1. The activities and subcellular distribution of enzymes implicated in the protection of cells from free-radical mediated damage were determined in liver biopsy specimens from control and iron-overloaded patients.

2. There was a small but insignificant decrease in the activity of glutathione reductase in patients with secondary iron overload due to multiple transfusion therapy for thalassaemia major.

3. The activities of superoxide dismutase, catalase and glutathione peroxidase were similar in both patient groups.

4. Subcellular fractionation studies indicated a major cytosolic localization of these enzymes with a minor mitochondrial component. The relative proportions of the enzymes in the two locations was similar in both control and iron-overloaded patients.

5. Approximately 80% of the hepatic glutathione was present in the reduced form in both patient groups and it is concluded that although free-radical mediated damage might be implicated in the pathogenesis of tissue damage due to iron overload no significant defect in these protective mechanisms can be demonstrated.

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