Purification and characterization of bovine placental lactogen

Bovine placental lactogen (bPL) was purified 4,200-fold from cotyledon homogenates by 1) salt precipitation and 2) ultrafiltration, followed by 3) gel filtration, 4) anion exchange, 5) hydroxylapatite, 6) chromatofocusing, and 7) final gel filtration chromatography. Purification was monitored by radioreceptor assay (RRA) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. During chromatofocusing, three distinct peaks of bPL were eluted: a small peak at pH 5.34 (bPL-1), a larger peak at pH 5.09 (bPL-2), and the largest peak at pH 4.84 (bPL-3). These three were further purified separately by the final gel filtration. The purified bPLs were compared with bovine (b) PRL (0.81 IU/mg) and bGH (30 IU/mg) by RRA. The bPRL-like activities were 67.1, 750, and 1,090 micrograms/mg, and the bGH-like activities were 74.4, 787, and 715 micrograms/mg for bPL-1, bPL-2, and bPL-3, respectively. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the molecular weights of all bPLs were the same, 31,100. In gel filtration under nondenaturing conditions, all bPLs had a 40,000 mol wt, with a Stokes radius of 2.70 nm. The isoelectric point (pI) of bPL-1 was higher than 5.85, the pI of bPL-2 was 5.52, and that of bPL-3 was 5.39. The amino acid compositions of bPL-2 and bPL-3 were virtually identical, except that bPL-3 had a few more acidic amino acids. Both bPL-2 and bPL-3 had an estimated 277 amino acid residues. Thus, bPL appeared to have both PRL- and GH-like activities equivalent to bPRL and bGH in the RRA on a mass basis, to be 50% larger than bPRL, bGH, and PLs in other species, and to occur in at least three different forms differing in their pI values and in their acidic amino acid compositions.