May play a role in the regulation of hemostasis. The soluble form may have inhibitory properties towards coagulation factors. May interact with cellular G-protein signaling pathways. May bind to the DNA 5'-GTCACATG-3'(CDEI box). Inhibits trypsin, chymotrypsin, plasmin, factor XIA and plasma and glandular kallikrein. Modulates the Cu/Zn nitric oxide-catalyzed autodegradation of GPC1 heparan sulfate side chains in fibroblasts (By similarity). Interacts with CPEB1. Interacts (via NPXY motif) with DAB2 (via PID domain); the interaction is impaired by tyrosine phosphorylation of the NPXY motif (By similarity). Interacts (via cytoplasmic domain) with APBB2/FE65L (PubMed:8855266). Interacts (via intracellular domain) with APBB3/FE65L2 (PubMed:10081969). Additional isoforms seem to exist. Expressed in placenta, brain, heart, lung, liver, kidney and endothelial tissues. The BPTI/Kunitz inhibitor domain is O-glycosylated. Belongs to the APP family.