**4. Degradation Products under Oxidative Stress**

#### *4.1. Proteins*

Protein carbonyls are degradation products of reactions between reactive species and proteins, resulting in a loss of function or aggregation. Quantification with 2,4-Dinitrophenylhydrazine products showed increased carbonylation in plasma and serum of RRMS patients [65–67]. The plasma carbonyl levels were elevated in SPMS and correlated with the EDSS and the Beck Depression Inventory [208]. The levels of carbonyl groups were elevated in serum of patients with RRMS and lowered in the group of RRMS patients treated with INF-β [74]. The levels of CSF carbonyl proteins measured were elevated in RRMS and progressive MS [209,210] (Tables 3 and 4).

A highly active RNS reacts with tyrosine residues of proteins to form nitrotyrosines, leading to the alternation of protein conformation function. The main product of tyrosine oxidation is 3-nitrotyrosine (3-NO-Tyr), formed by the substitution of a hydrogen by a nitro group in the phenolic ring of the tyrosine residues. The content of 3-NO-Tyr is assessed by western blotting, high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC/MS), and enzyme-linked immunosorbent assay (ELISA) [72]. Mean 3-NO-Tyr was observed significantly higher in plasma and serum of RRMS and SPMS patients and significantly higher 3-NO-Tyr was found in SPMS than RRMS [66,68,69]. Decreased mean 3-NO-Tyr was reported following a relapse and corticosteroid treatment [70]. In the serum of MS patients, 3-NO-Tyr was found significantly lower following INF-β1b treatment [71]. Following GA treatment, 3-NO-Tyr was found significantly reduced in peripheral leukocytes [69] (Tables 3 and 4).

Protein glutathionylation is a redox-dependent posttranslational modification that results in the formation of a mixed disulfide between GSH and the thiol group of a protein cysteine residue [211]. Protein glutathionylation is observed in response to oxidative or nitrosative stress and is redox-dependent, being readily reversible under reducing conditions. Extracellular SOD, α1-antitrypsin and phospholipid transfer protein were found glutathionylated at cysteine residues in CSF of MS Patients, witnessing the footprints of oxidative assault of MS [210].

Oxidative environments generate oxidized tyrosine orthologues such as o-tyrosine, m-tyrosine, nitrotyrosine, and dityrosine. Dityrosine was elevated in serum of RRMS patients [73,74]. Advanced oxidation protein products (AOPPs) are uremic toxins produced in reaction of plasma proteins with

chlorinated oxidants such as chloramines and hypochlorous acid (HClO) [212]. The levels of AOPPs were significantly higher in plasma of MM patients [213]. The levels of AOPPs were significantly higher in plasma and CSF of CIS and RRMS patients than healthy controls, and the AOPPs levels were significantly higher CIS than RRMS. Furthermore, the levels of AOPPs were significantly higher in patients with higher EDSS scores than lower ones [50]. The AOPPs levels were decreased in serum of RRMS patients treated with IFN-β [74] (Tables 3 and 4).

AGEs are a group of glycotoxins produced in reaction of free amino groups of proteins, lipids, or nucleic acids and carbonyl groups of reducing sugars. The AGEs can accumulate in tissues and body fluids, resulting in protein malfunctions, reactive chemical production, and inflammation [214]. The levels of AGEs were significantly elevated in serum of RRMS patients, but no significant change was observed after IFN-β treatment [74]. The concentrations of AGEs were significantly higher in brain samples of MS patients, compared to nondemented counterparts. The levels of free AGEs were correlated in CSF and plasma samples of MS patients, but not protein-bound AGEs [75] (Tables 3 and 4).
