Based upon recent evidence that the secreted form of APP can cause the release of cytokines and elicit other biological activities, we sought to identify whether a receptor could be identified on the surface of cells. The secreted amyloid precursor protein containing the Kunitz domain (scAPP751) is identical to protease nexin II, a protease inhibitor which has been shown to form complexes with labeled EGF binding protein that subsequently binds to cells. Results of [125I]scAPP751-trypsin complex incubated with intact fibroblast cells show that the complex appears to bind in a saturable time-dependent and reversible manner. The kinetic constants from the binding studies demonstrate a k1 = 2.5 x 10(7) M-1 s-1 and k2 = 4.7 x 10(-4) s-1 and thus a KD (= k2/k1) = 20 pM. Furthermore, the complex formation of [125I]scAPP751 with a protease appears to be a requirement for optimal binding. The binding affinity of secreted APP demonstrated in this study is consistent with its potency in eliminating a range of biological efforts that have been documented.