The purification and characterization of rabbit placental lactogen

Rabbit placental lactogen, a polypeptide hormone functionally related to the growth hormone/prolactin family, was isolated from placenta by (NH4)2SO4 precipitation, gel filtration and ion-exchange chromatography on DEAE-and CM-cellulose. The hormone was purified to more than 90% homogeneity, as determined by end-group analysis. On disc gel electrophoresis at pH9.0 it migrates as a pair of closely spaced bands with mobilities of 0.489 (minor band) and 0.511 (major band), and its isoelectric point is 6.1. Its mol.wt. is 20600, as determined by sedimentation--equilibrium centrifugation, and 24200, as estimated by gel electrophoresis in sodium dodecyl sulphate. Its amino acid composition resembles that of rabbit growth hormone and rat prolactin, except for a lower glutamic acid and leucine content. Like the prolactins, rabbit placental lactogen has two tryptophan and six cysteine residues, and its N-terminus, valine, is identical with that for human placental lactogen. By radioimmunoassay, it does not cross-react with antisera to either rat growth hormone or rat prolactin; in addition, it does not cross-react with antisera to bovine placental lactogen by double immunodiffusion. The similarity of the biochemical characteristics of rabbit placental lactogen to the other non-primate placental lactogens lends further support to the hypothesis that these molecules occupy a more central position in the growth hormone/prolactin "tree" than do their primate counterparts.