Cysteine and Cystine

Cysteine is readily converted to the corresponding disulfide, cystine, even under mild oxidative conditions, such as treatment with I2 or potassium hexacyanoferrate (III). Reduction of cystine to cysteine is possible using sodium borohydride or thiol reagents (mercaptoethanol, dithiothreitol)  

Reaction of cysteine with alkylating agents yields thioethers. lodoacetic acid, iodoacetamide, dimethylaminoazobenzene iodoacetamide, ethyl-enimine and 4-vinylpyridine are the most commonly used alkylating agents  

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Production by Isolation. Natural cysteine and cystine have been manufactured by hydrolysis and isolation from keratin protein, eg, hair and feathers. Today the principal manufacturing of cysteine depends on enzymatic production that was developed in the 1970s (213). 

The advantages of this method are a short reaction time and the nonfluorescence of the OPA reagent. Therefore, excess reagent must not be removed before the chromatography stage. Using this method, it is possible to measure tryptophan, but not secondary amino acids such as proline or hydroxyproline. Cysteine and cystine can be measured, but because of the low fluorescence of their derivatives, they must be detected using an UV system, or alternatively oxidized to cysteic acid before reaction. 

The -ATPase gene sequence indicates the presence of eight cysteine residues in the molecule [37,38]. In order to ascertain the chemical state of these cysteine residues, direct chemical studies with established cysteine and cystine reagents were carried out [44]. Titrations with the cysteine reagent, dithiobisnitrobenzoate, and the cystine reagent, nitrothiosulfobenzoate, indicated the presence of six free cy-... 

Cysteine and cystine has been determined in seawater by a method based on cathode stripping voltammetry of the copper complex [327]. 

The amounts of single amino acids excreted in urine in the conjugated form, as determined independently by Stein and Muting, are given in Tables 1 and 2. According to Stein, glycine, glutamic acid, aspartic acid, histidine, and proline are quantitatively the most important amino acids liberated in the course of urine hydrolysis. Serine, lysine, tyrosine, cysteine and cystine, threonine, alanine, valine, phenylalanine, and leucine are... 

A biological example of this relationship is provided by cysteine and cystine ... 

The actual formation of pyruvic acid from various mercapturic acids upon which these formulae for cysteine and cystine were founded, was only shown later by Baumann s pupils, Konigs, Brenzinger and Schmitz, and in conjunction with Suter s observation that a-thiolactic acid was formed by the hydrolysis of horn, this formula for cystine was accepted. The results obtained, however, scarcely justified this formula as pointed out by Friedmann in 1902, who showed conclusively that the cystine, obtained from proteins, had not this constitution. 

Under these reducing conditions of hydrolysis of tryptophan peptides, cystine is reduced to cysteine and its coelution with proline using standard buffer gradients, makes quantitation difficult. Thus, cysteine and cystine are generally derivatized prior to acid hydrolysis by oxidation to cysteic acid with performic acid 21 or alkylation, upon reduction in the case of cystine, with iodoacetic acid 21 or, more appropriately, with 4-vmylpyridine)22 23 50 Conversion of cysteine into 5- 3-(4-pyridylethyl)cysteine bears the additional advantage of suppressing epimerization via the thiazoline intermediate, thus allowing for standardization of the acid-hydrolysis dependent racemization of cysteine in synthetic peptides)24 ... 

Except for glycine, they are chiral molecules with the l or S (or R, for cysteine and cystine) configuration at the a C. 

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /Mactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 

Cysteine and cystine are relatively insoluble and are toxic in excess.450 Excretion is usually controlled carefully. However, in cystinuria, a disease recognized in the medical literature since 1810,451 there is a greatly increased excretion of cystine and also of the dibasic amino acids.451 452 As a consequence, stones of cystine develop in the kidneys and bladder. Patients may excrete more than 1 g of cystine in 24 h compared to a normal of 0.05 g, as well as excessive amounts of lysine, arginine, and ornithine. The defect can be fatal, but some persons with the condition remain healthy indefinitely. Cystinuria is one of several human diseases with altered membrane transport and faulty reabsorption of materials from kidney tubules or from the small intestine. Substances are taken up on one side of a cell (e.g., at the bottom of the cell in Fig. 1-6) and discharged into the bloodstream from the other side of the cell. In another rare hereditary condition, cystinosis, free cystine accumulates within lyso-somes.453... 

The amino acids cysteine and cystine are interconverted by oxidation-reduction reactions, as shown by... 

