Interaction of anions with iron transferrin chelate complexes

Preliminary evidence suggested that phosphate or borate destabilize iron.ovotransferrin.nitrilotriacetate complexes in the absence of added bicarbonate. The iron.ovotransferrin.EDTA complex was prepared in the absence of bicarbonate, and a number of anions, including phosphate, sulfate, and citrate, were found to perturb the visible absorbance [λ(max) = 490 nm] of this complex. Other anions, such as chloride, nitrate, and perchlorate, had little or no effect on the spectrum. Also, when bicarbonate was added to a solution of the iron.transferrin.EDTA complex (A₅₁₅ = 0.45), within 2 min, the visible absorbance had decreased to A₅₁₅ = 0.13. Slowly a new peak appeared [λ(max) = 470 nm], evidently the iron.transferrin.CO₃ complex. When these spectral changes were monitored in detail, the lack of an isosbestic point indicated the existence of one or more intermediates in the conversion of iron.transferrin.EDTA complex to the iron.transferrin.CO₃ complex. Experiments using ternary complexes containing either Fe⁵⁹ or [C¹⁴]EDTA show that both iron and EDTA nearly completely dissociate from the protein (most likely concomitantly) within 2 min after bicarbonate is added. These observations are best explained by a paradigm which includes anion binding to the apoprotein. It is clear that there is an intimate relationship between anions and the binding of iron chelates by transferrin.