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

A rapid method requiring 30–45 min is described for preparing the complex of CoIII with serum transferrin from man, mouse and rat. It relies on first generating the citrate chelate of CoIII 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 CoIII-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 CoIII-transferrin is, by and large, kinetically inert. No difference was found between CoIII–and FeIII–transferrin with respect to intrinsic fluorescence. Reduction of CoIII–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.