Hirudin or hirudin-like factor - that is the question: insights from the analyses of natural and synthetic HLF variants.

The hirudin-like factor 1 (HLF1) of Hirudo medicinalis belongs to a new class of leech-derived factors. In previous investigations, HLF1 did not exhibit anticoagulatory activities. Here, we describe the analysis of natural and synthetic variants of HLF1 and HLF-Hyb, a yet uncharacterized member of the HLF family. Modifications within the N terminus of HLF1 have a strong impact on its activity. Some variants of HLF1 exhibit thrombin-inhibiting activity comparable to hirudins, whereas others have reduced or no activity. The analyses of HLF-Hyb variants revealed a strong impact of the central globular domain on activity. Our results indicate a comparable mode of action of hirudins and thrombin-inhibiting HLF variants. Finally, we propose and discuss criteria for classifying hirudins and HLFs.