Mast cell protease inhibitor, trypstatin, is a fragment of inter-alpha-trypsin inhibitor light chain.

The mast cell protease inhibitor trypstatin was purified from rat peritoneal mast cells (Kido, H., Yokogoshi, Y., and Katunuma, N. (1988) J. Biol. Chem. 263, 18104-18107). We noticed that trypstatin could possibly be identical with the second half of inter-alpha-trypsin inhibitor light chain (ITI-LC), because only 5 amino acid residues of trypstatin are different from the sequence deduced from the recently reported rat ITI-LC mRNA. Southern blot analysis revealed that a 170-base pair probe corresponding to the second half of rat ITI-LC hybridized to the digested rat genomic DNAs only at single bands even in low stringency conditions. Similarly, a 170-base pair probe corresponding to the human counterpart, which differs by 10 deduced amino acids from the rat probe, also hybridized to the digested rat genomic DNAs only at single bands at the same positions and same conditions. These results suggest that rat trypstatin is genetically identical with rat ITI-LC. ITI-LC/trypstatin mRNA was not detected in rat peritoneal mast cells by RNA blot nor by reverse transcription-polymerase chain reaction analysis. Since trypstatin was purified from rat peritoneal mast cells and the cells were immunohistochemically positive with anti ITI-LC antibody, ITI-LC/trypstatin may be taken up into mast cell granules from the serum but may not be generated by mast cells themselves.