Isolation and characterization of multiple forms of bovine placental lactogen from secretory granules of the fetal cotyledon.

Previous work has shown that, unlike other species, placental lactogen (PL) in the bovine (bPL) has a mol wt of approximately 32,000 and exists in several different forms with different isoelectric points. This study was carried out to develop a more rapid purification scheme, whereby the yield of bPL obtained was increased while at the same time the possibility of artifacts from a prolonged purification protocol was decreased. A procedure was developed in which a fraction enriched in bPL-containing granules was obtained after gentle disruption of the binucleate cells of the fetal cotyledon. The fetal portion of the placentomes from midpregnant cows was minced with scissors and vigorously stirred in order to remove and disrupt binucleate cells within the fetal villi. The supernatant from this step was fractionated by differential centrifugation followed by a four-step discontinuous Percoll gradient of 1.03-1.08 g/ml. A granule-enriched fraction was isolated from the 1.04 g/ml zone from which membrane-enclosed protein was released by freezing and thawing. Membranes and insoluble proteins were sedimented by high speed centrifugation to yield an extract which contained approximately 20% of the hormone initially in the tissue. Two subsequent chromatographic steps, gel filtration on Sephadex G-75 and high performance reversed phase chromatography with a C-4 column, resulted in a preparation of greater than 98% homogeneity. Two-dimensional gel electrophoresis of purified bPL revealed at least nine protein spots in the 31,000-33,000 mol wt range with isoelectric points ranging from 4.85-6.3. All forms exhibited parallel dilution curves in a RIA for bPL. It would appear, therefore, that multiple forms of bPL exist and that they are not artifacts of the prolonged purification protocol previously used.