BACKGROUND AND PURPOSE: Based on their ability to chelate metals, hydroxamate 
molecules and siderophores have been successfully used as metalloenzyme 
inhibitors. As the anthrax toxin lethal factor (LF) is a zinc 
(Zn)-metallopeptidase, an investigation of the ability of some small 
non-siderophore hydroxamate compounds, 5 hydroxamate-containing siderophores, 
and 1 catecholate siderophore was undertaken to determine whether these 
compounds would inhibit LF. In addition, salmon sperm protamine and 
ethylenediaminetetraacetic acid were investigated.
METHODS: A spectrophotometric assay of LF activity, based on its reaction with 
the substrate 
(Ac-gly-tyr-betaala-arg-arg-arg-arg-arg-arg-arg-arg-val-leu-arg-p-nitroanilide), 
was used to assess the degree of inhibition of LF by the putative inhibitors. 
Procedures were implemented to avoid iron contamination of the test solutions 
and non-ferrated siderophores and hydroxamates were used as potential 
inhibitors.
RESULTS: The hydroxamate-containing siderophores displayed limited capacities to 
inhibit LF, as did the low molecular weight hydroxamate compounds. In contrast, 
the catecholate siderophore enterobactin and the cationic polyamine salmon sperm 
protamine demonstrated notable inhibition of LF at concentrations ranging from 
approximately 10 to 200 microM.
CONCLUSIONS: The polyamine salmon sperm protamine which mimics the target site 
of proteins cleaved by LF, was the most effective inhibitor of the molecules 
examined, while the small molecule hydroxamates and the hydroxamate siderophores 
were among the poorest. If chelation of the Zn of LF results in LF inhibition by 
the molecules examined, it is most likely secondary to binding of the putative 
inhibitors to the active site of LF.