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Resolution of protein disulphide-isomerase and glutathione-insulin transhydrogenase activities by covalent
Abstract:
1. Protein disulphide-isomerase (EC 5.3.4.1) and glutathione-insulin transhydrogenase (EC 1.8.4.2) were resolved by covalent chromatography. Both activities, in a partially purified preparation from bovine liver, bind covalently as mixed disulphides to activated thiopropyl-Sepharose 6B, in a new stepwise elution procedure protein disulphide-isomerase is displaced in mildly reducing conditions whereas glutathione-insulin transhydrogenase is only displaced by more extreme reducing conditions. 2. This together with evidence for partial resolution of the two activities by ion-exchange chromatography, conclusively establishes that the two activities are not alternative activities of a single bovine liver enzyme. 3. Protein disulphide-isomerase, partially purified by a published procedure, has now been further purified by covalent chromatography and ion-exchange chromatography. The final material is 560-fold purified relative to a bovine liver homogenate; it has barely detectable glutathione-insulin transhydrogenase activity. 4. The purified protein disulphide-isomerase shows a single major band on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis corresponding to a mol.wt. of 57000. 5. The purified protein disulphide-isomerase has Km values for 'scrambled' ribonuclease and dithiothreitol of 23 microgram/ml and 5.4 microM respectively and has a sharp pH optimum at 7.5. The enzyme has a broad thiol-specificity, and several monothiols, at 1mM, can replace dithiothreitol. 6. The purified protein disulphide-isomerase is completely inactivated after incubation with a 2-3 fold molar excess of iodoacetate. The enzyme is also significantly inhibited by low concentrations of Cd2+ ions. These findings strongly suggest the existence of a vicinal dithiol group essential for enzyme activity. 7. When a range of thiols were used as co-substrates for protein disulphide-isomerase activity, the activities were found to co-purify quantitatively, implying the presence of a single protein disulphide-isomerase of broad thiol-specificity. Glutathione-disulphide transhydrogenase activities, assayed with a range of disulphide compounds, did not co-purify quantitatively.