SQUAMOUS-CELL CARCINOMA ANTIGEN-2 IS A NOVEL SERPIN THAT INHIBITS THECHYMOTRYPSIN-LIKE PROTEINASES CATHEPSIN-G AND MAST-CELL CHYMASE

The squamous cell carcinoma antigen (SCCA) serves as a serological mar ker for more advanced squamous cell tumors. Molecular cloning of the S CCA genomic region revealed the presence of two tandemly arrayed genes , SCCA1 and SCCA2. Analysis of the primary amino acid sequences shows that both genes are members of the high molecular weight serpin superf amily of serine proteinase inhibitors. Although SCCA1 and SCCA2 are ne arly identical in primary structure, the reactive site loop of each in hibitor suggests that they may differ in their specificity for target proteinases. SCCA1 has been shown to be effective against papain-like cysteine proteinases. The purpose of this study was to determine wheth er SCCA2 inhibited a different family of proteolytic enzymes. Using re combinant DNA techniques, we prepared a fusion protein of glutathione S-transferase and full-length SCCA2. The recombinant SCCA2 was most ef fective against two chymotrypsin-like proteinases from inflammatory ce lls, but was ineffective against papain-like cysteine proteinases. Ser pin-like inhibition was observed for both human neutrophil cathepsin G and human mast cell chymase. The second order rate constants for thes e associations were on the order of similar to 1 x 10(5) M(-1) s(-1) a nd similar to 3 x 10(4) M(-1) s(-1) for cathepsin G and mast cell chym ase, respectively. Moreover, SCCA2 formed SDS stable complexes with th ese proteinases at a stoichiometry of near 1:1. These data showed that SCCA2 is a novel inhibitor of two physiologically important chymotryp sin-like serine proteinases.