Protein disulfide isomerase or PDI (EC 5.3.4.1) is an enzyme in the endoplasmic reticulum in eukaryotes or periplasmic space of prokaryotes that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold. This allows proteins to quickly find the correct arrangement of disulfide bonds in their fully folded state and therefore the enzyme acts to catalyze protein folding. Conversely, reduced (dithiol) form of PDI is able to catalyse a reduction of mispaired thiol residues of a particular substrate, acting as an isomerase. Therefore, PDI is capable of catalyzing the post-translational disulfide exchange. Such exchange reactions can occur intramolecularly, leading to the rearrangement of disulphide bonds in a single protein. The Entrez logo The Entrez Global Query Cross-Database Search System allows access to databases at the National Center for Biotechnology Information (NCBI) website. ... 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Another major function of PDI relates to its activity as a chaperone, i.e. it aids wrongly folded proteins to reach a correctly folded state without the aid of enzymatic disulfide shuffling.

PDI has been found to be involved in the breaking of bonds on the HIV gp120 protein during HIV infection of CD4 positive cells and is required for HIV infection of lymphocytes and monoctyes. Some studies have shown it to be avialable for HIV infection on the surface of the cell clustered around the CD4 protein yet conflicting studies have shown that it is not available on the cell surface but instead is found in significant amounts in the blood plasma.

Other functions

PDI helps load antigenic peptides into MHC class I molecules. These molecules (MHC I) are related to the peptide presentation by APC in the immunity response.

Assays used for PDI activity

Insulin Turbidity Assay: PDI breaks the two disulfide bonds between two insulin (a and b) chains that results in precipitation of b chain. This precipitation can be monitored at 620 nm which is indirectly used monitor PDI activity (1). Sensitivity of this assay is in micromolar range.

ScRNase assay: PDI converts scrambled (inactive) RNase into native (active) RNase that further acts on its substrate (2). The sensitivty is in micromolar range.

Di-E-GSSG assay: This is the flurometric assay that can detect picomolar quantities of PDI and therefore is the most sensitive assay to date for detecting PDI activity (3). Di-E-GSSG has two eosin molecules attached to oxidized glutathione (GSSG). The proximity of eosin molecules leads to the quenching of its fluorescence. However, upon breakage of disulfide bond by PDI, fluorescence increases 70-fold.

1)Lundstrom J., Holmgren A. Protein disulfide-isomerase is a substrate for thioredoxin reductase and has thioredoxin-like activity. J. Biol. Chem. 1990;265:9114–9120.

2)Lyles M. M., Gilbert H. F. Catalysis of the oxidative folding of ribonuclease A by protein disulfide isomerase: dependence of the rate on the composition of the redox buffer. J. Biol. Chem. 1991;30:613–619

3)Raturi A., Mutus B. Characterization of redox state and reductase activity of protein disulfide isomerase under different redox environments using a sensitive fluorescent assay Free Radic Biol Med. 2007 ;43(1):62-70.