At the indicated times, reactions were quenched for 30 min at room temperature with an equal amount of SDSPAGE sample buffer containing either 10 mM AMS (Invitrogen) or 50 mM NEM. prevalent in the ER. The stable regulatory disulphides were only partially reduced by PDI (E180 mV), suggesting either that this is a mechanism for preventing excessive Ero1 activity and oxidation of PDI or that additional factors are required for Ero1 activation within the mammalian ER. Keywords:disulphide formation, Ero1, protein disulphide isomerase, reduction potential == Introduction == The endoplasmic reticulum (ER) of mammalian cells contains a number of enzymes, the role of which is to ensure efficient formation of native disulphide bonds in proteins entering the secretory pathway. Central to the formation of disulphides is the initial oxidation of cysteine residues catalysed by protein disulphide isomerase (PDI) (Freedman, 1995). PDI is one of a large family of homologous oxidoreductases present within the ER (Ellgaard and Ruddock, 2005). Oxidation by PDI results in the introduction of a disulphide into the substrate protein with the concomitant reduction of a disulphide within one of the active sites in PDI (Kulpet al, 2006). A second enzyme, Ero1 or Ero1 in mammalian cells (Cabibboet al, 2000;Paganiet al, 2000), is thought to catalyse the reoxidation of the PDI active site (Sevier and Kaiser, 2002). The Ero1 enzymes are flavoproteins that can couple the introduction of a disulphide into PDI with the reduction of oxygen to liberate hydrogen peroxide (Grosset al, 2006). Hence, for every disulphide introduced into a substrate protein one molecule of hydrogen peroxide theoretically can be produced by Ero1, providing a potential source of reactive oxygen species (ROS) and ER-generated oxidative stress (Chakravarthiet al, 2006). To prevent the deleterious accumulation of ROS in the ER lumen, the enzymatic activity of Ero1 needs to be tightly regulated. Ero1 and Ero1 levels are regulated by hypoxia (Gesset al, 2003) and the unfolded protein response (UPR) (Paganiet al, 2000), respectively, Jervine providing a mechanism for responding to changing conditions within the ER. Yeast Ero1 (Ero1p) is also regulated by the UPR (Frand and Kaiser, 1998;Pollardet al, 1998) but, in addition, is regulated post-translationally by the formation of non-catalytic disulphides that need to be reduced to activate the enzyme (Sevieret al, 2007). Jervine Jervine Such a mechanism allows the enzyme to be activated during conditions when the ER is more reducing but inactivated when oxidising conditions prevail. It is highly likely that a similar system of regulation exists with the mammalian Ero1 proteins; however, the cysteine residues outside the active site are not conserved between the Jervine yeast and human Ero1s (Sevier and Kaiser, 2008) making it difficult to predict which cysteines form the regulatory disulphides. In addition Jervine to enzymes catalysing the oxidation of disulphide bonds, the ER lumen also contains enzymes that catalyse the reduction or isomerisation of non-native disulphides (Jansenset al, 2002). PDI is able to catalyse both the formation and reduction of disulphides (Freedman, 1995), whereas other members of the PDI family of oxidoreductases such as ERp57 may exert an effect primarily to resolve non-native disulphides (Jessopet al, 2007). To catalyse these reactions, the active site disulphides within these enzymes need to be reduced. Indeed, most of the PDI family members, apart from PDI, which is partially oxidised, are reduced at steady state (Jessop and Bulleid, 2004). Glutathione (GSH) is Rabbit Polyclonal to GPR34 required to maintain such a reduced state, depletion of GSH does not prevent disulphide formation but can lead to a defect in folding due to a lack of resolution of non-native disulphides (Chakravarthiet al, 2006). Hence, the ratio of reduced to oxidised GSH (GSH/GSSG) must be sufficiently reducing to maintain the ER oxidoreductases in a reduced state, while allowing correct disulphide formation in substrate proteins. Ero1p can oxidise PDI even at quite high GSH/GSSG ratios (Tuet al, 2000) potentially explaining how both oxidation and reduction reactions can occur within the same intracellular organelle. Interestingly, the phenotype of the temperature-sensitiveero1-1mutant can be suppressed by perturbing the biosynthesis of GSH, suggesting that the presence of this low molecular weight thiol increases the load on Ero1p (Cuozzo and Kaiser, 1999). Although GSH is a poor substrate for Ero1p, it can reduce oxidised PDI (Gilbert, 1989;Darby and Creighton, 1995), hence Ero1 could oxidise GSH indirectly through the oxidation of PDI or other PDI family members. In this study, we set out to identify the regulatory disulphides within human Ero1 and to evaluate the ability of GSH and PDI to activate the enzyme. To be able to characterise Ero1, we purified full-length recombinant protein expressed inEscherichia coliand characterised its activity towards thioredoxin and PDI. We found that although thioredoxin was an excellent substrate for the enzyme, PDI required the presence of GSH for sustained activity. In addition, the regulatory disulphides were shown to have a midpoint reduction.