Thermodynamic study of dihydrofolate reductase inhibitor selectivity

The thermodynamic parameters of the binding of some folate analogues (methotrexate, trimetrexate and trimethoprim) to dihydrofolate reductases from different species have been measured with a flow microcalorimetric method at 37 degrees C. In the absence of NADPH, the three inhibitors exhibited a higher affinity for E. coli DHFR than for vertebrate DHFRs. This selectivity in favor of bacterial DHFR is entropy driven and is correlated with a weaker conformational change for bacterial DHFR than for vertebrate DHFRs, and with additional hydrophobic contacts, provided by this enzyme to the ligands. In presence of NADPH, as reported in the literature, trimetoprim shows a high selectivity in favor of bacterial DHFR, contrarily to methotrexate and trimetrexate, whose affinities are elevated and highly similar for mammalian and bacterial enzymes. The positive cooperative effect of NADPH, which has an enthalpic origin, fluctuates widely with inhibitor structure and with enzyme species. For trimethoprim, the cooperative effect is much more pronounced for bacterial DHFR than for vertebrate DHFRs. But the role of NADPH is not to induce a selectivity: it only increases the selectivity that trimethoprim already presented in absence of NADPH. Inversely, for methotrexate and trimetrexate, the cooperative effect is stronger for vertebrate enzymes than for the bacterial enzyme, and thus, NADPH cancels the selectivity the two antifolic compounds had, in the absence of NADPH, for the bacterial enzyme.