Location of the protease-inhibitory region of secretory leukocyte protease inhibitor

Secretory leukocyte Protease inhibitor (SLPI) is a two-domain protein that inhibits a wide range of proteases including chymotrypsin, leukocyte Elastase, and trypsin. Based on its homology to other Protease Inhibitors and on x-ray crystallography of an SLPI-chymotrypsin complex it has been proposed that the Elastase and chymotrypsin-inhibitory site is in the COOH-terminal domain and that the trypsin-inhibitory site is in the NH2-terminal domain. We have prepared muteins of SLPI by site-directed mutagenesis of a synthetic gene for the protein, followed by expression in Escherichia coli. The protease-inhibitory activities of these muteins indicate that leucine 72 in the COOH-terminal domain is at the inhibitory site for Elastase and chymotrypsin. Unexpectedly, our measurements indicate that the trypsin-inhibitory site is not in the NH2-terminal domain. Instead they suggest that leucine 72 is also the inhibitory site for trypsin, even though the amino acid residues at the inhibitory sites of other trypsin inhibitors are almost always either lysine or arginine.