Cystine constitution

The first investigations on the constitution of cystine are those of Dewar and Gamgee in 1871, who, on treating cystine with nitrous acid, obtained an acid which they thought was pyruvic acid, CH3. CO. COOH,... 

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. 

The formation of bromophenylmercapturic acid from bromobenzene and cystine in the organism, if it had the formula given it by Baumann now seemed scarcely possible, unless an isomeric a-thio-y8-aminopropionic acid were also present in the protein molecule together with the di-/8-thio-a-aminopropionic acid or cystine. The investigation of their constitution was therefore taken up by Friedmann in 1904, who succeeded in showing that they were also derived from -thio-a-aminopropionic acid and not from the isomeric a-thio-/8-aminopropionic acid. 

Thus, the work of Friedmann on the constitution of cystine, its synthesis by Erlenmeyer jun. and by Fischer, definitely show that it has the composition... 

J. Dewar and A. Gamgee. Researches on the Constitution and Physiological Relations of Cystine (CjHjNOjS). J. of Anat. and Physiol., 1871, 5, 142-149. 

A variant of this procedure is provided by the preparation of S-benzyl-l-cysteine (Expt 5.206). The required thiolate salt is prepared by the reductive cleavage with sodium in liquid ammonia of the disulphide linkage in the amino acid, (S)-cystine, and is alkylated in situ with benzyl chloride. The preparation of this S-benzyl derivative constitutes a method of protection of the thiol grouping in cysteine. 

L-Cysteine desulfhydrase in leaves of cucurbit plants is a constitutive enzyme whose activity can be enhanced by preincubation of leaf discs with L-cysteine, D-cysteine, or structural analogs of L-cysteine at millimolar concentrations preincubation with cystine does not affect the activity of the enzyme (20.261. Although the stimulation of L-cysteine desulfhydrase activity is... 

When ferredoxin was obtained in crystalline form, one of the basic questions was whether the unstable form of sulfur was a special constitutent of ferredoxin or whether it arose from the half-cystine residues. This question has not been answered unequivocally, but available evidence supports the presence of two types of sulfur in ferredoxin half-cystine sulfur and inorganic sulfide. This conclusion is based mainly on the specificity of the method used for determination of inorganic sulfide, on amino acid and elemental analyses, and on mercurial titration data. 

Probably the most important achievement in insulin research was the determination of its primary structure by Frederick Sanger and his associates in Cambridge, England. In order to elucidate the amino acid sequence of the hormone it was necessary to separate the two chains constituting the molecule. This was accomplished by oxidation with performic acid. This operation cleaved the three disulfide bridges by converting each cystine to two cysteic acid residues ... 

E. Brand, M. Sandberg, The labihty of the sulfur in cystine derivatives and its possible bearing on the constitution of insuUn. J. Biol. Chem. 70 381-395 (1926)... 

Ibere is no doubt that such a reaction, catalysed by a pyridoxal phosphate-dependent enzyme, can occur in biological systems. It is quite possible, however, that this only represents a side reaction of other enzymes. Qrstathionase, for example, will act on cystine with the elimination of a cysteine persulphide and pyruvate. The persulphide then reacts with cysteine to eliminate sulphide and re nerate cystine. The complete cycle would constitute a cysteine desulphydrase activity. 

Since biological value is dependent primarily upon essential amino acid constitution, it would seem logical to assess the nutritive value of a protein by determining its essential amino acid constitution and then comparing this with the known amino acid requirements of a particular class of animal. Application of modern chromatographic techniques coupled with automated procedures allows relatively quick and convenient resolution of mixtures of amino acids. However, the acid hydrolysis used to produce such mixtures from protein destroys practically all the tryptophan and a considerable proportion of the cystine and methionine. Tryptophan has to be released by a separate alkaline hydrolysis, and cystine and methionine have to be oxidised to cysteic acid and methionine sulphone to ensure their quantitative recovery. Losses of amino acids and the production of artefacts, which are greater with foods of high carbohydrate content, are reduced if the hydrolysis is carried out in vacuo. Evaluations of proteins in terms of each individual amino acid would be laborious and inconvenient, and several attempts have been made to state the results of amino acid analyses in a more useful and convenient form. 

Four amino acids (glycine, tryptophan, proline, and, particularly, half-cystine) tend to be conserved. The half-cystine residues constituting the intrachain disulfide loops occur at corresponding locations in all human and Vx sequences and are used as a basis for alignment. There are 7 glycine residues, at positions 16, 41, 57, 64, 68, 99, and 101 which are present in nearly all k and X chains (34). 

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