Ferrimycobactin reductase activity from grown under iron-deficient conditions had a for ferrimycobactin of less than 4 μm and a for NADH of 1.75 mM. Salicylate (0.12 mM), which is synthesized by this bacterium, could substitute for EDTA as an acceptor of Fe in the assay system. Reagents which react with thiol groups (HgCl, -ethylmaleimide) at 0.1 mM inhibited activity by about 40%; other inhibitors (KCN, NaN, carbonyl cyanide -chlorophenylhydrazone and 2,4-dinitrophenol) were less effective (though these inhibited active iron transport, which uses exochelin and not mycobactin; Stephenson & Ratledge, 1979). The rate of ferrimycobactin reduction in extracts was about ten times faster than the rate observed following a shift-up of low-iron cells to high-iron status. Reduction of iron in ferri-ferrioxamine B, ferriexochelin MS and ferric ammonium citrate occurred at comparable rates to ferrimycobactin reduction.

Ferrimycobactin reductase activity was undiminished in grown under iron-sufficient conditions. A similar activity was found in extracts of and where it still required NADH. Yeast alcohol dehydrogenase reduced ferrimycobactin independently of NADH; this activity was attributed to the free thiol groups of this protein. Reductase activity therefore may be associated with a protein whose principal function need not be that of a siderophore reductase.


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