Purification and characterization of dihydroorotase from Pseudomonas putida

Dihydroorotase was purified to homogeneity from Pseudomonas putida. The relative molecular mass of the native enzyme was 82 kDa and the enzyme consisted of two identical subunits with a relative molecular mass of 41 kDa. The enzyme only hydrolyzed dihydro-L-orotate and its methyl ester, and the reactions were reversible. The apparent Km and Vmax values for dihydro-L-orotate hydrolysis (at pH 7.4) were 0.081 mM and 18 mumol min-1 mg-1, respectively; and those for N-carbamoyl-DL-aspartate (at pH 6.0) were 2.2 mM and 68 mumol min-1 mg-1, respectively. The enzyme was inhibited by metal ion chelators and activated by Zn2+. However, excessive Zn2+ was inhibitory. The enzyme was inhibited by sulfhydryl reagents, and competitively inhibited by N-carbamoylamino acids such as N-carbamoylglycine, with a Ki value of 2.7 mM. The enzyme was also inhibited non-competitively by pyrimidine-metabolism intermediates such as dihydrouracil and orotate, with a Ki value of 3.4 and 0.75 mM, respectively, suggesting that the enzyme activity is regulated by pyrimidine-metabolism intermediates and that dihydroorotase plays a role in the control of pyrimidine biosynthesis.