Phosphate inhibits in vitro Fe3+ loading into transferrin by forming a soluble Fe(III)–phosphate complex: A potential non-transferrin bound iron species

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In chronic kidney diseases, NTBI can occur even when total iron levels in serum are low and transferrin is not saturated. We postulated that elevated serum phosphate concentrations, present in CKD patients, might disrupt Fe3+ loading into apo-transferrin by forming Fe(III)–phosphate species. We report that phosphate competes with apo-transferrin for Fe3+ by forming a soluble Fe(III)–phosphate complex. Once formed, the Fe(III)–phosphate complex is not a substrate for donating Fe3+ to apo-transferrin. Phosphate (1–10mM) does not chelate Fe(III) from diferric transferrin under the conditions examined. Complexed forms of Fe3+, such as iron nitrilotriacetic acid (Fe3+-NTA), and Fe(III)–citrate are not susceptible to this phosphate complexation reaction and efficiently deliver Fe3+ to apo-transferrin in the presence of phosphate. This reaction suggests that citrate might play an important role in protecting against Fe(III), phosphate interactions in vivo. In contrast to the reactions of Fe3+ and phosphate, the addition of Fe2+ to a solution of apo-transferrin and phosphate lead to rapid oxidation and deposition of Fe3+ into apo-transferrin. These in vitro data suggest that, in principle, elevated phosphate concentrations can influence the ability of apo-transferrin to bind iron, depending on the oxidation state of the iron.