Comparison of iron binding and uptake from FeCl3 and Fe-citrated by Neisseria meningitidis

Iron-starve Neisseria meningitidis SD1C took up Fe added as FeCl3 by a two-step process: a rapid phase occurring during the 1st min after Fe addition and unaffected by 0.5 mM KCN and a slower secondary phase of uptake sensitive to various metabolic inhibitors. The rate and extent of energy-dependent Fe uptake from stage dicitrate and nitrilotriacetate Fe3+ complexes were essentially identical to FeCl3, indicating the presence of a high-affinity Fe acquisition system. The sulfonated siderophore Desferal (deferrioxamine beta-mesylate) effectively prevented any uptake of Fe from the citrate complex and was unable to remove that Fe already bound by either poisoned or active cells. The energy-independent system rapidly deferrated Fe3+ dicitrate and bound Fe to a finite number of cellular sites. Fe derived from FeCl3 was associated with these same sites as well as with a large number of apparently nonspecific sites. The extent of low-affinity, nonspecific binding was concentration dependent. Both energy-independent and energy-dependent systems involved in the uptake of Fe from the Fe3+ dicitrate were inactivated by 5 min at 60 degrees C, but not by 45 degrees C. The citrate carrier itself was recycled, being neither bound separately nor in concert with Fe3+ by these sites.