Properties of Purified Quinonoid Dihydropterin Reductase

Quinonoid dihydropterin reductase has been purified to homogeneity from sheep liver, sheep brain, and beef adrenal medulla. Each of these enzymes has a molecular weight of about 45 000–55 000, and is composed of two subunits of half that weight. The subunits of the sheep liver reductase have identical charge, size, and N-terminal amino acid residue. The reductase exists in solution over a wide range of concentrations as the dimer. A dimer covalently linked by dimethylsuberimidate retains full activity. A number of kinetic properties of quinonoid dihydropterin reductase, including inhibition by thiol reagents and by pterin analogues, are reported.