Furin is a ubiquitous prototypical mammalian kexin/subtilisin-like endoproteinase that is involved in the proteolytic processing of a variety of proteins in the exocytic and endocytic pathways, with cleavage occurring at the C terminus of the minimal consensus furin recognition sequence Arg-Xaa-Xaa-Arg. In this study, human proteinase inhibitor 8 (PI8), a widely expressed 45-kDa ovalbumin-type serpin that contains two sequences homologous to the minimal sequence for recognition by furin in its reactive site loop, was tested for its ability to inhibit a recombinant soluble form of human furin. PI8 formed an SDS-stable complex with furin and inhibited its amidolytic activity via a two-step mechanism with a kappa assoc of 6.5 x 10(5) M-1 S-1 and an overall Ki of 53.8 pM. Thus, PI8 inhibits furin in a rapid, tight binding manner that is characteristic of physiological serpin-proteinase interactions. PI8 is not only the first human ovalbumin-type serpin to demonstrate inhibitory activity toward furin, but it is also the first significant inhibitor of furin identified that is not a serpin reactive site loop mutant, either naturally occurring or engineered.