Purification and partial characterization of hamster placental lactogen

A radioreceptor assay for PRL-like activity was used to monitor the purification of a lactogenic protein, hamster placental lactogen (haPL), from late pregnant hamster placentae. Investigations of the PRL-like activity in placental extracts demonstrated that haPL is subject to disulfide-related aggregation phenomena that are not observed for mouse PL. By inclusion of 2-mercaptoethanol in the buffers used for purification, monomeric haPL was obtained. A 750-fold purification was achieved by ammonium sulfate precipitation and chromatography on phenyl-Sepharose, TSK diethylaminoethyl-650S, hydroxylapatite, and Sephadex G-100. This procedure yielded 6.2 mg (by dry wt) of purified haPL from 286 g 15-day pregnant hamster placentae, with an overall yield of 20%. The purified haPL has a mol wt of 25,200 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and an isoelectric point of 8.8. haPL is lactogenic as judged by its ability to compete for lactogen binding sites on rabbit mammary gland membranes and to stimulate secretion of alpha-lactalbumin by cultured mouse mammary gland epithelial cells.