Fate of Labeled Hydroxamates During Iron Transport from Hydroxamate-Iron Chelates

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Abstract
The fate of the hydroxamic acid-iron transport cofactors during iron uptake from the 59 Fe 3+ chelates of the 3 H-labeled hydroxamates schizokinen and aerobactin was studied by assay of simultaneous incorporation of both 59 Fe 3+ and 3 H. In the schizokinen-producing organism Bacillus megaterium ATCC 19213 transport of 59 Fe 3+ from the 3 H-schizokinen- 59 Fe 3+ chelate at 37 C was accompanied by rapid uptake and release (within 2 min) of 3 H-schizokinen, although 3 H-schizokinen discharge was temperature-dependent and did not occur at 0 C. In the schizokinen-requiring strain B. megaterium SK11 similar release of 3 H-schizokinen occurred only at elevated concentrations of the double-labeled chelate; at lower chelate concentrations, 3 H-schizokinen remained cell-associated. Temperature-dependent uptake of deferri (iron-free) 3 H-schizokinen to levels equivalent to those incorporated from the chelate form was noted in strain SK11, but strain ATCC 19213 showed only temperature-independent binding of low concentrations of deferri 3 H-schizokinen. These results indicate an initial temperature-independent binding of the ferric hydroxamate which is followed rapidly by temperature-dependent transport of the chelate into the cell and an enzyme catalyzed separation of iron from the chelate. The resulting deferri hydroxamate is discharged from the cell only when a characteristic intracellular concentration of the hydroxamate is exceeded, which happens in the schizokinen-requiring strain only at elevated concentrations of the chelate. This strain also appears to draw the deferri hydroxamate into the cell by a temperature-dependent mechanism. The aerobactin-producing organism Aerobacter aerogenes 62-1 also demonstrated rapid initial uptake and temperature-dependent discharge of 3 H-aerobactin during iron transport from 3 H-aerobactin- 59 Fe 3+ , suggesting a similar ferric hydroxamate transport system in this organism.