Chemistry Reference
In-Depth Information
Migration
0.2
µ
M 3-CP
2
µ
M 3-CP
20
µ
M 3-CP
200
µ
M 3-CP
2000
µ
M 3-CP
w/o nitroxide
0.2
µ
M TEMPO
2
µ
M TEMPO
20
µ
M TEMPO
200
µ
M TEMPO
2000
µ
M TEMPO
w/o MbFe
III
/H
2
O
2
R L S
Figure 17.3
DNA scission induced by MbFe
IV
and MbFe
IV
in the absence and presence of nitroxides. The
MMbFe
III
,2mMH
2
O
2
,20
systemcontained1
MEDTAin2mMphosphate
bufferpH7.6.Thereactionwasstartedbytheadditionof2mMH
2
O
2
.Following30minincubationthesamples
weretreatedwithproteinaseKtodegradetheproteinandsubjectedtoelectrophoresisonagarose.Themigrating
bands of supercoiled circularly closed, relaxed nicked circular, and the linear DNA are denoted S, R, and L
respectively.
µ
g/mLpUC19plasmidDNA,40
µ
µ
In several experimental systems nitroxides provided significant protection in a concentration-dependent
manner, even at
M levels against radical-induced damage to various biological targets.
21,54,56 - 60
The
protective effect of nitroxides against damage induced by H
2
O
2
in the presence of heme-protein also
demonstrates their catalytic nature at significantly low concentration (unpublished results). Incubation of
metmyoglobin (MbFe
III
µ
with H
2
O
2
can lead to biological damage (e.g., scission of single and double DNA
strand breaks) mediated by ferrylmyoglobin (MbFe
IV
)
and, particularly, by the globin radical (
•
MbFe
IV
).
Nitroxides were shown to prevent DNA breakage in a dose-dependent manner where even 2
)
M TEMPO
provided significant protection (Figure 17.3). In this reaction system the nitroxide is being recycled by
H
2
O
2
, as seen in Scheme 17.3.
In fact, several pathways are suggested for nitroxide recycling (Scheme 17.4).
µ
(i) Reduction of the oxoammonium cation back to the parent nitroxide via one-electron transfer mecha-
nism. This has been demonstrated in the case of H
2
O
2
(Equation 17.6)
19
and in the case of 3-CP
+
reacting with CH
3
OH.
21
(ii) Reduction of the oxoammonium cation to the respective hydroxylamine via two-electron transfer
mechanism as in the case of NAD(P)H
19,46,61
and piperidine derivatives, for example, TEMPO
+
,
reacting with alcohols.
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