Cystin

As constituents of proteins the amino-acids are important constituents of the food of animals. Certain amino-acids can be made in the body from ammonia and non-nitrogenous sources others can be made from other amino-acids, e.g. tyrosine from phenylalanine and cystine from methionine, but many are essential ingredients of the diet. The list of essential amino-acids depends partly on the species. See also peptides and proteins. 

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. 

A) Amino-aliphatic carboxylic acids. Glycine, tyrosine cystine. ... 

Tyrosine and cystine have been included primarily for students of biochemistry, physiology and medicine these two amino-acids might, however, be omitted by other students. 

Glycine or amino-acetic acid, NHjCHjCOOH tyrosine, />-HOC<.H4CH2CH(NH2)COOH cystine, [ SCH2CH(NH2)COOH]2. 

A) AMINO-ALIPHATIC CARBOXYLIC ACIDS. Glycine, tyrosine, 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. ... 

Action of nitrous acid. To a few ml. of 20% NaNO, solution add a few drops of cold dil. acetic acid. Pour the mixture into a cold aqueous solution of glycine, and note the brisk evolution of nitrogen. NH CH COOH -h HNO2 = HO CH2COOH + N + H O. Owing to the insolubility of cystine in acetic acid use a suspension in dU. acetic acid for this test. In each case care must be taken not to confuse the evolution of nitrogen with any possible thermal decomposition of the nitrous acid cf. footnote, p, 360). 

Tyrosine and cystine are insoluble in water therefore place about 0 2 g. in the test-tube A, dissolve in the dil. NaOH solution, add phenolphthalein as before and then add dil. HCl until pink colour is iust not discharged then proceed as above. 

Many amino-acjds are easily identified as their 3,5-dinitroben2oyl derivatives (Saunders, Biochem. Jour., 1934, 28, 580 1942, 36, 368). Cystine, however, forms a gelatinous derivative, but is readily identified by special tests. Tyrosine does not form a derivative under these conditions. 

Niiroprusside test. Dissolve about o-i g. of cystine in a few ml. of dilute ammonia and then add a few drops of potassium cyanide solution. This reduces cystine to cysteine,... 

Almost insoluble in cold water. Higher alcohols (including benzyl alcohol), higher phenols (e.g., naphthols), metaformaldehyde, paraldehyde, aromatic aldehydes, higher ketones (including acetophenone), aromatic acids, most esters, ethers, oxamide and domatic amides, sulphonamides, aromatic imides, aromatic nitriles, aromatic acid anhydrides, aromatic acid chlorides, sulphonyl chlorides, starch, aromatic amines, anilides, tyrosine, cystine, nitrocompounds, uric acid, halogeno-hydrocarbons, hydrocarbons. 

Peptide chain with two cystine residues (R = -CHgSH)... 

Tertiary structure also refers to the overall shape of a molecule, especially to structures stabilized by disulfide bridges (cystine) formed by the oxidation of cysteine mercapto groups. 

Keratin is the protein of hair and wool. These proteins are insoluble because of the disulfide cross-linking between cystine units. Permanent waving of... 

LPC Product Quality. Table 10 gives approximate analyses of several LPC products. Amino acid analyses of LPC products have been pubhshed including those from alfalfa, wheat leaf, barley, and lupin (101) soybean, sugar beet, and tobacco (102) Pro-Xan LPC products (100,103) and for a variety of other crop plants (104,105). The composition of LPCs varies widely depending on the raw materials and processes used. Amino acid profiles are generally satisfactory except for low sulfur amino acid contents, ie, cystine and methionine. 

The sulfur amino acid content of soy protein can be enhanced by preparing plasteins from soy protein hydrolysate and sources of methionine or cystine, such as ovalbumin hydrolysate (plastein AB), wool keratin hydrolysate (plastein AC), or L-methionine ethyl ester [3082-77-7] (alkaU saponified plastein) (153). Typical PER values for a 1 2 mixture of plastein AC and soybean, and a 1 3 mixture of alkah-saponified plastein and soybean protein, were 2.86 and 3.38, respectively, as compared with 1.28 for the soy protein hydrolysate and 2.40 for casein. 

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