The inhibitory specificity of human proteinase inhibitor 8 is expanded through the use of multiple reactive site residues.

Serine proteinase inhibitors function as regulators of serine proteinase activity in a variety of physiological processes. Proteinase inhibitor 8 (PI8) is a 45 kDa member of the ovalbumin family of serpins that is an inhibitor of trypsin-like proteinases through the use of Arg339 as the inhibitory P1 amino acid residue in its reactive site loop. In this study, we have described the inhibitory mechanism of recombinant human PI8 towards chymotrypsin. PI8 formed an SDS-stable complex with and inhibited the amidolytic activity of chymotrypsin via a two-step mechanism with an overall equilibrium inhibition constant of 1.7 nM and an overall second-order association rate constant of 1.0 x 10(4) M-1s-1, utilizing Ser341 as the P1 residue. The use of separate reactive site loop residues by PI8 to inhibit distinctly different classes of proteinases not only supports the hypothesis of the existence of the serpin reactive site as a highly mobile and flexible loop, but also suggests an evolved function in which separate amino acid residues can be used to broaden the inhibitory specificity of PI8.