Isolation, purification, and characterization of mouse placental lactogen

Mouse placental lactogen was purified 1840-fold from BALB/c placentae from days 14-18 of gestation with an overall yield of 29%. The purification procedure included alkaline homogenization and extraction, ammonium sulfate precipitation, hydrophobic interaction chromatography on Phenyl-Sepharose, ion-exchange chromatography on CM- and DEAE-cellulose, and gel exclusion chromatography on Sephadex G-100. On 10% alkaline polyacrylamide gels, mouse placental lactogen had an Rf of 0.19. Electrophoresis in gels containing NaDodSO4 showed a single band with a mobility corresponding to a Mr of 23,000 +/- 1000. The isoelectric point, determined by isoelectric focusing in 8 M urea/5% 2-mercaptoethanol, was 7.1. When tested in the pigeon crop sac assay, 10 micrograms of mouse placental lactogen produced stimulation comparable with that evoked by 10 micrograms of ovine prolactin. In the rabbit mammary gland radioreceptor assay, mouse placental lactogen was 150% more potent than ovine prolactin in displacing 125I-labeled ovine prolactin from rabbit mammary gland membranes. Iodinated purified mouse placental lactogen could be displaced from rabbit mammary gland membranes by mouse placental lactogen, mouse prolactin, and ovine prolactin. Ovine prolactin was 45% as avid as mouse placental lactogen is displacing 125I-labeled mouse placental lactogen from rabbit mammary gland membranes. Mouse placental lactogen did not crossreact with antisera to mouse prolactin or mouse growth hormone in a radioimmunoassay.