Interaction of mouse prolactin with mouse hepatic receptors

Lactogenic receptors are usually studied in heterologous systems where prolactin is derived from one species and receptors prepared from another. In such systems the foreign prolactin could be seen as a growth hormone by the host tissue. We have therefore developed a homologous radioreceptor assay using secreted mouse prolactin (smPRL) and mouse hepatic receptors. In this system, monovalent anions augment the smPRL-receptor interaction in the order F- greater than Cl- greater than Br- greater than I-. Divalent cations (Mg2+, Ca2+, Sr2+), phosphate and acetate also increase smPRL binding. Temperature and pH optima are at 8 degrees C and pH 8.3, respectively. Under optimum conditions, the percent total, specific and nonspecific binding are 55%, 45% and 10%, respectively. At infinite receptor concentration the maximum specific bindability of labeled smPRL is 50%. The effects of ions on binding of smPRL to the receptor show that hydrophobic forces participate in smPRL-receptor coupling. The biphasic dissociation kinetics show initial and final rate constants of 1.56 X 10(-4)/s and 7.62 X 10(-6)/s, respectively. The lactogenic receptor does not bind mouse growth hormone; however, it binds both mouse placental lactogen (mPL) and smPRL with equilibrium association constants of 3.90 X 10(8) M-1 and 2.25 X 10(8) M-1, respectively, suggesting that smPRL and mPL share biological roles by acting through the same receptor.