Immunological studies of rodent placental lactogens

Structural relationships among the placental lactogens (PLs) from the mouse, the rat, and the hamster were examined by evaluating the cross-reactions between the PLs with a polyclonal antiserum to each PL. Two techniques, competitive radioimmunoassay displacement curves and immunoblotting of SDS-polyacrylamide gels, were used to evaluate the cross-reactions. By the first technique, a greater general similarity is found between mouse PL and rat PL than between either of these hormones and hamster PL. In contrast, the immunoblotting technique shows a greater similarity between mouse PL and hamster PL than between either of these hormones and rat PL. In the radioimmunoassays, the antibodies are reacting with the three-dimensional surfaces of the native PLs, while in the immunoblots the PLs have been denatured and unfolded. Thus, the different cross-reactions found by the two methods may reflect a dichotomy between the relationships in the primary and tertiary structures of the rodent PLs. Incomplete displacement by rat PL in the mouse PL radioimmunoassay suggests that mouse PL and hamster PL appear to share at least one surface feature that is completely absent on rat PL. All three anti-PL antisera react with multiple molecular weight bands in preparations of mouse PL and rat PL (but not hamster PL), indicating that mouse PL and rat PL are composed of several different molecular weight species while hamster PL consists of a single molecular weight form.