Plasminogen activation occurs through conversion of plasminogen to plasmin by plasminogen activators. In adult mammals, liver has been the only known site for plasminogen synthesis. Seminiferous tubules secrete plasminogen activator, but are behind a barrier that excludes the entrance of many macromolecules to this tissue. Therefore, it became of interest to study the existence of plasminogen in this system. After metabolic labeling, a 90 kDa lysine-binding polypeptide was found. This protein could be immunoblotted by anti-plasminogen antibody, and was shown to possess urokinase-dependent proteolytic activity. The findings suggest that plasminogen is synthesized by seminiferous tubules. We propose that local plasminogen synthesis may occur in tissues which are separated by specific anatomical barriers.
