Differences Between the Binding Sites for Iron Binding and Release in Human and Rat Transferrin

Abstract

Iron release from human and rat transferrin was studied using a system in which the release mediated by 2,3-diphosphoglycerate in the presence of desferrioxamine was measured by the change in absorbance at 295 nm. With both proteins iron release could be resolved into two components that represented release from the two iron binding sites at markedly different rates. The rates of iron release were directly proportional to the hydrogen ion concentrations. No difference was found between the rates of iron release from the two isotransferrins isolated from rat plasma. Measurement of the rates of iron release was used to determine at which site iron was bound after addition to the apotransferrins. When iron was added at pH 6.0, it preferentially bound to the more slowly releasing site of both species of transferrin. However, when added at pH 7.4 the iron of ferrous ammonium sulfate and ferric chloride bound equally to the two binding sites, but that of ferric nitrilotriacetate and ferric citrate bound preferentially to the slow-releasing site. Radioactive iron bound to the slow- or fast-releasing sites was taken up at similar rates by reticulocytes. It is concluded that the two sites of human and rat transferrin differ in the process of iron binding and in the rate of iron release but that this is unlikely to be of functional importance.