Differences in subunit composition and iron content of isoferritins.

Horse spleen ferritin was fractionated into its constituent isoferritins by isoelectric focusing. Separated isoferritins were stable and showed no tendency to redistribute when re-examined by analytical gel focusing. All of the isoferritins were immunologically indistinguishable when tested with antibodies raised against unfractionated horse spleen ferritin. The separated isoferritins also had similar conformations as determined by circular dichroism. Iron distribution studies, however, revealed a wide disparity among the isoferritins. The most acidic components had the lowest iron content but the iron content did not vary systematically throughout the isoferritin spectrum. Natural apoferritin, isolated from the ferritin by density gradient centrifugation, focused exclusively as the acidic moieties, whereas apoferritin prepared by reduction of native ferritin exhibited a banding pattern similar to that of unfractionated ferritin. The subunit structure of the isoferritins was examined by gel electrophoresis in sodium dodecyl sulfate or acidic urea systems. Multiple subunit types were demonstrated by both methods. The relative proportion of these subunit types varied progressively through the isoferritin spectrum. This difference in subunit population appears to be the basis for much of the structural heterogeneity in the apoferritin shells.