M. Schara*, M. Nemec*, S. Pečar*,**
* J. Stefan Institute, Jamova 39, SI-1001 Ljubljana, Slovenia
** Faculty of Pharmacy, University of Ljubljana, Aškerčeva 7, SI-1000 Ljubljana, Slovenia
The reaction kinetics, measured by electron paramagnetic resonance (EPR) was used to study the involvement of uric acid in the reactions of nitroxide radical (TEMPONE) scavenging by ascorbate (PBS, pH 7.4, and T=370 C). In absence of iron ions, the oxygen reoxidation of hydroxylamine was slow and the addition of uric acid did not change the observed kinetics. On the other hand, in presence of iron ions the reoxidation starts to recover the nitroxide radical. This reaction rate is strongly enhanced by uric acid. In fact there are two hypotheses, the first ascribes more efficient iron complexes with uric acid, by which the reoxidation rate increases. Especially the competition between the phosphate and uric acid ligands might be important. However, uric acid could be directly involved in scavenging the hydroxyl radicals, and herewith influence the reaction rate. No scavenging activity of uric acid in UV irradiated aqueous solutions was found. The trapped adducts of the hydroxyl and the superoxide radicals show the same concentration in presence and absence of uric acid.