Radiochemical Studies of Iron Binding and Stability in Ferrimycobactin S

Abstract

Measurements of iron uptake, ionic dissociation and radiolytic stability of the mycobacterial iron complex ferrimycobactin S (FeMy) were carried out using ''Fe as a radioactive tracer. The effect of gamma irradiation was studied using labeled FeMy prepared by reaction of mycobactin S with s 'FeCl 3 . The major effect of radiation is release of iron from FeMy (21% loss at 4.4 MGy), probably due mainly to reduction of Fe (III) by radiationgenerated electrons. Transfer of iron to and from mycobactin S in chloroform solutions was followed by extraction procedures using acidic aqueous phases, under either isohydric or isochronal conditions. The basic mechanism of iron uptake and release is shown to be governed by the competition of the Fe 3+ ions with the acidic protons in combining with the mycobactin complexation sites. For an overall description of the data, however, it is necessary to invoke an additional process based on the association of Fe 3+ with the CI" counterions to yield neutral FeCl 3 molecules which inhibit incorporation of Fe into mycobactin S.

Iron Acquisition and Metabolism by Mycobacteria

“…Although a formation constant of 10 30 M Ϫ1 is widely quoted, the only direct measurements have monitored the dissociation of 59 Fe-labeled ferri-MB S under strongly acidic conditions. From this data, one can extrapolate a formation constant of approximately 4 ϫ 10 26 M Ϫ1 at pH 7 (41). Coordination to ferrous iron by MB is much less effective, allowing release of iron from a ferri-MB complex by reduction.…”

Characterization of Fe(iii) sequestration by an analog of the cytotoxic siderophore brasilibactin A: Implications for the iron transport mechanism in mycobacteria

“…The stability constant for the Fe(III)-Bbtan complex (log b 110 = 26.96) compares favorably to the stability constant for the Fe(III) complex of mycobactin S, log b 110 = 25.5, which is structurally similar to Bbtan. 10 The small difference in determined stability constant may arise from the difference in techniques used. Due to the low solubility of mycobactin S in aqueous solution, organic-aqueous extraction techniques with 59 Fe were used over a range of acid concentrations to determine the percent iron uptake, which was extrapolated to determine the mycobactin Fe(III) affinity.…”

“…It is also important to note that the type of mycobactin used by one species of mycobacterium can be lethal to other mycobacteria. 9 As of this writing, the only mycobactin siderophore for which its Fe(III) affinity has been characterized is mycobactin S. 10 Brasilibactin A is a siderophore produced by the actinomycete, Nocardia brasiliensis, that features a mycobactin backbone tethered to the cell by a long carbon chain (Fig. 1, top).…”