Serine protease inhibitor which exhibits anti-trypsin activity (PubMed:4054311). In the pancreas, protects against trypsin-catalyzed premature activation of zymogens (By similarity). In the male reproductive tract, binds to sperm heads where it modulates sperm capacitance by inhibiting calcium uptake and nitrogen oxide (NO) production. This protein is unusual, with respect to the other mammalian SPINK1 proteins, in having an extra N-terminal residue.