Transferrin Biphasic kinetics

The biphasic kinetic pattern described for the removal of iron from transferrin by pyrophosphate can be ascribed to the two different iron-containing sites in transferrin/ Various anions and acids can assist such removal/ Details of iron removal have been probed by studying the kinetics of metal removal from transferrin derivatives containing iron and cobalt variously distributed between the two inequivalent binding sites, and from transferrins containing iron in only one of the two sites. The kinetics of iron removal from the Fee sites show a first-order dependence on pyrophosphate concentration, from the FeN sites show saturation kinetics. The current situation with respect to mechanisms of iron removal from, and incorporation into, transferrin have been reviewed. ... 

Iron(III)-pyrophosphate looks promising as an alternative to iron(III)-carbohydrate preparations for parenteral administration for treatment of anemia.Kinetics of removal of iron from transferrin (tf) by pyrophosphate (pp) were found to be biphasic under certain conditions, with the rapid first phase attributed to the formation of a pp—Fe—tf—CO intermediate.A later study of the kinetics of removal of iron from transferrin employed pyrophosphate and tripodal phosphonates such as nitrilotris(methylenephosphonic acid), N(CH2P03H2)3. For the tripodal ligands there are parallel first-order and saturation pathways, with the latter dominant (contrast the exclusively first-order reaction of ferritin with nitrilotriacetate) for pyrophosphate the paths are roughly equal in importance. The saturation kinetics suggest that tfiFe-phosphonate intermediates play an important role in the kinetics. 

The two sites also differ in their pH stability towards iron release. Experiments on serum transferrin showed that one site loses iron at a pH near 6.0, and the other at a pH nearer 5.0 (203, 204), giving a distinctly biphasic pH-induced release profile (Fig. 28). The acid-stable A site was later shown to be the C-terminal site (202). It is this differential response to pH, together with kinetic effects (below), that enables N-terminal and C-terminal monoferric transferrins to be prepared (200). Although the N-terminal site is more labile, both kinetically and to acid, the reasons are not necessarily the same the acid stability may depend on the protonation of specific residues (Section V.B) and is likely to differ somewhat from one transferrin to another in response to sequence changes. The biphasic acid-induced release of iron seen for transferrin is not shared by lactoferrin. Although biphasic release from lactoferrin, in the presence at EDTA, has been reported (205), under most conditions both sites release iron essentially together at a pH(2.5-4.0) several units lower than that for transferrin (Fig. 28). 

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