Two distinct placental lactogen-like substances in serum during mid-pregnancy in the rat

Two forms of placental lactogen (PL), designated PL-I and PL-II, have been reported in the serum of pregnant rats PL-I was secreted during mid-pregnancy and PL-II was secreted near term. In the present study, we found that two distinct forms with PL-like activity were secreted into the blood during mid-pregnancy. Serum from day-12 pregnant rats was subjected to high-performance liquid chromatography (HPLC) gel-filtration (TSK-G3000SswXL column), and PL activity was assayed by radioreceptor assay using ¹²⁵I-labeled rat PRL and hepatocyte plasma membrane from pregnant rats. HPLC-gel filtration separated the PL activity into two peaks with mol wt of 55-60 K and 45-50 K, as estimated by reference to the elution volume of standard proteins. We tentatively designated these peaks PL-α and PL-β, respectively. Considering these mol wt, PL-β seemed to be identical to PL-I. mRNA was extracted from samples of placenta obtained each day from day 8 to day 12 of pregnancy and analyzed by means of a translation system involving micro-injection into Xenopus oocytes. The time of appearance of the mRNAs corresponding to PL-α and PL-β did not correspond and differed according to the day of pregnancy, suggesting that there are individual mRNAs for each PL in rat placenta. Treatment of PL-α and PL-β with peptide: N-glycosidase F completely abolished the binding activity to their receptors. Also since they were sensitive to glycosidases such as endoglycosidase H, endoglycosidase F and neuraminidase, these PLs were considered to possess N-linked glycoresidue(s) and to be sialylated. Based on the results, there are two distinct PLs, which are secreted at mid-pregnancy, are glycosylated and have different mRNAs.