Apo-transferrin

As noted above, iron-loaded serum transferrin has the role of transporting Fe(III) to cells requiring it. Once it reaches a target cell, it binds to the transferrin receptor (TfR) on the cell outer surface. TfR is a disulfide-linked dimer that binds two transferrin molecules. At neutral pH, apo transferrin does not bind to the transferrin receptor. Once formed, the transferrin receptor complex becomes detached from the cell membrane and enters the cell enclosed in a clathrin-coated vesicle. Uncoating of the vesicle generates an endosome in which the pH is lowered to 5.5, promoting release of iron. At this pH, the transferrin remains bound to its receptor, and... 

Figure 4. Cycle-1 chromatograms of polyproline (10 nmols), ovalbumin (5 nmols), and apo-transferrin (5 nmols) showing the identification of the C-terminal thiohydantoin-proline residue. Approximate 10-20% initial recoveries are found.

Transferrin is mainly synthesized in the hepatocytes. There are about 20 known variants. Iron is transported by transferrin (approx. 30% of transferrin is saturated with iron). With the help of a membrane receptor, the iron-transferrin complex is taken up and released in the liver cell, where it is immediately bound (because of its toxicity) to ferritin. The liver cells take up iron predominantly from transferrin, to a lesser degree also from haptoglobin, haemopexin, lactoferrin and circulating ferrin. Transferrin, which is mainly formed in the hepatocytes, may also bind and transport, in decreasing order, chromium, copper, manganese, cobalt, cadmium, zinc and nickel. The half-life of transferrin is 1 - 2 hours, which is very short in view of its total blood concentration of 3-4 mg. Approximately 0.4 g ferritin iron is stored in the liver. In the case of transferrin deficiency, its bacteriostatic and fungistatic effects are also reduced. Transferrin without iron saturation is known as apo-transferrin. (31, 66, 67)... 

Saturation of ritn ironsfcrrin. This test measures the proportion of transferrin occurring in the apo-transferrin form. Apo-transferrin is transferrin lacking iron,... 

Injected I.V. ferrous iron disappears rapidly from the circulation in the absence of ferroxidase activity and does not bind as readily to apo-transferrin as iron injected in the ferric form. In other words, in the absence of adequate ferroxidase, while there was no diflFerence in the levels maintained when Fe(III) is injected, there was a 50% reduction in serum iron levels when Fe(II) was injected. This is interpreted as a demonstration of the direct physiological role of Cp in the control of serum Fe. 

The specificity of the iron mobilization response in the perfusate system was also studied. Only Cp among the compounds tested proved to have any activity in the perfused livers. No activity was shown by 30 fxM apo-transferrin, HCOa", 10 /xM CUSO4, 5 mM glucose, 0.6 mM fructose, 120 iiM citrate, or 36 fxM bovine serum albumin, 21 fxM CUSO4. Further experiments designed to test apo-Cp, other copper oxidases, and other iron or copper binding compounds are in progress. 

Figure 14 Iron release from transferrin. Iron is coordinated through four protein residues (D63, H249, Y95, and Y188) and the synergistic bidentate carbonate anion. The lobe continually samples the open and closed conformations. Partial loss of the carbonate, aspartate, and histidine are the first steps in the process. Coordination of the anion or chelator (A/C) forms a quaternary complex between the protein, iron, carbonate and the chelator/anion. Decay of the quaternary complex yields apo transferrin.

Figure 9 (A) Reflector MALDI mass spectrum of an in situ digest of apo-transferrin taken from the 2D map of rat sera displayed in Figure 4, which were alkylated with do-acrylamide and ds-acrylamide and mixed in a 30/70% ratio. (B) and (C) are two short intervals taken from (A), and are associated with the two indicated peptide sequences. (Reproduced from Gehanne S, Cecconi D, Carboni L, et al. (2002) Quantitative analysis of two-dimensional gel-separated proteins using isotopically marked alkylating agents and matrix-assisted laser desorption/ionization mass spectrometry. Rapid Communications in Mass Spectrometry 16 1692-1698.)...

Bedford, E.E., Boujday, S., Humblot, V., Gu, F.X., Pradier, C.-M., 2014. Effect of SAM chain length and binding functions on protein adsorption P-lactoglobulin and apo-transferrin on gold. Colloids Surf. B Biointerfaces 116, 489—496. 

Gel prepared from N-[(2-hydroxy-3-trimethylammonium) propyl] chitosan chloride (HTCC) crosshnked with sodium tripolyphosphate and conjugated with the apo-transferrin These gels showed triggered release of methotrexate disodium (MTX) at low tumor pH. [24]... 

A likely difference in the metabolism of free and chelated sources lies in post-absorptive processes, particularly transport in the bloodstream. The study of the interactions of insulin-enhancing compounds with serum proteins has been almost exclusively studied by EPR. Apo-transferrin and albumin have been implicated in the transport of vanadyl ions in the blood, and these proteins represent a significant metal-binding capability in the blood. Considerable interest in the role of these proteins in the transport and biotransformation of administered vanadium compounds has been evident in the recent literature. 

Chasteen and coworkers conducted extensive studies on the interactions of VOSO4 with apo-transferrin (apo-Tf) and albumin (HSA) by EPR [31,50,74-77]. This body of work underpins all current studies of these proteins with insulinenhancing compounds but will not be reviewed here. 

Figure /. EPR spectra of (top) apo-transferrin, 0.29 mM, vanadyl sulfate, 1.11 mM (middle) apo-transferrin, 0.13 mM, BMOV, 0.11 mM and (bottom) BMOV, L10mM(T==298K,pH7.4, 0.16MNaCl).

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