Characterization of an inhibitor for luteinizing hormone receptor site binding

An inhibitor for luteinizing hormone (LH) receptor site binding has been partially purified from aqueous extracts of ovaries from pseudo-pregnant or pregnant rats. The LH receptor binding inhibitor (LH-RBI) was heat-resistant, partially inactivated by trypsin or pronase digestion, and had a molecular weight of approximately 3800 daltons. The LH-RBI was not found in the ovary of mature non-pregnant rats or immature rats, nor was it found in testis or liver extracts. The LH-RBI strongly inhibited the in vitro binding of 125I-labeled ovine LH to ovarian LH receptors but did not inhibit 125I-labeled ovine prolactin binding to ovarian PRL receptors.