Microcystins cannot be broken down by standard proteases like pepsin, trypsin, collagenase, and chymotrypsin due to their cyclic chemical nature. They are hepatotoxic, i.e., able to cause serious damage to the liver. Once ingested, microcystin travels to the liver via the bile acid transport system, where most is stored, though some remains in the blood stream and may contaminate tissue.
Acute health effects of Microcystin-LR are abdominal pain, vomiting and nausea, diarrhea, headache, blistering around the mouth, and after inhalation sore throat, dry cough, and pneumonia.
Studies suggest that the absorption of microcystins occurs in the gastrointestinal tract. Furthermore, it was found that these hepatotoxins inhibit the activity of protein enzymes phosphatase PP1 and PP2A causing hemorrhagic shock and were found to kill within 45 minutes in mice studies.
There appears to be inadequate information to assess the carcinogenic potential of microcystins by applying EPA Guidelines for Carcinogen Risk Assessment. A few studies suggest a relationship may exist between liver and colorectral cancers and the occurrence of cyanobacteria in drinking water in China. Evidence is, however, limited due to limited ability to accurately assess and measure exposure.