Chemistry Reference
In-Depth Information
suggest capability of oxidizing or reducing superoxide equally and the role of
water in the catalytic activity of SOR [115]. A recent study on the catalytic
mechanism of the reaction of
O
•−
with SOR suggests both ferrous iron-
superoxo and ferric hydroperoxide species as intermediates of the reactions.
Significantly, lysine 48 plays an important role in controlling the evolution of
iron peroxide intermediate to yield H
2
O
2
[127].
In summary, superpoxide is relatively unreactive with amino acids except
cys, Trp, His, and Phe. These amino acid residues as well as their locations play
a significant role in the inactivation of selected dehydrogenases by
O
•−
[4].
Superoxide is reactive with complexes of transition metal ions such as cu, Ni,
Fe, and Mn. Several evidences have been given for possible targets of super-
oxide reactivity
in vivo
. In the research on the catalytic oxidation of superox-
ide, it was determined that the specificity for metals can differ with the type
of SOD, and differences exist in the interaction between metals and proteins
[128]. Research is in progress on understanding the mechanisms causing the
change in the metal-specific activity. More spectroscopic and structural studies
are thus needed on mutants of FeSOD and MnSOD. Mechanism studies on
NiSOD are also required to unravel the inherent complexity of making nickel
redox active with superoxide [31]. A study on the immobilization of SOD on
organo-functionalized mesoporous silica nanoparticles has been conducted to
determine the activity and structural changes of SOD upon immobilization
[129]. Results showed that the immobilized SOD had a higher activity than
the free enzymes and the structure of SOD did not change. More work in this
area may provide information on designing drugs to reduce dangers from
superoxide to the cells.
4.2 SINGLET OXYGEN
Formation of singlet oxygen (molecular oxygen in the
1
Δ
g
state;
1
O
2
) has been
observed in dark- as well as in light-mediated processes in the presence of
endogenous and exogenous sensitizers [13, 130-136]. The nonlight processes
include biological systems such as peroxides (e.g., horseradish peroxidase,
lactoperoxidase, MPO, and chloroperoxidase) and lipoxygenase-mediated
reactions [137-141]. The generation of
1
O
2
was also observed in several stimu-
lated cell types (e.g., eosinophils and macrophases) [142, 143]. The glyoxal-
peroxynitrite system generates
1
O
2
[144]. Reactions of ozone with biological
molecules and plant leaves produced
1
O
2
[145, 146].
1
O
2
was observed during
the reactions of H
2
O
2
with sodium molybdate and HOcl [147, 148]. Steady-
state irradiation of equilibrium mixtures of the retinal lipofuscins, 2-[2,
6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetraenyl]-
1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,
5E-hexatrienyl]-pyridinium (A2E) and double-bond isomer of A2E (iso-A2E)
also generates
1
O
2
[140]. The self-reaction of sec-peroxy radicals has been
shown to yield
1
O
2
[149]. In photochemical reactions, the incidence of UV or
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