N,N'-BIS-DIBENZYL ETHYLENEDIAMINEDIACETIC ACID (DBED) - A SITE-SPECIFIC HYDROXYL RADICAL SCAVENGER ACTING AS AN OXIDATIVE STRESS ACTIVATABLE IRON CHELATOR IN-VITRO

During oxidative stress, iron traces are supposed to be released from normal storage sites and to catalyse oxidative damage by Fenton-type r eactions. This type of damage is difficult to prevent in vivo except b y the use of strong iron chelators such as deferoxamine (affinity cons tant for Fe(III): log K = 30.8). However, strong iron chelating agents are also suspected to mobilize iron from various storage and transpor t proteins thereby leading to toxic effects. In contrast, N,N'-bis-dib enzyl ethylenediaminediacetic acid (DBED) is an iron chelator with rel atively low affinity for iron (affinity constant for Fe(III): log K < 15). In the present paper, we show that, in situations mimicking oxida tive stress in vitro, DBED is site-specifically oxidized into new spec ies with strong iron binding capacity. Indeed, in the presence of asco rbate as a reductant, the iron chelate of DBED reacts with H2O2 in aqu eous solution to yield a purple chromophore with minor release of free HO. in the medium, as measured by aromatic hydroxylation assay. The f ormation of these purple species is not suppressed by the presence of HO. scavengers at high concentration. The visible spectrum of these sp ecies is consistent with a charge transfer band from a phenolate ligan d to iron. N-2-hydroxybenzyl N'-benzyl ethylenediaminediacetic acid (H BBED) was identified in the medium as one of the oxidation products of DBED. Therefore, these results suggest that the iron chelate of DBED undergoes an intramolecular aromatic hydroxylation by HO. leading to 2 -OH derivatives and hence that DBED is a site-specific HO. scavenger. Moreover, since the measured affinity for Fe(III) of HBBED (log K = 28 ) is at least 13 orders of magnitude higher than that of DBED and sinc e ferric HBBED chelate is not a catalyst of Fenton chemistry, DBED may be looked as an ''oxidative stress activatable'' iron chelator, e.g. which increase in affinity for iron is triggered in the presence of H2 O2 and an electron donor. Therefore it is proposed that DBED and relat ed derivatives may be interesting as protective compounds against oxyg en radicals toxicity, especially for chronic use.