Cysteine decomposition

Variations Between Lakes. Results of a study to evaluate sulfide production variation with water depth is given in Table V. In this experiment, samples were taken from five different sediment depths over a two-day period at each lake in early October. At both lakes sulfate reduction exceeded putrefaction by a factor of approximately 2 with overall mean rates of 0.55 and 0.29 mg S L-kH1 respectively. Sulfate reduction exceeded cysteine decomposition in all samples except one collected from Third Sister Lake at 17 m. Results of this study snow a good correlation at Third Sister Lake between percent hydrogen sulfide production attributable to putrefaction and depth of sampling station (r=0.94) and oxidation-reduction potential (r=0.98). This correlation was not observed at Frains Lake. A possible factor m differences observed may be the physical nature of the sediment at Frains which was less dense and more flocculent than thatofTliird Sister. 

Fig. 5.17. Cysteine decomposition by a Strecker degradation mechanism formation of H2S (I) or 2-mer-captoethanal (II)...

The deterruination of amino acids in proteins requires pretreatment by either acid or alkaline hydrolysis. However, L-tryptophan is decomposed by acid, and the racemi2ation of several amino acids takes place during alkaline hydrolysis. Moreover, it is very difficult to confirm the presence of cysteine in either case. The use of methanesulfonic acid (18) and mercaptoethanesulfonic acid (19) as the protein hydroly2ing reagent to prevent decomposition of L-tryptophan and L-cysteine is recommended. En2ymatic hydrolysis of proteins has been studied (20). 

If reaction 49 is responsible for the high decomposition yield of ASCO, it can be understood why it does not occur for PSCO, since the C—H bond strength in the allyl compound is weaker than in the propyl derivative due to the resonance stabilization of the radical60. However, the yield of alanine was found to be 1.97 in the case of radiolysis of PCSO and almost zero for ACSO. Thus reaction 49 does not occur for the case of ACSO. Since only the yields of cysteine (0.98 for ACSO and 0 for PCSO) are given, no explanation can be proposed for the high decomposition yield of ACSO. 

The fast interaction of O2 with Fe(II)-cysteine complexes to give an oxygen adduct which rapidly undergoes one-electron breakdown to an Fe(III)-cysteine complex and -OJ has been examined by stopped-flow spectrophotometry at 570 nm . Subsequent decomposition of the Fe(IlI) complex to yield Fe(II) and the disulphide, cystine, was much slower. Both mono- and bis-complexes of Fe(Il) are involved and the reaction is first-order in both Fe(II) complex and O2 k (mono) = (5 +1) x 10 l.mole ksec" and k (bis) = (2 0.5) x lO l.mole . sec at 25 °C, corresponding to factors of 10 and 10 times faster than the analogous reactions with sulphosalicylic acid complexes of Fe(II), a feature attributed to Fe(ll)-S bonding in the cysteine complexes. ... 

Peroxynitrite easily oxidizes nonprotein and protein thiyl groups. In 1991, Radi et al. [102] have shown that peroxynitrite efficiently oxidizes cysteine to its disulfide form and bovine serum albumin (BSA) to some derivative of sulfenic acid supposedly via the decomposition to nitric dioxide and hydroxyl radicals. Pryor et al. [124] suggested that the oxidation of methionine and its analog 2-keto-4-thiomethylbutanic acid occurred by two competing mechanisms, namely, the second-order reaction of sulfide formation and the one-electron... 

The other simple peptide complex e.g. [Fe(Z-Cys-Ala-OMe)4]2- did not exhibit such a reversible redox couple under similar conditions. The Fe(lll) complexes of simple peptide thiolates or cysteine alkyl esters are found to be thermally quite unstable and decompose by oxidaticxi at the thiolate ligand by intramolecular electron transfer. Thus the macro-ring chelation of the Cys-Pro-Leu-Cys ligand appears to stabilize the Fe(in) state. The stability of the Fe(ni) form as indicated by the cyclic voltamnoogram measurements and by the visible spectra of the Fe(in) peptide complexes suggests that the peptide prevents thermal and hydrolytic decomposition of the Fe-S bond because of the hydrophobicity and steric bulk of the Pro and Leu residues (3,4). 

Baumann next found that cystine, on reduction with zinc and hydrochloric acid, was converted into a new base, which he called cysteine this gave the same products on decomposition as cystine, into which it was easily reconverted by oxidation. He therefore recognised that these compounds were related to each other, as a mercaptan is to a disulphide consequently the formula... 

Kanner et al. (1984) further showed that cysteine-Fe—NO is inhibitory to hemin or cysteine-Fe catalyzed oxidaticrn of /3-carotene. They explained the antioxidant activity of cysteine-Fe-NO and hemin-NO as the simultaneous quenching of free radicals [Eq. (1)] and decomposition of hydroperoxides (Eq. (2)]. The free radicals formed during decomposition of hydroperoxides are... 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

The decomposition mode of the adducts has early been noticed to be associated with redox reactions (60,77), and is currently under scrutiny because of its great bioinorganic relevance. It has been shown that the reduction of NP to the EPR-active [Fe(CN)5NO]3 ion occurs in the reaction with cysteine, which is oxidized to cystine (60). In this reaction, NP showed to behave catalytically with respect to the autoxidation of cysteine to cystine, provided enough oxygen was present. A recent kinetic and mechanistic study has thrown more light on the complex mechanistic details comprising the decompositions of the red adducts formed by NP with cysteine, A-acetylcysteine, ethyl cysteinate, and glutathione (120). Under conditions of excess of NP, in anaerobic medium, the reversible adduct formation step is shown by Eq. (26) ... 

The kinetics of reaction of a number of A-nitrosothiols (334) in water with mercury(II) salts have been reported. Reaction is first order in both reactants and the products are nitrous acid and the corresponding thiol-Hg2+ complex. The mechanism involves slow attack by water at the nitrogen atom in the complex.300 The same group has also studied the copper(II)-catalysed decomposition of the, -nitrosothiols derived from penicillamine, cysteamine, thiomalic acid, A -acetylpenicillaminc, and cysteine.301... 

S-Nitrosocysteine decomposition has been shown to exhibit first- and second-order dependence on total cysteine concentration. A decomposition pathway has been established via intramolecular nitroso group transfer to give a primary A-nitrosamine that decomposes to give the corresponding diazonium salt (Scheme 71). The nitroso group transfer reaction occurs intermolecularly for the pathway exhibiting a quadratic dependence on cysteine concentration.108... 

L-Cysteine Monohydrochloride occurs as a white, crystalline powder. It is freely soluble in water and in alcohol. The anhydrous form melts with decomposition at about 175°. 

Such chemical changes may lead to compounds that are not hydrolyzable by intestinal enzymes or to modifications of the peptide side chains that render certain amino acids unavailable. Mild heat treatments in the presence of water can significantly improve the protein s nutritional value in some cases. Sulfur-containing amino acids may become more available and certain antinutritional factors such as the trypsin inhibitors of soybeans may be deactivated. Excessive heat in the absence of water can be detrimental to protein quality for example, in fish proteins, tryptophan, arginine, methionine, and lysine may be damaged. A number of chemical reactions may take place during heat treatment including decomposition, dehydration of serine and threonine, loss of sulfur from cysteine, oxidation of cysteine and methio-... 

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