References for OH-Radical Probes

For setting up a probe system for use in biological systems, it is required that it provides OH but also adequately behaves with respect to competition kinetics. The Fenton system seems to fulfill the first criterion in that it produces the required products in good yields but certainly not the second one. As can be seen from Table 3.7, the measured yields and the calculated ones [based on competition kinetics, Eq. (43), and established rate constants] dramatically disagree. The reason for this is not yet known, but it is evident that this system cannot be used with advantage as a reference for OH production. 

Similar effects have been observed when OH was generated using the xan-thine/xanthine oxidase system in the presence of EDTA-complexed iron, a similar Fenton-type system (Owen et al. 1996). The efficiency of suppression of the formation of the dihydroxybenzoic acids by OH scavengers increases from 

These dramatic discrepancies mentioned above may also be of some relevance in biological systems, where these probes have been widely used (see below). If OH were formed in biological systems by Fenton-type reactions, these probe molecules could strongly overestimate (or underestimate) the OH production in these systems. 

A more reliable means of providing a reference of -OH in a biological system maybe by means of irradiation with ionizing radiation (von Sonntag et al. 2000). The action of ionizing radiation on an aqueous medium gives rise to OH whose yield/dose relationship (G value) is known (Chap. 2). Apart from this, since biological media are concentrated solutions the formation of the indicator product, e.g., a phenol (ArOH), via the direct effect [expressions (69) and (70)] must in principle be taken into account as well. It can be shown that with k4i [probe]/ k42 [cellular components] above 10 4 the direct effect contributes less than 10% 

The various procedures mentioned above may be calibrated by conducting the incubations for the duration of equal times t and then subject the sample ex vivo to different doses D of ionizing radiation (y, e-beam). The intrinsic rate of QH-formation v(OH)intrinsic without irradiation can then be calculated from a 

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