Cobalt(III)–transferrin: production and some of its properties

Abstract

A rapid method requiring 30–45 min is described for preparing the complex of Co^III with serum transferrin from man, mouse and rat. It relies on first generating the citrate chelate of Co^III with H₂O₂ and then transferring the metal from the chelate to the protein at pH 9.0. The molar absorption coefficient of the chromoprotein thus obtained at 405 nm was 9040 ± 240 l mol^–1 cm^–1, which within experimental error is the same as that reported in the literature for Co^III-transferrin obtained by directly reacting H₂O₂, CoCl₂ and transferrin, but involving considerably longer preparation times (1 to 7 d). Pyrophosphate (10 mmol l^–1) released Co from transferrins with half-lives ranging from 21 to 31 h, thus confirming earlier reports that the cobalt in Co^III-transferrin is, by and large, kinetically inert. No difference was found between Co^III–and Fe^III–transferrin with respect to intrinsic fluorescence. Reduction of Co^III–transferrin by tritiated tetrahydroborate showed that regardless of the method used to make it, the product contained close to three times more reducible sites than transferrin not exposed to H₂O₂.