Cysteine from cystine

Synthesis of 1-cysteic acid and 1-cysteine from cystine RSSR —> RSO3H ... 

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

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). 

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

Fig. 3. Decay of the H202 concentration versus time during the anaerobic oxidation reaction with cysteine in the presence of CuS04. First stage of constant rate (first-order in [Cu]) during the period of oxidation, second stage of increasing rate after completion of the oxidation of cysteine to cystine. Reprinted from Journal of Molecular catalysis, vol. 11, Zwart, J. van Wolput, J. H. M. C. van der Cammen, J. C. J. M. Koningsberger, D. C. Accumulation and Reactions of H202 During the Copper Ion Catalyzed Autoxidation of Cysteine in Alkaline Medium, p. 69, Copyright (2002), with permission from Elsevier Science.

Sanchez-Cano et al. have proposed paired synthesis for obtaining L-cysteic acid and L-cysteine from L-cystine which greatly improves the economical parameters [57], The global process-flow for the paired synthesis, with L-cystine and water as starting materials is shown in Fig. 3. Table 2 compares the results for the paired (B) and the individual syntheses (A, C). 

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. 

In contrast to the syntheses described above, which all start from cystine derivatives to form lanthionine, the lanthionine syntheses in this section all start from a protected cysteine as the nucleophilic precursor, which is then allowed to react with any of a variety of different substrates. These subsequent reactions are the Michael addition with dehydroalanine, the nucleophilic substitution of halo amino acids, or the ring-opening reaction of serine p-lactones and aziridines, respectively. However, it must be emphasized that the Michael... 

Experimental observations indicate that the oxidation of cobalt (II) to cobalt (III) and the formation of ethylenediamine from N-hydroxyethylethylene-diamine occur simultaneously. This is quite the opposite to what is usually assumed in other instances of transition metal catalysis of organic reactions—for example, the catalytic effect of manganese in the oxidation of oxalic acid (7, 8), of iron in the oxidation of cysteine to cystine (22) and of thioglycolic acid to dithioglycolic acid (5, 23), of copper in the oxidation of pyrocatechol to quinone and in the oxidation of ascorbic acid (29, 30), and of cobalt in the oxidation of aldehydes and unsaturated hydrocarbons (4). In all these reactions the oxidation of the organic molecule occurs by the abstraction of an electron by the oxidized form of the metal ion. 

Disulphide bonding. The two principal caseins, asl and / , contain no cysteine or cystine but the two minor caseins, as2 and k, each contains two cysteines per mole which normally exist as intermolecular disulphide bonds. Under non-reducing conditions, ocs2-casein exists as a disulphide-linked dimer (previously known as as5 casein) while K-casein exists as a series of disulphide-linked molecules ranging from dimers to decamers. 

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... 

L-Cysteine L-Cystine 10 Extraction from human hair Limited substitute for methionine... 

The molecular weight of 320,000 obtained for the muscle enzyme from sedimentation-diffusion data at 2-6 mg/ml and v = 0.75 (132) is to be compared with 270,000 obtained by Wolfenden et al. from s20,w = 11.1 S and D2 ,w = 3.75 X 10 7 cm2 sec1, and v = 0.731 calculated from the amino acid content (92). The rabbit muscle enzyme has a normal amino acid content, that is, no unusually low or large amount of a particular amino acid was found. Of the 32 cysteine/half-cystine residues per mole based on a molecular weight of 270,000, 6.2 were rapidly titrated with p-mercuribenzoate (92). Typical protein absorption spectra were reported for elasmobranch fish (126), carp (125), rat (127), and rabbit muscle enzyme (68). An E m at 280 nm = 9.13 has been reported for the rabbit muscle enzyme (133). The atypical absorption spectrum with a maximum at 275-276 nm observed by Lee (132) is indicative of contaminating bound nucleotides. 

L Sottrup-Jensen. Determination of half cystine in proteins as cysteine from reducing hydrolysates. Biochem Mol Biol Int 30 789-794, 1993. 

That summary is based on the reports of a well-conceived and carefully executed research program carried out by Rohan. Mohr et al. (7) extended these studies and was able to draw additional conclusions. First, without exception, free amino acids are much more sensitive to destruction in this system than the peptide-bound amino acids. Second, differences in the stability of amino acids under these conditions are not great —from 25% loss for isoleucine to 68.5% for lysine, over a relatively short period of time. In this system the reducing sugars must be the limiting factor, since the glucose and fructose are completely destroyed or removed. Third, neither cystine nor cysteine are reported to be present, and the only other sulfur-containing amino acid, methionine, is present at a much lower concentration than any other amino acid. Clearly, as we shall see later, cocoa would probably have a considerably different flavor if cysteine or cystine were present in the fermented beans. 

