Expression of an active proteinase inhibitor, α₁-antichymotrypsin, by human breast epithelial cells

The synthesis of an active proteinase inhibitor, gp 66, by human breast epithelial cells is reported. This glycoprotein is identical to serum α₁-antichymotrypsin, which inhibits proteinases that cleave at hydrophobic residues. Immunohistological studies show the in vivo expression on normal secretory and ductal epithelial cells and on primary and metastatic adenocarcinomas. Immunoaffinity-purified gp 66 from MCF-7 culture supernatants is an active inhibitor of chymotrypsin as determined in a fluorogenic enzyme assay and can form stable 88 kDa enzyme-inhibitor complexes. The synthesis of a functional inhibitor may represent the epithelial cell's attempt to stabilize its extracellular milieu.