Iron binding to Azotobacter salinestris melanin, iron mobilization and uptake mediated by siderophores

Department of Microbiology, University of Alberta, Edmonton, Canada.
BioMetals (Impact Factor: 2.5). 02/1995; 8(1):59-64. DOI: 10.1007/BF00156159
Source: PubMed


Iron-sufficient Azotobacter salinestris cells bound large amounts of 55Fe to cell-associated catechol melanin in an energy-independent manner. Iron was mobilized from the cell surface by citric acid and transported into the cell in a process that was inhibited by azide, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), KCl or RbCl, the latter two known to inhibit Na(+)-dependent activities in A. salinestris. Iron-limited cells produced a hydroxamate compound (HDX) which promoted 55Fe-uptake into iron-limited cells in a two step process. Initial uptake was inhibited by azide or CCCP, but not by KCl, while subsequent uptake was blocked by all inhibitors. Citric acid also mediated energy-dependent 55Fe-uptake in iron-limited cells, but initial iron-uptake was less sensitive to CCCP than HDX-mediated iron-uptake. The results show that melanin serves as an iron trap, probably to protect the cells from oxidative damage mediated by H2O2 and the Fenton reaction. A model for HDX siderophore-mediated iron-uptake is proposed which requires energy to concentrate iron in the periplasm and H+/Na(+)-dependent events to bring iron into the cell.

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    • "The contribution of melanin to cell wall charge has been studied in Cryptococcus neoformans (Nosanchuk and Casadevall 1997). An investigation of iron binding by the melanin of a bacterium , Azotobacter salinestris (Page and Shivprasad 1995), may provide hints of a similar function for fungal melanins. These authors proposed that the bacterial melanin acts as an iron trap to protect cells from oxidative damage by hydrogen peroxide generated by iron-mediated Fenton reactions. "
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