In neutral solutions, desferrioxamine (Desferal) can react with the superoxide free radical, O2.- (possibly through its protonated form HO2.), to form a relatively stable nitroxide free radical, which can have a half-life of approx. 10 min at room temperature. The formation of the radical can be largely prevented by the presence of superoxide dismutase. The radical reacts rapidly with cysteine, methionine, glutathione, vitamin C and a water-soluble derivative of vitamin E. It also reacts rapidly with alcohol dehydrogenase, causing a loss of enzyme activity. The implications of these findings for mechanistic free-radical biochemistry and iron-chelation therapy could be considerable.

The principal mechanism of H2O2 toxicity is thought to involve the generation of hydroxyl (HO.) radicals through its interactions with Fe2+ ions by the Fenton reaction. Of particular interest has been the demonstration by Ward, Blakely & Joner [(1985) Radiat. Res. 103, 383-392] that the cytotoxicity of H2O2 is diminished at low temperature. We have now examined this phenomenon further with a mammalian epithelial cell line (CNCMI-221). Resistance of these cells to 100 microM-H2O2 added extracellularly exhibits a transition in the temperature range between 27 degrees C and 22 degrees C. We have found that the low-temperature resistance to cytotoxic concentrations of H2O2 is abolished by preincubation of cells with reductants such as ascorbate or reduced lipoic acid. This implies that the low-temperature resistance to H2O2 cytotoxicity may be due to inhibition of cellular reductive processes. The restoration of the cytotoxic action of H2O2 at 4 degrees C by ascorbate is prevented by pre-exposure of cells to desferrioxamine. This is evidence that transition-metal ions (such as iron ions) are involved in the cytotoxicity and is consistent with a mechanism of cell damage that depends on the Fenton reaction and a metal ion in the reduced state. Restoration of H2O2 cytotoxicity at low temperature by ascorbate is consistent with the artificial production of an intracellular reducing environment that at normal temperatures is sustained by cellular metabolism.

The activities of 6-phosphogluconate dehydrogenase and glucose-6-phosphate dehydrogenase have been measured in squamous epithelial cells of the uterine cervix from normal patients and cases of cervical intraepithelial neoplasia (CIN). A biochemical cycling method, which uses only simple equipment and is suited to routine use and to automation, was applied to cells separated by gradient centrifugation. In addition, cells were examined cytochemically, and the intensity of staining in the cytoplasm of single whole cells was measured using computerised microcytospectrophotometry. Twenty per cent of cells in samples from normal patients (n=61) showed staining intensities above an extinction of 0.15 at 540 nm, compared to 71% of cases of CIN 1 (n=14), 91% of cases of CIN 2 (n=11) and 67% of cases of CIN 3 (n=15). The cytochemical data do not allow definitive distinctions to be made between different grades of CIN whereas the biochemical assay applied to cell lysates shows convincing differences between normal samples and cases of CIN. There are no false negatives for CIN 3 (n=14) and CIN 2 (n=10) and 11% false negatives for CIN 1 (n=9) and 14% of false positives for normal cases (n=21). The results of this preliminary study with reference to automation are discussed [corrected]. Images Figure 1

Uptake of iron by a mammalian epithelial cell line (CNCM I-221) was shown to be dependent on the nature of the iron complex. Iron uptake was demonstrated by cytochemical staining and determination of redox-reactive iron in cell lysates. Three classes of ligands were investigated: (i) low molecular weight hydrophilic compounds, represented by ethylenediamine-tetraacetic acid (EDTA) and other charged ligands such as adenosine phosphates (ATP, ADP, AMP) and diethylenetriaminepentaacetic acid (DTPA), (2) low-molecular weight lipophilic ligands such as 8-hydroxyquinoline (8-HQ) and (3) a high molecular mass ligand, dextran. Iron complexed to 8-HQ accumulated intracellularly, the uptake rate of iron being 4.16 fmoles cell-1 h-1 of exposure at 37 degrees C or 3.86 fmoles cell-1 h-1 at 4 degrees C. Iron-dextran was endocytosed and retained in phagosomes. The uptake rate of iron following exposure to iron dextrans was found to be 5.6 fmoles cell-1 h-1 of exposure at 37 degrees C. In contrast to iron/8-HQ, uptake of iron dextran by cells was inhibited at 4 degrees C. Iron complexed to low molecular weight hydrophilic ligands was not taken up by cells. Cytotoxicity was measured by reduction of plating efficiency or tritiated thymidine incorporation. These tests showed that toxic effects of added iron were demonstrable only in cells exposed to the complex with 8-HQ.

The effect of a range of iron chelates on the cytotoxicity of H2O2 was studied on a mammalian epithelial cell line. Iron complexes which were internalised enhanced the cytotoxicity of H2O2 measured by delayed thymidine incorporation. Iron complexed to 8-hydroxyquinoline (Fe/8-HQ) potentiated the cytotoxicity of 50 microM by 38% and Fe/dextran by 23%. Pre-exposure of cells to Fe/dextran at 4 degrees C did not result in any potentiation of H2O2-induced cytotoxicity which we ascribe to failure of the Fe/dextran to be endocytosed at low temperature. Iron complexes which are slowly taken up or remain extracellular protected the cells from H2O2-induced cytotoxicity. Thus, Fe/EDTA inhibited the cytotoxicity of 50 microM H2O2 by 33%; Fe/ADP by 80% and Fe/ATP by 88%, suggesting mutual extracellular detoxification.