PBP binds ATP, opioids and phosphatidylethanolamine, exhibiting a lower affinity for phosphatidylinositol and phosphatidylcholine. This serine protease inhibitor inhibits thrombin, neuropsin and chymotrypsin but not trypsin, tissue type plasminogen activator and elastase. PBP contains hippocampal cholinergic neurostimulating peptide (HCNP), which may be involved in the function of the presynaptic cholinergic neurons of the central nervous system. HCNP increases the production of choline acetyltransferase but not acetylcholinesterase.