**3.1 Free radical cause peroxynitrites**

The combined effects of peroxynitrite and sulfur-containing amino acids have been implicated in Parkinson's disease by performing the synergistic toxicity to a neuronal cell line [46]. The protein nitration in the form of nitrotyrosine residues is by the actions of peroxynitrite. Increased concentrations of both protein carbonyls and 3-nitrotyrosine have been reported in various neurodegenerative diseases like Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) [47]. The decreased activity of antioxidant enzymes with the simultaneous increase in the concentrations of peroxynitrites leads to disturbances of oxidative phosphorylation was observed in the brain mitochondria [48]. The modification of the monomers, destabilize microtubules, and major alterations in neurodegenerative diseases is performed by peroxynitrites [49]. The effects of S-nitrosylation on neuronal excitation are due to stimulation of ionotropic glutamate receptors and toxic Aβ peptides in Alzheimer's disease [50].
