The squamous cell carcinoma antigen (SCCA) 1 and its homologous molecule, SCCA2, belong to the ovalbuminserpin family. Although SCCA2 inhibits serine proteinases such as cathepsin G and mast cell chymase, SCCA1 targets cysteine proteinases such as cathepsin S, K, L, and papain. SCCA1 is therefore called a cross-class serpin. The inhibitory mechanism of the standard serpins is well characterized; those use a suicide substrate-like inhibitory mechanism during which an acyl-enzyme intermediate by a covalent bond is formed, and this complex is stable against hydrolysis. However, the inhibitory mechanism of cross-class serpins remains unresolved. In this article, we analyzed the inhibitory mechanism of SCCA1 on a cysteine proteinase, papain. SCCA1 interacted with papain at its reactive site loop, which was then cleaved, as the standard serpins. However, gel-filtration analyses showed that SCCA1 did not form a covalent complex with papain, in contrast to other serpins. Interaction with SCCA1 severely impaired the proteinase activity of papain, probably by inducing conformational change. The decreased, but still existing, proteinase activity of papain was completely inhibited by SCCA1 according to the suicide substrate-like inhibitory mechanism; however, papain recovered its proteinase activity with the compromised level, when all of intact SCCA1 was cleaved. These results suggest that the inhibitory mechanism of SCCA1 is unique among the serpin superfamily in that SCCA1 performs its inhibitory activity in two ways, contributing the suicide substrate-like mechanism without formation of a covalent complex and causing irreversible impairment of the catalytic activity of a proteinase.The serpins (serine proteinase inhibitors) are a superfamily of proteinase inhibitors characterized by a conserved structure and employing a suicide substrate-like inhibitory mechanism (1, 2). The structure of the serpins consists of three  sheets (A-C), nine ␣ helices (A-I), and the reactive site loop (RSL) 1 composed of ϳ17 amino residues (1). The inhibitory mechanism of the serpin is well characterized (2). The exposed RSL of the serpin is recognized by the proteinase, and an initial noncovalent Michaelis encounter complex is formed. Then, in the inhibitory pathway, a "bait" peptide bond (P1-P1Ј) that mimics the normal substrate of the proteinase is attacked by the active serine residue of the proteinase, subsequently forming an acylenzyme intermediate linked by an oxy-ester bond. In the cleaved form, the P side of the RSL inserts into the body of the protein, which dramatically changes the conformations of the serpin and the proteinase, making it impossible for the ester bond to hydrolyze (3). In the non-inhibitory or substrate pathways, the serpin is cleaved by the proteinase just as the substrate of the proteinase after the Michaelis encounter complex is formed. It has been revealed that the serpins are involved in various kinds of biological functions: fibrinolysis, coagulation, inflammation, tumor cell invasion, cellular differentiat...