Novel fungicide quinofumelin shows selectivity for fungal dihydroorotate dehydrogenase over the corresponding human enzyme

The species selectivity of class 2 dihydroorotate dehydrogenase (DHODH), a target enzyme for quinofumelin, was examined. The Homo sapiens DHODH (HsDHODH) assay system was developed to compare the selectivity of quinofumelin for fungi with that for mammals. The IC(50) values of quinofumelin for Pyricularia oryzae DHODH (PoDHODH) and HsDHODH were 2.8 nM and >100 µM, respectively. Quinofumelin was highly selective for fungal over human DHODH. Additionally, we constructed recombinant P. oryzae mutants where PoDHODH (PoPYR4) or HsDHODH was inserted into the PoPYR4 disruption mutant. At quinofumelin concentration of 0.01–1 ppm, the PoPYR4 insertion mutants could not grow, but the HsDHODH gene-insertion mutants thrived. This indicates that HsDHODH is a substitute for PoDHODH, and quinofumelin could not inhibit HsDHODH as in the HsDHODH enzyme assay. Comparing the amino acid sequences of human and fungal DHODHs indicates that the significant difference at the ubiquinone-binding site contributes to the species selectivity of quinofumelin.