Iron entry route in horse spleen apoferritin

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Abstract
Apoferritin has been selectively labeled with a maleimide nitroxide derivative at Cys‐126, located in the hydrophilic 3‐fold channels. Titration of this derivative with Fe(II), which gives rise to the initial Fe(III)‐apoferritin complex, produces, at low metal‐to‐protein ratios, a decrease of the intensity of the label EPR signal due to the occurrence of a magnetic dipolar interaction. A label‐metal distance ranging between 8–12 Å can be estimated from titrations performed with VO(IV), which is known to bind in the 3‐fold channels, and likewise produces a decrease in the label EPR signal. The present findings indicate that iron binds in the hydrophilic channels in its higher oxidation slate and that these channels represent the metal entry route at least at low metal‐to‐protein ratios.