A new factor Xa inhibitor (lefaxin) from the Haementeria depressa leech

The salivary complex of the leech Haementeria depressa produces potent anti coagulant components. Among them, a protein named lefaxin inhibits factor X a (FXa). Lefaxin was purified to homogeneity from dissected salivary comple xes by gel filtration in Sephadex G-150 followed by two ion exchange chroma tography steps in Mono-Q. Inhibition of FXa by lefaxin was demonstrated by the inhibition of its amidolytic activity, measured with chromogenic substr ate S-2765 (apparent K-1 of 1 nM), and of its ability to inhibit thrombin g eneration in the prothrombinase complex (EC50 of 40 nM). Lefaxin has a mole cular weight of 30 kDa and an isoelectric point of 5.7. It is made of a pol y peptide chain whose N-terminal sequence shows no similarity with that of other FXa inhibitors (antistasin and ghilianten) isolated from leech saliva , On the other hand, the N-terminal sequence of lefaxin presents significan t sequence similarity with nitric oxide carrier proteins myohemerythrin fro m the annelid Nereis diversicolor and prolixin S from the tick Rhodnius pro lixus. Interestingly, prolixin S also proved to be an anticoagulant protein acting on FXa.