INHIBITION OF SOLUBLE RECOMBINANT FURIN BY HUMAN-PROTEINASE-INHIBITOR-8

Furin is a ubiquitous prototypical mammalian kexin/ subtilisin-like en doproteinase that is involved in the proteolytic processing of a varie ty of proteins in the exocytic and endocytic pathways, with cleavage o ccurring 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 t wo sequences homologous to the minimal sequence for recognition by fur in in its reactive site loop, was tested for its ability to inhibit a recombinant soluble form of human furin. PI8 formed an SDS-stable comp lex with furin and inhibited its amidolytic activity via a two-step me chanism with a k(assoc) of 6.5 x 10(5) M-1 S-1 and an overall K-i of 5 3.8 pm. Thus, PI8 inhibits furin in a rapid, tight binding manner that is characteristic of physiological serpin-proteinase interactions. PI 8 is not only the first human ovalbumin-type serpin to demonstrate inh ibitory activity toward furin, but it is also the first significant in hibitor of furin identified that is not a serpin reactive site loop mu tant, either naturally occurring or engineered.