Oxidation of Cystine Sulfur

Conversely, according to observations of du Vigneaud, Loring, and Craft (133) with rabbits, the sulfur of acetyl and, formyl derivatives of l-cystine is oxidized just about as in the case of i-cystine itself. However, the sulfur of the corresponding derivatives of d-cystine is considerably more resistant to oxidation than the sulfur of d-cystine. 

The oxidation of cystine sulfur to taurine is likewise a classical fact since the work of von Bergmann (9) and of Foster, Hooper, and Whipple (40), confirmed by Virtue and Doster-Virtue (134), has demonstrated that simultaneous ingestion of cystine and cholic acid in dogs with bile fistulas produces a considerable increase in the amount of tauroeholic acid synthesized in the animals. In these experiments, the starvation nature of the diet to which the animal had been submitted before is of great importance. One must employ dogs, as was done by Virtue and Doster-Virtue, which have been suitably fasted prior to ingestion of the compounds in question. 

MECHANISMS OF OXIDATION OF CYSTINE SULFUR 1. Prior Reduction to Cysteine 

To a lesser extent, the reduction of cystine to cysteine may result from a hydrolysis which is probably not enzymic. This produces a molecule of sulfenic acid and one molecule of cysteine according to  

The possibility of such a hydrolysis has been particularly discussed by Medes and Floyd (86). 

---

The oxidation of cystine sulfur in the animal leads to the formation of sulfate ari3 taurine, among other products. 

The hydrolysis reaction discussed above leads to a primary direct oxidation of cystine sulfur, which is converted to cysteinesulfenic acid. We have discussed the fate of this acid on page 386. 

Contrary to what takes place in the case of methionine, the direct oxidation of the sulfur of cysteine plays a very important role in the metabolism of this compound. This oxidation leads, depending on conditions, to cystine, cysteinesulfenic and cysteinesulfinic acids, or to the corresponding sulfonic acid, cysteic acid. 

The most convenient oxidant for the preparation of cysteic acid from cystine is aqueous bromine.1 Iodine2 and hydrogen peroxide3 also bring about the reaction, but with both substances some of the sulfur is split off as sulfuric acid. 

Other than water, protein is the major constituent of meat averaging nearly 21% in heef or chicken meat, with fat varying fiom 4.6 to 11.0% in beef and fiom 2.7 to 12.6% in chickoi. The principal radiolytic reactions of aqueous solutions of aliphatic amino acids are reductive deamination and decarboxylation. Alanine yields NH3, pyruvic add, acetaldehyde, propionic acid, CO2, H2, and ethylamine (6). Sulfur-containing amino adds are espedally sensitive to ionizing radiation. Cysteine can be oxidized to cystine by the hydroxyl radical or it can react with the hydrated electron and produce... 

This reaction is catalyzed by manganese ions at pH values from 6 to 7.5. S02 can also react with cystine to yield a series of oxidation products. Some of the possible reaction products resulting from the oxidation of sulfur amino acids are listed in Table 3-11. Nielsen et al. (1985) studied the reactions between protein-bound amino acids and oxidizing lipids. Significant losses occurred of the amino acids lysine, tryptophan, and histidine. Methionine was extensively oxidized to its sulfoxide. Increasing water activity increased losses of lysine and tryptophan but had no effect on methionine oxidation. 

Hydrogen cyanide is metabolized through several pathways. In the major metabolic pathway (60-80% of absorbed cyanide), cyanide is converted to thiocyanate in a reaction that is catalyzed by rhodanase or 3-mercaptopyruvate sulfur transferase (Baumann et al. 1934 Himwich and Saunders 1948 Wood and Cooley 1956 Singh et al. 1989). Minor pathways include the oxidation of hydrogen cyanide or thiocyanate to carbon dioxide, reaction with cystine to form 2-aminothiazoline-4-carboxylic acid and 2-imnothizolidine-4-carboxylic acid, reaction with hydroxocobalamine to form cyanocobalamin, and conversion of hydrogen cyanide to formic acid, which enters one-carbon metabolism in the body (Wood and Cooley 1956 Boxer and Rickards 1952 Ansell and Lewis 1970 Baumeister et al. 1975). 

Oxidation of Sulfur-Containing Amino Adds Methionine/ Cystine/Cysteine) (W3)... 

Organic phosphorus compounds, primarily inositolhexaphosphates (probably more than 50% of all organic phosphates), occiu in soils. The parent cyclic polyol, inositol, exists in numerous stereoisomeric configurations, of which myo-, scyllo-, neo-, and cZZ-inositol have been isolated from soils as phosphate esters. The hexaphosphate of myoinositol (myo-IHP), phytic acid, occurs in plant tissues. It often occurs as phytin, the calcium magnesium salt. Esters of myo-IHP are readily adsorbed in acidic soil solution by clay minerals and finely divided hydrated oxides of iron and aluminum. Organic sulfur compounds present in soils probably occur primarily as amino acids—e.g., cysteine, cystine, and methionine. 

This method involves oxidation of hair using peracetic acid. Analysis of the gamma-keratose from human hair indicates a higher proportion of sulfur compared to the other keratose fractions or to whole fiber [42]. Cor-held et al. [93] have isolated matrix material from merino wool by this procedure. Chemical analysis shows a relatively high proportion of sulfur and a correspondingly greater proportion of cystine compared to the other fractions or to whole fiber [42]. 

The actual presence of large amounts of cysteic acid in bleached hair had at one time been in doubt [55, 56]. It had been theorized that the cysteic acid found in bleached hair hydrolysates was formed by decomposition of intermediate oxidation products of cystine during hydrolysis prior to the analytical procedure [55]. However, differential infrared spectroscopy [4] and electron spectroscopy for chemical analysis by Robbins and Bahl [5] on intact unhydrolyzed hair have conclusively demonstrated the existence of relatively large quantities of cysteic acid residues in chemically bleached hair. Evidence for other sulfur acids (e.g., sulfinic or sulfenic acids) in bleached hair has not been provided. It is unlikely that these amino acids exist in high concentrations in hair because these species are relatively unstable. 

Among sheep with dietary insufficiencies, the minerals copper and zinc, when supplemented to the diet, have been shown to be important to wool fiber growth. Their effectiveness is attributed to the important roles these minerals play in sulfur amino acid metabolism copper serves to catalyze the oxidation of cysteine to cystine during fiber synthesis [109]. Zinc is required for cell division to occur, and it also appears to play a role in protein metabolism [110]. 

---