The best known salivary product of the leech is
hirudin, one of the most potent natural anticoagulants.
Hirudin was first isolated and named by Haycroft
in 1884.28 Its chemical analysis was completed
in 1955, and it was identified as a 65 amino acid
polypeptide with specific activity as a direct inhibitor
of thrombin.29 Hirudin has a low molecular
weight of 9000, and is a specific non-enzymatic inhibitor
of thrombin. Thrombin has two main roles in
the coagulation process: it stimulates the release of
platelet activation factor (PAF) and is the enzyme
responsible for the universal final step in the coagulation
cascade. In this step, fibrinogen is converted
to fibrin, which acts to stabilise clots by forming
cross-linking strands. Hirudin inactivates thrombin
by blocking substrate-binding groups, which
prevents other molecules binding to its saturable
sites.