The out-exchange kinetics of tritium from apoferritin, ferritin of various iron contents, and apoferritin subunits were examined. The exchange kinetics indicated no detectable conformational differences in the tetracosamer with and without hydrous ferric oxide in the internal cavity of the molecule. The data for apoferritin subunits were markedly different from those for the tetracosameric state. The exchange kinetics for apoferritin were consistent with a rapid exchange of water between the internal cavity of the protein and the bulk solvent outside the protein shell.
