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

^a Department of Microbiology, University of Mississippi School of Medicine, Jackson, Mississippi 39216

ABSTRACT

The fate of the hydroxamic acid-iron transport cofactors during iron uptake from the ⁵⁹Fe³⁺ chelates of the ³H-labeled hydroxamates schizokinen and aerobactin was studied by assay of simultaneous incorporation of both ⁵⁹Fe³⁺ and ³H. In the schizokinen-producing organism Bacillus megaterium ATCC 19213 transport of ⁵⁹Fe³⁺ from the ³H-schizokinen-⁵⁹Fe³⁺ chelate at 37 C was accompanied by rapid uptake and release (within 2 min) of ³H-schizokinen, although ³H-schizokinen discharge was temperature-dependent and did not occur at 0 C. In the schizokinen-requiring strain B. megaterium SK11 similar release of ³H-schizokinen occurred only at elevated concentrations of the double-labeled chelate; at lower chelate concentrations, ³H-schizokinen remained cell-associated. Temperature-dependent uptake of deferri (iron-free) ³H-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 ³H-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 ³H-aerobactin during iron transport from ³H-aerobactin-⁵⁹Fe³⁺, suggesting a similar ferric hydroxamate transport system in this organism.

¹ Present address: Department of Microbiology, St. Louis University School of Medicine, St. Louis, Mo.

² Present address: Department of Biochemistry, University of Alberta School of Medicine, Edmonton, Alberta, Canada.

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