More specifically, said protein, in the presence of dissolved oxygen, an oxidized glutathione (GSSG)-reduced glutathione (GSH) mixture or a reducing agent such as dithiothreitol (DTT), 2-mercaptoethanol or ascorbic acid, acts on a reduced protein to catalyze a reaction by which natural type disulfide bonds are formed, or acts on an oxidized protein having disulfide bonds, specifically non-natural