Thiol-protein disulphide oxidoreductases. Differences between protein disulphide-isomerase and glutathione-insulin transhydrogenase activities in ox liver

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Abstract
Protein disulfide-isomerase [EC 5.3.4.1] and glutathione-insulin transhydrogenase activities were assayed in parallel through a conventional purification of protein disulfide-isomerase from ox liver. Throughout a series of purification steps (differential centrifugation, acetone extraction, (NH4)2SO4 precipitation and ion-exchange chromatography), the 2 activities appeared in the same fractions but were purified to different extents. The final sample was 143-fold purified in protein disulfide-isomerase but only 10-fold purified in glutathione-insulin transhydrogenase; nevertheless the 2 activities in this preparation were not resolved by high-resolution isoelectric focusing and both showed pI 4.65. In a partially purified preparation containing both activities, glutathione-insulin transhydrogenase was far more sensitive to heat denaturation than was protein disulfide-isomerase; conversely protein disulfide-isomerase was more sensitive to inactivation by deoxycholate. The data are inconsistent with a single enzyme being responsible for all the protein disulfide-isomerase and glutathione-insulin transhydrogenase activity of ox liver. Several similar thiol-protein disulfide oxidoreductases of overlapping specificities may better account for the data.

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