresis. Somewhat larger estimates were obtained upon Sephadex gel filtration columns. The isoelectric point of the purified hormone as determined by analytical polyacrylamide gel isoelectric focusing is 5.5. The binding and displacement curves of radioiodinated bPL and hGH in the RRA for GH are superimposable, whereas the displacement curve of bPL in the RRA using the particulate fraction of a pregnant rat liver homogenate is parallel to the ovine PRL standards. Purified bPL is capable of modulating hGH-specific receptor sites in cultured human IM-9 lymphocytes. Stimulation of the growth of quiescent NB2 node lymphoma cells in culture demonstrates that bPL and bovine PRL are equipotent in this bioassay.