Acrosin is the major protease of mammalian spermatozoa. It is a serine protease of trypsin-like cleavage specificity, it is synthesized in a zymogen form, proacrosin and stored in the acrosome. Preferential cleavage: Arg-|-Xaa, Lys-|-Xaa. Inhibited by SERPINA5. Heavy chain (catalytic) and a light chain linked by two disulfide bonds. Forms a heterodimer with SERPINA5 (By similarity). X's in position 19 to 22 were introduced by homology with the pig sequence. Belongs to the peptidase S1 family.