Hamster placental lactogens: gestational profiles and high molecular weight forms

We previously reported the purification of a lactogenic protein, hamster placental lactogen (haPL; now designated haPL-II) from late pregnant hamster placentas. In this study the lactogenic factors in maternal serum during the second half of pregnancy were measured using RIAs for haPL-II and haPRL and a RRA for total lactogenic activity. haPL-II was first detectable on day 10 of pregnancy and reached a maximal concentration [12.3 +/- 0.8 microgram/ml (mean +/- SEM); n = 6] on day 16 (term). PRL concentrations were relatively low during the second half of gestation. Significant amounts of lactogenic activity that could not be ascribed to haPL-II or haPRL were detected throughout this period, with maximal concentrations on days 10-12. Gel filtration chromatography of day 10 serum gave an apparent mol wt of 35,000 for this lactogenic factor. A lactogenic factor with a similar mol wt (37,000) was detected in an extract of day 10 placenta. This lactogenic factor was designated haPL-I. The most prominent forms of haPL-II in day 16 serum were several disulfide-bonded forms with mol wt greater than 200,000. Smaller quantities of lower mol wt forms, including monomeric haPL-II, were present, but circulated as noncovalently bound complexes with mol wt greater than 100,000. Complete conversion to monomeric haPL-II was found only after treatment with both sodium dodecyl sulfate and 2-mercaptoethanol. The high mol wt forms of haPL-II in maternal serum were composed of two different monomeric species of haPL-II.