INHIBITION OF HUMAN SERINE PROTEASES BY SPAAT, THE C-TERMINAL 44-RESIDUE PEPTIDE FROM ALPHA(1)-ANTITRYPSIN

SPAAT has previously been shown to be a competitive inhibitor of the m odel 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 mu M, respectively. The mechani sm of this inhibition was investigated by comparing the relative effec tiveness of the 23-residue N-terminal fragment of SPAAT (N-SPAAT) to i nhibit 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 mu M versus 7.5 mu M). In contrast, this fragment, which conta ins the primary human neutrophil elastase cleavage site, was found to competitively inhibit human neutrophil elastase with a K-i of 24 mu M which was comparable to that of SPAAT (K-i = 15-20 mu M). Thus it appe ars 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 ca scade of decreasingly potent, but increasingly specific SPAAT-related inhibitory peptides could be generated. These results further substant iate the view that SPAAT may play a role in vivo in the protection of extracellular proteins from inappropriate attack by proteases which ar e elevated during various pathophysiological conditions. (C) 1997 Else vier Science B.V.