Wahome and N. The A subunit (StxA) shows limited homology with the A subunit of ricin (RTA), although the two proteins catalyzes the same depurination reaction (Calderwood et al., 1987, Endo et al., 1988, Strockbine et al., 1988). Stx-producing (STEC) strains such as O157:H7, cause gastrointestinal illnesses including bloody diarrhea, hemorrhagic colitis, and life-threatening hemolytic uremic syndrome (HUS). For either ricin or Stx exposure, treatment is strictly supportive; there are currently no specific antidotes against these toxins (Audi et al., 2009; Challoner and McCarron, 1990; Quiones et al., 2009; Serna and Boedeker, 2008). RTA is usually linked via a single disulfide bond to the B subunit (RTB), a galactose-specific lectin that facilitates Walrycin B binding of ricin to host cell surfaces (Baenziger and Fiete, 1979). On binding to cognate cellular glycoprotein and glycolipid receptors, ricin is usually internalized by endocytosis and Walrycin B then trafficked via the retrograde pathway to the Golgi apparatus and the endoplasmic reticulum (ER) (Sandvig and van Deurs, 2000; Sandvig et al., 2002). The toxin is usually processed in the ER, and RTA is usually translocated to the cytoplasm, where a fraction of it escapes degradation by proteosomes and is able to target the host protein biosynthetic machinery (Sandvig and van Deurs, 2000; Sandvig et al., 2002). Walrycin B Stx, following association with its cognate receptor globotriaosylceramide (Gb3), follows a similar intracellular route. Once in the cytoplasm, both StxA and RTA selectively inactivate 28S rRNA (Sandvig and van Deurs, 2000). Ricin’s cytoxicity is due to a combination of protein synthesis Walrycin B arrest and triggering of intracellular stress-activated pathways; the result is the induction of apoptosis, with the release of pro-inflammatory mediators (Gonzalez et al., 2006; Hughes et al., 1996; Yoder et al., 2007). Because all of these effects are initiated following ribosome arrest, the most effective inhibitors of ricin and Stx are likely to be those that directly interfere with the toxins’ active sites. The X-ray structure of RTA was solved to resolutions of 2.8? and 2.5 ? by Montfort et al. (1987) and Rutenber et al. (1991), respectively. Those studies, in combination with site-directed mutagenesis experiments, GPIIIa enabled the identification of the key active site residues, including Glu177, Arg180, Tyr80, Tyr123, and Typ211. Monzingo and Robertus proposed that depurination of adenine entails: Protonation of adenine (N3) by Arg180; delocalization of ring electrons, causing cleavage of C1-N9 glycosidic bond; and generation of an oxacarbenium ion at C1. The latter is usually stabilized by a hydroxide ion that is generated when Glu177 abstracts a proton from a free water molecule in the active site (Monzingo and Robertus, 1992). The authors also reported the crystal structures of RTA bound to two substrate analogues, formycin monophosphate (FMP) and a dinucleotide ApG. The structures of these complexes revealed important ionic and hydrophobic interactions that promote binding of Walrycin B the substrate and its analogues in the active site of RTA (Monzingo and Robertus, 1992). The active site of Stx has key residues that are conserved within the family of ribosome inactivating protein (RIP) and is analogous to the active site of RTA (Fraser et al., 1994; Katzin et al., 1991, Monzingo and Robertus, 1992). There have been numerous attempts to identify active-site inhibitors of RTA, with the long-term goal of using these molecules as therapeutics against both ricin and Stx. Virtual screening (Shoichet, 2004) has recognized substrate analogues and derivatives of pterin, pyrimidine, and guanine as poor to modest RTA inhibitors, with IC50 values ranging from 200 to >2000 M (Bai et al., 2009; Monzingo et al., 1992; Robertus et al., 1996; Yan et al., 1997). For example, pteroic acid (PTA) and 8-methyl-9-oxaguanine were identified using this method and were confirmed by kinetic measurements to be modest inhibitors of RTA, with respective IC50 values of 0.6 and 0.4 mM (Miller et al.,.