Cystine solubility

HSCH -CHNHj-COjH. Cysteine is a reduction product of cystine. It is the first step in the breakdown of cystine in the body, one molecule of cystine splitting to give two molecules of cysteine. Cysteine is soluble in water but the solution is unstable, and is reoxidized to cystine. 

Tyrosine and cystine are colourless solids almost insoluble in water gfid in ethanol (tyrosine dissolves in hot water). They are readily soluble in dilute caustic alkali solution, in ammonia and mineral acids, but not in acetic acid. They are also classed as neutral ampholytes. ... 

The presence of sparingly soluble components in human urine, such as calcium oxalate, calcium phosphate, magnesium ammonium phosphate, uric acid and l-cystine. Kidney stones are composed mainly of these compounds. 

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... 

Oxidative bleaching of wool is invariably carried out with hydrogen peroxide. The active species involved is likely to be the same as on cellulosic substrates but specific reactions with wool amino acid residues must be considered. The primary reaction is oxidation of cystine disulphide bonds leading to the formation of cysteic acid residues (Scheme 10.41). The rupture of disulphide crosslinks, with attendant increase in urea-bisulphite and alkali solubility values, adversely affects fibre properties. As the severity of bleaching conditions increases, the urea-bisulphite solubility remains little changed but the relationships between alkali solubility and cysteic acid (Figure 10.36) and between cystine and cysteic acid (Figure... 

Cystinuria. Penicillamine reduces excess cystine excretion in cystinuria. Penicillamine with conventional therapy decreases crystalluria and stone formation, and may decrease the size of or dissolving existing stones. This is achieved by disulfide interchange between penicillamine and cystine, resulting in a substance more soluble than cystine and readily excreted. 

Dithiothreitol (DTT) and dithioerythritol (DTE) are the trans and cis isomers of the compound 2,3-dihydroxy-1,4-dithiolbutane. The reducing potential of these versatile reagents was first described by Cleland in 1964. Due to their low redox potential (—0.33 V) they are able to reduce virtually all accessible biological disulfides and maintain free thiols in solution despite the presence of oxygen. The compounds are fully water-soluble with very little of the offensive odor of the 2-mercaptoethanol they were meant to replace. Since Cleland s original report, literally thousands of references have cited the use of mainly DTT for the reduction of cystine and other forms of disulfides. 

Small molecules containing disulfide bonds (such as cystine-containing peptides) may be reduced and isolated simply by removing the immobilized reductant. Separation of reduced molecules from reductant is much more difficult if a soluble reducing agent is used with low-molecular-weight disulfides. 

The method of protein hydrolysis was important acid hydrolysis caused destruction of tryptophan but alkaline treatment gave even greater losses of other amino acids especially cystine. The amino acids were usually separated by then standard chemical procedures based on differences in solubility, selective precipitation by agents such as Reinecke salt (proline and hydroxyproline), or flavianic acid (arginine). 

Hu Z, Guan WC, Tang XY, Huang LZ, Xu H (2007) Synthesis of water-soluble cystine C derivative with catalyst and its active oxygen radical scavenging ability. Chin. Chem. Lett. 18 51-54. 

Carta, R. and Tola, G. Solubilities of L-cystine, L-tyrosine, L-leucine, and glycine in aqueous solutions at various pHs and NaCl concentrations, J. Chem. Eng. Data, 41(3) 414-417, 1996. 

D-penidllamine can promote the elimination of copper (e.g., in Wilson s disease) and of lead ions. It can be given orally. Two additional uses are cystinu-ria and rheumatoid arthritis. In the former, formation of cystine stones in the urinary tract is prevented because the drug can form a disulfide with cysteine that is readily soluble. In the latter, penicillamine can be used as a basal regimen (p. 320). The therapeutic effect may result in part from a reaction with aldehydes, whereby polymerization of collagen molecules into fibrils is inhibited. Unwanted effects are cutaneous damage (diminished resistance to mechanical stress with a tendency to form blisters), nephrotoxicity, bone marrow depression, and taste disturbances. 

Three of the amino acids, naniely, tyrosine, cystine and diaminotrioxy-dodecanic acid are characterised by their extremely slight solubility in neutral aqueous solutions. They are therefore easily obtained after hydrolysis by acids by neutralising and concentrating the solution, when they crystallise out. 

The separation of cystine and tyrosine as they are obtained by hydrolysis with hydrochloric acid was described by Morner in I901. The protein—hair, keratin from horn, eggshells, etc.—was boiled with five times its quantity of 13 per cent hydrochloric acid under a reflux condenser on a water bath for six to seven days. The solution was then decolorised with charcoal and evaporated in vacuo, and the residue dissolved in 60 to 70 per cent, alcohol. The two acids then crystallised out on neutralising with soda, and were separated by fractional crystallisation from ammonia if much tyrosine was present it separated out first, but if cystine exceeded tyrosine in quantity this compound crystallised out first the remainder was only separated with difficulty. Embden separated the mixture of the two acids by means of very dilute nitric acid, in which tyrosine is very easily soluble, but cystine with difficulty. Their separation may also be effected by precipitation with mercuric sulphate in 5 per cent, sulphuric acid solution in which the mercury compound of tyrosine is soluble (Hopkins and Cole). 

The amino acid composition of keratin, the protein of hair and wool, includes a greater-than-average proportion of the sulphur-containing amino acid, cystine. Since this is the least soluble of the protein amino acids it can readily be isolated after carefully neutralising an acid hydrolysate of hair (Expt 5.187). Protein hydrolysis is usually effected by boiling for about 10-20 hours with 20 per cent hydrochloric acid. The hydrolysis of hair for the isolation of cystine is, however, best achieved using a mixture of hydrochloric and formic acids. 

Macy et al. (38-39) later quantitatively studied the reactivity of water soluble constituents of meat during cooking. Significantly, some sugars and cystine disappeared completely during meat cooking. 

A number of amino acid transport disorders may be associated with one or several of the systems described in Table 20.4. These are characterized by the excretion of amino acids in the urine but no increase in amino acid levels in the bloodstream. They are usually of hereditary origin. The most common disorder is cystinuria, characterized by the excretion of cystine. Because cystine is only slightly water soluble, cystinuria is often accompanied by the deposition of cystine-containing stones in the genitourinary tract. Cystinuria is apparently caused by a defect in the cationic amino acid transport system. Another disease that affects this system is lysinuric protein intolerance, which is associated with a failure to transport lysine, ornithine, arginine, and citrulline across membranes. Citrulline and ornithine are urea cycle intermediates (see later), and a disruption of their interorgan traffic results in hyperammonemia. 

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