Hirudin is a compound made by medicinal leeches that interferes with the body’s ability to form blood clots. Leeches have been used in medicine for centuries. Currently, they are used for various conditions that benefit from the removal of blood clots. Such anticoagulation involves the inhibition of the protein thrombin, which catalyzes blood clot formation. The use of live leeches risks the introduction of bacteria into the wound, and hirudin is generally used by itself to prevent blood from clotting.
Medicinal leeches, Hirudo medicinalis, produce a number of compounds in their salivary glands that help with the process of bloodletting from their hosts, known as hirudotherapy. They produce anesthetics, so the host will be less likely to notice as the leech attaches to them. Also in the mixture are compounds that keep the blood from clotting, so it will flow more freely into the leech. The blood continues to remain unclotted, even after the leech has detached. There are some disadvantages to using live leeches for anticoagulation, since they can introduce bacteria into the wound.
The compound in the saliva that is the most specific for anticoagulation is the peptide hirudin. This is a chain of 65 amino acids, first discovered in 1884. It was not until 1976 that the structure was determined. Leeches only have a small amount of this compound in their saliva, and there are several different forms of hirudin mixed together. Most of the pure compound used medically is derived from genetically engineered peptide.
Blood clotting involves the conversion of soluble protein called fibrinogen to the insoluble mesh fibrin. This reaction is catalyzed by an enzyme called thrombin, which is a protease and can cleave other proteins. When thrombin is inactivated, fibrin does not form and the blood no longer clots.
There are different types of thrombin. Hirudin inactivates only the type of thrombin that is the most specific for fibrinogen, and therefore it is the most potent compound that inhibits thrombin. It is even more specific than the body’s own thrombin inhibitor, antithrombin III.
Conditions that benefit from anticoagulation treatment include varicose veins and genetic disorders in which the blood coagulates too much. Hirudin can act on thrombin that has formed a mesh and become a clot. The condition of having such clots inside blood vessels is known as thrombosis. This peptide can even act on complexed thrombin, and this ability gives it an advantage over the drug heparin, also used as an anticoagulant.
