Liver depot iron can be divided into two fractions: ferritin iron and non-ferritin depot iron. Three methods intended to measure the non-ferritin depot iron in the rat liver were compared using livers of normal rats and livers of rats loaded with iron by transfusion of erythrocytes. Liver depot iron varied between 75 and 850 microgram Fe/g liver. Non-ferritin depot iron, measured as the iron fraction sedimentable at 10 000 x g, was in the range 4--22 microgram Fe/g liver. This fraction did contain ferritin. When measured as the difference between total liver depot iron and heat-stable iron (ferritin iron), the range was 10--270 microgram Fe/g liver but this fraction also includes some ferritin iron. The values derived with both methods were linearly proportional to the total liver depot iron values. Non-ferritin depot iron, when measured as the difference between total liver depot iron and total ferritin iron, ranged from 0 to 190 microgram Fe/g liver. In this last method no ferritin iron is included. This method provides the best estimate of the non-ferritin depot iron fraction. The concentrations obtained with this method were not always linearly proportional to the total liver depot iron concentration. Intravenous injection of rat liver ferritin resulted in a rapid accumulation of ferritin iron in the liver, together with an increase of the non-ferritin depot iron fraction from 18 microgram Fe/g liver to 55 microgram Fe/g liver. This confirms a relationship between ferritin catabolism and the non-ferritin depot iron fraction.
