Thrombospondin is a high-molecular weight, heparin-binding glycoprotein constituent of human platelets (1). Ranging from 30-50 µg per 109 platelets, thrombospondin is one of the most abundant proteins in the platelet a-granule (2,3). Thrombospondin was initially termed "thrombin-sensitive protein" based upon its release by thrombin-activated platelets (4,5). Structurally, thrombospondin is a 450,000 molecular weight glycoprotein (1) consisting of three, identical, disulfide-linked polypeptide chains (6-8). The binding of thrombospondin to the surface of both resting and thrombin-activated platelets has been reported (9). A thrombospondin-specific membrane receptor has also been partially characterized (10). Functionally, platelet-derived thrombospondin may play a role in platelet adherence and aggregation (11).

Thrombospondin is not an exclusive product of platelets and megakaryoctes. The synthesis of thrombospondin by endothelial cells (12), fibroblasts (13), monocytes and macrophages (14), and osteoblasts (15) has been reported. Thrombospondin is also an integral component of the basement membrane in a number of different tissues (16). Thrombospondin interacts with a variety of extracellular macromolecules including heparin (1,17), collagen (18), fibrinogen and fibronectin (19), plasminogen (20), plasminogen activator (21), and osteonectin (22). Through the collective efforts of a number of different investigators employing both peptide chemistry and cDNA analytical techniques, distinct heparin, Ca2+-ion, platelet, and protein binding domains within thrombospondin have been identified (23). Based upon its specific interactions with both cells and extracellular matrix components, thrombospondin has been hypothesized to be a member of a class of extracellular proteins which may modulate cell-matrix interactions (24).

Our human thrombospondin (TSP-1) is prepared from the supernatant of activated platelets by heparin-agarose affinity and gel filtration chromatography (25). The purified protein is supplied in 50% vol/vol glycerol/H2O and should be stored at -20°C. Purity is assessed by SDS-PAGE analysis.