PAI-1 NBD-P9 mutant is created via mutagenesis of the P-9 serine residue (Ser338) on the reactive center loop to cysteine. This promoted the availability of a free thiol group for incorporation of NBD, a fluorescent probe highly sensitive to changes in dissolution and hydrophobic environment. The fluorescence emission of PAI-1 NBD-P9 is enhanced 6-7 fold upon insertion of the reactive center loop into beta-sheet A following complex formation with proteases, formation of the latent species, or cleavage by elastase. This modified PAI-1 is nearly as active as wild type PAI-1 and is more resistant to the spontaneous latency reaction making it an excellent tool for monitoring reaction rates of PAI-1. PAI-1 NBD-P9 has been utilized in a number of studies to determine the rates of loop insertion and SERPIN reaction mechanisms when reacted with various proteinases, inactivating antibodies and conformational changes imposed by the binding of vitronectin.