The purification and characterization of ovine placental lactogen

Ovine placental lactogen (oPL), has been purified approximately 1,000-fold from sheep cotyledons using conventional protein purification procedures. Radioreceptor assays using rabbit liver particulate fractions for growth hormone (RRA-GH) and using rabbit mammary gland particulate fractions for prolactin (RRA-PRL) were employed to monitor the hormonal activities. The molecular weight of oPL is approximately 22,000 as determined by gel filtration on Sephadex G-100, and its isoelectric point as determined by isoelectric focusing is 8.8. In the two RRA's, the displacement curve of oPL is parallel to bovine growth hormone (bGH) and ovine prolactin (oPRL) standards and the ratio of GH-activity to PRL-activity of oPL is 1:2. In a body weight gain assay using hypophysectomized rats, oPL has a growth-promoting potency of 1.3 U/mg. In rabbit mammary explants, oPL stimulates casein synthesis. In a receptor assay for growth hormone using human liver, oPL and hGH are equipotent in competing for receptor sites, suggesting that oPL and hGH have common structural features that are lacking in other non-primate hormones.