Iron chelating agents are essential for treating
iron overload in diseases such as
beta-thalassemia and are potentially useful for
therapy in non-
iron overload conditions, including
free radical mediated tissue injury.
Deferoxamine (DFO), the only
drug available for
iron chelation therapy, has a number of disadvantages (e.g., lack of intestinal absorption and high cost). The tridentate
chelator pyridoxal isonicotinoyl hydrazone (PIH) has high
iron chelation efficacy in vitro and in vivo with high selectivity and affinity for
iron. It is relatively non-toxic, economical to synthesize and orally effective. We previously demonstrated that submillimolar levels of PIH and some of its analogues inhibit lipid peroxidation, ascorbate oxidation,
2-deoxyribose degradation, plasmid
DNA strand breaks and 5,5-dimethylpyrroline-N-oxide (DMPO) hydroxylation mediated by either Fe(II) plus H(2)O(2) or
Fe(III)-EDTA plus ascorbate. To further characterize the mechanism of PIH action, we studied the effects of PIH and some of its analogues on the degradation of
2-deoxyribose induced by
Fe(III)-EDTA plus ascorbate. Compared with
hydroxyl radical scavengers (
DMSO,
salicylate and
mannitol), PIH was about two orders of magnitude more active in protecting
2-deoxyribose from degradation, which was comparable with some of its analogues and DFO. Competition experiments using two different concentrations of
2-deoxyribose (15 vs. 1.5 mM) revealed that
hydroxyl radical scavengers (at 20 or 60 mM) were significantly less effective in preventing degradation of
2-deoxyribose at 15 mM than
2-deoxyribose at 1.5 mM. In contrast, 400 microM PIH was equally effective in preventing degradation of both 15 mM and 1.5 mM
2-deoxyribose. At a fixed Fe(III) concentration, increasing the concentration of
ligands (either
EDTA or NTA) caused a significant reduction in the protective effect of PIH towards
2-deoxyribose degradation. We also observed that PIH and DFO prevent
2-deoxyribose degradation induced by
hypoxanthine, xanthine oxidase and
Fe(III)-EDTA. The efficacy of PIH or DFO was inversely related to the
EDTA concentration. Taken together, these results indicate that PIH (and its analogues) works by a mechanism different than the
hydroxyl radical scavengers. It is likely that PIH removes Fe(III) from the chelates (either
Fe(III)-EDTA or
Fe(III)-NTA) and forms a Fe(III)-PIH(2) complex that does not catalyze oxyradical formation.