Pregnant mare serum gonadotropin. An in vitro biological characterization of the lutropin-follitropin dual activity.

The interaction of highly purified pregnant mare serum gonadotropin (PMSG or eCG) has been studied in rat and equine testis homogenates. In the rat studies, unlabeled PMSG was shown to possess both high levels of intrinsic follitropin-like and lutropin-like activity when the inhibition of radioiodinated follitropin and lutropin binding by unlabeled PMSG was studied. Radioiodinated PMSG exhibited only lutropin activity in this system. Upon iodination of intact PMSG, only the a subunit incorporates iodine. In addition to being more effective in the radioligand binding assays, partially desialylated PMSG was also more effective in other in vitro bioassays specific for either follitropin or lutropin activity. Although the rat follitropin receptor would recognize the follitropin information i PMSG, the equine follitropin receptor would not perceive PMSG as a follitropin, as evidenced by the inability of high levels of PMSG to significantly inhibit the specific binding of equine follitropin to equine testes homogenates. Upon reassociation of PMSG a and subunits, the lutropin activity was restored to a greater extent than the follitropin activity, suggesting that highly purified PMSG may be a heterogeneous population of modified and unmodified molecules or that the conformational requirements for the expression of follitropin activity by a reassociated complex of PMSG subunits are more extensive than those necessary for the expression of lutropin activity.