https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5372539/
"Both NK and lumbrokinase (derived from earthworms), unlike most proteins, are more resistant to the highly acidic gastric fluids in the stomach and can be absorbed in the later sections of the digestive tract. In 1995, Fujita and colleagues demonstrated that NK could be absorbed from the rat intestinal tract in an intact form and degraded fibrinogen in plasma blood samples [3]. Subsequently, in 2013, a research team in the United States detected intact NK in the serum of healthy humans after they were administrated a single, oral dose of NK (2000 FU/100 mg) in a capsule [16]. Other studies have also shown that oral administration of NK can enhance fibrinolytic activity in plasma [3,16]. The mechanism by which NK is transported from the digestive tract into the circulatory system still needs to be elucidated. NK can resist high temperature (50 °C) and pH (to 10), which certainly contributes to the ability of this enzyme to remain intact in the gastrointestinal tract [6]."
