Comparison of injectable iron complexes in their ability to iron load tissues and to induce oxidative stress.

Iron and copper homeostasis have been studied in various tissues after iron-loading with the polynuclear ferric hydroxide carbohydrate complexes, iron dextran, iron polymaltose, iron sucrose and iron gluconate for four weeks. There were significant increases in the iron content of the different rat tissues compared to controls, with the exception of the brain, which showed no change in its iron content following iron loading. However, the level of iron loading in the different tissues varied according to the preparation administered and only iron dextran was able to significantly increase the iron content of both broncho-alveolar macrophages and heart. The hepatic copper content decreased with iron loading, although this did not reach significance. However the copper content did not alter in the iron loaded broncho-alveolar macrophages. Despite such increases in hepatic iron content, there was little evidence of changes in oxidative stress, the activities of cytosolic (apart from iron dextran) or mitochondrial hepatic superoxide dismutase, SOD, were similar to that of the control rats, confirming the fact that the low reduction potential of these compounds prevents the reduction of the ferric moiety. It was not necessary for macrophages to significantly increase their iron content to initiate changes in NO* release. Iron gluconate and iron sucrose increased NO* release, while iron polymaltose and iron dextran decreased NO* release although only the latter iron preparation significantly increased their iron content. It may be that the speciation of iron within the macrophage is an important determinant in changes in NO* release after ex vivo stimulation. We conclude that tissues loaded with iron by such polynuclear iron complexes have variable loading despite the comparable iron dose. However, there was little evidence for participation of the accumulated iron in free radical reactions although there was some evidence for alteration in immune function of broncho-alveolar macrophages.

The influence of iron homoeostasis on macrophage function.

Iron loading of alveolar macrophages in vivo significantly altered their ability to respond to various inflammatory stimuli. This was exemplified by reduced synthesis of inducible nitric oxide synthase after stimulus with lipopolysaccharide and interferon gamma, and an enhanced activation of nuclear factor kappa B in the absence of tumour necrosis factor alpha stimulation, and enhanced production of reactive oxygen species after activation with activated zymosan and PMA. Such results may indicate an imbalance in the production of reactive oxygen and reactive nitrogen species generated by the iron-loaded macrophages after an appropriate stimulus.

Does the haemosiderin iron core determine its potential for chelation and the development of iron-induced tissue damage?

Haemosiderin, the major iron storage protein in tissues of iron-loaded tissues shows heterogeneity with respect to both its iron mineralisation product and associated protein. Such mineralisation products have been characterised by a variety of physical techniques including Mössbauer spectroscopy, electron diffraction and EXAFS, and are closely related to the mineral ferrihydrite. A wide range of iron chelators are being developed for the treatment of abnormal haemoglobinopathies, predominantly beta-thalassaemia, which may show greater chelator efficacy for particular mineralisation products of haemosiderin. Even though the tissue iron loadings achieved in different iron-loading syndromes are similar, e.g. naturally occurring iron loading, genetic haemochromatosis and thalassaemia, it is clear that the iron loading in thalassaemic causes extensive damage. The explanation for this could relate to the distribution of iron within different cell types, predominantly reticuloendothelial, its rate of deposition and the mineralisation product of its haemosiderin iron core, goethite.

Effect of chronic chloroquine administration on iron loading in the liver and reticuloendothelial system and on oxidative responses by the alveolar macrophages.

The ability of chloroquine to alter iron loading in the liver, spleen, and alveolar macrophages was investigated in iron-loaded or -depleted rats. Chloroquine significantly reduced incorporation of iron into the liver, spleen, and alveolar macrophages of animals loaded in vivo with iron dextran. The ability of these macrophages to respond to oxidative stress was assayed by their capacity to release reactive nitrogen intermediates after lipopolysaccharide (LPS) stimulation. A significant reduction in nitrite release was observed in primary cultures of macrophages isolated from chloroquine/iron dextran-administered rats in comparison to macrophages lavaged from rats iron-loaded alone. Macrophages isolated from iron-deficient rats showed a significant increase in nitrite after LPS stimulation, whereas nitrite release in the macrophages lavaged from the rats which had also received chloroquine during the iron depletion regime was much lower. These results indicate that the use of agents which decrease the iron content and diminish the oxidative response of the cell to altered iron status may be of therapeutic value in patients with iron loading, particularly of the reticuloendothelial system.