Iron lactoferrin

Lactoferrin is a glycoprotein found in mammalian milk that tightly binds two ferric ions producing an iron complex more physically and chemically stable than the uncomplexed protein. Bovine lactoferrin inhibited oxidation in com oil-in-water emulsions and lecithin liposome systems (Table 10.8). At the same molar concentration, lactoferrin was less effective than EDTA in inhibiting hydroperoxide formation in a com oil emulsion. This lower antioxidant activity of lactoferrin may be explained by its partial iron saturation and lower affinity for ferric ions. The formation constant for ferric-EDTA is 1.3 x 10 compared to 10 ° for the ferric-lactoferrin complex. Lactoferrin was a better iron chelator in the liposome than in the emulsion systems. Inhibition in liposomes with iron-lactoferrin mixtures was in the order 1 2 > 1 1 > 2 1. This order suggested that lactoferrin also chelated metal impurities as well as added iron to inhibit lipid oxidation. Lactoferrin did not inhibit the copper-catalysed... 

Lactoferrin j Iron-binding protein May inhibit growth of certain bacteria by binding iron and may be involved in regulation of proliferation of myeloid cells... 

Gutteridge, J.M.C. (1987). Bleomycin-detectable iron in knee-joint synovial fluid from arthritic patients and its relationship to the extracellular activities of caeruloplasmin, transferrin and lactoferrin. Biochem. J. 245, 415-421. 

Monteiro, H.P. and Winterbourne, C.C. (1988). The superoxide-dependent transfer of iron from ferritin to transferrin and lactoferrin. Biochem. J. 256, 923-928. 

Aruoma, 0.1. and Halliwell, B. (1987). Superoxide-dependent and ascorbate-dependent formation of hydroxyl radicals from hydrogen peroxide in the presence of iron. Are lactoferrin and transferrin promoters of hydroxyl radical generation. Biochem. J. 241, 273-278. 

Iron is, as part of several proteins, such as hemoglobin, essential for vertebrates. The element is not available as ion but mostly as the protein ligands transferrin (transport), lactoferrin (milk), and ferritin (storage), and cytochromes (electron transport) (Alexander 1994). Toxicity due to excessive iron absorption caused by genetic abnormalities exists. For the determination of serum Fe a spectrophoto-metric reference procedure exists. Urine Fe can be determined by graphite furnace (GF)-AAS, and tissue iron by GF-AAS and SS-AAS (Alexander 1994 Herber 1994a). Total Iron Binding Capacity is determined by fuUy saturated transferrin with Fe(III), but is nowadays mostly replaced by immunochemical determination of transferrin and ferritin. 

Iron transport agents may belong to the protein or non-protein class. In the former group are found the animal proteins transferrin (25), lactoferrin (26) and conalbumin (27). The low molecular weight iron carrying compounds from microorganisms, the siderochromes, may occur with or without a bound metal ion. Typically, severe repression of biosynthesis of these substances can be expected to set in at an iron concentration of ca. 2 x 10-5 g atoms/liter (28). Most, but not all, of these substances can be described as phenolates or hydroxamates (4). 

FhuA and FepA will prove to be the reference structures for a large group of bacterial outer-membrane transporters that take up bacterial Fe3+-siderophores, Fe3+ released from host transferrin and lactoferrin, haem, and haem released from haemoglobin and haemopexin. It is assumed that all iron sources are transported... 

The majority of Fur-regulated gene products are involved in iron uptake. Genes for transport and biosynthesis of enterobactin have been studied in E. coli K-12 (Earhart, 1996). It is assumed that this system is found in nearly every E. coli strain. Also the ferrichrome transport system seems to have a very broad distribution. The ferric citrate transport system (fee), however, is only present in some E. coli strains and may be part of a pathogenicity island. The aerobactin and yersiniabactin biosynthesis and transport systems are not found in all E. coli strains and are integrated into pathogenicity islands (Schubert et al., 1999). The ability to utilize haem seems also to be a specific pathogenicity-related adaptation. Haem transport systems are used in the animal or human host, where transferrin and lactoferrin create an iron-poor environment for bacteria. 

Figure 5.1 Schematic diagram of the lactoferrin molecule. The positions of carbohydrate attachment are marked with a star. O, ovotransferrin T, human serotransferrin L, human lactoferrin R, rabbit serotransferrin M, melanotransferrin A, the connecting helix B, the C-terminal helix. The disulfide bridges are indicated by heavy bars, and the iron and carbonate binding sites by filled or open circles, respectively. Reprinted from Baker et al., 1987. Copyright (1987), with permission from Elsevier Science.

The iron-binding sites have been characterized by crystallographic studies on several transferrins, and in Figure 5.7 (Plate 7) that of the N-lobe of human lactoferrin is presented. The 3+ charge on the ferric ion is matched by the three anionic ligands Asp-63, Tyr-95 and Tyr-188 (the fourth, His-249, is neutral), while the charge on the carbonate anion is almost matched by the positive charge on Arg-124 and the... 

Transferrin iron uptake via receptor-mediated endocytosis has clearly appeared fairly late in evolution, when we consider that the bilobal iron-binding protein is found only as far back as insects . As we have seen in the preceding chapters, iron-uptake mechanisms involving the synthesis of more or less specific siderophores have evolved together with strategies implying the solubilization of insoluble ferric iron by the combined effects of pH and reduction, and even the development of receptor proteins capable of taking up transferrin-, lactoferrin- or haem-bound iron from specific hosts. 

The Gram-positive bacterium Streptococcus pneumoniae is an important cause of respiratory tract infections, bacteremia, and meningitis. In this strain, the cell wall anchored pneumococcal surface protein A (PspA) has been demonstrated to bind lactoferrin [181]. PspA and closely related proteins in a variety of pneumococcal isolates are most likely involved in the sequestration of iron from lactoferrins, and finally contribute to the virulence of these bacteria. However, the means by which the pneumococcus acquires iron at the mucosal surface during invasive infection is not well understood at the molecular level [182],... 

So called because they are found in milk, where the iron-binding protein lactoferrin sequesters iron so tightly... 

---