Iron release kinetics

The kinetics are, however, considerably more complex. Both pH and salt affect the two sites differently (Section IV.D.2), so that iron release kinetics are very different at pH 6 compared with those at pH 7.4, for example. The kinetics are also dependent on the particular chelator... 

Most of the data in this chapter was obtained from laboratory experiments in which the dissolution kinetics were followed by monitoring the change in the level of iron released into solution. The dissolution rate and mechanism are often established on the basis of data corresponding to the first few percent of the reaction, (e.g. Stumm et ak, 1985). To insure that the initial stages are in fact representative of the behaviour of the bulk oxide ( and not an impurity, for example), a complete dissolution curve should be obtained in any investigation. 

Karathanasis, A. D., Y. L. Thompson, and V. P. Evangelou. 1991. Kinetics of aluminum and iron released from acid mine-drainage contaminated soil and spoil materials. J. Environ. Qual. 19 389-395. 

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). 

Many studies have noted weak cooperativity between the sites during iron release (3). One recent analysis used mixed-metal transferrins, with kinetically inert Co3+ in one site and Fe3+ in the other (221,224). With pyrophosphate, release of iron from the C-site was accelerated by the presence of a metal in the N-site, but no corresponding effect was seen for iron release from the N-site. The cooperative effects were also weaker and somewhat different for different chelators (221). 

Osaki, 1974 Osaki, 1966 Osaki et al., 1966). Using the kinetic values given above, they estimated that without the ferroxidase activity of Cp in the plasma 80% of the iron released from erythrocyte turnover would accumulate as non-Tf-bound Fe(ll) and thereby would be unavailable for reabsorption by the reticuloendothelial system. Furthermore, this free Fe(II) could catalyze the formation of reactive oxygen species via the Fenton reaction. This, in turn, could lead to a subsequent organismal pathophysiology (Miyajima et al., 1996 Nakano, 1993). This inference has been strikingly confirmed by research over the past 6 years in both yeasts and mammals this research has directly tested the hypothesis that multicopper oxidase-dependent ferroxidase activity is essential to eukaryotic iron homeostasis (Askwith et al., 1996 Harris et al., 1995 1998 Wessling-Resnick, 1999). 

Most kinetic studies of iron release have focused on pathways involving the use of chelate ligands such as EDTA (217), pyrophosphate 218-220), phosphonates 220,221), catecholates 108,216), hydroxa-mates 120), and nitrilotriacetate (221). In many cases, simple saturation kinetics are observed, and interpreted in terms of the formation of a quaternary complex, ligand-Fe-transferrin-COs (120,122). The failure to observe this complex spectroscopically [in contrast to iron uptake studies (120)] has been explained in terms of a rate-limiting conformational change, giving a basic three-step mechanism, which is essentially the reverse of that given for iron uptake (Section V.A.l). 

Prior reports have demonstrated the transfer of iron from holo frataxin to nucleation sites on ISU as a prerequisite step for [2Fe-2S] cluster formation on ISU 10), The time course of the cluster assembly reaction is conveniently monitored from the 456 nm absorbance of holo ISU formed during the [2Fe-2S] cluster assembly reaction (Figure 4). A kinetic rate constant A obs 0.126 min was determined with 100 pM ISU, 2.4 mM Na2S, and 40 pM holo frataxin in 50 mM Hepes buffer (pH 7.5) with 5 mM DTT. Similar rates were obtained for IscS/Cys-mediated sulfur delivery, consistent with iron release from frataxin as a rate-limiting step in the cluster assembly reaction. 

Dubey NB, Windhab El (2013) Iron encapsulated microstructured emulsion-particle formation by prhhng process and its release kinetics. Journal of Food Engineering 115 198-206. 

Iron(III) complexes of hexadentate siderophores are kinetically and thermodynamically stable, which although being ideal for their scavenging roles, presents problems to the organism with respect to iron release. Redox pro-... 

Burger RM, Projan SJ, Horwitz SB, Peisach J. The DNA cleavage mechanism of iron-bleomycin. Kinetic resolution of strand scission from base propenal release. J Biol Chem. 1986 261 15955-15959. 

In this study, the dispersion of SEs and DEs during spraying has been studied extensively and quantified in such a way that the reduction of drop size can be either pre-estimated or can be fully or partially prevented by choosing proper process conditions for air-assisted and rotary atomization. The impact of emulsion structure in the prills, prill particle size, and shelf life on the release kinetics of a model nutrient (iron) was quantified exemplarily. 

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