Antithrombin III, the major modulator of intravascular coagulation, is synthesized by human endothelial cells.

Human endothelial cells in culture is shown to synthesize antithrombin III (At-III). The endothelial cell At-III (EC-At-III) consists of a small fraction similar to plasma At-III and a larger fraction with decreased heparin-binding as tested by crossed immunoelectrophoresis. However, both the anti-Xa and thrombin-neutralizing activities of the EC-At-III were rapid and active even in the absence of added heparin. It is concluded that the major portion was probably bound to endogenous heparin-like substance, thus accounting for its decreased exogenous heparin binding. The presence of At-III and other antithrombotic factors in the vascular endothelium offer protection against thrombosis and possibly atherosclerosis.