M. Buzuk et al.
reactivity [2, 3]. As cysteine-dependent proteins appear to
be intimately involved in the neurodegenerative processes,
a proper redox environment for their activity can provide
evidence for their redox control in neurodegenerative dis-
eases [4]. Cysteine has been shown to be the major
extracellular antioxidant and major component of various
physiologically antioxidant systems and also play an
important role in the production of reactive sulfur species
(RSS). Beside these species, as a consequence of the
imbalance in the metabolism of reactive intermediates,
disulfide stress, as a specific type of oxidative stress asso-
ciated with oxidation of the pair cysteine/cystine, was
recently reported [5].
measurements were performed under a nitrogen atmo-
sphere. They also found that the excess of cysteine has
significant effect on the improved stability of the cuprous
bis–cysteine complex—[(RS-)Cu?(-SR)]- in solutions
equilibrated with the atmospheric O2. These experiments
were performed by spectrophotometric and electron para-
magnetic resonance (EPR) analysis. Rigo et al. [6]
investigated the stability of cuprous complexes, in anaer-
obic conditions, by spectroscopy, NMR and electron spin
resonance (ESR), at various Cu? (or Cu2?): cysteine ratios.
They reported the stability of Cu?(-SR) complex (char-
acterized with stoichiometry Cu2?:cysteine = 1:1.2, owing
to the formation of polymeric species with bridging thiolate
sulfur), up to Cu2?: cysteine ratio of 0.45. When Cu2?
exceeds this ratio (even in trace amounts), a fast and
complete oxidation of the complex occurs. Nevertheless, it
was previously reported [18] that cystine (produced by
decomposition of cuprous cysteine complex) could slow
down the terminal oxidation of Cu? (produced by
decomposition of cuprous cysteine complex), which was
not reported by the Pecci et al. [17]. Begiyan et al. [19]
investigated the kinetic order of the catalytic oxidation of
thiol compounds by molecular oxygen in an aqueous
solution, in the presence of various metals ion, at various
pH values. The authors concluded that, in the neutral
medium, oxidation rates depended only on the concentra-
tion of variable-valence metal ions. Moreover, the same
authors emphasize that, in an alkaline media, with the
presence of a large excess of thiol compounds with respect
to Cu2?, predominant complex is cuprous bis–cysteine
complex—[(RS-)Cu?(-SR)]-. The copper-catalyzed oxi-
dation of cysteine to cystine, in an alkaline solution, was
investigated by Cavallini et al. [20] and they reported
cysteine–Cu2? complex, with stoichiometry of cysteine to
Cu2?—2:1. Hanaki and Kamide [21] proposed a direct and
indirect mechanism of cysteine oxidation, based on results
of oxidation kinetics of cysteine with H2O2 in the presence
of Cu2? obtained by the spectrophotometric method. Zwart
et al. [22] suggested that, during the catalytic oxidation, at
least two different copper complexes are operative.
Masoud and El-Hamid [23] describe the synthesis of a pale
green Cu2?–cysteine complex in distilled water with a
stoichiometry of 1:1.2. Their findings suggest that both
carboxylate and thiol groups are involved in the copper
binding. However, Dokken et al. [24] suggested that they
probably synthesized oxidized Cu?–cysteine complex or
an impure Cu2?–cystine complex similar to that reported
by Gale and Winkler [25].
Copper is known to be the cofactor of enzymes involved
in respiration processes or in the removal of reactive
oxygen species. In addition, copper can be found in some
proteins such as hemocyanin, plastocyanin, ceruloplasmin.
Since copper ions can easily cycle between cuprous ion and
cupric ion, they could take part into non-enzymatic redox
processes, thus altering the cell status by acting on intra-
cellular redox potential [6]. In the presence of oxygen,
copper ions influence the catalytic oxidation of biomole-
cules. Thus, they influence the production of reactive
oxygen species (ROS) and consequently the level of
oxidative stress. The interactions of cysteine and other
biological thiols with various transition metals have been
given a considerable amount of attention for a long period
of time.
The term ‘‘self-oxidation of thiols’’ can be found in
some articles from the 1950s [7], although this behavior
was first noticed by Mathews and Walker [8]. They
reported acceleration in the atmospheric cysteine oxidation
when small quantities of metals are present in the solution.
Warburg and Sakuma explained that ‘‘autoxidation’’ of
cysteine is catalyzed by metals, assuming the formation of
an autoxidisable intermediate complex [9]. In addition,
some authors found that the oxidation of cysteine is cat-
alyzed by cystine, probably by the formation of an
intermediate cysteine–cystine complex [10]. Other authors
[11, 12] found that metals can accelerate the oxidation of
cysteine, even in anaerobic conditions. Heretofore, oxida-
tions of thiol compounds have been the subject of
numerous scientific investigations.
However, the available data are too contradictory or
scarce to estimate the mechanism of the proposed reaction.
These include the dependence of the oxidation rate of thiol
compounds on pH [13], the inertness of thiol to oxygen in
aqueous alkaline solution and the oxidation of aminothiols
in the presence of various variable-valence metals [14–16].
For example, Pecci et al. [17] have proposed simpli-
fied reactions when cupric ions have been added in a
solution of cysteine at pH 7.4 and found high stability of
cuprous bis–cysteine complex—[(RS-)Cu?(-SR)]-, when
As it is presented above, many contradictory findings
concerning the mechanism, condition and role of specific
species in the oxidation of thiol compounds can be noticed.
In addition, all used techniques were time-consuming,
expensive or improper for continuous measurement and
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