Serpin receptor 1: a hepatic receptor that mediates the clearance of antithrombin III-proteinase complexes.

Antithrombin III (ATIII) clearance from blood occurs by redistribution into the extravascular compartment and by binding to the endothelial surface. When, however, ATIII reacts with a proteinase such as alpha-thrombin, the complex is rapidly cleared from the circulation (half-life is approximately five minutes) by a receptor present on hepatocytes. This receptor binds a number of other serine proteinase inhibitors that are members of the class designated as the "serpins." ATIII, alpha 1-proteinase inhibitor, heparin cofactor II, and alpha 1-antichymotrypsin proteinase complexes bind to the same hepatic receptor, now designated as serpin receptor 1. Proteinase complexes with alpha 2-antiplasmin, another member of the serpin class, do not bind to serpin receptor 1. Recent studies suggest that the specificity of the receptor for serpins may reside in the so-called D helix (nomenclature based on the structure of alpha 1-proteinase inhibitor). The presence of ATIII on the surface of endothelial cells offers a unique mechanism for regulating proteinases formed during coagulation. Since this ATIII is probably associated with heparin-like substances and exists in a high-affinity state, the inhibitor rapidly binds proteinases such as alpha-thrombin. Once the complex forms, its affinity for heparinoids is decreased compared with ATIII, allowing the complex to dissociate from the endothelial surface for rapid clearance by the liver.