水蛭素是一种 7kDa 大小的酸性蛋白，含有 65 个氨基酸残基，等电点为 3.5-4.0。它未被糖基化，不含色氨酸、精氨酸和甲硫氨酸残基。在 pH 2-12 和高达 80°C 的温度下化学性质稳定。

水蛭素这种抗凝血剂是可溶性和血凝块结合性凝血酶最强效的天然抑制剂。它与凝血酶的结合具有高亲和力，可覆盖其表面积的 20% 以上，并封闭活性位点和外部位点 I（纤维蛋白原和 PAR 识别位点）。水蛭素可阻滞血栓生长及血小板活化。它在人体血流中不发生代谢反应，通过肾脏过滤原样排除。

The anticoagulant, hirudin, is the most potent natural inhibitor of both soluble and clot-bound thrombin. It forms a high-affinity 1:1 complex with thrombin, occluding both the proteolytic site and exosite I (fibrinogen and PAR recognition site). Hirudin blocks thrombus growth and platelet activation, and has been suggested as the clinically preferred anticoagulant (over heparin and citrate) for its specific mode of action and absence of side effects. It is not metabolized in the bloodstream of humans and is eliminated unchanged via kidney filtration.

Protein determined by Lowry.

One unit (ATU) will neutralize one NIH unit of thrombin at 37 °C, based on direct comparison to an NIH thrombin reference standard.