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Squamous cell carcinoma (SCC) antigen was tested, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, for its ability to inhibit the activity of serine proteases, i.e., trypsin, chymotrypsin and elastase. We demonstrated that the serine protease inhibitor (serpin) of SCC antigen is specific for chymotrypsin. Preincubation of chymotrypsin with recombinant SCC antigen inhibited chymotryptic digestion of gelatin and ovalbumin through the formation of sodium dodecyl sulfate-stable complexes. These findings promote understanding of the biological functions of SCC antigen as serpin in the stratification of the normal squamous cells and in the malignant behavior of the tumor cells.

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We investigated the "cross-class" interaction between cysteine proteinases and a novel inhibitory serpin, recombinant squamous cell carcinoma (rSCC) antigen-1, which inhibits a serine proteinase, chymotrypsin. rSCC antigen-1 inhibited the cysteine proteinases, papain, papaya proteinase IV and cathepsin L. Interestingly, although rSCC antigen-1 formed sodium dodecyl sulfate (SDS)- and heat-stable complexes with chymotrypsin, rSCC antigen-1 gave the 40 kDa fragment and small molecular mass peptide by incubation with papain without forming an SDS- and heat-stable complex. The cleavage was observed between the Gly353-Ser354 bond, indicating that rSCC antigen-1 interacts with cysteine proteinases not at the predicted reactive site P1-P1' portion (Ser354-Ser355), but at the Gly353-Ser354 of the P2-P1 portion. These findings promote understanding of the "suicide inhibition" mechanism of SCC antigen-1 against cysteine proteinases.