A maltol-ammonia browning reaction produced thirteen pyrazines, two pyrroles, two oxazoles, and one pyridine (12). The major products of this system were 2-ethyl-3-hydroxy-6-methylpyridine and 2-ethyl-3,6-dimethylpyrazine. It is difficult to construct possible formation mechanisms for these compounds from maltol and ammonia. All the carbon atoms must come from maltol. It is possible, then, that maltol degrades into smaller carbon units and that these fragments recombine to form larger carbon units, producing these compounds. Recently, the formation of thiophenones and thiophenes from the reaction of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and cysteine or cystine was reported (13. 14). All these reaction mixtures were reported to possess a cooked meat-like flavor. 

During the period of time when the nature of the 19-nortestosterone acetate-dependent photoinactivation was under investigation, a new bacterial steroid isomerase was obtained from extracts of Pseudomonas putida (Biotype B) in nearly homogeneous form and some of its physical and enzymatic properties were characterized (64, 65, 66). The putida isomerase is similar in its molecular weight and quaternary structure to the testosteroni isomerase. Chemically, the most striking difference between the two isomerases is the presence of four residues of cysteine per polypeptide chain of the putida isomerase whereas no cysteine or cystine is present in the testosteroni isomerase. N-Terminal sequence analysis of the putida isomerase demonstrated substantial sequence homology between the two enzymes. 

Samples from TSK column chromatography and some chemical procedures contained non-volatile contaminants which we usually removed by dilution to below 5% CH CN with aqueous 0.1% TFA followed by trapping on C g Sep-Pak as previously indicated (1) each desalting yields the test material in 4 x 0.3 ml of volatile solvents, which are suitable aliquots for vacuum evaporation. We were interested to obtain evidence for or against the existence of cysteine or cystine in the allatostatins, and devised a small-scale... 

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... 

Luse and M(iLaren (1963) have reviewed published research on the photolysis products and quantum yields tor the destruction of amino acids and have attributed the photochemical inactivation of the enzymes chymo-trypsin, lysozyme, ribonuclease, and trypsin by UV light at 254 m i primarily to destruction of the cystyl and tryptophyl residues. The destruction of these residues in proteins was suggested to be a function of the product of the number of residues present, the molecular extinction coefficient, and the quantum yield for destruction of each residue. Cysteine and tryptamine were identified among the irradiation products from cystine and tryptophan, respectively. Tyrosine, histidine, and phenylalanine were also shown to be degraded by UV, histidine yielding histamine, urocanic acid, and other imidazole derivatives, and phenylalanine yielding tyrosine and dihydroxyphenylalanine. Destruction of these three amino acids was not considered to contribute appreciably to the enzyme inactivation. 

Cysteine is converted into the 5-(tert-butylsulfanyl) derivative by reaction with tert-h x-tylsulfinic acid tert-butyl thioester, or preferably by exploiting the steric barriers toward dimerization of 2-methylpropane-2-thiol by air oxygen. The latter approach makes it possible to drive the thiol/disulfide exchange reaction between cystine (5) and 2-methylpropane-2-thiol in favor of the desired product, i.e. H-Cys(StBu)-OH (6), in aqueous alkaline media because the cysteine intermediate is reoxidized to cystine and transformed further (Scheme 19) [1,239] same principle has also been conveniently exploited for the conversion of short dimeric cystine peptides, prepared directly from cystine, into the related S-protected... 

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. 

Although the precision of analysis with our present technology is often 1 to 3 %, the quantitative release of many amino acids and amino acid derivatives from proteins is often difficult and lowers the overall precision. For example, if constant-boiling HCl (about 5.7 N) is used to hydrolyze a protein in vacuo at 110°C for 24 hr (these conditions are those most commonly used), the amounts of aspartic acid, asparagine, serine, threonine, glutamic acid, glutamine, valine, isoleucine, methionine, tyrosine, tryptophan, cysteine and cystine present in the... 

These 3 sulfur-containing amino acids and their derivatives are susceptible to oxidation and other destructive reactions. Even when great care has been taken to remove all oxygen from hydrolysis tubes, considerable losses of cysteine and cystine are found after acid hydrolysis, and this usually prevents direct quantitation of these amino acids in proteins. However, total cysteine plus half-cystine content may be determined as cysteic acid after performic acid... 

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