The von Willebrand factor-reducing activity of thrombospondin-1 is located in the calcium-binding/C-terminal sequence and requires a free thiol at position 974.

Plasma von Willebrand factor (VWF) is a multimeric protein that mediates adhesion of platelets to sites of vascular injury; however, only the very large VWF multimers are effective in promoting platelet adhesion in flowing blood. The multimeric size of VWF can be controlled by the glycoprotein, thrombospondin-1 (TSP-1), which facilitates reduction of the disulfide bonds that hold VWF multimers together. The TSP family of extracellular glycoproteins consists of 5 members in vertebrates, TSP-1 through TSP-4 and TSP-5/COMP. TSP-1 and TSP-2 are structurally similar trimeric proteins composed of disulfide-linked 150-kDa monomers. Recombinant pieces of TSP-1 and TSP-2 incorporating combinations of domains that span the entire subunit were produced in insect cells and examined for VWF reductase activity. VWF reductase activity was present in the Ca(++)-binding repeats and C-terminal sequence of TSP-1, but not of TSP-2. Alkylation of Cys974 in the C-terminal TSP-1 construct, which is a serine in TSP-2, ablated VWF reductase activity. These results imply that the reductase function of TSP-1 centers around Cys974 in the C-terminal sequence.