Evidence for a High Molecular Weight Follicle-Stimulating Hormone Binding Inhibitor in Bovine Testis

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Abstract
Cytosol prepared after centrifugation (100,000 .times. g) of homogenates of immature bovine testis inhibited binding of iodinated human FSH to membrane-bound receptors (FSH-BI) from the same source in a dose-related manner. FSH-BI in the cytosol passed H1P100 (MW > 100,000) and was retained by H1P5 hollow fiber membranes (MW > 5000) (Fraction F1). FSH-BI in F1 was adsorbed by DEAE-32 ion-exchange fibers (pH 8.3) and could be eluted at 0.1 M NaCl (F2-1) and 0.2 M NaCl (F2-2). Filtration of either F2-1 or F2-2 through Sephadex-G75 (0.05 M ammonium acetate, pH 6.3) resolved a single region (Ve/Vo = 1.37) having FSH-BI activity (Fraction F3). Based on its elution volume (Ve), FSH-BI fraction F3 was estimated to have a MW of about 33,000 and a Stokes radius of about 25 .ANG.. When testes were collected under sterile conditions there was no difference in the yield of binding inhibitor compared to testes collected at the abattoir, indicating that bacterial contamination was not the source of FSH-BI. No FSH-like activity was detectable in either F1 or F3 by radioimmunoassay. Specific FSH-BI activity of F1 per mg of preparation did not differ between immature bovine testis (3-6 g), mature bovine testis (200-400 g) or mature ovine testis. However, as testis size increased, total FSH-BI per testis increased. A large MW FSH-BI was present in ovine and bovine testis. An increase of total FSH-BI activity in mature testis apparently is due to an increase in testis size rather than an increased production of inhibitor per mg of tissue protein.