Nattokinase is a serine protease enzyme isolated from natto, a traditional Japanese food made from fermented soybeans. Dr. Hiroyuki Sumi discovered it in 1980 while screening clot-degrading compounds. Unlike most cardiovascular supplements that work upstream by improving vessel dilation or reducing inflammation, nattokinase works downstream through a process called fibrinolysis. It breaks down fibrin, the protein mesh that holds blood clots together. It also appears to inactivate plasminogen activator inhibitor-1 (PAI-1), the body’s own brake on clot dissolution.