Penicillamine-cysteine mixed disulfide

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] ... 

Fig. 23. Reaction of penicilloic acid (/) with cystine to yield penamaldic acid-cysteine mixed disulfide (//). Subsequent hydrolysis of II yields penicillamine-cysteine mixed disulfide (III) and penaldic acid (IV)...

D-Penicillamine may arise directly from penicilloic acid or through formation of a penicillamine-cysteine mixed disulfide, following a penamaldate rearrangement (Levine 1960 c Yemal et al. 1978). This reaction also occurs with functional derivatives of the alpha-carboxylic group of penicilloic acid (Schneider et al. 1973). The penicillamine determinant may therefore arise in vivo from any penicilloyl conjugate. 

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. 

The rationale for the use of penicillamine in cystinuria is that penicillamine reacts with the poorly soluble cysteine in a thiol-disulfide exchange reaction and forms a relatively water-soluble cysteine-penicillamine mixed disulfide. In cystinuria, the urinary excretion of cystine is used to adjust dosage, although 2 g/day in 4 divided doses usually is employed. 

Free thiol groups react with cysteinyl-disulfide residues in proteins to form a new disulfide link with the concomitant release of a free cysteine residue within the protein (Scheme 12, pathway A). No metabolic activation is required for the covalent binding process to occur. Thiols are also readily oxidized to sulfenic acids, which react with cysteinyl residues and reduced GSH to form mixed disulfides (Scheme 12, pathway B) (Migdalof et al., 1984 Coleman et al., 1988). Methimazole, penicillamine, and captopril are examples of free thiol-containing drugs wherein evidence has been presented that traces toxicity of these compounds with oxidation of the thiol group. 

Thiols disappear from urine in contact with air, forming both homo- and mixed-disulfides. Rapid losses have been reported for cysteine, D-penicillamine, and thiomalate. It is therefore essential to acidify urine immediately after it has been voided. Collection of urine into 10 mL of 6 mol L hydrochloric acid per litre of urine inhibits thiol oxidation for at least 12 h at room temperature. Although addition of EDTA and storing at —20 °C inhibits thiol degradation, further loss is still possible. Dithiothreitol (DTT) can be used to maintain thiols in the reduced form prior to analysis, but this procedure can give erroneous results since DTT might release thiols from mixed disulfides. Furthermore, DTT often interferes with HPLC-ED methods. 

The radiochemlstty of the drugs themselves is of interest. Thiyl radicals formed by the radiolysis of cysteine, M A, and the related disulfides in aqueous solution can build up a steady-state concentration and "scramble" many disulfide groups.The radlolysls of aqueous mixtures of cystine and penicillamine disulfide gave a spectrum of products very similar to that obtained from the mixed disulfide of the two components. Cystine alone gave sulfonic acids, while penicillamine difulside, which is not radioprotective, gave none. Perhaps the ability to form sulfonic acids is characteristic of protective disulfides.68... 

Although such reactions have been known for a long time, it appears a somewhat neglected area of study. Most attention has been on cysteine and its oxidation to the disulfide which is catalyzed by metal ions, in particular CuI[ (see Section 20.2.2.2.2) and FeI,[.81 The likely intermediates in these reactions are metal-cysteine complexes which undergo internal electron transfer. As noted earlier (Section 20.2.2.2.2), penicillamine differs from cysteine in its reactivity and gives rise to mixed valence species. More recently Mn11 has also been found to catalyze the oxidations of Cys and Pen. 

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