Cysteine, with peroxidizing lipid

Some amino acids may be oxidized by reacting with free radicals formed by lipid oxidation. Methionine can react with a lipid peroxide to yield methionine sulfoxide. Cysteine can be decomposed by a lipid free radical according to the following scheme ... 

Despite the conclusions in the cited literature about direct MT interaction with free radicals, the mechanism of MT antioxidant activity remains obscure. Markant and Pallauf [339] concluded that cysteine groups and not zinc are responsible for the inhibition of lipid peroxidation in hepatocytes. Maret and Vallee [340,341] also questioned the possibility of direct scavenging of free radicals by MT and suggested that zinc release is a major mechanism of antioxidant effects of metallothioneins. 

Reported rate constants for the reaction of 02 with GSH have varied from 102 to > 105 M 1 s. A re-examination of this reaction by spin trapping with DMPO established that earlier studies had been confounded by the direct reduction of the DMPO/ OOH adduct to DMPO/ OH by GSH. Taking account of this reaction, the revised rate constant was reported to be 200 M-1 g-i.25i.2S2 other workers have examined, for example, the effects of GSH and N-acetyl-L-cysteine on lipid peroxidation 253 and the role of GS in the toxicity of the diabetogenic agent alloxan.254 Direct EPR has been used to detect binuclear Cu(II) complexes of homocysteine. The interactions of such complexes with blood-vessel linings may account for the link between elevated homocysteine and atherosclerosis.255... 

Peroxynitrite is a nonspecific oxidant that reacts with all classes of biomolecules depleting low-molecular-weight antioxidants, initiating lipid peroxidation, damaging nucleic acids and proteins. Its reactions are much slower than those of the hydroxyl radical but are faster than those of hydrogen peroxide. Comparison of peroxynitrite reactivity with various amino acid residues of human serum albumin have shown that cysteine, methionine, and tryptophan are the most reactive... 

Protein cross-links may be also produced in reaction of 4-hydroxynonenal with lysine, histidine, serine, and cysteine residues, primarily via Michael addition (J5, R7, U8). These reactions occur spontaneously, but also may be catalyzed by certain glutatione 5-transferases. The glutathione transferase A4-4, which unlike other alpha-class glutathione transferases, shows high catalytic activity toward lipid peroxidation products such as 4-hydroxynon-2-enal, is the key enzyme for these reactions (B31). Products of protein coupling with aldehydes secondary to lipid peroxidation have a specific fluorescence, which can herald the protein oxidative modification process (CIO). 

In experiments, SAMe was found to prevent lipid peroxidation and to normalize the reduced glycogen content of hepatocytes in fiver damage. In further studies, its cytoprotective effect was also confirmed. (163, 166—171, 173, 177—181) This protective effect likewise applied to preneoplastic cell damage. The results concerning the prevention of cholestasis were impressive. In cirrhosis or severe fiver disease, there is a reduction in SAMe synthetase, glutathione, cysteine and phospholipid methyltransferase (together with a simultaneous deficiency in phosphatidylcholine formation, accompanied by disturbed membrane fluidity and decreased activity of Na /K -ATPase and Ca -ATPase). 

Wuweizisu B from Schisandra chinensis showed strong protective effects on lipid peroxidation damage to the surface of cultured hepatocytes of rats treated with Fe2+/cystein and also scavenger free radical properties [240], On the other hand, the oral administration of wuweizisu B increases the activities of antioxidant enzymes. 

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