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The Kinetics and Mechanism of Iron(III) Exchange between Chelates and Transferrin

I. THE COMPLEXES OF CITRATE AND NITRILOTRIACETIC ACID

From the ¹ From the Graduate Program of Biochemistry, University of Southern California, Los Angeles, California 90007

The rate at which iron(III) is transferred from various chelating agents to the metal-binding sites in human transferrin is a function of the chemical nature of the complexing agent. Although equilibrium concentrations are related to the magnitude of the relative stability constants, the kinetics of transfer appears to be regulated by steric factors. The time required for equimolar concentrations of three typical chelates to half-saturate the sites is nitrilotriacetate, 3 sec; citrate, 8 hours; ethylenediaminetetraacetate, about 4 days.

The kinetics of ferric citrate transfer is complicated by the polynuclear behavior of this chelate. At a citrate to iron ratio of 1:1, the rate-limiting step of the metal binding is depolymerization to a low molecular weight, reactive complex. At ratios above 20:1, polymerization is suppressed by the formation of ferric dicitrate, which reacts rapidly with the protein to release the metal via the formation of a reactive ferric citrate complex.

A reaction sequence is proposed involving the formation of a ternary complex of chelate-iron-transferrin as an intermediate. Ionic Fe(III) does not appear to be involved in the transfer.

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