| Abstract: | Dihydroorotase (4,5-L-dihydroorotate amidohydrolase (EC 3.5.2.3], which catalyzes the reversible cyclization of N-carbamyl-L-aspartate to dihydro-L-orotate, has been purified to homogeneity from an over-producing strain of Escherichia coli. Treatment of 70 g of frozen cell paste produces about 7 mg of pure enzyme, a yield of about 35%. The native molecular weight, determined by equilibrium sedimentation, is 80,900 +/- 4,300. The subunit molecular weight, determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis is 38,400 +/- 2,600, and by amino acid analysis is 41,000. The enzyme is thus a dimer and contains 0.95 +/- 0.08 tightly bound zinc atoms per subunit when isolated by the described procedure, which would remove any loosely bound metal ions. Isoelectric focusing under native conditions yields a major species at isoelectric point 4.97 +/- 0.27 and a minor species at 5.26 +/- 0.27; dihydroorotase activity is proportionately associated with both bands. The enzyme has a partial specific volume of 0.737 ml/g calculated from the amino acid composition and a specific absorption at 278 nm of 0.638 for a 1 mg/ml solution. At 30 degrees C, the Michaelis constant and kcat for dihydro-DL-orotate (at pH 8.0) are 0.0756 mM and 127 s-1, respectively; for N-carbamyl-DL-aspartate (at pH 5.80), they are 1.07 mM and 195 s-1. |
