Phosphate facilitates Fe(II) oxidative deposition in pea seed (Pisum sativum) ferritin

The iron core within phytoferritin interior usually contains the high ratio of iron to phosphate, agreeing with the fact that phosphorus and iron are essential nutrient elements for plant growth. It was established that iron oxidation and incorporation into phytoferritin shell occurs in the plastid(s) where the high concentration of phosphate occurs. However, so far, the role of phosphate in iron oxidative deposition in plant ferritin has not been recognized yet. In the present study, Fe(II) oxidative deposition in pea seed ferritin (PSF) was aerobically investigated in the presence of phosphate. Results indicated that phosphate did not affect the stoichiometry of the initial iron(II) oxidation reaction that takes place at ferroxidase centers upon addition of ≤48 Fe(II)/protein to apoferritin, but increased the rate of iron oxidation. At high Fe(II) fluxes into ferritin (>48 Fe(II)/protein), phosphate plays a more significant role in Fe(II) oxidative deposition. For instance, phosphate increased the rate of Fe(II) oxidation about 1-3 fold, and such an increase depends on the concentration of phosphate in the range of 0-2 mM. This effect was attributed to the ability of phosphate to improve the regeneration activity of ferroxidase centers in PSF. In addition, the presence of phosphate caused a significant decrease in the absorption properties of iron core, indicating that phosphate is involved in the formation of the iron core.