Exchange of iron by gallium in siderophores

Abstract

Siderophores are iron transport compounds produced by numerous microorganisms and which strongly chelate Fe(III), but not Fe(II). Other trivalent metals, such as Al(III), Cr(III), or Ga(III), are not capable of significantly displacing iron from siderophores. However, I demonstrate here that Ga(III) can effectively displace iron under reducing conditions. With ascorbate as reductant and ferrozine as Fe(II) trapping agent, the kinetics of reductive displacement of iron by Ga(III) were followed spectroscopically by the increase of absorbance at 562 nm due to formation of the Fe(II)-ferrozine complex. No significant reduction of siderophore occurred in the absence of Ga(III). With excess Ga(III), the displacement was quantitative and very rapid. The rate of metal exchange was pseudo first order with respect to Ga(III) concentration and highly pH dependent, suggesting that siderophore ligands are displaced from the iron in a concerted mechanism by Ga(III) and protonation to expose the Fe(III) to reduction by ascorbate. Reaction rates were dependent upon the structure of the siderophore, being greatest for ferric rhodotorulic acid and slowest for ferrichrome A at pH 5.4. The pH profile for ferric rhodotorulic acid was unusual in that it showed a maximum at pH 6.5, while all other siderophores examined showed an increase in rate as pH was lowered from 7.0. The physiological significance of this reaction to the clinical use of gallium is discussed.

Reductive and Non-reductive Mechanisms of Iron Assimilation by the Yeast Saccharomyces cerevisiae

“…Emery & Hoffer (1980) showed that gallium analogues of siderophores could be taken up by micro-organisms in an active transport process indistinguishable from that of ferri-siderophores. In addition, it has been demonstrated that Ga3+ can displace iron from siderophores under reducing conditions (Emery, 1986). This was confirmed for the reduction of FOB by S. cerevisiae: the rate of formation of the Fe (I1) 1 mwferrozine and 5% (w/v) glucose was increased by more than 50% by the addition of 500 p~-G a ( N 0~)~.…”

“…In contrast, Ga3+ had little or no effect. Emery (1986) showed that citrate could inhibit the reductive exchange reaction between Ga3+ and FOB, and that acidic conditions were required for optimum reaction rate. Citrate buffer, pH 6.5, was therefore replaced by acetate buffer, pH 5.2, for studying the effects of Ga3+ and Ga(II1)-DFOB on FOB uptake rate as a function of FOB concentration.…”

Siderophores for molecular imaging applications

“…Gallium is an isosteric diamagnetic substitute for Fe(III) [39] and, thus, the affinity constants of many siderophores for gallium are in the range of their iron counterparts. At that time, it was also demonstrated that under reducing conditions, Ga(III) can rapidly displace Fe(III) from siderophores, whereas without concerted reduction of the iron no significant exchange was observed [40]. Emery and Hoffer [41] have used 67 Ga to study the uptake mechanisms for different siderophores in Ustilago sphaerogena and found this energy-dependent process to be indistinguishable from that of its Fe(III) counterpart.…”

Siderophore uptake and use by the yeast Saccharomyces cerevisiae

“…Genetic studies are also not simple : a mutation that affects some step of nonreductive iron assimilation in S. cerevisiae will be difficult to detect because iron can still enter the cells by the reductive mechanism. Emery (1986) used gallium analogues of siderophores, which cannot be reduced, to distinguish between reductive and non-reductive uptake of siderophores by U. sphaerogena. We used mutants of S. cerevisiae in which the FET3 and FET4 genes, encoding components of the high-affinity and low-affinity reductive uptake systems, respectively (Askwith et al, 1994 ;Dix et al, 1994), had been deleted.…”