Interaction of anions with iron-transferrin-chelate complexes.

Preliminary evidence suggested that phosphate or borote 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 (lambdamax = 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 (lambdamax = 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 59Fe or [14C]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.