Dynamic relaxometry: application to iron uptake by ferritin

We introduce dynamic relaxometry as a novel technique for studying biochemical reactions, such as those leading to mineral formation (biomineralization). This technique was applied to follow the time course of iron oxidation and hydrolysis by the protein ferritin. Horse spleen apoferritin was loaded with single additions of 4, 10, 20, 40, and 100 ferrous ions per protein, and with multiple additions of 4, 10, 20, and 100 ferrous ions. The NMR T2 relaxation time was then measured sequentially and continuously for up to 24 h. At low loading factors of 4-10 Fe atoms/molecule, the iron is rapidly bound and oxidized by the protein on a time scale of approximately 15 s to several minutes. At intermediate loading factors (10-40), rapid initial oxidation was observed, followed by the formation of antiferromagnetic clusters. This process occurred at a much slower rate and continued for up to several hours, but was inhibited at lower pH values. At higher loading factors (40-1000), iron oxidation may occur directly on the core, and this process may continue for up to 24 h following the initial loading. Dynamic relaxometry appears to be a potentially powerful technique that may well have applications beyond the study of iron uptake by the ferritin protein.