Subunit dimers in sheep spleen apoferritin. The effect on iron storage

Ferritin with high and low iron content, 2000 and 790 iron atoms/molecule, was isolated from the spleens of copper-poisoned and control lambs, respectively. Differences in the iron content in vivo were reflected in the properties of the apoferritin protein shells, since the apoprotein from the low iron ferritin took up iron relatively more slowly (0.52 +/- 0.09) and released it more rapidly (1.68 +/- 0.06) in vitro. Although the two types of apoferritin were indistinguishable in terms of surface charge (pI range 4.98-5.43) and in consisting of both heavy and light subunits, the subunit interactions differed markedly; 40-50% of the subunits of low iron ferritin were in dimers stable to reduction and carboxylmethylation, 4% mercaptoethanol, 8% sodium dodecyl sulfate, and 100 degrees C for 30 min, 70% formic acid, and 30% methanol. Subunit dimers were also observed in liver ferritin from mouse and neonatal pig and were enriched in a low iron fraction of horse spleen ferritin. Based on cyanogen bromide fragmentation and NH2-terminal analysis, the natural and chemically cross-linked subunit dimers had two peptides in common; natural subunit dimers also appeared to have a second region cross-linked, suggesting the possibility of both intra- and intersubunit links in the natural dimers. In sheep spleen ferritin, both heavy and light subunits appeared to participate in subunit dimerization. Natural subunit dimers were enriched in low iron ferritin fractions of all ferritin preparations tested (linear correlation = 0.94) and can explain, at least in part, the previously observed effects of iron core size on the apoferritin shell. Whether the subunit cross-links represent part of the subunit assembly process subsequently cleaved by iron (or copper) or whether the cross-links form after iron core formation in vivo has yet to determined. In either case, it is clear that such post-translational variations can affect iron uptake and release and emphasize the importance of the protein shell in determining the iron storage properties of ferritin.