Purification and Characterization of an FSH Releasing Protein from Porcine Ovarian Follicular Fluid

Abstract

A variety of hypophysiotropic peptides or proteins have been reported to be present in mammalian gonads. Inhibin, a hormone that under most circumstances selectively suppresses the secretion of follicle-stimulating hormone (FSH) but not luteinizing hormone (LH), has been isolated from the gonadal fluids of several species1–5 and characterized as a heterodimeric protein consisting of α- and β-polypeptides associated by disulphide bonds. The complete amino-acid sequences of the precursors of porcine6,7 and human7 inhibin α-subunits and two distinct porcine inhibin β-subunits (βA and βB)6 have been deduced from complementary DNA sequences. Gonadotropin releasing peptides have also been found in the gonad and have generally been shown to be active in radioreceptor assays for gonadotropin releasing hormone (GnRH) but to exhibit different chromatographic and immunological characteristics from those of GnRH8–14. During our purification of inhibin from porcine follicular fluid, we noted fractions that could stimulate the secretion of FSH by cultured anterior pituitary cells3. We report here the purification of an FSH releasing protein (FRP) and its characterization by SDS-polyacrylamide gel electrophoresis under non-reducing and reducing conditions and by partial sequence analysis. Our results indicate that porcine gonadal FRP is a homodimer consisting of two inhibin βA-chains linked by disulphide bonds. FRP is highly potent (50% effective concentration (EC50)∼25 pM) in stimulating the secretion and biosynthesis of FSH but not of LH or any other pituitary hormone. In contrast to the effects of GnRH and other reported gonadal gonadotropin releasing fractions, the action of FRP is not mediated by GnRH receptors.