Drug selectivity is determined by coupling across the NAD(+) site of IMP dehydrogenase

Drug resistance often results from mutations that are located far from the drug-binding site. The effects of these mutations are perplexing. The inhib ition of IMPDH by MPA is an example of this phenomenon. Mycophenolic acid ( MPA) is a species-specific inhibitor of IMPDH; mammalian IMPDHs are very se nsitive to MPA, while the microbial enzymes are resistant to the inhibitor. MPA traps the covalent intermediate E-XMP* and binds in the nicotinamide h alf of the dinucleotide site. Previous results indicated that about half of the difference in sensitivity derives from residues in the MPA-binding sit e [Digits, J. A., and Hedstrom, L. (1999) Biochemistry 38, 15388-15397]. Th e remainder must be attributed to regions outside the MPA-binding site. The adenosine subsite of the NAD(+) site is not conserved among IMPDHs and is, therefore, a likely candidate. Our goal is to examine the coupling between the nicotinamide and adenosine sites in order to test this hypothesis. We performed multiple inhibitor experiments with the Tritrichomonas foetus and human type 2 IMPDHs using tiazofurin and ADP, which bind in the nicotinami de and adenosine subsites, respectively. For T. foetus IMPDH, tiazofurin an d ADP are extraordinarily synergistic. In contrast, these inhibitors are vi rtually independent for the human type 2 enzyme. We suggest that the differ ence in coupling of the nicotinamide and adenosine subsites accounts for th e remaining difference in MPA affinity between T. foetus and human IMPDH.