Degradation pathway and generation of monohydroxamic acids from the trihydroxamate siderophore deferrioxamine B

Abstract

Siderophores are avid ferric ion-chelating molecules that sequester the metal for microbes. Microbes elicit siderophores in numerous and different environments, but the means by which these molecules reenter the carbon and nitrogen cycles is poorly understood. The metabolism of the trihydroxamic acid siderophore deferrioxamine B by a Mesorhizobium loti isolated from soil was investigated. Specifically, the pathway by which the compound is cleaved into its constituent monohydroxamates was examined. High-performance liquid chromatography and mass-spectroscopy analyses demonstrated that M. loti enzyme preparations degraded deferrioxamine B, yielding a mass-to-charge (m/z) 361 dihydroxamic acid intermediate and an m/z 219 monohydroxamate. The dihydroxamic acid was further degraded to yield a second molecule of the m/z 219 monohydroxamate as well as an m/z 161 monohydroxamate. These studies indicate that the dissimilation of deferrioxamine B by M. loti proceeds by a specific, achiral degradation and likely represents the reversal by which hydroxamate siderophores are thought to be synthesized.

FIG. 1.

Hypothesized pathways of DFB degradation to monohydroxamates by the DFB hydrolase preparation. Mass to charge ratios of DFB and the hypothesized intermediates and products would be increased by 1 to represent the additional H⁺ of these ions. Angled arrows represent proposed sites of initial hydrolysis. Compound I, DFB; Compound II, MH₁; Compound III, MH₂; Compound IV, DH.

FIG. 2.

Electrospray mass spectra (positive mode) of DFB after 45 min of degradation by the DFB hydrolase preparation. The sample had been incubated for 45 min prior to being acidified, passed through a 3,000-NMWCO filter, and then injected into the MS.