Local conformational changes in ferritins with variable iron content.
Conformational changes were induced in human spleen ferritin by partial or complete removal of iron, and the immunoreactivity of the ferritin samples with variable iron content was analyzed. We established that a decrease in iron content resulted in bimodal changes in immunoreactivity of the epitopes recognized by the monoclonal antibodies G10 and F11. Immunoreactivity demonstrated a 3-6-fold decrease on lowering iron content from 800 to 40 atoms per protein molecule, followed by a sharp (4-14-fold) increase that was observed when low-iron ferritin was converted to iron-free apoferritin. These bimodal changes suggest the presence of more than two conformational states of ferritin with local alterations of the epitopes recognized by the monoclonal antibodies. The global conformation of ferritin, however, remained essentially unaltered, as demonstrated by ferritin interaction with polyclonal antibodies. Together, the results indicate that local conformational changes in the ferritin protein shell occur on progressive iron removal that results in low-iron and iron-free forms of ferritin. These changes are most clearly seen in apoferritin when compared to low-iron ferritin.