Nitrosative stress results in irreversible inhibition of purified mitochondrial complexes I and III without modification of cofactors

The effects of both nitric oxide (NO) and peroxynitrite on complexes I (NADH dehydrogenase) and III (cytochrome c reductase) isolated from bovine heart have been examined. EPR signals (“g = 2.01”) previously detected in association with loss of complex I and III activities in cultured cells and isolated mitochondria subjected to nitrosative stress are shown not to arise from these particular enzymes. Neither NO nor peroxynitrite (ONO2−) reacts to any appreciable extent with the oxidized forms of flavin mononucleotide, iron-sulfur clusters, or heme moieties found in complexes I and III. However, ONO2− is readily able to abstract electrons from the reduced forms of both complexes I and III, without any apparent modification of the enzyme cofactors. While no attempt was made in the present study to catalog all the possible modifications, it is clear that ONO2− can react with the protein moieties of the enzymes. For example, when added in excess, ONO2− derivatizes a select few tyrosine residues in both complexes I and III forming 3-nitrotyrosine as detected by immunoblots. In the case of complex I, we find a minimum of 3 out of the 46 subunits present were modified (49, ∼18, and ∼15 kDa); whereas in complex III, 4 out of the 13 subunits stained for 3-nitrotyrosine (46, 27, 7, and 6 kDa). Significant irreversible inhibition of activity required the addition of >102-fold excesses of ONO2− to the enzymes. At 103-fold excess of added ONO2−, the activity of complex I was only diminished by ∼18%, while a 60% loss of activity was observed for complex III.