The purification and characterization of rabbit placental lactogen

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Abstract
Rabbit placental lactogen, a polypeptide hormone functionally related to the growth hormone[GH]/prolactin [PRL] 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 pH 9.0 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 MW is 20,600, as determined by sedimentation-equilibrium centrifugation, and 24,200, as estimated by gel electrophoresis in sodium dodecyl sulfate. Its amino acid composition resembles that of rabbit GH and rat PRL, except for a lower glutamic acid and leucine content. Like PRL, rabbit placental lactogen has 2 tryptophan and 6 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 GH or rat PRL; 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 GH/PRL tree than do their primate counterparts.