title: A direct, continuous, sensitive assay for protein disulphide-isomerase based on fluorescence self-quenching.
abstract: PDI (protein disulphide-isomerase) activity is generally monitored by insulin turbidity assay or scrambled RNase assay, both of which are performed by UV-visible spectroscopy. In this paper, we present a sensitive fluorimetric assay for continuous determination of disulphide reduction activity of PDI. This assay utilizes the pseudo-substrate diabz-GSSG [where diabz stands for di-(o-aminobenzoyl)], which is formed by the reaction of isatoic anhydride with the two free N-terminal amino groups of GSSG. The proximity of two benzoyl groups leads to quenching of the diabz-GSSG fluorescence by approx. 50% in comparison with its non-disulphide-linked form, abz-GSH (where abz stands for o-aminobenzoyl). Therefore the PDI-dependent disulphide reduction can be monitored by the increase in fluorescence accompanying the loss of proximity-quenching upon conversion of diabz-GSSG into abz-GSH. The apparent K(m) of PDI for diabz-GSSG was estimated to be approx. 15 muM. Unlike the insulin turbidity assay and scrambled RNase assay, the diabz-GSSG-based assay was shown to be effective in determining a single turnover of enzyme in the absence of reducing agents with no appreciable blank rates. The assay is simple to perform and very sensitive, with an estimated detection limit of approx. 2.5 nM PDI, enabling its use for the determination of platelet surface PDI activity in crude sample preparations.
authors.affiliation: Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Ave., Windsor, ON, Canada N9B 3P4.
authors.forename: Arun
authors.initials: A
authors.lastname: Raturi
authors.forename: Panayiotis O
authors.initials: PO
authors.lastname: Vacratsis
authors.forename: Dana
authors.initials: D
authors.lastname: Seslija
authors.forename: Lana
authors.initials: L
authors.lastname: Lee
authors.forename: Bulent
authors.initials: B
authors.lastname: Mutus
mesh_terms: D001792:Blood Platelets / Q000201:enzymology; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism; D005453:Fluorescence; D005978:Glutathione / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D019803:Glutathione Disulfide / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008565:Membrane Proteins / Q000032:analysis / Q000378:metabolism; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011485:Protein Binding; D019704:Protein Disulfide-Isomerases / Q000032:analysis* / Q000378:metabolism; D012680:Sensitivity and Specificity; D013379:Substrate Specificity
chemical_list: D004220:Disulfides; D008565:Membrane Proteins; D019704:Protein Disulfide-Isomerases; D005978:Glutathione; D019803:Glutathione Disulfide