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Figure 11<br />

Two-dimensional structure of SPADNS, a<br />

micromolar inhibitor of T.brucei PGK.<br />

Page 384<br />

discussed in the literature. Suramin inhibits glycosomal PGK of T. brucei with a K i of 8.0 μM [69]. In<br />

addition a number of yeast PGK inhibitors are known: gallic acid with a K i = 0.4 mM [86],<br />

hydroxyethylidene biphosphate with a K i = 24 mM [87]. 1,3,6-naphthalenetrisulphonic acid with a K i =<br />

5.5 mM, and 2-(p- sulphophenylazo)-1,8-dihydroxy-3,6-disulphonic acid, also known as SPADNS<br />

(Figure 11), with a K i = 126 μM [87]. None of the four yeast PGK inhibitors are potent, but, for<br />

SPADNS, the binding mode has been further characterized. Studies <strong>by</strong> Williams et al. [87] have<br />

demonstrated that SPADNS is directly competitive with both enzyme substrates, 3-phosphoglycerate<br />

and ATP. Moreover, <strong>by</strong> 600 MHz 1H-NMR it was shown that SPADNS interacts with the nucleotide<br />

binding site while the conformation of the enzyme changes substantially [87].<br />

Since the four yeast PGK inhibitors are commercially available it was logical to test them for T. brucei<br />

PGK inhibition. The first three compounds were active in the millimolar range. However, SPADNS<br />

exhibited a K i of 10.0 μM in these in preliminary tests [88]. Moreover, when assayed against a<br />

commercially available rabbit muscle PGK, SPADNS had no influence on the enzyme kinetics up to a<br />

concentration of 250 μM [88]. In conclusion, SPADNS appears to be an excellent lead because of its<br />

potency and selectivity. Crystallographic experiments to determine its binding mode to T. brucei PGK<br />

are underway.<br />

IV. Lead Optimization: Glycosomal Gapdh<br />

From the selectivity point of view the adenosine binding site of GAPDH is attractive for drug design, as<br />

we explained in Section II.B. Unfortunately, inhibition studies on T. brucei and L. mexicana GAPDH<br />

revealed the poor affinity of our natural lead adenosine with IC 50 values of 100 mM and 50 mM,<br />

respectively. Moreover, adenosine is an “antiselective” lead because the IC 50<br />

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