Cloned cDNAs have been isolated that encode a variant of hirudin, a potent thrombin inhibitor that is secreted by the salivary glands of the medicinal leech, Hirudo medicinals. This variant probably corresponds to a form that has been purified from leech heads but differs in amino acid sequence from the hirudin purified from whole leeches. There are at least three hirudin transcripts detectable in leech RNAs that are different in size, site of synthesis, inducibility by starvation, and relationship to hirudin activity. The new hirudin variant predicted by the cDNA and the heterodisperse transcription products suggest a hirudin protein family. The hirudin cDNA was expressed in Esckerichia coli under the control of the bacteriophage X PL promoter. The recombinant product is biologically active, inhibiting the cleavage by thrombin of fibrinogen and a synthetic tripeptide substrate.Leech hirudin is the most potent natural inhibitor of coagulation known (1-4). A very stable noncovalent 1:1 complex is rapidly and specifically formed with a-thrombin, thereby abolishing its ability to cleave fibrinogen (4). To date there is no evidence that it can interact with other components of the human coagulation cascade (5, 6).Hirudin is a polypeptide of 65 amino acids that is stable to extremes of pH and heat (4). It contains six cysteine residues grouped in the NH2-terminal half of the protein, an acidic COOH-terminal half, and one sulfated tyrosine (7). A hirudin form with isoleucine at the NH2 terminus was first purified from leech heads (H. medicinalis) (4, 8) in which activity was found to be concentrated in the salivary glands. Subsequently, Bagdy et al. (9) adopted new purification schemes using whole leeches instead of heads, yielding a form with Val-Val as the first two NH2-terminal amino acids. The amino acid sequence of the "whole body form" has been determined by independent groups (8, 10), and valine residues at positions 1 and 2 have been confirmed. Both forms had a specific activity of around 8000-10,000 antithrombin units/mg. However, more recently, Baskova et al. (11) Hirudin Activity. Antithrombin activity in leech or bacterial extracts was measured in a clotting assay (4) using citrated human platelet-poor plasma as a fibrinogen source or in a colorimetric assay using the thrombin chromogenic substrate Tos-Gly-Pro-Arg-p-nitroanilide (Chromozym TH, Boehringer Mannheim; Tos = tosyl) (7). Standard curves Abbreviation: kb, kilobases. tPresent address:

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