Protein disulfide-isomerase (PDI) was studied for the first time as a stimulator of refolding of recombinant proteins produced in bacteria. Experiments with reduced or scrambled ribonuclease as a substrate showedthat PDI was active in low concentrations of the protein denaturants which were generally used in the refolding systems of recombinant proteins. Crude human pro-urokinase cloned and expressed in Escherichia coli was tested as an example of a recombinant protein, and PDI accelerated its refolding. The time needed to reach 50%maximumpro-urokinase refolding was shortened by half by the addition ofPDI. Reduced pro-urokinase was further stimulated to refold by this enzyme. Evidence that PDI catalyzes the refolding of pro-urokinase through thiol-disulfide
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