OPN cleavage by thrombin, plasmin, and cathepsin D. Western blot analysis using a polyclonal OPN antibody. Lane 1, molecular weight standards. Lane 2, full-length OPN purified from milk (fraction II in Fig. 1B). Lane 3, N-terminal OPN fragments purified from human milk (fraction I in Fig. 1B). Lane 4–6, full-length OPN digested with thrombin (lane 4), plasmin (lane 5), and cathepsin D (lane 6).