Iron chelators naturally occurring

Lebedev, A.V., Ivanova, M.V., and Levitsky, D.O., Echinochrome, a naturally occurring iron chelator and free radical scavenger in artificial and natural membrane systems. Life Sci., 76, 863, 2005. 

Most of the naturally occurring chelating agents are substituted hydroxamates which are produced by a variety of protista so that iron(III) subsequently becomes available for biochemical processes. Neilands (73) has suggested that the hydroxamates facilitate the transport of iron across cell membranes. The distribution of hydroxamates in the biosphere appears limited. However, if there was a wider distribution of hydroxamates in the environment then the management of actinide wastes could become a problem of horrifying dimensions if these chelators facilitated the transport of actinides across cell membranes. 

Bleomycin is a naturally occurring fermentation product of Streptomyces verticillus. It is a basic glycoprotein, complexed with Cu++. It intercalates between DNA base pairs, and it also chelates iron, generating oxygen radicals which further damage the DNA. It is the only cell-cycle specific agent among the antibiotics as it causes accumulation of cells in the G2 phase of the cell cycle. 

Desferrioxamine B (Desferal), a chelating agent that occurs naturally in certain microorganisms, has a very high affinity for ferric ion but only a relatively weak affinity for most other cations. Under physiological conditions it can complex free iron and competitively remove iron from ferritin and haemosiderin. It also binds,... 

A comparison of the stability constants of the naturally occurring siderophores uncovers a difference of f7 orders of magnitude between enterobactin (K most stable hydroxamate complex, ferrioxamine E. Using the more comparable pM values, enterobactin remains stiU eight orders of magnitude more effective than ferrioxamine E. Enterobactin has the highest affinity for Fe ion of any biological iron chelator tested so far. 

The chelate effect of DFB causes the formation constant to be greater than that of the monomeric acetohydroxamic acid by two orders of magnitude. Table 1. Other naturally occurring iron chelators, called siderochromes, include various modifications of DFB, ferrichrome (II), rhodotorulic acid (III) and enter-obactin (IV), all of which possess high formation constants... 

The catalytic effect of iron was also shown on the thermal regeneration of granular activated carbon, with an increase of mass loss from 4.2 to 5.7% in the presence of chelated Fe(III) but without an alteration of the pore structure. However, this catalytic effect of iron is no longer significant in the presence of sulfur at concentrations (0.01-0.03%) naturally occurring in granular activated carbon [42],... 

Again, these ferric complexes are very stable (/>K 28) as are the natural iron chelating compounds and they bind ferrous iron relatively weakly (50). In an acid medium (pH 2—6) the hydroxamic acid forms a deep purple 1 1 complex with ferric iron while in neutral or basic solution a brown-red 3 1 complex is formed in which the hydroxamates act as three bidentate ligands which occupy the six octahedral positions about the iron. The 1 1 purple complex has an absorption maximum at 5000—5200 A depending on the nature of the hydroxamic acid ligand. The 3 1 red-brown complex (it becomes yellow-orange on dilution) has an absorption maximum usually between 4250—4400 A with many of the same spectral features as the naturally occurring complexes of ferric iron. 

Takagi, S., 1976. Naturally occurring iron-chelating compounds in oat- and rice-root washings, i. Activity measurement and preliminary characterization. Soil Sci. Plant Nutr. 22, 423 33. 

Fig. 1. Some naturally occurring iron chelators. Structures are numbered for reference in the text. I, desferrioxamine B II, enterobactin VI, rhodotorulic acid VIII, desferrithiocin IX, maltol...

With repect to metal chelates, the use of naturally occurring iron-containing antibiotics (the siderophores) has been known for some time. The structures of many of these compounds and the mechanisms by which they may act have been reviewed recently [34]. In principle, iron depletion... 

Studied for their antibacterial activity and in this case a metal-sequestering (iron) mechanism is indicated, analogous to naturally-occurring antibiotics. Other chelating agents such as 8-hydroxyquinoline may chelate iron and activate oxygen as a mode of action. 

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