Hirullin P18 is a 61-amino acid hirudin-related protein having potent antithrombin activity. Similar to hirudin, it contains a highly acidic C-terminus, but has a significantly different sequence from any other known hirudin variant. The present study demonstrates that the C-terminal fragment acetyl-hirullin P1840–61 possesses an antithrombin potency similar to that of acetyl-desulfatohirudin45–65. Additionally, like the hirudin fragment analog, it inhibits fibrin-clot formation by binding to a non-catalytic site on thrombin. Sequential shortening of the hirullin P18 C-terminal fragment demonstrates the critical nature of Phe51, which corresponds to the important Phe56 residue of hirudin. Although the sequences of hirullin P1854–61 and hirudin59–65 have substantial differences, the C-terminal functional domain represented by hirullin P1850–61 appears to be comparable to hirudin55–65 in terms of its functional role in antithrombin activity.
