| Here , we report that TFPI binds specifically and saturably to thrombospondin 1 ( TSP 1 ) purified from platelet alpha granules with an apparent K ( D ) of approximately 7 . 5 nm . ^^^ Binding is inhibited by polyclonal antibodies against TFPI and partially inhibited by the B 7 monoclonal anti TSP 1 antibody . ^^^ TFPI bound to immobilized TSP 1 remains an active proteinase inhibitor . ^^^ Additionally , in solution phase assays measuring TFPI inhibition of factor VIIa / tissue factor catalytic activity , the rate of factor Xa generation was decreased 55 % in the presence of TSP 1 compared with TFPI alone . ^^^ Binding experiments done in the presence of heparin and with altered forms of TFPI suggest that the basic C terminal region of TFPI is required for TSP 1 binding . ^^^ |