The novel anticancer drug candidate brequinar sodium (DuP 785, NSC 368390, 
6-fluoro-2-(2'-fluoro-1,1'-biphenyl-4-yl)-3-methyl-4-quinoline- carboxylic acid 
sodium salt) was shown previously to be an inhibitor of dihydroorotate 
dehydrogenase, the fourth enzyme of the de novo pyrimidine biosynthetic pathway. 
Brequinar sodium inhibits the activity of this enzyme isolated from mammalian 
sources only but not those forms isolated from yeast or bacteria, which also use 
ubiquinone as the cofactor. Brequinar sodium also does not inhibit the activity 
of a soluble Zymobacterium oroticum dihydroorotate dehydrogenase which uses NAD+ 
as a cofactor. Brequinar sodium inhibits L1210 dihydroorotate dehydrogenase with 
mixed inhibition kinetics with respect to either the substrate (dihydroorotate) 
or the cofactor (ubiquinone Q6) with Ki' values in the 5-8 nM range. Our results 
suggest that brequinar sodium inhibits dihydroorotate dehydrogenase by binding 
to the enzyme at a unique site that is distinct from the dihydroorotate or the 
ubiquinone-binding site. This binding site appears to be unique to the mammalian 
enzyme, because brequinar sodium does not inhibit the yeast, Escherichia coli, 
or Z. oroticum forms of the enzyme.