Appearance of nitrite reducing activity of cytochrome c upon heat denaturation

The appearance of NO2- reducing activity of cytochrome c (Cyt c) upon heat denaturation was investigated with equine heart Cyt c. Denatured equine heart Cyt c (dCyt c), which was treated at 100 degrees C for 30 min, had NO2- reducing activity in the presence of dithionite and methylviologen in an aqueous solution under anaerobic conditions. In contrast, hemoglobin and myoglobin had no such activity under the same conditions. Using spectroscopic methods, we found that the appearance of this activity in the Cyt c was due to the following intramolecular changes: unfolding of the peptide chain, exposure of the heme, dissociation of the sixth ligand methionine sulfur, and appearance of autoxidizability. The dCyt c catalyzed NO2- reduction to NH4+ via ferrous-NO complexes, and this reaction was a 6-electron and 8-proton reduction. Sepharose-immobilized dCyt c had activity similar strength to that in solution. The resin retained the activity after five uses and even after storage for 1 year. On the basis of these results, we concluded that Cyt c acquired a new catalytic activity upon heat treatment, unlike to other familiar biological molecules.