Reversible interconversion between sulfo and desulfo xanthine oxidase in a system containing rhodanese, thiosulfate,...

The desulfo form of milk xanthine oxidase (xanthine: oxygen oxidoreductase, EC 1.2.3.2) was reactivated by incubation with rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1), thiosulfate, and sulfhydryl reagent; 50% of full activity was recovered. No further reactivation occurred with additional incubation. It was also found that native enzyme in the sulfo form with full activity was inactivated by incubation with the same system, down to half of full activity and no further inactivation occurred. After these incubations the enzyme was found to be a mixture of functional and nonfunctional enzymes based on spectral changes with xanthine, on [14C]oxipurinol equilibration, and on steady-state kinetics. The 35S of [35S]thiosulfate was incorporated into desulfo xanthine oxidase in parallel with an increase in catalytic activity. Most of the 35S was cyanolysable but was protected from cyanolysis by pretreatment with allopurinol. The 35S was released from 35S-labeled reconstituted xanthine oxidase upon incubation with the rhodanese system containing unlabeled thiosulfate. However, catalytic activity remained unchanged, indicating that the sulfur atom was exchanged during the incubation.