Serine protease inhibitor which exhibits anti-trypsin activity (PubMed:7142173). 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 is the function of SPINK1 (serine peptidase inhibitor Kazal type 1, Ensembl gene identifier ENSG00000164266).