The amide bonds in peptides and proteins can be hydrolyzed in strong acid or base Treatment of a peptide or protein under either of these conditions yields a mixture of the constituent amino acids. Neither acid- nor base-catalyzed hydrolysis of a protein leads to ideal results because both tend to destroy some constituent ammo acids. Acid-catalyzed hydrolysis destroys tryptophan and cysteine, causes some loss of serine and threonine, and converts asparagine and glutamine to aspartic acid and glutamic acid, respectively. Base-catalyzed hydrolysis leads to destruction of serine, threonine, cysteine, and cystine and also results in racemization of the free amino acids. Because acid-catalyzed hydrolysis is less destructive, it is often the method of choice. The hydrolysis procedure consists of dissolving the protein sample in aqueous acid, usually 6 M HC1, and heating the solution in a sealed, evacuated vial at 100°C for 12 to 24 hours. 

Incomplete oxidation can be a problem. Higher recoveries of cysteine and cystine have been achieved by reduction of those amino acids with 2-mercaptoethanol followed by incubation with 4-vinylpyridine. This converts cysteine and cystine to S-(4-pyridylethyl)-L-cysteine, a derivative that can be separated by ion-exchange chromatography. Performic acid oxidation of methionine in the presence of phenol is a suitable method for analysis of cysteine. 

There are two pyridoxal phosphate-requiring enzymes in the homocysteine degradation pathway, which are associated with genetic diseases. In homo-cystinuria, cystathionine synthase is defective, and large amounts of homocystine are excreted in the urine. Some homocystinurics respond to the administration of large doses of vitamin B6. In cystathioninuria, cystathionase is either defective or absent. These patients excrete cystathionine in the urine. Cystathionase is often underactive in the newborns with immature livers, and cysteine and cystine become essential amino acids. Human milk protein is especially rich in cysteine, presumably to prepare the newborn for such a contingency. 

Table 3-8 Cysteine and Cystine Content of Some Proteins (g Amino Acid/100 g Protein)...

From a comparison of the spectra of cysteine and cystine it is immediately clear that oxidation of SH to disulfide in peptides or proteins can result... 

Low yields of cysteine and cystine are often encountered in acid hydrol-yzates, especially when carbohydrates are present. For this reason determination of these amino acids as cysteic acid in acid hydrolyzates of oxidized proteins is required for accurate results (Schram et al, 1954 Moore, 1963). Often small losses of cysteic acid are encountered, but these can be estimated by time studies as described above. 

The cystine residues, because they may act as cross-linkages between protein chains, have been studied more closely than other residues in keratin. Burley s (1956a) use of the concept of thiol-disulfide interchange to explain the effects of chemical modification on the physical properties of wool fibers has stimulated further work on cysteine and cystine residues. 

Cysteine and cystine. Most cytosolic proteins lack disulfide bonds, whereas extracellular proteins usually contain them. Why ... 

The high growth temperatures of archaeal thermophiles raise questions not only about the stability of the protein conformation but also about the protection of the peptide chain from covalent damage which occurs in mesophilic proteins. As shown by several authors [22-28] these chemical modifications mainly comprise, (a) deamidation of Asn and, to a minor extent. Gin (b) hydrolysis of Asp-containing peptide bonds (limited to the acidic pH range) and Asn-X bonds (c) destruction of cysteine and cystine residues. 

Pos et ai, 1998). Colored dissolved organic material is frequently involved in such processes. Laboratory irradiation has confirmed that cysteine and cystine are efficient precursors of CS2 and that OH radicals are likely to be important intermediates (Xie et ai, 1998), but it is also likely that some of these sulfur gases are produced directly by biological processes. Ocean waters appear to be supersamrated in carbon disulfide and capable of yielding —0.1 Tg(S) yr (Xie and Moore, 1999). 

Seitz, P., Godel, H., Quantitation of Cystein and Cystine Hewlwtt Packard Application Note 12- 5901-0775E (1991). 

About 70% of the filtered Na and K is resorbed in the proximal tubule. Sodium is cotransported with glucose and the amino adds, resulting in the resorption of over 99% of these nutrients. Cotransport of Na with bicarbonate results in resorption of 80 to 90% of the filtered bicarbonate. The uptake of these nutrients at the apical membrane occurs by Na mino acid and Na-glucose cotransport systems. One of two sodium ions are cotransported with each glucose. Seven Na-amino acid cotransport systems have been found in the renal tubule. These include systems specific for acidic amino acids, basic amino acids, glycine, neutral amino acids, cysteine, and cystine. One or more sodium ions are cotransported with each amino acid. 

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