Inhibition of human serine proteases by SPAAT, the C-terminal 44-residue peptide from α1-antitrypsin

Abstract SPAAT has previously been shown to be a competitive inhibitor of the model serine protease, chymotrypsin. We now present evidence that SPAAT is likewise a competitive inhibitor of human neutrophil elastase and cathepsin G with K i 's of 15–20 and 40 μ M, respectively. The mechanism of this inhibition was investigated by comparing the relative effectiveness of the 23-residue N-terminal fragment of SPAAT (N-SPAAT) to inhibit chymotrypsin and human neutrophil elastase. N-SPAAT, which does not contain the primary chymotrypsin cleavage site, was approximately 10-fold less effective as an inhibitor of chymotrypsin than SPAAT ( K i of 65 μ M versus 7.5 μ M). In contrast, this fragment, which contains the primary human neutrophil elastase cleavage site, was found to competitively inhibit human neutrophil elastase with a K i of 24 μ M which was comparable to that of SPAAT ( K i =15-20 μ M). Thus it appears that SPAAT is a reversible inhibitor of these enzymes rather than an irreversible, stoichiometric one like its parent protein, AAT. Such fragmentation of AAT, however, might provide a mechanism whereby a cascade of decreasingly potent, but increasingly specific SPAAT-related inhibitory peptides could be generated. These results further substantiate the view that SPAAT may play a role in vivo in the protection of extracellular proteins from inappropriate attack by proteases which are elevated during various pathophysiological conditions.