Kinetics of iron complexing and metal exchange in solutions by rhizoferrin, a fungal siderophore

Rhizoferrin, a siderophore produced by Rhizopus arrhizus, has been shown in previous studies to be an outstanding Fe carrier to plants. Yet, calculati ons based on stability constants and thermodynamic equilibrium lead to cont radicting conclusions. In this study a kinetic approach was employed to elu cidate apparent contradictions and to determine the behavior of rhizoferrin under conditions representing soil and nutrient solutions. Stability of Fe 3+ complexes in nutrient solution, rate of metal exchange with Ca, and rate of Fe extraction by the free ligand were monitored for rhizoferrin and oth er chelating agents by Fe-55 labeling. Ferric complexes of rhizoferrin, des ferri-ferrioxamine-B (DFOB) and ethylenediamine-di(o-hydroxyphenylacetic ac id) (EDDHA) were found to be stable in nutrient solution at pH 7.5 for 31 d , while ferric complexes of ethylenediaminetetraacetic acid (EDTA) and mugi neic acid (MA) lost 50% of the chelated Fe within 2 d. Iron-calcium exchang e in Ca solutions at pH 8.7 revealed rhizoferrin to hold Pe at nonequilibri um state for 3 to 4 wk at 3.3 mM Ca and for longer periods at lower Ca conc entrations. Ethylenediaminetetraacetic acid lost the ferric ion at a faster rate under the same conditions. Iron extraction from freshly prepared Fe h ydroxide at pH 8.7 and with 3.2 mM Ca was slow and followed the order. DFOB > EDDHA > MA greater than or equal to rhizoferrin > EDTA. Based on these r esults we suggest that a kinetic rather than equilibrium approach should be the basis for predictions of Fe chelates' efficiency. We conclude that the nonequilibrium state of rhizoferrin is of crucial importance for its behav ior as an Fe carrier to plants.