Characterization of rat placental lactogen-alpha (PL-alpha) with an antipeptide antibody directed against rat PLs

We have already reported that the serum of the mid-pregnant rat contains placental lactogen-alpha (PL-alpha), which has a higher molecular weight than PL-I when analysed by gel-filtration chromatography. In order to characterize PL-alpha, we prepared antipeptide antisera directed against the hydrophilic regions of authentic PLs and developed RIAs with them. With these RIAs, antiserum AI-86 reacted only with PL-I, whereas antiserum AII-86 reacted with both PL-alpha and PL-I, which indicates that the antibody recognition sites of these molecules are not identical and their core proteins differ distinctly. As the use of the RIAs with AI-86 and AII-86 in combination enabled PL-alpha and PL-I to be discriminated, the glycoresidues and hydrophobicity of PL-alpha were analyzed further. Analysis with hydrophobic chromatography revealed that PL-alpha was less hydrophobic than PL-I and, furthermore, PL-alpha, but not PL-I, bound specifically to a wheat germ agglutinin (WGA) lectin column. Therefore, in view of their different amino acid sequences and glycoresidues, PL-alpha and PL-I are distinct molecular entities.