Mixed disulfide

The thiol form (12) is susceptible to oxidation (see Fig. 2). Iodine treatment regenerates thiamine in good yield. Heating an aqueous solution at pH 8 in air gives rise to thiamine disulfide [67-16-3] (21), thiochrome (14), and other products (22). The disulfide is readily reduced to thiamine in vivo and is as biologically active. Other mixed disulfides, of interest as fat-soluble forms, are formed from thiamine, possibly via oxidative coupling to the thiol form (12). 

II. Thiol oxidants cystaminc (mixed disulfide formation), diamide, t-BHP, menadione, diquat... 

The special properties of lipoic acid arise from the ring strain experienced by oxidized lipoic acid. The closed ring form is approximately 20 kj higher in energy than the open-chain form, and this results in a strong negative reduction potential of about —0.30 V. The oxidized form readily oxidizes cyanides to isothiocyanates and sulfhydryl groups to mixed disulfides. 

Figure 12.12 Coupled SEC-RPLC separation of Plioflex rubber stock (a) SEC (b) RPLC ti ace of fraction 1, Wingstay 100 (Eive-peak pattern is representative of diarylphenylenedi-amine isomers) (c) RPLC ti ace of fraction 2, mixed disulfide and MBTS (2,2 -thiobis (ben-zothiazole)). Obtained under the same conditions as given for Eigure 12.11. Reprinted from Journal of Chromatography, 149, E. L. Johnson et al, Coupled column chromatography employing exclusion and a reversed phase. A potential general approach to sequential analysis , pp. 571-585, copyright 1978, with permission from Elsevier Science.

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... 

Kamura T Tsuda H., Yae Y., et al. An abnormal fibrinogen Fukuoka II (Gly-B(315- CY5) characterized by defective fibrin lateral association and mixed disulfide formation. J Biol Chem 1995 270,29392-9. 

Xu et al. [5] described the effect of (z>)-penicillamine on the binding of several antiacetylcholine receptor monoclonal antibodies to the Torpedo acetylcholine receptor. Penicillamine is covalently incorporated into the acetylcholine receptor through SS exchange at the cysteine residues of the a-subunit, altering the antigenic structure of the receptor. This effect on the structure of the native receptor at the neuromuscular junction may be responsible for the establishment of the autoimmune response to the acetylcholine receptor in (i))-penicillamine-induced myasthenia gravis. Cysteine and penicillamine interact to form penicillamine-cysteine mixed disulfide complexes [6] ... 

Rabenstein and Yamashita [52] determined penicillamine and its symmetrical and mixed disulfides by HPLC in biological fluids. Plasma and urine were deproteinized with trichloroacetic acid, and HPLC was performed on a column (25 cm x 4.6 mm) or Biophase ODS (5 pm) with a mobile phase comprising 0.1 M phosphate buffer (pH 3) and 0.34 mM Na octylsulfate at 1 mL/min. Detection was with a dual Hg-Au amalgam electrode versus a Ag-AgCl reference electrode. (z>)-penicillamine and homocysteine were determined at the downstream electrode at +0.15 V, and homocystine, penicillamine-homocysteine, and penicillamine disulfides were first reduced... 

The second mode of toxicity is postulated to involve the direct interaction of the epidithiodiketopiperazine motif with target proteins, forming mixed disulfides with cysteine residues in various proteins. Gliotoxin, for example, has been demonstrated to form a 1 1 covalent complex with alcohol dehydrogenase [13b, 17]. Epidithiodi-ketopiperazines can also catalyze the formation of disulfide bonds between proxi-mally located cysteine residues in proteins such as in creatine kinase [18]. Recently, epidithiodiketopiperazines have also been implicated in a zinc ejection mechanism, whereby the epidisulfide can shuffle disulfide bonds in the CHI domain of proteins, coordinate to the zinc atoms that are essential to the tertiary structure of that domain, and remove the metal cation [12d, 19],... 

Figure 1.74 Thiol-containing disulfide reductants reduce disulfide groups through a multi-step process producing a mixed disulfide intermediate.

Figure 1.75 DTT is highly efficient at reducing disulfides, since a single molecule can reduce the intermediate mixed disulfide by forming a ring structure.

The reduction of disulfides by 2-mercaptoethanol proceeds through a mixed disulfide intermediate. 

FIGURE 4.90 Oxidation pathways of thiols. In vivo the major product is a mixed disulfide as shown for captopril. 

Y. Mely and D. Gerard, Structural and ion-binding properties of an SlOOb protein mixed disulfide Comparison with the reappraised native SlOOb protein properties, Arch. Biochem. Biophys. 279, 174-182 (1990). 

N. F. Tabachnik, P. Blackburn, C. M. Peterson, A. Cerami, Protein Binding of JV-2-Mer-captoethyl-2,3-diaminopropane via Mixed Disulfide Formation after Oral Administration of WR 2721 , J. Pharmacol. Exp. Ther. 1982, 220, 243 - 246. 

DeLucia, A. J., M. G. Mustafa, M. Z. Hussain, and C. E. Cross. Ozone interaction with rodent lung. III. Oxidation of reduced glutathione and formation of mixed disulfides between protein and nonprotein sulfhydryls. J. Gin. Invest. 55 794-802, 1975. 

The proposed metabolic pathway in strawberries is shown in Figure 3. Five compounds mixed disulfide, sulfide, sulfenic acid, thiophosgene and the GSH-re-action product have not been identified as a strawberry metabolite but their involvement is very likely based on the formation of bis-(fluorodichloromethyl)... 

Cysteamine (/3-mercapto-ethylamine) is used for the treatment of nephropathic cystinosis. Cysteamine converts within lysosomes cystine into cysteine and cysteine-cysteamine mixed disulfide, both of which can exit the lysosome thus removing the extra cystine. After oral administration peak plasma levels are reached at about 1.4 hours post dose. It is eliminated as a sulfate in the urine with a half-life of 4-5 hours. The most frequent adverse reactions seen involve the gastrointestinal and central nervous systems. Side effects include abdominal pain, diarrhea, drowsiness, fever, loss of appetite, nausea or vomiting and skin rash. Confusion, dizziness and headache may occur. 

Mechanism of Action Asulfhydryl compound with similar properties to those of penicillamine and glutathione that undergoes thiol-disulfide exchange with cysteine to form tiopronin-cysteine, a mixed disulfide. This disulfide is water soluble, unlike cysteine, and does not crystallize in the kidneys. May break disulfide bonds present in bronchial secretions and break the mucus complexes. Therapeutic Effect Decreases cysteine excretion. 

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