Squamous cell carcinoma antigen is a potent inhibitor of cysteine proteinase cathepsin L

Abstract A squamous cell carcinoma antigen (SSCA), which is a member of the serpin family of proteinase inhibitors, was purified from sera of cancer patients. It did not inhibit serine proteinases. However, it non-competitively inhibited human cathepsin L with a K i of 0.064 nM, but not cathepsins B and H among cysteine proteinases. These results indicated that SCCA is a non-functional serpin that inhibits cathepsin L in cancer cells.