PDI assists in redox protein folding, involving oxidation, multiple intramolecular thiol-disulphide exchanges, and isomerization (reduction) activities and it is highly specific in its interaction with different substrates. Since most cellular compartments are reducing environments, protein disulphide bonds are usually unstable in the cytosol, although there are exceptions ( Frand et al., 2000). Disulphide bonds play an important role in the folding and stability of proteins and they are present in more than 30% of all human proteins that traverse the secretory pathway ( Fewell et al., 2001). It is also responsible for the isomerization, formation, and rearrangement of protein disulphide bonds, thereby providing another mechanism by which native protein conformation is maintained. PDI is induced during endoplasmic reticulum (ER) stress ( Wilkinson and Gilbert, 2004) and it serves as a vital cellular defense against general protein misfolding via its chaperone activity. Protein disulphide isomerase (PDI) was the first folding catalyst isolated from rat liver ( Goldberger et al., 1963) and it is found abundantly in many tissues, accounting for 0.8% of total cellular protein ( Freedman et al., 1994).
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