Disulphide exchange

Miller, R.M., Sies, H., Park, E.-M. and Thomas, J.A. (1990). Phosphorylase and creatine kinase modification by thiol-disulphide exchange and by xanthine oxidase-initiated S-thiolation. Arch. Biochem. Biophys. 276, 355-363. 

Figure 3.23. Diagram representing the molecular process of intra-chain (a) and inter-chain (b) disulphide exchange. Refer to text for specific details. (-0-=amino acid residues in polypeptide)...

Disulphide exchange can also sometimes occur and prompt a reduction in biological activity (Figure 3.23). Intermolecular disulphide exchange can result in aggregation of individual polypeptide molecules. 

Decomposition of the protein starts in both cases with evolution of H2S+. The narrow shape of the thermogram for the condensed state in comparison with the adsorbed state, suggests an important role and co-operative nature of thiol-disulphide exchange in BSA thermolysis. This mechanism was discussed... 

Probably the most frequently used spectrophotometric method to detect thiol groups, both for non-protein and protein sulphydryl groups involves the use of Ellman s reagent (Scheme 7.5). 5,5 -Dithiobis(2-nitrobenzoic acid), (DTNB) (Ellman 1959) undergoes disulphide exchange with thiol groups and the formation of 5-thio-2-nitrobenzoate anion (TNB) (Scheme 7.6). 

The so-called disulphide exchange reaction is also accepted today. Wool is constructed of a matrix and microfibrils. The crystalline substance of the microfibrils is embedded in the matrix, the amorphous substance. The two morphological components, differ distinctly in sulphur content. During the kier decatizing, it is assumed that this disulphide exchange, the splitting of disulphide bridges, takes place... 

Other chemical degradation pathways include -elimination, which can lead to racemization and disulphide exchange reactions. The amino acids that may undergo P-elimination include Cys, Ser, Thr, Phe, and Lys (40) and occur especially at alkaline pH (64). The reduction and oxidation of the disulphide bonds are often accompanied by a considerable change in the protein conformation (23,33). An example is the secondary structure of insulin that is disrupted or completely lost when the disulphide bonds are broken (23). The disulphide bond disruption or interchange can also result in an altered three-dimensional structure and therefore a possible loss of activity (40) or aggregation (29). For further details, the reader is directed to the following reviews and book chapters (30,40,62,65). 

Thiol-disulphide exchanges to form unsymmetrical disulphide bonds have been exploited extensively both in protein and peptide chemistry. King et aL (13) have used an aromatic thiol, the 4-dithiopyridyl group, as a cysteinyl-activating group to cross-link proteins via intermolecular disulphide bonds. Similarly, other aromatic thiols such as 2-thiopyridyl and nitropyridyl sulphenyl (Npys) have been used in peptide synthesis to form unsymmetrical disulphides. These and other similar methods can be applied to the ligation of unprotected peptides. 

The second approach is reported by Kent et at. (34) and differs only by the use of a peptidyl thiocarboxylic acid as precursor and the peptidyl thioester is obtained via thioalkylation or disulphide exchange. This method requires an extra step but the ligation mechanism is similar to that described above. 

Because of its dithiol/disulphide exchange activity, thioredoxin determines the oxidation state of protein thiols. This small ( 12 kDa) protein is evo-lutionarily conserved between prokaryotes and eukaryotes from yeast to plants and animals. A characteristic feature of most thioredoxins is the presence of a conserved catalytic site Trp-Cys-Gly-Pro-Cys-Lys in a protrusion of the three-dimensional structure of the protein. The two cysteine residues of the site can be reversibly oxidies to form a disulphide bridge and, thereafter, ne re-... 

Covalent reaction by thiol-disulphide exchange with a macroporous silica support bearing an activated thiol group... 

Aralkyl trimethylsilyl sulphides RSSiMej give disulphides with sulphenyl chlorides, and symmetrical diselenides are formed from the selenium analogues with Ij (PhSeSiMcj - PhSeSePh + MejSil). The formation of unsymmetrical disulphides from steroidal A -diene-S-thiones and 2-nitrobenzenesulphenyl chloride is in accordance with the concept of hard and soft acids and bases, the soft thione nucleophile reacting with the soft sulphenyl halide, but not with the hard sulphonyl chloride. The resulting A -trienyl 2-nitrophenyl disulphides participate in thiol-disulphide exchange (see next section). 

The emphasis here is on the properties of glutathione (or cysteine if data for cysteine, but not GSH are available). Dithiols which form cyclic or stabilized disulphide radical anions, such as dithiothreitol (//ireo-l,4-dimercapto-2,3-butanediol) [29-33], or the reduced form of lipoic acid (6,8-dithiooctanoic acid) [15, 34-37], in effect have uncharacteristically high values of to be good models for glutathione. In addition, biological effects may be complicated by thiol/disulphide exchange since these thiols will reduce many disulphides [38-40]. Arenethiyl radicals (e.g. phenylthiyl or derived from 2-mercaptoimidi-azole) are closer to phenoxyl radicals in nature than aliphatic thiyl radicals [41-44]. 

A logical extension of this type of study, because of the natural presence of bile salts in the intestine and the presence of synthetic surfactants in formulations, is the consideration of bile salt-surfactant mixtures. One such study [118] has considered the effect of sodium glycocholate and its mixtures with NaLS and polysorbate 80 on the absorption and metabolism of a thiamine disulphide derivative, in rats (see Scheme 7.1). It had previously been shown that surfactants altered the reaction rates of the thiol-disulphide exchange reaction that these compounds undergo [119] and that o-benzoyl thiamine disulphide interacts with the lauryl sulphate anion to form a 1 2 complex this complex is broken up by sodium glycocholate to form new mixed micelles of thiamine derivatives and the surfactants. NaLS decreases /c and /c promoting the conversion of V to VII. The reduction in absorption and the decreased enzymatic deacylation are both explained by complex formation, although inactivation of intestinal esterase by... 

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