Recombinant mouse placental lactogen-I binds to lactogen receptors in mouse liver and ovary: partial characterization of the ovarian receptor.

The binding of recombinant mouse placental lactogen-I (mPL-Ir) to liver and ovarian membranes was investigated in virgin and pregnant mice. Competitive binding assays demonstrated that mPL-Ir, mouse placental lactogen-II (mPL-II), and mouse PRL (mPRL) bind to the same receptors in ovarian membranes. The relative abilities of the three hormones to displace [125I]iodo-mPL-Ir from the ovarian lactogen receptors was mPL-II greater than mPL-Ir much greater than mPRL. Scatchard analysis of mPL-Ir binding to ovarian membranes from day 10 pregnant mice showed a Ka of 2.0 x 10(9) M-1 and a binding capacity of 3.2 x 10(-14) mol/mg membrane protein. The specific binding of [125I]iodo-mPL-Ir to ovarian membrane preparations was significantly higher on day 17 than on day 10 of gestation. Dissociation of endogenous hormones with 4 M MgCl2 increased the binding of [125I]iodo-mPL-Ir to ovarian membranes but not to liver membranes. Affinity cross-linking of [125I]iodo-mPL-Ir to liver and ovarian membranes resulted in the specific labeling of proteins with receptor mol wt (Mr) of 44K and 40K (nonreduced) and 50K and 42K (reduced), as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The lactogen receptors from liver and ovary appeared structurally homologous, producing fragments with similar Mr when treated with proteolytic enzymes and undergoing similar reductions in Mr when treated with glycolytic enzymes. The ability of mPLs to bind specifically and with high affinity to receptors in mouse ovarian membranes suggests that these hormones may regulate ovarian function during gestation.