The Hydroxyl Radical - Free-Radical-Induced DNA Damage and Its Repair - page 61

erroneous assignment of • OH formation. Moreover, this approach is not directly
applicable to the in vivo study of biological oxidations (Kaur and Halliwell 1996),
at least not in humans because spin traps are poisonous (Pou et al. 1989). Also, it
is considered that the spin-trap method is neither sufficiently sensitive (Mason
and Knecht 1994; Rosen et al. 1994), i.e., requiring the presence of micromolar
concentrations of the radical species being measured (Ste-Marie et al. 1996), nor
practical (Coudray et al. 1995). Thus, it remains to be seen whether a GC/MS
technique (Castro et al. 1997) or electrochemical detection (Floyd et al. 1984) can
improve the sensitivity situation. The problem of the competition with the O 2 • −
also seems now on the way to be overcome, and N -oxides have been designed
that are more specific for • OH (Rosen et al. 1994), and this may be especially true
for 2,2-dimethyl-4-methoxycarbonyl-2 H -imidazole-1-oxide (Tsai et al. 1999).
The detection of secondary radicals such as • CH 3 from dimethylsulfoxide has
been employed to measure • OH (Rosen et al. 1994). Complications can also arise
in vivo because the spin trap (such as N -oxides) may undergo metabolic reduc-
tion (Belkin et al. 1987).
3.5.3
Miscellaneous
There are • OH reactions, however, that allow the detection of the product of the
reaction without a further transformation. A case in point is the formation of
methanesulfinic acid from DMSO [Babbs and Gale Steiner 1990; reactions (14)
and (15)], a nontoxic xenobiotic (Tsay et al. 1998). Methanesulfinic acid has been
used as an • OH-marker in vivo (Tsay et al. 1998) (detected ex vivo), and ex vivo
systems (Fukui et al. 1993). In this context, it may be worth mentioning that
methanesulfinic acid can be readily oxidized, and in the presence of O 2 it un-
dergoes a very efficient chain reaction, methanesulfonic acid being the product
(Sehested and Holcman 1996; Flyunt et al. 2001). The other product of reaction
(15) is the methyl radical. In the presence of O 2 , it is converted to a large extent
into formaldehyde (Schuchmann and von Sonntag 1984), and it has been sug-
gested (Klein et al. 1981) that this product be used as a marker for • OH in biologi-
cal DMSO-containing systems.