The serpins (serine proteinase inhibitors) are a superfamily of proteins (350 -500 amino acids in size) that fold into a conserved structure and employ a unique suicide substrate-like inhibitory mechanism. The serpins were last reviewed in 1994 (1). More recent studies show: 1) an expanded distribution within the kingdoms of metazoa and plantae, as well as certain viruses, 2) a surprising effect on the covalently bound target proteinase, and 3) novel biochemical and biological functions.Most serpins inhibit serine proteinases of the chymotrypsin family. However, cross-class inhibitors have been identified. The viral serpin CrmA and, to a lesser extent, PI9 (SERPINB9) inhibit the cysteine proteinase, caspase 1 (2), and SCCA1

A surprising recent finding is that thyroxine binding globulin (TBG) and cortisol binding globulin (CBG), are members of the serine protease inhibitor (serpin) superfamily. Apparently evolution has completely adapted the serpin structure for its new role in these proteins as a transport agent, as there is no evidence of any retained protease inhibitory activity. This drastic change in function raises the question as to why such a complex molecular framework has been selected for the relatively simple task of hormone transport? To function as inhibitors the serpins have a native stressed (S) conformation that makes them vulnerable to proteolytic cleavage, the cleavage being accompanied by an irreversible transition to a stable relaxed (R) form. We demonstrate here that TBG and CBG have retained the stressed native structure typical of the inhibitor members of the family and we provide evidence that the S-R transition has been adapted to allow altered hormone delivery at inflammatory sites.

The human squamous cell carcinoma antigens (SCCA) 1 and 2 are tandemly arrayed genes that encode two high-molecular-weight serine proteinase inhibitors (serpins). Although these proteins are 92% identical, differences in their reactive site loops suggest that they inhibit different types of proteinases. Our previous studies show that SCCA2 inhibits chymotrypsin-like serine proteinases [Schick et al. (1997) J. Biol. Chem. 272, 1849-1855]. We now show that, unlike SCCA2, SCCA1 lacks inhibitory activity against any of the more common types of serine proteinases but is a potent cross-class inhibitor of the archetypal lysosomal cysteine proteinases cathepsins K, L, and S. Kinetic analysis revealed that SCCA1 interacted with cathepsins K, L, and S at 1:1 stoichiometry and with second-order rate constants >/= 1 x 10(5) M-1 s-1. These rate constants were comparable to those obtained with the prototypical physiological cysteine proteinase inhibitor, cystatin C. Also relative to cystatin C, SCCA1 was a more potent inhibitor of cathepsin K-mediated elastolytic activity by forming longer lived inhibitor-proteinase complexes. The t1/2 of SCCA1-cathepsin S complexes was >1155 min, whereas that of cystatin C-cathepsin complexes was 55 min. Cleavage between the Gly and Ser residues of the reactive site loop and detection of a stable SCCA1-cathepsin S complex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis suggested that the serpin interacted with the cysteine proteinase in a manner similar to that observed for typical serpin-serine proteinase interactions. These data suggest that, contingent upon their reactive site loop sequences, mammalian serpins, in general, utilize their dynamic tertiary structure to trap proteinases from more than one mechanistic class and that SCCA1, in particular, may be involved in a novel inhibitory pathway aimed at regulating a powerful array of lysosomal cysteine proteinases.