Evidence for a proteinase inhibitor binding component associated with murine spermatozoa

The supernatants of frozen-thawed murine epididymal sperm suspensions contain a heat-labile component capable of binding a low molecular weight, acid-stable proteinase inhibitor of seminal vesicle origin. The substance has a molecular weight of approximately 15,000 and can be isolated by affinity chromatography using the inhibitor as the ligand. Although the substance has no inherent enzymatic properties, it will decrease the activity of the seminal inhibitor in the standard N-benzoyl-DL-arginine-p-nitroanilide (BAPNA) assay. Enzyme-linked immunosorbent assays (ELISA) indicate that the substance, when bound to microtiter plates, is capable of binding the seminal vesicle inhibitor. Turkey egg white trypsin inhibitor will decrease the amount of the seminal inhibitor that will bind to the substance, while noninhibitor proteins, e.g., bovine serum albumin or insulin, have no effect. Turkey egg white and lima bean trypsin inhibitor will also decrease the amount of seminal vesicle inhibitor capable of binding to washed sperm. These data indicate the presence of an inhibitor acceptor site associated with murine epididymal spermatozoa.