Cysteine peroxide

This enzyme releases S° from cysteine with formation of alanine464-466 as is shown in Eq. 14-34 for release of Se° from selenocysteine. As with rhodanese an active site cysteine accepts the departing S° of cysteine to form an enzyme-bound persulfide. This protein may in turn transfer the sulfur into the forming Fe-S or Fe-S-Mo clusters.464 Three PLP-dependent persulfide-forming sulfurtransferases related to the NifS protein have been found in E. coli. Similar enzymes are present in other organisms.4663 1 A sulfur atom may be transferred from the bound persulfide anion to acceptor proteins involved in metal cluster formation. Some members of the nifS-like family act on cystine to release free thiocysteine (cysteine peroxide), which may also serve as a sulfur atom donor 466e... 

Chlorine Benzoyl peroxide L-Ascorbic acid L-Cysteine... 

FIGURE 4-18 Permselective coatings flow injection response of a poly(l,2-diaminoben-zene)-coated electrode to the following a, hydrogen peroxide (1 mM) b, ascorbic acid (1 mM) c, uric acid (1 mM) d, L-cysteine (1 mM) and e, control human serum. (Reproduced with permission from reference 63.)... 

Primary and secondary products, and end-products of lipid peroxidation have all been shown to accumulate in senile cataracts (Babizhayev, 1989b Simonelli et al., 1989). Accumulation of these compounds in the lenticular epithelial membranes is a possible cause of damage preceding cataract formation. In senile cataracts there is also extensive oxidation of protein methionine and cysteine in both the membrane and cytosol components (Garner and Spector, 1980), while in aged normal lenses a lesser extent of oxidation was confined to the membrane. The authors therefore suggested that oxidation of membrane components was a precataract state. 

It has been proposed [91] that nitric dioxide radical formation during the oxidation of nitrite by HRP or lactoperoxidase (LPO) can contribute to tyrosine nitration and be involved in cell and tissue injuries. This proposal was supported in the later work [92] where it has been shown that N02 formed in peroxide-catalyzed reactions is able to enter cells and induce tyrosyl nitration. Reszka et al. [93] demonstrated that N02 mediated the oxidation of biological electron donors and antioxidants (NADH, NADPH, cysteine, glutathione, ascorbate, and Trolox C) catalyzed by lactoperoxidase in the presence of nitrite. 

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. 

Figure 6.20 Oxidation of (a) methionine and (b) cysteine side chains, as can occur upon exposure to air or more potent oxidizing agents (e.g. peroxide, superoxide, hydroxyl radicals or hypochlorite). Refer to text for specific details...

Benzoyl peroxide may be used to treat superficial inflammatory acne. It is a nonantibiotic antibacterial that is bacteriostatic against P. acnes. It is decomposed on the skin by cysteine, liberating free oxygen radicals that oxidize... 

Kinetic studies in the physiological pH range (6.5 to 7.8) provided consistency with the above results in that the accumulation of H202 was also observed and the stoichiometry of the reaction depended on the conditions applied (66-69). However, a simple 2 1 stoichiometry was confirmed between cysteine consumed and hydrogen peroxide formed in dilute solution. The reaction followed Michaelis-Menten kinetics with... 

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

The acid-catalysed oxidation of a protected S-methyl cysteine, which gives poor diastereoselectivity when oxidized in conventional solvents, shows density-dependent diastereoselectivity as shown in scheme 6.1 [8], Here, tert-butyl hydrogen peroxide (TBHP) is used as the oxidant and the reaction is catalysed by an Amberlyst resin (a solid acid). By tuning the pressure at which this reaction was carried out, almost 100% selectivity to one diastereomer could be achieved (Figure 6.4). 

Polyaniline-modified electrodes allow electrometric determination of hydrogen peroxide produced in aminooxidase systems, without interference of electroreactive amino acids, such as cysteine, histidine, methionine, tyrosine and tryptophan359. 

The oxidation of cysteine, as well as other amino acids, was studied by Mudd et a/. Individual amino acids in aqueous solution were exposed to ozone the reported order of susceptibility was cysteine, methionine, tryptophan, tyrosine, histidine, cystine, and phenylalanine. Other amino acids were not affected. This order is similar to that for the relative susceptibility of amino acrids to radiation and to lipid peroxides. Evaluation of the ozonization products revealed that cysteine was converted to cysteic acid, as well as cystine methionine to methionine sulfoxide tryptophan to a variety of pioducrts, including kynurenine and N-formylkynurenine tyrosine also to a variety of products, includiitg dihydroxyphenylalanine histidine to ammonia, proline, and other compounds and cystine in part to cysteic acid. In some cases, the rate and end products depended on the pH of the solution. 

There is some information concerning the reaction of ozone with chemicals under aqueous conditions. The information available suggests that double-bond cleavage takes place, just as it does under nonaqueous conditions, except that ozonides are not formed. Instead, the zwitterionk intermediate reacts with water, producing an aldehyde and hydrogen peroxide. In addition to double-bond cleavage, a number of other oxidations are possible. Mudd et showed that the susceptibility of amino acids is in the order cysteine, tryptophan, methionine. 

Site-specific cleavage of a protein has been achieved by attaching a —CH2CONHC6H4CH2CH- CH2N(CH2C02 )2 2 group at a cysteine residue, followed by complexation with Fe " ", then treatment with hydrogen peroxide. " ... 

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