Nitration of specific tyrosines in F_oF₁ ATP synthase and activity loss in aging

It has been reported that C-nitration of proteins occurs under nitrative/oxidative stress; however, its role in pathophysiological situations is not fully understood. In this study, we determined that nitration of Tyr ³⁴⁵ and Tyr³⁶⁸ in the β-subunit of the mitochondrial F_oF₁-ATPase is a major target for nitrative stress in rat liver under in vivo conditions. The chemical characteristics of these Tyr make them suitable for a facilitated nitration (solvent accessibility, consensus sequence, and pK_a). Moreover, β-subunit nitration increased significantly with the age of the rats (from 4 to 80 weeks old) and correlated with decreased ATP hydrolysis and synthesis rates. Although its affinity for ATP binding was unchanged, maximal ATPase activity decreased between young and old rats by a factor of two. These changes directly impacted the available ATP concentration in vivo, and it was expected that they would affect multiple cellular ATP-dependent processes. For instance, at least 50% of available [ATP] in the liver of older rats would have to be committed to sustain maximal Na ⁺-K⁺-ATPase activity, whereas only 30% would be required for young rats. If this requirement was not fulfilled, the osmoregulation and Na⁺-nutrient cotransport in liver of older rats would be compromised. On the basis of our studies, we propose that targeted nitration of the β-subunit is an early marker for nitrative stress and aging.