Chemistry Reference
In-Depth Information
The involvement of singlet O
2
(O
2
1
∆
g
) has been ascertained by showing that the
rate of cross-linking increases when the solvent water is replaced by D
2
O. G has
the by far highest singlet O
2
rate constant among the DNA constituents (
k
(DNA)
= 7
10
5
;
k
(dGMP) = 5
10
6
;
k
(dAMP) = 7.5
10
4
;
k
(CMP) = 2.4
10
4
;
k
(TMP)
×
×
×
×
10
5
dm
3
mol
−1
s
−1
; Wilkinson et al. 1995), and a damaged G may be a re-
sponsible for the cross-linking reaction.
Peroxidized proteins (cf. Gebicki et al. 2002; Simpson et al. 1992; Gebicki
and Gebicki 1993; Gieseg et al. 2000; for a review see Dean et al. 1997) in their
reaction with DNA yield DNA
= 1.5
×
protein cross-links (Gebicki 1997; Gebicki and
Gebicki 1999; Luxford et al. 1999). Complexation of transition metal ions by, e.g.,
desferal reduces their yield but does but fully prevent cross-linking. DNA strand
breakage was not observed under these conditions. Mechanistically, it is not yet
understood, how protein hydroperoxides interact with DNA.
An important contributor to DNA
−
protein cross-links may be the reaction of
DNA products with proteins. A case in point is the cross-linking of the relatively
abundant DNA damage 2-dRL with a lysine moiety [reaction (39); Hashimoto et
al., 2001].
−
This type of reaction inhibits the repair by endonuclease III from
E. coli
, and
Lys-120 is responsible for this cross-linking reaction. Under the conditions of
these experiments, the cross-linking efficiency is 20%. It is expected that such
cross-linking reactions are more general and that more examples will be de-
tected.
DNA-protein cross-links were observed in mammalian cells upon ionizing
radiation (150 per Gy and cell with a considerable background level of
10
3
per cell; Ramakrishnan et al. 2003) but also upon addition of Fe
2+
(but not with
Cu
2+
) to the culture medium (Altman et al. 1995). The Thy-tyrosine cross-link
shown below has been identified by GC/MS of enzyme-digested material, but
mechanistic details as to the formation of this interesting cross-link are as yet
not known.
∼
6
×
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