Human Follicular Fluid Contains a Follicle-Stimulating Hormone (FSH) Receptor Binding Inhibitor Which Has FSH Agonist Activity, is Immunologically Similar to FSH, But Can Be Distinguished from FSH*

Abstract

We have previously reported the presence in human follicular fluid (hFF) of a high (greater than 5000) mol wt FSH receptor binding inhibitor (FSH-BI). This hFF FSH-BI was further purified by removal of material insoluble in acidified acetone (pH 4.1) but soluble in diethyl ether (pH 10.5), followed by molecular sieving through Sephacryl S-100. FSH-BI activity eluted from S-100 with an elution volume similar to that of hFSH, but could be distinguished from hFSH on the basis of a differential sensitivity to acid inactivation. Human FSH was inactivated in acetone at pH 4.1 (1 h, 25 C), whereas hFF FSH-BI retained activity under these conditions. Human FF FSH-BI also demonstrated FSH-like agonist activity, defined as the ability to stimulate basal levels of estradiol synthesis in cultured rat Sertoli cells. Human FSH-BI strongly cross-reacted to a commercially available monoclonal antibody used to measure serum hFSH. Indeed, recovery of FSH immunologic activity was significantly greater (134-fold on a mass basis) after partial purification, indicating that antibody recognition sites were apparently masked in unfractionated hFF. In summary, large mol wt hFSH-BI has been partially purified from hFF and found to be similar in size to pituitary hFSH and to have FSH-like agonist activity in vitro. Although distinguishable from pituitary hFSH on the basis of stability to acid, hFSH-BI appears immunologically related to pituitary hFSH so that measurements of hFSH levels in hFF using immunologic techniques should be interpreted with caution.