Low molecular weight FSH binding inhibitor in bovine testis

Purification of low Mr (less than 5000) inhibitors of FSH binding to receptors has been hampered by their low concentration in tissue extracts and physiologic fluids. The calf testis represents an accessible and economic source of large quantities of tissue, and was therefore studied as a source of FSH binding inhibitors. Supernatants (27,000 X g) of calf testis homogenates inhibited binding of [125I]-hFSH as well as [125I]-hCG to membrane-bound receptor from the same source. FSH binding inhibitor was concentrated from large volumes of testis supernatant by precipitation of inert material with metaphosphoric acid, concentration/desalting of the resulting supernatant by ultrafiltration (Amicon UM-05 membranes) and lyophilization. Separation of FSH binding inhibitor and LH(hCG) binding inhibitor activities could be achieved by molecular sieving on Sephadex G-25. A partially purified fraction with inhibitors of FSH binding activity (ED50 = 44 micrograms protein) and free of LH(hCG) binding inhibitor activity (no activity at 800 micrograms protein) emerged with a Ve/Vo of 1.9, reflecting an apparent Mr of about 1500. Inhibitors of LH(hCG) binding activity emerged with the column outer volume. Rechromatography of the FSH binding inhibitor fraction on G-25 indicated two closely associated peaks of activity. These could be further resolved by gel filtration through BioGel P2, to give a salt-free fraction with an FSH binding inhibitor activity (ED50) of 24 micrograms protein. The inhibitor was heat-labile, losing 80% of its activity after 2 hours at 100 C. The testicular low molecular weight FSH binding inhibitor is similar to bovine follicular fluid and serum FSH binding inhibitor by several parameters.(ABSTRACT TRUNCATED AT 250 WORDS)